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W T Garvey

Publications and source records attributed to W T Garvey.

At least 73 records · Page 4Linked to original sources
PubMed1989

Glucose transport in primary cultured neurons.

UNLABELLED: In this study, we have examined glucose uptake and its regulation by insulin in primary cultured neurons. Glucose transport was assessed by measuring the initial rate of uptake of 3H-2-deoxyglucose, a glucose analog that is transported and phosphorylated but not further metabolized. The uptake of 2-deoxyglucose was saturable; measurements of the intracellular concentration of 2-deoxyglucose and 2-deoxyglucose-6-phosphate revealed that hexokinase activity rather than membrane transport is the rate-limiting step for glucose uptake. Insulin had no effect on 2-deoxyglucose uptake at low (0.2 mM) or high (20 mM) concentrations of substrate. The order of potency of other hexoses to competitively inhibit the accumulation of 2-deoxyglucose was D-glucose (0.2 mM) = D-mannose (0.2 mM) greater than 3-0-methylglucose (9 mM) greater than D-galactose (90 mM). Cytochalasin B was a potent inhibitor of 2-deoxyglucose uptake (IC50 = 500 nM) and phloretin was more potent than ploridzin in inhibiting uptake. The structure of glucose transporters was examined by photoaffinity labeling using 3H-cytochalasin B and by immunologic detection using antibodies raised against the human erythrocyte transporter. 3H-cytochalasin B labeled two proteins of 55 kDa and 43 kDa and the antibody recognized primarily a 43 kDa protein. The subcellular distribution of glucose transporters, estimated by measuring the number of specific cytochalasin B binding sites in subfractions of neuronal homogenates, showed 3.62 pmol/mg protein in the 11,000g pellet and 1.34 pmol/mg protein in the 200,000g pellet. IN CONCLUSION: 1) Neuronal glucose transport is not acutely regulated by insulin. 2) The kinetics of 2-deoxyglucose uptake into neurons are determined largely by hexokinase activity rather than membrane transport. 3) The apparent molecular weight of neuronal glucose transporters is similar to transporters in other tissues. 4) The number of glucose transporters per milligram of protein is relatively low in neurons compared to other tissues.

Animals↗
PubMed1989

Dexamethasone regulates the glucose transport system in primary cultured adipocytes: different mechanisms of insulin resistance after acute and chronic exposure.

We have studied the ability of dexamethasone to regulate the glucose transport system in primary cultured adipocytes and delineated the mechanisms of insulin resistance after both acute and chronic treatment. Acutely, 20 nM dexamethasone led to a 65% decrease in basal and a 31% decrement in maximally insulin-stimulated glucose transport (ED50 = 3-4 nM; t1/2 = 50 min). These effects were maximal by 90-120 min, and a plateau was maintained over an additional 1-1.5 h. Chronic dexamethasone exposure (24 h) led to a more profound decrease in basal (77%; ED50 = 0.4 nM) and maximally stimulated (55%; ED50 = 1.0 nM) rates of glucose transport and shifted the transport: insulin dose-response curve to the right by increasing the half-maximally effective insulin concentration from 0.2 to 0.4 ng/ml. Dexamethasone did not affect cell surface insulin binding over 24 h. Both the short and long term effects of dexamethasone were partially blocked by the combined presence of insulin during preincubation and were not modulated by glucose. We also assessed effects on the number and cellular distribution of glucose transporter proteins using the cytochalasin-B binding assay. After 2 h, dexamethasone (30 nM) decreased the number of glucose transporters in plasma membranes by 30% in basal cells and by 41% in maximally insulin-stimulated cells, while increasing the number of low density microsomal transporters by 22-23% (P = NS). Transporter number in a total cellular membrane fraction was unaltered by short term dexamethasone. Chronic dexamethasone exposure (24 h) decreased plasma membrane and low density microsomal transporters by 30-50% in both basal and insulin-stimulated cells and depleted transporters by 43% in a total cellular membrane fraction. In conclusion, 1) dexamethasone induces progressive insulin resistance by sequentially regulating multiple aspects of the insulin-responsive glucose transport system. At early times (2 h) dexamethasone impairs insulin's ability to translocate intracellular glucose transporters to the cell surface and with more chronic exposure (24 h), depletes the total number of cellular transporters. 2) Glucose modulates desensitization of the glucose transport system by insulin, but not by dexamethasone, and thus, there are both glucose-dependent and -independent mechanisms of insulin resistance. 3) Insulin can heterologously inhibit dexamethasone's effects on glucose transport at both early and late phases of desensitization. These studies highlight the complex hormonal regulation at the glucose transport system.

