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Perspectives in Clinical Diabetes, Obesity, and Atherosclerosis: Type 2 Diabetes as "Glucose Insensitivity"

The "Stuffed-Cell" (c) Hypothesis, the "Shifted Glucose-ATP/ ATP-Insulin Dose Response Curves" Hypothesis, and the "Gluconeogenic Suppression Fallacy" - Putative Relationships to Genetically Derived -Cell and Target Organ Susceptibilities to Glucose Toxicity



Ronald J. Innerfield, M.D.-

-- Knowledge cannot spring from experience alone, but only from the comparisons of the inventions of the intellect with the observed fact. -Albert Einstein

After basking for some time in the intense light of medical knowledge, our serpentine concepts of disease tend to exchange their old external habiliments for newer and [hopefully] more compatible ones. Thus "Uremic Poisoning" has been shed for "Bright's Disease of the Kidneys" which in turn has molted to "Chronic Pyelonephritis" and "Chronic Glomerulonephritis". On the one hand, "Chronic Pyelonephritis" has been considered the end-stage of more acute "Interstitial Nephritides" including "Analgesic Abuse Nephropathy" and toxicities secondary to other pharmacologic agents as well as secondary to infectious agents , metabolic deficiencies, or vascular insufficiencies. "Chronic Glomerulonephritis", on the other hand, is currently perceived as a final common pathway of expression for immunologic (Systemic Lupus Erythematosus, Acute Diffuse Post-Streptococcal Nephritis), infiltrative (Amyloidosis, either primary or secondary to Rheumatoid Arthritis or Tuberculosis), metabolic (Diabetic Glomerulosclerosis or Kimmelstiel-Wilson Disease), or idiopathic disorders (Membranous Glomerulonephritis, Minimal-Change Nephropathy).

Although viewed by astute clinicians (Himsworth, H., Lancet 1:127, 1936\;Farber SJ, personal communication, 1969) for quite some time as a conglomeration of distinct pathophysiologic disorders culminating in fasting hyperglycemia, diabetes mellitus has only recently absorbed any particularly formal recognition of its incipient complexity. In the otherwise unremarkable year of 1979, the National Diabetes Data Group published its "Classification and Diagnosis of Diabetes Mellitus and Other Categories of Glucose Intolerance" (Diabetes 28(12):10039, 1979). That landmark "consensus" divided overt diabetic territory into two more homogeneous areas - Type 2 Diabetes (Non-Insulin Dependent Diabetes Mellitus) and Type 1 Diabetes (Insulin Dependent Diabetes Mellitus), and one heterogeneous gemisch ignominiously labeled - "Other".

Type 1 Diabetes does appear to represent the most homogeneous of these three categories, but only in so far as its definition comprises absolute, isolated -cell insulin secretory failure (Srikanta S, Ganda OP, Gleason RE, Kaldany A, Garovoy MR, Milford EL, Carpenter CB, Soeldner JS, and Eisenbarth GS, Ann Int Med 99:320, 1983). Most of the patients in this category are young and manifest their disease on the basis of a specific immunologic attack upon the pancreatic -cell of the islets of Langerhans (Cudworth AG, Diabetologia 14:281, 1978).

 

The "Other" category includes generalized islet failure, usually in the setting of pancreatic exocrine deficiency and chronic pancreatitis of varied etiologies. It includes as well, however, resistance to the action of insulin caused by certain genetic disorders or by excess concentrations of known counter-regulatory hormones, such as epinephrine, cortisol, glucagon, and growth hormone.

Type 2 Diabetes accounts for 90-95% of the total number of patients with diabetes mellitus (Harris MI, in National Diabetes Data Group, eds. Diabetes in America:diabetes data compiled in 1984. Bethesda, Md.:National Institutes of Health, Chapter VI, 1985: publication no. 85-1468).

It is certainly less homogeneous than Type 1 Diabetes, although the majority (70%) of patients who comprise Type 2 Diabetes manifest obesity as some basis of their pathophysiology. The evidence for this is: (1) not only are they obese at diagnosis (Everhart J, Knowler WC, and Bennett PH, in National Diabetes Data Group, eds. Diabetes in America: Chapter IV, loc.cit.), but (2) their disease dissipates with weight loss significant to the order of 5.00 to 10.00 kg-2 of Body Mass Index (BMI) (Bistrian BR, Blackburn GL, Flatt JP, Sizer J, Scrimshaw NS, and Sherman M, Diabetes 25:494, 1976\; Genuth SL, Am. J. Clin. Nutr 32:2579, 1979\; Hughes TA, Gwynne JT, Switzer BR, Herbst C, and White G, Am J Med 77:7, 1984\; Hadden DR, Montgomery DAD, Skelley RJ, Tremble RR, Weaver SA, Wilson FA, and Buchanan KD, Br med J 3:276, 1975\; Bogardus C, Ravusin E, Robbins DC, Wolfe RR, Horton ES, and Sims EAH, Diabetes 33:311, 1984) and (3) it is more difficult for obese Type 2 Diabetes patients to lose weight than their obese non-diabetic counterparts (Henry RR, Wiest-Kent TA, Scheaffer L, Kolterman OG, and Olefsky JM, Diabetes 35:155,1986). Nutritional status is therefore a key pathogenic component of a majority of patients with Type 2 Diabetes.

Nevertheless, insufficient insulin secretion to overcome hepatic [and peripheral] insulin resistance seems to be the common pathophysiologic thread weaving its way throughout disorders currently classified as Type 2 Diabetes (Haffner SM, Stern MP, Hazuda HP, Mitchell BD, and Patterson JK, NEJM 319:1297, 1988). Since the genetic/concordance data are much stronger for Type 2 Diabetes, than for Type 1 Diabetes (Newman B, Selby JV, King MC, Slemenda C, Fabsitz R, and Friedman GD, Diabetologia 30:763, 1987\; Barnett AH, Eff C, Leslie RD, and Pyke DA, Diabetologia 20:87, 1981), investigators are feverously searching for the molecular basis of the defect [or defects] in Type 2 Diabetes. Obesity itself, though, has also been shown to have a very strong genetic predisposition (Bouchard C, Tremblay A, Despres JP, Nadeau A, Lupien PJ, Theriault G, Dussault J, Moorjani S, Pinault A, and Fournier G, NEJM 322:1477, 1990\; Stunkard AJ, Harris JR, Pedersen NL, and McClearn GE, NEJM 322:1483, 1990).

Given the above data relating obesity to Type 2 Diabetes, perhaps a significant genetic link which has been observed in nondiabetic offspring of Type 2 Diabetes patients might associate more closely with obesity [which in turn correlates with hyperinsulinemia] than with glycemia, per se (Haffner et al, Table 1, loc.cit). In any case, what might make a particular obese individual predisposed to Type 2 Diabetes? In addition, is it possible that the route to Type 2 Diabetes could represent a normal physiologic escape from what otherwise might develop into the pathophysiologic state of "morbid obesity?" Are there any lines of evidence which might lend support to such a hypothesis? Finally, or, perhaps first of all, what is the nature of the relationships which exist between insulin resistance, hyperinsulinemia, and obesity?