Adipose Tissue↗
PubMed1989

Mechanism of insulin resistance induced by sustained levels of cytosolic free calcium in rat adipocytes.

We have recently provided evidence that elevated levels of cytosolic free Ca2+ ([Ca2+]i) decreased insulin-stimulated glucose uptake in isolated rat adipocytes. To investigate the mechanism of Ca2+ action, we examined the effects of elevated levels of [Ca2+]i on insulin binding, autophosphorylation, and tyrosine kinase activity (TKA) of insulin receptors as well as basal and insulin-stimulated cellular distribution of glucose transporters. The latter was assessed by cytochalasin-B binding to plasma membrane and cytosolic fractions. Elevated concentrations of [Ca2+]i were maintained by incubating adipocytes with a depolarizing concentration of K+ (40 mM). Basal nonstimulated glucose uptake was not altered by increased levels of [Ca2+]i. Adipocytes with higher [Ca2+]i (220 +/- 15 nM) showed 30% reduction in insulin-stimulated 2-deoxyglucose uptake compared with control cells ([Ca2+]i, 140 +/- 18 nM). Moreover, adipocytes with higher levels of [Ca2+]i demonstrated an approximately 10% reduction in autophosphorylation and TKA of insulin receptors without a change in insulin binding. Both basal and insulin-stimulated distributions of glucose transporters were unaffected by sustained levels of [Ca2+]i. The effects of elevated [Ca2+]i were not mimicked by protein kinase-C activation. These observations suggest that 1) elevated or sustained levels of [Ca2+]i impair insulin-stimulated glucose uptake; and 2) Ca2+-induced impairment appears to reside at the postbinding steps of insulin action and probably interferes with the TKA of insulin receptors and the intrinsic activity of glucose transporters.

Adipose Tissue↗
PubMed1989

Expression of a glucose transporter gene cloned from brain in cellular models of insulin resistance: dexamethasone decreases transporter mRNA in primary cultured adipocytes.

In two cellular models of insulin resistance we measured glucose transport activity, total glucose transporter number using the cytochalasin B binding assay, and expression of a transporter mRNA species specifically hybridizing with cDNA cloned from brain. In primary cultured adipocytes, chronic exposure to glucose plus insulin (24 h), but neither agent alone, markedly decreased (less than 50%) glucose transport activity; however, neither glucose nor insulin regulated the number of glucose transporters or levels of transporter mRNA whether normalized per total RNA, RNA per cell, or as a fraction of CHO-B mRNA. On the other hand, chronic treatment with 30 nM dexamethasone (24 h) decreased basal and maximal transport rates (approximately 75%), led to a 40% depletion in total cellular glucose transporters, and decreased transporter mRNA by 57-59% (t 1/2 = 10 h; ED50 = 4-5 nm). Dexamethasone's effects to decrease transport rates, transporter protein, and mRNA were inhibited by coincubation with insulin. Dexamethasone did not alter the degradation rate of transporter mRNA relative to that in control cells indicating a lack of effect on mRNA stability. Also, suppression of transporter mRNA did not appear to require ongoing protein synthesis since the effect was observed when dexamethasone was added to cycloheximide-treated cells; however, cycloheximide per se specifically increased transporter mRNA 4-fold. We conclude in adipocytes: 1) glucose and insulin (24 h) do not regulate the total number of glucose transporters or expression of mRNA encoding a transporter species cloned from brain. 2) Long-term dexamethasone treatment reduces the cellular abundance of both glucose transporters and the specific transporter mRNA; these effects may be due to inhibition of gene transcription since dexamethasone does not influence transporter mRNA stability. 3) Insulin heterologously inhibits regulation of the glucose transport system by dexamethasone. 4) Dexamethasone-mediated insulin resistance is due in part to regulation of a glucose transporter species encoded by cDNA cloned from brain. These observations may be relevant to mechanisms of insulin resistance in clinical states of hypercortisolism.

Adipose Tissue↗
PubMed1988

Cellular mechanisms of insulin resistance in non-insulin-dependent (type II) diabetes.

Recent studies have led to an enhanced understanding of cellular alterations that may play an important role in the pathophysiology of non-insulin-dependent diabetes mellitus (NIDDM). The insulin receptor links insulin binding at the cell surface to intracellular activation of insulin's effects. This transducer function involves the tyrosine kinase property of the beta-subunit of the receptor. It was found that adipocytes from subjects with NIDDM had a 50 to 80 percent reduction in insulin-stimulated receptor kinase activity compared with their non-diabetic counterparts. This defect was relatively specific for the diabetic state since no decrease was observed in insulin-resistant non-diabetic obese subjects. The reduction in kinase activity was accounted for by changes in the ratio of two pools of receptors, both of which bind insulin but only one of which is capable of tyrosine autophosphorylation and subsequent kinase activation; 43 percent of the receptors from non-diabetic subjects were capable of autophosphorylation compared with only 14 percent in the NIDDM group. A major component of cellular insulin resistance in NIDDM involves the glucose transport system. Exposure of cells to insulin normally results in enhanced glucose transport mediated by translocation of glucose transporters from a low-density microsomal intracellular pool to the plasma membrane. It was found that cells from NIDDM subjects had a marked depletion of glucose transporters in both plasma membranes and low-density microsomes, relative to obese non-diabetic control participants. Obese non-diabetic persons had a normal number of plasma membrane transporters but a reduced number of low-density microsome transporters in the basal state compared with lean control volunteers; insulin induced the translocation of relatively fewer transporters from the low-density microsome to the plasma membrane in the obese subgroups. In addition to the diminished number of glucose transporters, cells from both NIDDM and obese subjects had impaired functional activity of glucose carriers since decreased whole-cell glucose transport rates could not be entirely explained by the magnitude of the decrement in the number of plasma membrane transporters. Thus, impaired glucose transport is due to both a numerical and functional defect in glucose transporters. The cellular content of high-density microsomal transporters was the same in lean and obese control volunteers and NIDDM subjects, suggesting that transporter synthesis is normal and that cellular depletion results from increased protein turnover once transporters leave the high-density microsomal subfraction.(ABSTRACT TRUNCATED AT 400 WORDS)

Adipose Tissue↗
PubMed1988

Role of glucose transporters in the cellular insulin resistance of type II non-insulin-dependent diabetes mellitus.

To examine the role of glucose transport proteins in cellular insulin resistance, we studied subcutaneous adipocytes isolated from lean control, obese control (body mass index [BMI] 33.4 +/- 0.9), and untreated obese non-insulin-dependent diabetes mellitus (NIDDM) patients (BMI 35.2 +/- 2.1; fasting glucose 269 +/- 20 mg/dl). Glucose transporters were measured in plasma membrane (PM), low-density (LDM), and high-density (HDM) microsomal subfractions from basal and maximally insulin-stimulated cells using the cytochalasin B binding assay, and normalized per milligram of membrane protein. In all subgroups, insulin led to an increase in PM glucose transporters and a corresponding depletion of transporters in the LDM. Insulin recruited 20% fewer transporters to the PM in the obese subgroup when compared with lean controls, and this was associated with a decline in LDM transporters with enlarging cell size in the control subjects. In NIDDM, PM, and LDM, transporters were decreased 50% in both basal and stimulated cells when compared with obese controls having similar mean adipocyte size. Cellular depletion of glucose transporters was not the only cause of insulin resistance, because the decrease in rates of [14C]-D-glucose transport (basal and insulin-stimulated) was greater than could be explained by reduced numbers of PM transporters in both NIDDM and obesity. In HDM, the number of transporters was not influenced by insulin and was similar in all subgroups. We conclude that (a) in NIDDM and obesity, both reduced numbers and impaired activity of glucose transporters contribute to cellular insulin resistance, and (b) in NIDDM, more profound cellular insulin resistance is associated primarily with a further depletion of cellular transporters.

Adipose Tissue↗
PubMed1988

Day-long integrated serum insulin and C-peptide profiles in patients with NIDDM. Correlation with urinary C-peptide excretion.