Epidemiology and Cell Biology

For both the power and direction of any light shed upon the world of Type 2 Diabetes to date, we are certainly indebted to the National Diabetes Data Group as a whole, and to such epidemiologists, particularly, as Kelly West, Maureen Harris, Peter Bennett, Clifton Bogardus, William C. Knowler, Stephen Lillioja, Michael Stern, Steven Haffner, Paul Zimmett, Gary Dowse, Susan Serjeantson, Harry Keen, Giancarlo Viberti, Per Bjorntrop, Jaako Tuomilehto, Ron Klein, Larry Rand, Zoltan Skrabalo, Jean-Pierre Felber, Eveline Eschwege, Maureen Harris, and Elizabeth Barrett-Connor, among others.

Without the light of the above epidemiologists, most of us would still be groping. Given the genetic predisposition of Type 2 Diabetes, both prospective and retrospective studies have focused, are focusing, and will likely continue to focus upon the following populations: 1) Pima Indians from the Gila River Valley 2) Nauruans from Polynesia and Micronesia 3) Mexican Americans from San Antonio 4) Mauritians from Mauritius 5) Maltese from Malta and Canada 6) Indians from Fiji, Asia and North America (Cherokee,Choctaw) 7) Aborigines from Australia 8) First degree relatives of patients with documented Type 2 Diabetes.

The two common denominators amongst all of the above groups has been (1) the statistically significant highest incidences and risks of development of Type 2 Diabetes seen in any of the world populations and (2) the presence of prandial hyperinsulinemia and/or insulin resistance in the normal-glucose-tolerant first degree relatives of patients with Type 2 Diabetes. Insulin-resistance is defined here as decreased whole body glucose disposal under euglycemic-hyperinsulinemic "clamped" conditions. The increased incidence of Type 2 Diabetes in these populations with a background of insulin resistance has been ascribed, not altogether facetiously, by Koessler and others, to "Coca-Colonization". Herein lies a good part of the genesis and rationale of the "stuffed cell hypothesis."

Once the high incidence of Type 2 Diabetes in these groups was noted by the respective epidemiologists, the genetic linkage was confirmed by documenting an even higher association with Type 2 Diabetes in one or two parents of [primarily those most obese] affected probands (Knowler WC, Pettitt DJ, Savage PJ, and Bennett PH, Am J Epidemiol 113:144, 1981). Obesity by itself is also considered a very "strong risk factor for Type 2 Diabetes" (Everhart, loc.cit.). Nevertheless, the single strongest predictor of Type 2 Diabetes in both univariate and multivariate analyses would appear to be postprandial glycemia or "Impaired Glucose Tolerance - IGT" (Bennett PH, Knowler WC, Pettitt DJ, Carraher MJ, and Vasquez B, In: Eschwege, E., ed., Advances in Diabetes Epidemiology. Amsterdam:Elsevier Biomedical, 1982, p.65).

In a multivariate analysis of Mexican-Americans, fasting glycemia in the presence of fasting hyperinsulinemia and IGT was the best predictor of Type 2 Diabetes (Haffner SM, Stern MP, Mitchell BD, Hazuda HP, and Patterson JK, Diabetes 39:283, 1990). Peter Bennett's study of the Pimas reported eight years previously (Bennett, 1982, loc.cit.) determined that those individuals who progressed from IGT to Type 2 Diabetes had not only elevated fasting levels of insulin and glucose, but significantly decreased postload insulin concentrations as well. Unfortunately the Haffner group did not measure postload insulin levels (loc.cit.,1990).

However Bennett's postprandial insulin observations have found ample confirmation elsewhere (Sicree, RA, Zimmett, P, King HOM, and Coventry S, Diabetes 36:179, 1987\; Charles MA, Fontbonne A, and Eschwege E, Diabetologia 31 [Suppl1]:479A, 1988\; Kadowaki T, Miyake Y, Hagura R, Akanuma H, Kuzuya N, Takaka F, and Kosaka K, Diabetologia 26:44, 1984). This kind of data allowed Bennett to postulate in 1982 (loc.cit.) that inadequate insulin secretion might be a significant factor in the transition from IGT to Type 2 Diabetes! Nonetheless, in only one long-term (10 year) clinical trial (Sweden/open/tolbutamide plus diet/Sartor, G, Scherten B, Carlstrom S, Melander A, Nordan A, and Persson, G, Diabetes 29:41, 1980) did intervention into IGT seem to negatively influence the development of Type 2 Diabetes, whereas two trials (Bedford/blind/tolbutamide or diet/Keen, H, Jarrett RJ, and McCartney P, Diabetologia 22:73, 1982\; Whitehall/ blind/phenformin/Jarrett RJ, Keen, H, McCartney P In:Eschwege, E. loc.cit., p.95) showed no influence of intervention whatsoever. [Tolbutamide enhances insulin release whereas phenformin improves peripheral insulin action.] If obesity, IGT (especially with fasting hyperinsulinemia and prandial hypoinsulinemia), and parental history predict Type 2 Diabetes in these already high at-risk groups, what are the factors which predict IGT and/or obesity? At this level we are essentially looking at probands with normal glucose tolerance who are not obese, yet who are at the highest risk of developing obesity, IGT, and ultimately Type 2 Diabetes. Haffner et al (loc.cit.,1988) evaluated non-diabetic Mexican-Americans and found elevated fasting and prandial insulins, which associated in step-wise fashion with the number of parents with Type 2 Diabetes. These authors state, though, that "it is surprising that there is relatively little or no variation in body-mass index or body-fat distribution across the various family history categories (Table I)." In point of fact, however, the data in their Table I demonstrate statistically and clinically significantly increased mean BMI's among probands with one or both Type 2 Diabetes parent[s] as compared to those with no Type 2 Diabetes parents. In any event, the main point this paper establishes is that in a non-diabetic population at high risk for Type 2 Diabetes, there is significant familial hyperinsulinemia already present. Keep in mind that this population comprises those non-diabetics with both normal and abnormal glucose tolerance, however.

It was Lillioja et al (loc.cit., 1987) who first elegantly demonstrated a profound degree of familial insulin resistance clustering among mostly glucose tolerant (only 10% had IGT ) non-diabetic Pima siblings. Measuring glucose disposal at two levels of hyperinsulinemia (MMax @ 400 mUm-2min-1 and MSubmax @ 40 mUm-2min-1) with euglycemia clamped, they showed that profound insulin resistance (MMax) was highly familial and independent of obeseness, VO2 max, age or sex. A lesser degree of insulin resistance (MSubmax) did correlate as well with degree of obesity, age and sex.

Finally, fasting insulin levels correlated with fasting glucose, as well as sibship, degree of obesity, age, and sex. Thus, at least three different levels of familial insulin resistance may be suggested by this data: (1) genotypic and profound, (2) hormonal, phenotypic, and less profound, and (3) deterioration secondary to consequences of the aforementioned and least profound of all. Bogardus C, Lillioja S, Nyomba BL, Zurlo F, Swinburn B, Esposito-Del Puente A, Knowler WC, Ravussin E, Mott DM, and Bennett PH (Diabetes 38:1423, 1989) have detected an apparent trimodal distribution of the Mmax defect in the Pimas. This has led him to postulate an autosomal co-dominant mode of transmission involving two alleles at a single locus. This kind of genetic defect should result in dose-dependent inhibition of glycogen synthase activity, if Bogardus is correct on all counts.

Jose Caro and his group described significantly reduced insulin- stimulatable glucose transport in muscle biopsies taken from morbidly obese, non-diabetic as well as diabetic patients in eastern South Carolina about to have some surgical procedure (Dohm GL, Tapscott EB, Pories WJ, Dabbs DJ, Flickinger EG, Meelheim D, Fushiki T, Atkinson SM, Elton CW ,and Caro JF, J Clin Invest 82:486,1988) . "Transport of 3-O-methylglucose and 2-deoxyglucose was stimulated approximately twofold by insulin in muscle from normal nonobese subjects and stimulation occurred in the normal physiological range of insulin concentrations.

In contrast to insulin stimulation of 3-O-methylglucose and 2-deoxyglucose transport in muscle from normal, nonobese subjects, tissue from morbidly obese subjects, with or without Type 2 Diabetes, were not responsive to insulin. Maximal 3-O-methylglucose transport was lower in muscle of obese than nonobese subjects. Morbidly obese patients, with or without Type 2 Diabetes, have a severe state of insulin resistance in glucose transport." This defect was also documented in individuals with normal glucose tolerance. However, a majority of these patients may not go on to develop Type 2 Diabetes and/or may not be related to patients who have Type 2 Diabetes. Is this then, perhaps, a manifestation of the Msubmax defect which associates with a "corpulent predisposition"? If so, is the linkage causal or consequential? Which of these individuals, if any, also have an Mmax defect? Finally, the crucial question: do the patients either who do already or who will eventually manifest Type 2 Diabetes have the Mmax defect so elegantly demonstrated in the Pimas?

The Pima Indian Study group at NIDDK had previously demonstrated that physiological insulin resistance in that patient population correlated anatomically with a defect in muscle biopsy glycogen synthase activity. "Muscle glycogen content and glycogen synthase activity were measured in percutaneous muscle biopsy samples obtained from the vastus lateralis muscle before and after the euglycemic clamp procedure. The results showed that muscle glycogen synthase activity at the end of the euglycemic clamp was well correlated with insulin-mediated glucose storage rates at both low (r = 0.50, P less than 0.02) and high (r = 0.78, P less than 0.0001) insulin concentrations\; and also correlated with M[submax] (r = 0.66, P less than 0.001) and M[max] (r = 0.76, P less than 0.0001). Similar correlations were observed between the change in muscle glycogen synthase activity and glucose storage rates and M[max and submax].

The change in muscle glycogen synthase activity correlated with the change in muscle glycogen content (r = 0.46, P less than 0.03) measured before and after the insulin infusions" (Bogardus C, Lillioja S, Stone K, and Mott D, J Clin Invest 73:1185, 1984). This group has subsequently ascribed this defect to a decrease in glycogen synthase phosphatase or possibly to some non-suppressiblity of cyclic AMP dependent ("A-") [protein] kinase (Kida Y, Esposito_Del Puente A, Bogardus C, and Mott DM, J Clin Invest 85:476, 1990\; Okubo M, Bogardus C, Lillioja S, and Mott DM, J Clin Endocrinol Metab 69:798, 1989) If this represents the source of primarily the Mmax defect, how is it different from the Msubmax defect which supposedly correlates with obesity.

Since this glycogen synthase defect does still significantly correlate with Msubmax [but less well so than with Mmax], there should be some correlation additionally with obesity. Is this association with obesity primary and, if so, is the association causal or consequential? The data are certainly compatible with a schema in which Mmax precedes and predisposes towards obesity which results in some defect in glucose transport or Msubmax. The data are also compatible with a schema in which the Law of Mass Action applies to the glucose back-up associated with a glycogen synthase defect which in turn decreases glucose transport which results in glucose shunting to the adipocytes and, ultimately, obesity.

Bogardus et al (Diabetes, 1989, loc.cit.) feels simply that the data "suggest that if there is a gene for insulin resistance in the Pima, those who carry it are more likely to become obese." Or rather might any association with obesity be a secondary reflection of a defect more proximally related to the insulin receptor? If the latter, is it upstream of the glucose transport signal [as potentially intimated by the data of Caro et al]? In this scenario, however, one might expect a much tighter correlation between Mmax and Msubmax, whether or not obesity were present\; one might expect that tighter correlation, that is, unless: 1) obesity itself were etiologic and made some underlying propensity towards impaired glucose transport more manifest and 2) Mmax and Msubmax were unevenly affected by the upstream defect Finally, are the two defects completely independent and additive? In an attempt to clarify this issue, Shulman GI, Rothman DL, Jue T, Stein P, DeFronzo RA, and Shulman RG, (NEJM 322:223,1990) have reported a study in patients with Type 2 Diabetes utilizing a technique as yet unsubstantiated and not generally accepted, i.e., nuclear magnetic resonance spectroscopy. This study, on the basis of many assumptions, calculations, and uneven weighting of the data, purports to show in vivo a defect in muscle glycogen synthesis present in lean Type 2 Diabetes patients not found in normal controls.

Perhaps more to the point, Eriksson et al have performed a nice study utilizing the hyperinsulinemic- euglycemic [and hyperglycemic] clamp in (1) Type 2 Diabetes patients, (2) first degree non-diabetic relatives of Type 2 Diabetes patients, and (3) healthy non-diabetic controls (Eriksson J, Franssila-Kallunki, A, Ekstrand A, Saloranta C, Widen S, Schalin C, and Groop L, NEJM 321:337, 1989). This time, however, relatives were divided into those with and without normal glucose tolerance and compared with Type 2 Diabetes kin and non-diabetic controls.

Insulin infusion rates were roughly equivalent to Msubmax in the Lillioja study. Total-body glucose metabolism was significantly reduced in all three metabolically related groupings compared to controls: Type 2 Diabetes: 3.20 0.19 mg kg-1 min-1 IGT: 3.81 0.29 mg kg-1 min-1 NGT: 4.38 0.34 mg kg-1 min-1 CONTROL: 6.64 0.55 mg kg-1 min-1 There were no differences between any of the groups with respect to baseline glucose oxidation rate (as determined by indirect calorimetry). Insulin-stimulated glucose oxidation rate only differed significantly between Type 2 Diabetes patients (2.03 0.10 mg kg-1 min-1) and controls (2.91 0.23 mg kg-1 min-1).