To determine whether non-insulin-dependent diabetes mellitus (NIDDM) is characterized by day-long hypoinsulinemia, we measured 24-h serum profiles for glucose, insulin, and C-peptide by use of a constant-rate blood-withdrawal technique in diabetic and control subjects fed isocaloric meals. When only lean subjects were considered, diabetic subjects (relative body weight 0.99 +/- 0.3) and control subjects (relative body weight 0.95 +/- 0.03) had similar 24-h integrated serum insulin concentrations (13.4 +/- 2.5 vs. 16.1 +/- 2.0 microU/ml, P NS) due to the offsetting effects of increased basal levels and decreased postprandial responses in NIDDM. In contrast, both basal and meal-stimulated insulin levels were decreased in obese NIDDM subjects (relative body weight 1.39 +/- 0.07) compared with obese control subjects (relative body weight 1.60 +/- 0.08), resulting in a 61% reduction in the 24-h integrated insulin value (18.7 +/- 1.5 vs. 48.4 +/- 13.7 microU/ml). Thus, the capacity to increase 24-h integrated serum insulin as a function of relative body weight was impaired in NIDDM subjects (r = 0.27, P NS) compared with control subjects (r = .70, P less than .01). In contrast, 24-h integrated C-peptide was decreased (P less than .01) in both lean (0.92 +/- 0.13 pM/ml) and obese (1.52 +/- 0.19 pM/ml) NIDDM patients compared with the respective control groups (1.50 +/- 0.13 and 3.03 +/- 0.44 pM/ml). The molar ratio of 24-h integrated C-peptide to insulin was diminished in lean but not obese NIDDM compared with control subjects. A 3-wk period of intensive insulin therapy led to normalization of the mean 24-h integrated insulin (but not integrated serum C-peptide) value in NIDDM compared with a control group that had an identical mean relative body weight. The 24-h urinary C-peptide measured on the same day as the serum profile was correlated (P less than .01) with both the 24-h integrated serum insulin (r = .69) and C-peptide (r = .67) concentrations in control subjects but not in NIDDM subjects (r = .20 and .04, respectively, P NS). Additionally, the urinary clearance of C-peptide was increased in NIDDM (38.1 +/- 7.8 vs. 20.4 +/- 1.7 ml/min in control subjects, P less than .05) and varied with treatment status (26.0 +/- 4.6 ml/min after insulin therapy).(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1987

Shunting of insulin from a retroendocytotic pathway to a degradative pathway by sodium vanadate.

Adipocytes route internalized insulin through two major pathways, a degradative pathway and a retroendocytotic pathway. To examine whether sorting of incoming insulin-receptor complexes can be altered, we assessed the effect of vanadate on the intracellular processing of both insulin and insulin receptors. After cells were pretreated with vanadate (1 mM for 30 min at 37 degrees C), 125I-insulin was loaded into the cell interior. When the net efflux of insulin from cells into the medium was then monitored, vanadate was found to slow the efflux of insulin from a t1/2 of 6.2 min (controls) to 11 min. Since efflux reflects both the rapid extrusion of intact insulin and the slower release of degradative products, we proposed that vanadate diverts more insulin into the degradative pathway. Further evidence in support of this idea included the following: 1) when intracellular degradation of insulin was impaired by chloroquine, undegraded insulin accumulated faster within vanadate-treated cells, consistent with greater flux through a degradative pathway; 2) vanadate increased the percentage of degraded insulin released from cells from 61 and 72%; and 3) under steady-state binding conditions, more insulin resided in the cell interior of vanadate-treated cells (44.8% versus 34.5%), and the time required for the intracellular pool to reach equilibrium was prolonged (t1/2 of 5.5 min versus 4.0). Neither insulin internalization nor degradation was impaired by vanadate alone. In related studies Tris was found to inhibit insulin-mediated receptor recycling by only 10%, whereas in the presence of vanadate (plus Tris) almost all incoming insulin receptors were prevented from recycling. Vanadate alone had no effect on the ability of insulin receptors to recycle. Based on these results we conclude that: 1) vanadate shunts incoming insulin from a more rapid retroendocytotic pathway to a slower degradative pathway and diverts insulin receptors from a Tris-insensitive recycling pathway to one that can be completely inhibited by Tris; 2) these effects are selective, in that vanadate impairs neither insulin degradation nor receptor uptake and recycling. Considered together, these findings support the idea that a sorting mechanism exists for the intracellular routing of incoming insulin-receptor complexes.