Subtracting presumed "oxidative glucose metabolism" from "total-body glucose metabolism" supposedly yields "non-oxidative glucose metabolism" which "almost completely accounted for" the defect in total-body glucose metabolism [assuming that the calorimetry data is reliable cf. Ferrannini E, Metabolism 37:287, 1988]: Type 2 Diabetes: 1.18 0.17 mg kg-1 min-1 IGT: 1.40 0.22 mg kg-1 min-1 NGT: 1.81 0.27 mg kg-1 min-1 CONTROL: 3.76 0.55 mg kg-1 min-1 This non-oxidative glucose metabolism defect persisted in normal glucose tolerant (NGT) first degree relatives of Type 2 Diabetes despite a totally normal first phase (0' -> 10') insulin secretion: Type 2 Diabetes: 7 7 U/ml 10 min IGT: 44 18 U/ml 10 min NGT: 110 13 U/ml 10 min CONTROL: 101 34 U/ml 10 min and an enhanced second phase (10' -> 120') insulin secretion: Type 2 Diabetes: 50 19 U/ml 110 min IGT: 324 50 U/ml 110 min NGT: 465 40 U/ml 110 min CONTROL: 325 93 U/ml 110 min.

Maximal stimulated insulin secretion mirrored second phase responses for all categories as well. First degree relatives who showed a defect in first phase insulin response, however, appeared to manifest at least IGT in these studies. Basal hepatic glucose outputs correlated positively with fasting glucose values (r = 0.538, P_0.001) in the subjects as a group, were significantly lower in all groups than in Type 2 Diabetes, Type 2 Diabetes: 2.62 0.13 mg kg-1 min-1 IGT: 1.90 0.10 mg kg-1 min-1 NGT: 1.89 0.06 mg kg-1 min-1 CONTROL: 1.89 0.06 mg kg-1 min-1, and were suppressible by Msubmax insulin infusion in all groups except Type 2 Diabetes (N.B. negative values): Type 2 Diabetes: 0.31 0.09 mg kg-1 min-1 IGT: -0.27 0.19 mg kg-1 min-1 NGT: -0.72 0.29 mg kg-1 min-1 CONTROL: -1.83 0.50 mg kg-1 min-1

The "Gluconeogenesis Suppression Fallacy"

It should be mentioned at this juncture that "hepatic glucose output" as described in most of the above studies, is a purely ersatz value which has been calculated based upon the so-called "Steele Equation". This equation in turn is predicated upon a few assumptions, i.e., that there is (1) but a single pool (2) which is some fraction (usually ) of the total extracellular fluid space through which a given compound is (3) rapidly and (4) evenly distributed, and that (5) all "specific" radioactivity within that pool at any point in time represents solely intact compound.

Given those caveats, the equation derived by Robert Steele and his cohorts at Brookhaven National Library and the New York University School of Medicine (Amer J Physiol 187:15,1956), states that:- 1) Ra = F - Bt( dit/dt) it and 2) Rd = Ra - dBt/dt where, Ra is the rate of appearance of compound into the pool, Rd is the rate of disappearance of the compound from the pool, F is the rate of compound isotope infusion, Bt is the total amount of compound in the pool at time "t", and it represents the specific activity of the pool at time "t". Under most conditions the rate of appearance of a compound should represent the rate of infusion of the compound, Ri, plus the intrinsic production rate of the compound, Rh, or: 3) Ra = Ri + Rh Rearranging, then: 4) Rh = Ra - Ri

If the compound under discussion is glucose, Rh should represent predominantly "hepatic glucose output" [as the liver is the major site of gluconeogenesis]. But how can an Rh be negative? That is to say, under conditions of equilibrium, (and remembering that appearance rates are independent from disappearance rates) how can glucose appear [into whatever its putative distribution space] at a rate slower than which it is infused? At equilibrium, when Rh = 0 and dit/dt = 0. 5) Ra = F-Bt(dit/dt) = Rh + Ri it 6) F/Ri = it 7) i = it [by definition] In order for Rh to be negative in 4), Ra has to be UNDERestimated [i.e., Ra < Ri] by either: a) if dit/dt <0, then EITHER its negativity must be UNDERestimated or the pool size (Bt) UNDERestimated or b) if dit/dt >0, then EITHER its positivity must be OVERestimated or the pool size (Bt) OVERestimated Under conditions where Rh is presumably equal to 0, a negative dit/dt must represent further dilution of specific activity by a positive gluconeogenesis which itself is either UNDERestimated or with a glucose pool size which is UNDERestimated. Since Rh becomes even more negative with higher rates of insulin infusion, either increasing insulin concentrations increase the total glucose available to the extracellular pool at any point in time (thereby leading to an UNDERestimate of pool size) OR increasing insulin concentrations magnify the ability of sampled slope estimates of glucose specific activity to UNDERapproximate actual slopes at any point in time. [If dit/dt were positive, then the specific activity of the pool sample must be increasing with time at the moment of sampling. "Delayed mixing" has been bandied about as some sort of rationalization for this kind of result.

Nevertheless, this type of explication fails to explain how Rh becomes even more negative with higher rates of insulin infusion. If supposition "b" is to be correct, then either the pool size for glucose decreases as insulin increases, or the specific activity of plasma glucose is enhanced by some non-glucose radioactive label which has been stimulated by insulin, e.g., 14CO2, or 14C-[tri]glyceride, or 3H20, or 3H2CO3 or etc.] Some investigators (e.g., Shulman et al, loc cit) who utilize the glucose clamp technique base their assumptions that hepatic glucose output is suppressed under conditions of hyperinsulinemia and hyperglycemia ("Under these conditions - constant hyperglycemia and hyperinsulinemia - hepatic glucose output is completely suppressed8....") upon DiFronzo RA, Ferrannini E, Hendler R, Wahren J, and Felig P, Proc Nat Acad Sci 75:5173, 1978. The superscript -8- here is just such a reference.

Nevertheless, a perusal of this citation reveals absolutely no support for such an assumption. Indeed, "splanchnic output" per se is reported under basal conditions, but only net "splanchnic uptake" is reported under conditions of hyperglycemia and hyperinsulinemia. Thus, it is impossible to reach any conclusions about hepatic glucose output under hyperglycemia and hyperinsulinemia from the cited data provided by DeFronzo et al. However, this particular reference does shed significant light upon hepatic ("splanchnic") glucose uptake under varied conditions of insulinemia, glycemia, and route of glucose administration (p.o. or i.v.).

Interestingly enough, intravenously induced hyperglycemic clamping at 223mg/dl without exogenous insulin [achieving comparable levels of endogenous peripheral insulinemia (55U/ml) to that after a 100g oral GTT] increased the total percentage of glucose taken up by the splanchnic bed from ~5% [seen under euglycemia/hyperinsulinemia) to ~14%. When this was repeated with exogenous hyperinsulinemic clamping at the same level of hyperglycemia, the splanchnic component decreased to ~9% of the total body glucose uptake. Now when the glucose was administered orally [as well as intravenously], net splanchnic uptake increased almost sixfold from basal (DeFronzo et al, ibid.) and amounted to almost 60% of the total body glucose uptake (Felig et al, Diabetes 24:468, 1975 and Diabetes 27:121, 1978). Yet, virtually all of the studies purporting to show that the primary site of insulin resistance in Type 2 Diabetes is in the periphery have been done under conditions of solely intravenously administered glucose.