Adipose Tissue↗
PubMed1987

Glucose and insulin co-regulate the glucose transport system in primary cultured adipocytes. A new mechanism of insulin resistance.

We have previously shown in primary cultured rat adipocytes that insulin acts at receptor and multiple postreceptor sites to decrease insulin's subsequent ability to stimulate glucose transport. To examine whether D-glucose can regulate glucose transport activity and whether it has a role in insulin-induced insulin resistance, we cultured cells for 24 h in the absence and presence of various glucose and insulin concentrations. After washing cells and allowing the glucose transport system to deactivate, we measured basal and maximally insulin-stimulated 2-deoxyglucose uptake rates (37 degrees C) and cell surface insulin binding (16 degrees C). Alone, incubation with D-glucose had no effect on basal or maximal glucose transport activity, and incubation with insulin, in the absence of glucose, decreased maximal (but not basal) glucose transport rates only 18% at the highest preincubation concentration (50 ng/ml). However, in combination, D-glucose (1-20 mM) markedly enhanced the long-term ability of insulin (1-50 ng/ml) to decrease glucose transport rates in a dose-responsive manner. For example, at 50 ng/ml preincubation insulin concentration, the maximal glucose transport rate fell from 18 to 63%, and the basal uptake rate fell by 89%, as the preincubation D-glucose level was increased from 0 to 20 mM. Moreover, D-glucose more effectively promoted decreases in basal glucose uptake (Ki = 2.2 +/- 0.4 mM) compared with maximal transport rates (Ki = 4.1 +/- 0.4 mM) at all preincubation insulin concentrations (1-50 ng/ml). Similar results were obtained when initial rates of 3-O-methylglucose uptake were used to measure glucose transport. D-glucose, in contrast, did not influence insulin-induced receptor loss. In other studies, D-mannose and D-glucosamine could substitute for D-glucose to promote the insulin-induced changes in glucose transport, but other substrates such as L-glucose, L-arabinase, D-fructose, pyruvate, and maltose were without effect. Also, non-metabolized substrates which competitively inhibit D-glucose uptake (3-O-methylglucose, cytochalasin B) blocked the D-glucose plus insulin effect.(ABSTRACT TRUNCATED AT 400 WORDS)

3-O-Methylglucose↗
PubMed1987

Xanthogranulomatosis in an adult: lipid analysis of xanthomas and plasma.

Xanthomatosis in the absence of hyperlipidemia is unusual but has been associated with compositional abnormalities of lipoprotein particles. An adult who developed juvenile xanthogranulomatosis in association with oral contraceptive ingestion is reported. Plasma lipids and lipoprotein electrophoresis were normal, as in a few other patients reported with this disorder. However, analysis of cutaneous xanthoma and plasma by thin-layer and gas-liquid chromatography revealed that cholesterol was the principal lipid in xanthoma and that there were no unusual sterols in plasma or tissue. Possible mechanisms of xanthoma formation are discussed. Thus juvenile xanthogranulomatosis should be considered in adults with normolipemic xanthomatosis.

Adult↗
PubMed1987

Human adipocyte glucose transport system. Biochemical and functional heterogeneity of hexose carriers.

We have investigated glucose transport proteins in isolated human adipocytes. Using the cytochalasin B binding assay to measure glucose transporters in subcellular membrane subfractions, we found that insulin induced translocation of intracellular glucose transporters to the cell surface. Isoelectric focusing of glucose transporters photolabeled with [3H]cytochalasin B revealed two distinct glucose transporter isoforms in low density microsomes focusing at pH 5.6 and pH 6.4, but only the pH 5.6 isoform was detectable in plasma membranes and only the pH 6.4 form was found in the high density microsomes. Insulin recruited only the pH 5.6 glucose transporter from the low density microsomes to the plasma membrane with no effect on the pH 6.4 transporter isoform. The results suggest that the pH 6.4 species is an immature form of the glucose transporter initially located in the high-density microsome fraction, which then migrates to the low-density microsomes where it matures (converted to pH 5.6 species) and becomes available for insulin-mediated recruitment to the plasma membrane.

Adipose Tissue↗
PubMed1986

Insulin induces progressive insulin resistance in cultured rat adipocytes. Sequential effects at receptor and multiple postreceptor sites.

We have examined the ability of insulin to regulate insulin action in primary cultured adipocytes, and found that insulin induces progressive insulin resistance in this target tissue. To assess effects at both receptor and postreceptor sites, we cultured cells in the absence (control) and presence of 100 ng/ml insulin, and, after various times, measured the dose response of insulin's ability to bind cell-surface receptors and stimulate 2-deoxyglucose transport. In control cells, insulin binding (0.2 ng/ml) was increased 10-13% due to an apparent increase in receptor affinity (6-24 h). A comparable increase in affinity was also observed in treated cells; however, concomitantly, insulin decreased the number of cell-surface receptors causing a slowly progressive net decrease in binding after a 6-10-h lag (maximal 30% at 24 h). When insulin action was assessed in control cells, the functional consequence of increased receptor binding was hypersensitization (i.e., increased insulin sensitivity) manifested by a leftward shift in the 2-deoxyglucose dose-response curve. On the other hand, in the treated cells, insulin produced insulin resistance initially by decreasing insulin sensitivity. The ED50 for insulin stimulation of glucose transport increased 84% from 0.31 to 0.57 ng/ml at 6 h without a net change in insulin binding; this was the result of a decrease in coupling efficiency between occupied receptors and the insulin effect. Receptor uncoupling progressively increased in severity, but before the full effect was reached insulin also caused a rapid decline in maximally stimulated glucose transport rates (between 6 and 10 h). This decrease in insulin responsiveness (maximal 52%) exacerbated overall insulin resistance, and was indicative of a postreceptor defect in the glucose transport system. Finally, insulin-induced receptor downregulation contributed, along with uncoupling, to a further decrease in insulin sensitivity, and constituted a more long-term regulatory mechanism. We also observed that insulin could regulate the basal glucose transport system by preventing a progressive rise in basal transport observed in control cells. In conclusion, primary cultured adipocytes can be used to study long-term regulation of insulin action. We found that insulin induces progressive insulin resistance with sequential effects at multiple sites in the insulin action pathway, including decreased coupling efficiency between occupied receptors and stimulated glucose transport, a postreceptor defect in insulin responsiveness of the glucose transport system, and receptor downregulation. These mechanisms may be relevant to the cellular defects in insulin action present i

3-O-Methylglucose↗
PubMed1985

Insulin receptor down-regulation is linked to an insulin-induced postreceptor defect in the glucose transport system in rat adipocytes.

We have examined the relationship between insulin-induced receptor downregulation and the induction of a postreceptor defect in the insulin-stimulated glucose transport system in rat adipocytes, and found that downregulation was linked to the expression of the postreceptor defect. When recycling of insulin receptors was inhibited by 20 mM Tris, insulin pretreatment (100 ng/ml) for 4 h at 37 degrees C induced both net loss (65%) of cell-surface receptors and a 63% decrease in maximal insulin responsiveness. In contrast, when cells were treated with insulin alone for 4 h at 37 degrees C so that receptors could recycle, or treated at 16 degrees C with Tris plus insulin to inhibit receptor internalization, neither receptor downregulation nor a postreceptor defect was observed. Induction of the postreceptor defect was specific for insulin under conditions when downregulation would occur, since treatment of cells with Tris and the insulin mimicker spermine did not result in receptor loss or the postreceptor defect. Other experiments revealed that receptor downregulation occurred first without loss of insulin responsiveness, but, once the postreceptor defect appeared, its severity was correlated to the degree of further receptor loss, as a function of insulin dose and exposure time. Tris (20 mM) alone acutely decreased maximally stimulated glucose transport rates slightly (22%), but this effect was rapidly reversible after Tris removal and could not have been directly responsible for the lasting and profound postreceptor defect seen after pretreatment with insulin plus Tris. Taken together, these data suggest that insulin-induced receptor loss is linked to the induction of the postreceptor defect. The postreceptor defect was due to an inability to maximally increase the maximum velocity of glucose transport. Furthermore, the expression of the postreceptor defect depended upon the extent to which the glucose transport system was allowed to deactivate; maintaining the glucose transport system in an activated state prevented its expression. Thus, the mechanism could involve rapid inactivation or sequestration of glucose transporters during deactivation such that they become refractory to the subsequent stimulatory effects of insulin. In conclusion, (a) insulin does not acutely induce a postreceptor defect in the glucose transport system of adipocytes without loss of cell-surface insulin receptors; (b) the defect in stimulated glucose transport has been induced distal to the insulin receptor via a mechanism linked to receptor loss; and (c) the postreceptor lesion is due to decreased number of intrinsic activity of glucose transporters on the cell-surface in the presence of a maximally effective insulin concentration. These data suggest that insulin receptor downregulation and postreceptor defects in insulin action, which frequently co-exist both in vivo and in vitro, may be linked mechanistically.