If hepatic uptake can amount to at least 60% of an orally administered glucose load, and glucose is almost always ingested orally, 1) what is the significance of resistance demonstrated after solely intravenously administered glucose? and 2) why have there not been more studies utilizing clamping after orally as well as intravenously administered glucose? Steele R, Bjerknes BA, Rathgeb I, and Altszuler N (Diabetes 17:415, 1978) have shown that under normal physiologic conditions during a 3 hour oral GTT in dogs, hepatic glucose output is 54% of basal\;whereas total body glucose disposal is 163% of basal. This amounts to a net decrease of 0.29 g/kg/3hrs in hepatic output (still positive at 0.34 g/kg/3hrs), and an increase of 0.40 g/kg/3hrs in total body uptake (1.03g/kg).

Now, if DeFronzo et al are correct in that the net splanchnic uptake during intravenously induced hyperglycemia was ~14%, and Steele at al are correct that under "supposedly" similar conditions of [peripheral] insulinemia, hepatic glucose output was 0.34 g/kg/3hrs, and if one can analogize somewhat from dog to man, then the hepatic glucose uptake per se during those conditions in DeFronzo could represent approximately 0.4842 g/kg/3hrs (14% of a total glucose uptake of 1.03 mg/kg/3hrs added to a hepatic glucose output of 0.34 mg/kg/3hrs) or 47% of total body glucose uptake! Felig et al (Diabetes 24:468, 1975\;Diabetes 27:121, 1978) show net splanchnic uptakes of ~60% during 100 g oral glucose loads in normal subjects. These would translate into hepatic output components [during much more normal physiologic manipulation] of approximately 93%! Bear in mind, however, that this figure is based upon a rather sizeable glucose output under these conditions as determined by Steele et al (loc. cit.).

Under conditions of exogenously clamped hyperinsulinemia and hyperglycemia in DeFronzo (loc. cit.), the net fractional splanchnic uptake dropped to ~9% [from 14%]. Even assuming (without the benefit of any experimental data Shulman to the contrary not withstanding) that hepatic glucose output is completely suppressed under these conditions [of hyperglycemia and hyperinsulinemia], then hepatic glucose uptake would be equal to the net splanchnic uptake, and decrease to approximately 9% (from 47%) of total body glucose uptake.

Thus, increasing insulin to maximum levels [roughly four times those seen endogenously during an oral glucose load] could appear to increase predominantly peripheral (i.e., non-hepatic) glucose uptake. Might one not wonder as well whether any "peripheral insulin resistance" determined under these somewhat artificial conditions were conceivably a bit over-represented - and "hepatic insulin resistance" likewise somewhat under-represented?

The net upshot of all of this is that:- 1) Gluconeogenesis may be [significantly] underestimated in some studies (v.s.) due to either inaccurate pool size estimates or inaccurate accounting of plasma isotope activity 2) Hepatic glucose uptake may be [significantly] under-represented in some studies (v.s.) due to non-physiologic excesses of insulin [glucose] and failure to accurately account for gluconeogenesis under all conditions 3) Peripheral glucose uptake may be [significantly] over- represented in some studies (v.s.) due to non-physiologic excesses of insulin [glucose] and failure to accurately account for gluconeogenesis under all conditions

To further obfuscate the picture DeFronzo RA, Ferrannini E, and Simonson DC have apparently calculated from their data (Metabolism 38:387, 1989) that basal hepatic glucose output is not significantly different in "mild" diabetics from non-diabetic controls. This is in distinct contrast to conclusions drawn by Efendic S, Karlander S, and Vranic M (J Clin Invest 81:1953, 1988) who showed markedly increased basal hepatic glucose outputs and markedly enhanced intrahepatic glucose cycling in their patients with mild diabetes. Moreover, young non-diabetic Australian aborigines apparently have significantly greater basal and significantly less suppressible hepatic glucose outputs in response to oral glucose loading than controls (Proietto J, Nankerois K, Traicenedes K, Rosella J, and O'Dea K, Diabetes Res Clin Pract 5 [Suppl.I]:263, 1988).

Finally, using glucose labeled with tritium, and assaying for glucose specific activity utilizing the perchloric acid precipitation technique of Best et al (Diabetes 30:847, 1981), Kwame Osei (Diabetes 39:597, 1990) has shown that hepatic glucose outputs were significantly greater (25%) in nondiabetic first-degree relatives than in controls (which correlated in statistically significant fashion with fasting plasma glucose and fasting plasma c-peptide). Remember, however, that there did not appear to be any differences in basal glucose outputs from Erikkson et al (loc cit) in either normal- (N = 13) or impaired- glucose tolerant (N = 13) first-degree relatives from controls (N = 14). Even though more statistical power (N = 27 proband vs. N = 16 control) was available to detect this difference in Osei (loc cit\; p _ 0.05), further corroboration is certainly desireable.

The data of Niewohner CB and Nuttall FQ (Diabetes 37:1559, 1988) in rats "suggest rapid bidirectional glucose flux" into and out of the liver following an oral glucose load, "but with active transport maintaining a modestly higher intracellular glucose concentration" [than the extracellular milieu].

The data supplied by Reaven's group at Stanford entitled "Effect of Central Obesity on Regulation of Carbohydrate Metabolism in Obese Patients with Varying Degrees of Glucose Tolerance (Golay A, Chen N, Chen YDI Hollenbeck C, and Reaven GM, J Clin Endocrinol Metab 71:1299, 1990) may shed additional support for this concept. In Figure 2 of that paper, there were 6 patients with glucose disposals (Rd's) in the range of 1.2 to 1.75 mmoles m-2 min-1: 3 patients with NGT and 3 patients with IGT. The means of both groups were roughly equal [if anything the IGT patients trended a bit higher]. Assuming the group diagnoses based on oral glucose tolerance characterizations to be correct in those 6 patients, if the glucose disposals were equivalent, then those of these patients with IGT must have manifested increased glucose outputs as the primary basis of their abnormal glucose tolerance.

Widening the Rd window from 1.0 to 2.0 mmoles m-2 min-1 reveals 11 patients, 5 with NGT, 5 with IGT, and even 1 with Type 2 Diabetes. Roughly equivalent (with NGT patients this time trending higher than IGT), the means of the Rd's again imply that the differences in glucose tolerance must be accounted for primarily by differences in glucose outputs - compatible with current concepts of Type 2 Diabetes, but a rather new observation in IGT. The three other NGT patients included in this figure had Rd's greater than 2.5 mmoles m-2 min-1 (four NGT patients were not plotted for some reason). Three other IGT patients, and 4 other Type 2 Diabetes patients (six not plotted) all had Rd's less than 1.0 mmole m-2 min-1. The means for the three groups as a whole with standard deviations, however, appeared to be as follows: Dx Rd NGT: 2.09 0.27 mmoles m-2 min-1 IGT: 1.14 0.12 mmoles m-2 min-1* Type 2 Diabetes: 0.80 0.09 mmoles m-2 min-1** *p < 0.01 compared to NGT **p < 0.001 compared to NGT.

Nevertheless, even assuming that these group means are accurate, the take home message here is that a significant percentage (_62.5%) of obese patients with IGT may have increased [hepatic or renal] glucose outputs as the primary basis for the impairment in glucose tolerance, i.e., assuming insulin responses are comparable. Even though individual points for insulin responses and Rd were not shown in this report, the total mean daily integrated insulin response for the IGT group as a whole (3967 394 pmol L-1 h-1) was almost twice as high as that of the NGT group (2418 243 pmol L-1 h-1).

Although certainly not conclusive, it is therefore quite likely that glucose output was the major contributor to the impaired glucose output seen in this obese population. [It might also be noteworthy here to point out that the above authors also appeared to be somewhat reluctant to accept the notion that hyperglycemic/hyperinsulinemic conditions necessarily shut-down glucose output:- "If endogenous glucose production is totally suppressed during the insulin clamp study," [author's note: no references cited] the infusion rate of glucose needed to maintain the plasma glucose at 14mmol/L provides an estimate of the glucose disposal rate." Therefore, these authors used glucose with tritium labeled in the C3 position (in conjunction with the Steele equation) to estimate glucose disposal rate or Rd.]

I believe the notion that: the percent of hepatic glucose output suppression EXCEEDS the percent of maximal peripheral glucose uptake at any given and equal concentration of insulin [and glucose] in portal and peripheral capillary beds should be termed the "gluconeogenesis suppression fallacy." In the first place, I know of no good evidence to support such a notion. In the second place, the liver usually sees much higher post-prandial concentrations of insulin than does the periphery [due to the physiology of insulin release directly into the portal circulation.] In the third place and in a purely teleological sense, it would seem prudent from the perspective of the body's major glucose utilizer, i.e., the CNS, for gluconeogenesis [glucose-6-phosphatase?] to be somewhat resistant to the insulin message.

Therefore, it would seem much more likely and physiologically appropriate for: the percent of maximal peripheral glucose uptake to EXCEED the percent of hepatic glucose output suppression at any given and equal concentration of insulin [and glucose] in peripheral and portal capillary beds. Epidemiological data (Keen, H, Jarrett RJ, and McCartney P, Diabetologia 22:73, 1982\; Jarrett RJ, Keen, H, McCartney P In:Eschwege, E. loc.cit., p.95.) support the notion that most Type 2 Diabetes (51%) and IGT patients [but not the Pimas] have a very high prevalence of Type IVa or IVb hyperlipoproteinemia (20% have IIa) which, in turn, correlates very strongly with macrovasculopathy and coronary mortality [but, again, not in the Pimas].

Reaven and others have attributed this phenomenon to increased hepatic synthesis of triglycerides due to enhanced insulinemia. However, hyperinsulinemia may increase VLDL clearance as well by increasing [non-myocyte] lipoprotein lipase (LPL) synthesis (Garfinkle AS, Nilsson-Ehle P, Schotz MC, Biochim Biophys Acta 424:265, 1976). Nevertheless, insulin may also suppress muscle LPL synthesis thereby shunting VLDL uptake to adipocytes (Dobs AS, Prev Cardiol Rpts 4:1, 1991)

The obvious question here relates to the possibility of differential insulin sensitivities as well as differential hepatic effects of insulin upon fatty acid, amino acid, and glucose metabolism, i.e., if hepatic insulin sensitivity were greater than peripheral sensitivity (relative to ambient insulin concentrations), then would not one expect hyperlipemia in the face of IGT? More especially would one not expect hyperlipemia and IGT during fasting conditions of basal [hyper]insulinemia? Differential hepatic effects of insulin upon glucose and lipids might also explain how hyperlipemia could associate with fasting hyperglycemia and Type 2 Diabetes (once fasting insulin levels drop sufficiently). On the other hand, if both hepatic and peripheral sensitivities were decreased more or less equally (relative to ambient insulin concentrations), then might one not expect IGT (or Type 2 Diabetes depending on basal levels of insulinemia) without hyperlipemia [as in the Pimas, or treated Type 1 Diabetes, or later stage Type 2 Diabetes patients]? It is also possible, however, that the Pimas could have a glycogen synthetase defect in the periphery and, potentially, liver, as well as having either a lipogenic defect alone in the liver or lack of insulin suppressibility of LPL activity in muscle. In summary, then, Caveat Emptor! Although the preponderance of current dogma [based on IV infusions of both glucose and insulin] might favor a primary defect in insulin resistance peripherally at the level of the myocyte and glycogen synthesis and/or glucose transport, there does not appear to be any convincing data that a primary defect might not occur centrally at the level of the hepatocyte with, perhaps, increased glucose cycling. A primary defect in both muscle and liver glycogen synthase activity [or in enhanced hepatic gluconeogenesis] could shunt more of the glucose which is then taken up by adipocytes [and hepatocytes] in the direction of enhanced lipogenesis. "Cell Stuffing","Left-Shifting" and "Uncoupling" - A Basis for Ongoing Scientific Investigations The concept that an "ingestion threshold" might exist at the cellular level is herein only somewhat facetiously referred to as "cell stuffing". There is no little experimental basis for this concept which, indeed, may turn out to be rather key in glucose homeostasis, particularly as relating to the physiology of glucose-dependent insulin secretion by the -cell. Increases in intra(-)cellular glucose concentrations are reflected by glycolytic and oxidative phosphorylations of ADP to ATP. Higher levels of ATP in turn inhibit potassium channels resulting in depolarization of the plasma membrane. When this occurs, calcium channels open and calcium influx (Arkhammar P\; Nilsson T\; Rorsman P\; Berggren PO, Inhibition of ATP-regulated K+ channels precedes depolarization-induced increase in cytoplasmic free Ca2+ concentration in pancreatic beta-cells, J Biol Chem 262 (12): p5448-54, 1987) is associated with [?activation of another G-protein and] phospholipase-C activation, conversion of phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol 1,4,5-trisphosphate (InsP3) and sn-1,2-diacylglycerol (DAG), activation of protein kinase C, threonine and serine phoshorylation, and a cascade ultimately resulting in insulin release and synthesis. It turns out that in the presence of protracted and relatively-mild hyperglycemia, this pathway becomes less sensitive to glucose (Sako Y\; Grill VE Coupling of beta-cell desensitization by hyperglycemia to excessive stimulation and circulating insulin in glucose-infused rats. Diabetes 39 (12): p1580-3, 1990). It further turns out that upon restoration of euglycemia, [and in the absence of genetically susceptible irreversibility and diabetes (Korsgren O\; Jansson L\; Sandler S\; Andersson A,Hyperglycemia-induced B cell toxicity. The fate of pancreatic islets transplanted into diabetic mice is dependent on their genetic background,J Clin Invest 86 (6): p2161-8, 1990)] this pathway can restore itself virtually completely to normal (Leahy JL Bonner-Weir S, and Weir GC, J Clin Invest 81:1407, 1988).

Referred to recently as "glucose toxicity", the limiting step in this process may not unlikely be an inverse function of the potential change from baseline to Vmax of ATP generation via glucose catabolism, i.e., the higher the baseline ATP generation, the lower the secretory response to a given glucose challenge. This same desensitization as a manifestation of the Pasteur effect could further feedback inhibit glucose uptake [with partially corrective repercussions]. "Glucose toxicity" apparently does not result in a malicious cycle of increasing decompensation as one might, perhaps, have otherwise expected (Weir GC, Joslin Symposium on Type 2 Diabetes, Boston, MA, October 1, 1990)

Now I must unfortunately disappoint Sir William of Occam by introducing a second hypothesis, i.e., "left-shifting of the glucose-ATP/insulin dose response curves". Usually, when one deals with the physiopathology of drug, hormone, ligand failures, one speaks in terms of "right-shifting of the dose response curves", i.e., it takes more of some agonist to induce a given effect. More correctly, the concentration of agonist resulting in half-maximal activity [Km] turns out to be higher, and, thus "right-shifted". Furthermore, pharmacologic intervention usually involves some attempt to "left-shift" the dose-response curve for some agonist which of necessity requires more potency in order to achieve its desired biochemical or physiologic effect.

Coming back to glucose, ATP, and the -cell, then, several questions arise: 1) Why do many patients who have had gestational diabetes manifest decreased 1st phase insulin responses when not pregnant? (Porte,D, Seminar at Molecular Diagnostics, West Haven, September 25, 1990) 2) Why are normal, i.e., non-diabetic pregnancies associated with such a marked lowering of basal glycemia in the face of quite markedly increased insulin resistance and comparably increased insulin requirements? 3) Why does reactive hypoglycemia seem to be associated with impaired glucose tolerance and early diabetes (Khurana et al, Postgrad. Med. 53: 118,1973), and more especially in obese, first degree relatives of patients with Type 2 Diabetes ? (Ensick JW and Williams RH, p.858 in Williams RH, ed., Textbook of Endocrinology, W.B. Saunders, Philadelphia, 1981 citing the work of LeFebvre et al in 1976) 4) Might there not be some common explanation for the above observations? 5) Assume, (for the sake of argument), that agonists such as (a) estrogen progesterone or (b) some as yet unidentified compound associated with obesity could induce a left-shifting of the glucose-ATP dose response curve. How might the natural history of subsequent events develop? | <= intra -cell [ATP] | x x | x | x | x | <--"threshold" level--> | x | x | x | x | x | | +------------------+----+---+----------------+------------- 7.5mM 15mM ambient extra -cell [glucose] x <=> "agonist" <=> "normal" (+ <=> glucose concentration at "threshold") Figure 1 The early natural history of this kind of defect should reveal lower basal levels of glycemia which border or even infringe upon the limits of hypoglycemia as seen in normal gestation. A strong predisposition for weight-gain should result. This could further exacerbate insulin resistance. If meals were eaten so frequently or insulin resistance increased to such an extent that resultant elevated ambient glucose concentrations reflected intra[]cellular ATP concentrations increasing above the dotted line (Fig.1), then the decreasing slope of the dose-response curve could represent the early limits of 1st phase insulin secretory failure. What is intriguing is that, as with gestation, the threshold glucose concentration for this secretory failure could be extremely low (on the order of 6mM/L or 102 mg/dL).

Thus, and in summary, a "left-shifting" of the glucose-ATP dose response curve should result in:

1) Hypoglycemia which is: a) Early and basal during gestational periods b) Reactive in the presence of obesity or insulin resistance

2) 1st phase insulin resistance [if ambient glucose concentrations increase above some (minimal) threshold] which is: a) Completely reversible if the curve can be "right-shifted" and the ambient glucose concentration lowered below the threshold, e.g., by reducing insulin resistance b) "AT-RISK" if the curve cannot be "right-shifted" and partially reversible if the ambient glucose concentration can be lowered below the threshold.

This is a situation like that in previously gestational diabetics in which: (i) Fasting glucose levels could still be well within normal limits (ii) High insulin levels would imply a significant degree of insulin resistance (iii) Persistent elevations in ambient glucose concentrations could possibly induce further "left-shifting" of the glucose-ATP dose-response curve and foster "uncoupling" ("right-shifting" with decreasing Vmax) of the ATP-insulin dose-response curve (Fig.2) (iv) Low insulin levels would particularly imply uncoupling of -cell ATP concentrations from ultimate insulin release]. | <= insulin release | o o | o | o | o | + + | o + | o | o | o + | o + +----------------v--v-------------------------------------- Km's -cell [ATP] o <=> "normal" -cell (function and mass) + <=> "abnormal" -cell (function or mass) AND prolonged above-threshold glycemia Figure 2 In order for there to be -secretory failure according to this model, then, two conditions would have to apply: 1) Left-shifting of the glucose-ATP dose-response curve (Fig. 1) 2) Uncoupling (right-shifting with increased Km and decreased Vmax) of the ATP-insulin dose response curve (Fig. 2) Thus glucose-insulin dose responses [I(g)] can be considered to be a function of both the glucose-ATP [A(g)] and ATP-insulin [I(A)] dose-responses, i.e., I(g) = A(g) + I(A) (Fig.3) | <= intra -cell [ATP] | o o | o | | o | + + | o + | | o + | o | o + Km | | +---------------------v-+--------------------+------------- 7.5mM 15mM ambient extra -cell [glucose] o <=> normal -cell (function and mass) + <=> abnormal -cell (function or mass) AND prolonged above-threshold glycemia Figure 3

If this model turns out to be correct, we should then devote our efforts to designing therapeutic modalities which "right-shift" and thereby raise the Km of glucose for -cell ATP generation. Metformin is one agent which could be a prototype or lead-structure. The net effect of such a strategy would be to increase reserve ATP generating capacity at any given effector concentration. This strategy should be combined with a search for an agent that recouples insulin secretion to ATP (e.g., sulfonylureas?) Does this concept that an "ingestion threshold" might exist at the cellular level apply to the periphery as well as well as it does to the -cell?

Prolonged hyperglycemia in adipocytes does seem to result in fairly substantial insulin resistance. Moreover, this significant degree of insulin resistance appears to be reversible with restoration of euglycemia (Traxinger RR and Marshall S, J Biol Chem 264:8156, 1989). In addition, this effect is significantly aggravated by glutamine in dose-dependent fashion (Traxinger RR and Marshall S, J Biol Chem 264:20910, 1989). Fasting and weight loss (which effect cellular "unloading" or "unstuffing") both improve insulin secretory responses AND/OR peripheral sensitivity to insulin in obese Type 2 Diabetes (Savage PS, Bennion LJ, Flock EV, Nagulesparan M, Mott D, Roth J, Unger RH, and Bennett PH) J Clin Endocrinol Metab 48:999, 1979\; Hidaka H, Nagulesparan M, Klimes I, Clark R, Sasaki H, Aronoff SL, Vasquez B, Rubenstein AH, and Unger RH, J Clin Endocrinol Metab 54:217, 1982\; Henry RR and Olefsky JM, Diabetes 35:990, 1986).

The hypothesis here is that - cellular stuffing of the -cell on the one hand, as well as of the myocyte, adipocyte, and hepatocyte on the other hand - could ultimately result in (1) decreased insulin secretory responses to glucose and (2) decreased peripheral glucose responses to insulin, respectively [and by potentially similar pathophysiologic mechanisms.] Indeed, in rats with already abnormal -cell function (2 to streptozotocin), high fat feeding alone without carbohydrate or protein [hepatocellular and adipocyte stuffing] resulted in significant fasting hyperglycemia (Pascoe WS and Storlien LH, Diabetes 39:226, 1990).

A cartoon speculating about the possible natural history of the evolution from insulin resistance to Type 2 Diabetes is presented as Figure 2. Vertically there are three columns: 1) The left-most column portrays events putatively transpiring in the insulin-responsive cellular compartment comprising predominantly hepatocyte, adipocyte, and myocyte. 2) The central column represents the physiologic state at each level of development. 3) The right-most column shows events as they are theorized to evolve in the -cell at any given stage. Horizontally 5 stages are depicted: 1) The top level represents normal glucose tolerance, insulin sensitivity, and insulin levels and responses. 2) The second level reveals the state of insulin resistance. 3) The third level describes the phase of impaired glucose tolerance (IGT). 4) The fourth level shows the stage of overt Type 2 Diabetes. 5) The final level attempts to portray metabolic deterioration in Type 2 Diabetes. The central theses implied herein are that: 1) Increased ingestion of carbohydrate and protein ultimately lead to increased presentation of glucose and glutamine to insulin-responsive cells. 2) These insulin-responsive cells have an "ingestion-threshold" which in turn is dependent on: a) Intracellular concentrations of ATP b) Intracellular ratios of creatine-phosphate to ATP ratios [myocytes] c) Total intracellular glycogen [myocytes and adipocytes] d) Total intracellular triglyceride [adipocytes] e) Total cell size [adipocytes] f) ?Intracellular concentrations of cyclic adenosine monophosphate (cAMP)

3) Once these thresholds have been reached and/or exceeded there is a right-shifting of the insulin-net glucose utilization dose-response curve ("insulin resistance").

4) Increased ingestion of carbohydrate also results in increased presentation of glucose to insulin-secretory (-) cells.

5) Sustained elevated ambient glucose concentrations (as well as female sex hormones and some obesogenic factors) result in increased -cell basal concentrations of ATP as well as a left-shifting of the glucose-ATP dose-response curve.

6) Sustained left-shifting of the glucose-ATP dose-response curves put a patient at increased risk for 1st phase insulin secretory defects.

7) Sustained increases in -cell basal ATP concentrations above a threshold level may cause "fatigue" or uncoupling of the ATP-insulin responses. The "apparent" Km should then increase with right-shifting and Vmax depression of the ATP-insulin dose-response curve. These changes ought inevitably to result in not only diminished 1st phase insulin secretory responses, but also in eventual -cell failure as well.

8) Hypersensitivity of this uncoupling phenomenon might very easily represent the primary -cell defect which is ultimately expressed as Type 2 Diabetes.

Insulin-Resistance

The phase of insulin resistance may be due to any number of factors, either acting singly, or in concert: 1) Some myocyte defect in the glycogen synthase pathway (as defined in Pimas by the NIDDK group in Arizona) 2) Some myocyte defect in glucose transport (as defined by the Caro group at East Carolina State) 3) Some adipocyte defect in glucose utilization perhaps secondary to glutamine suppression (as described by Marshall's group in Memphis) 4) Some possible prandial defect in suppression of hepatic glucose output (? by glutamine or lactate) 5) Presence of "ingestion threshold" conditions (v.s.) 6) Excess insulinemia with normal to low glucose levels due to compensated left-shifting of the glucose-ATP/ATP-insulin dose- response curves The manifestations of such defects would be: 1) Decreased glycogen synthesis by myocytes 2) Increased lipogenesis and VLDL clearance by adipocytes and hepatocytes 3) Increased post-prandial insulinemia 4) Prandial and fasting euglycemia

Impaired Glucose Tolerance (IGT)

The development of impaired glucose tolerance from insulin resistance is probably due to: 1) Progression of insulin resistance 2) A 1st phase insulin secretory defect possibly resulting from increased -cell basal ATP concentrations above a threshold level and uncoupling of insulin secretion from ATP generation The manifestation of these progressive defects would be: 1) Decreasing lipogenesis and VLDL clearance by adipocytes 2) Increasing lipolysis and free fatty acid (FFA) production and release by adipocytes 3) Decreasing hepatic glycogen synthesis 4) Increasing hepatic re-esterification of FFA to triglyceride with enhanced VLDL production rates 5) Decreasing post-prandial insulinemia 6) Elevated fasting insulinemia 7) Prandial hyperglycemia with fasting euglycemia

Non-Insulin-Dependent Diabetes Mellitus (Type 2 Diabetes)

The evolution of Type 2 Diabetes from IGT may initially be reversibly due to consequences of glucose toxicity by: 1) Progression of factors in Insulin Resistance and IGT 2) Approaching the Vmax of the glucose-ATP/ATP-insulin dose-response curves in the -cell 3) Further left-shifting of the glucose-ATP and uncoupling of the ATP-insulin dose-response curves in the -cell 4) Progressively enhanced hepatic resistance to the insulin message possibly due to a combination of: a) Increasing free fatty acid levels b) Increasing lactate levels c) Progressive exceeding of "ingestion threshold" limits 5) Fasting insulin levels insufficient to overcome hepatic resistance The manifestations of Type 2 Diabetes are: 1) Greatly increasing lipolysis and fasting FFA production by adipocytes 2) Increasing hepatic fasting and prandial gluconeogenesis 3) Decreasing fasting insulinemia 4) Prandial and fasting hyperglycemia

Metabolic Decompensation

Severe metabolic decompensation may be progressively due to the unopposed effects of "cell-stuffing" and "glucose toxicity" upon both insulin target organs and insulin producing cells. It is manifested by increasing fasting and prandial glucose levels in the face of decreasing fasting and prandial insulin levels. Amylin - an insulin-antagonistic peptide co-secreted physiologically with insulin in a molar ratio of approximately 1:100 - may play a later role in a progressive, irreversible -cell failure seen in the presence of amyloid deposition and ultimate -cell necrosis.


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