Adipose Tissue↗
PubMed1985

Modulation of insulin secretion by insulin and glucose in type II diabetes mellitus.

We studied the dose-response characteristics of insulin's ability to modulate its own secretion in normal and type II diabetic (NIDDM) subjects by measuring suppression of serum C-peptide levels during insulin infusions with the plasma glucose level held constant. In normal subjects at euglycemia, primed continuous insulin infusion rates of 15, 40, 120, and 240 mU/M2 X min acutely raised serum insulin to steady state levels of 37 +/- 2 (+/- SE), 96 +/- 6, 286 +/- 17, and 871 +/- 93 microU/ml, respectively. During each infusion, maximal suppression of C-peptide to 30% of basal levels occurred by 130 min. At the higher insulin levels (greater than or equal to 100 microU/ml), C-peptide levels fell rapidly, with an apparent t1/2 of 13 min, which approximates estimates for the t1/2 of circulating C-peptide in man. This is consistent with an immediate 70% inhibition of the basal rate of insulin secretion. At the lower insulin level (37 +/- 2 microU/ml), C-peptide levels fell to 30% of basal values less rapidly (apparent t1/2, 33 min), suggesting that 70% inhibition of basal insulin secretion rates was achieved more slowly. In NIDDM subjects, primed continuous insulin infusion rates of 15, 40, 120, and 1200 mU/M2 X min acutely raised serum insulin to steady state levels of 49 +/- 7, 93 +/- 11,364 +/- 31, and 10,003 +/- 988 microU/ml. During studies at basal hyperglycemia, only minimal C-peptide suppression was found, even at pharmacological insulin levels (10,003 +/- 988 microU/ml). However, if plasma glucose was allowed to fall during the insulin infusions, there was a rapid decrease in serum C-peptide to 30% of basal levels, analogous to that in normal subjects. Three weeks of intensive insulin therapy did not alter C-peptide suppression under conditions of hyperinsulinemia and falling plasma glucose. The following conclusions were reached. 1) In normal subjects, insulin (40-1000 microU/ml) inhibits its own secretion in a dose-responsive manner; more time is required to achieve maximal 70% suppression at the lower insulin level (40 microU/ml). 2) In NIDDM studied at basal hyperglycemia, insulin has minimal ability to suppress its own secretion. Thus, impaired feedback inhibition could contribute to basal hyperinsulinemia. 3) Under conditions of hyperinsulinemia and falling plasma glucose, insulin secretion is rapidly suppressed in NIDDM (analogous to that in normal subjects studied during euglycemia.

Adult↗
PubMed1985

The effect of insulin treatment on insulin secretion and insulin action in type II diabetes mellitus.

We have studied the effects of 3 wk of continuous subcutaneous insulin infusion (CSII) on endogenous insulin secretion and action in a group of 14 type II diabetic subjects with a mean (+/-SEM) fasting glucose level of 286 +/- 17 mg/dl. Normal basal and postprandial glucose levels were achieved during insulin therapy at the expense of marked peripheral hyperinsulinemia. During the week of posttreatment evaluation, the subjects maintained a mean fasting glucose level of 155 +/- 11 mg/dl off insulin therapy, indicating a persistent improvement in carbohydrate homeostasis. Adipocyte insulin binding and in vivo insulin dose-response curves for glucose disposal using the euglycemic clamp technique were measured before and after therapy to assess the effect on receptor and postreceptor insulin action. Adipocyte insulin binding did not change. The insulin dose-response curve for overall glucose disposal remained right-shifted compared with age-matched controls, but the mean maximal glucose disposal rate increased by 74% from 160 +/- 14 to 278 +/- 18 mg/m2/min (P less than 0.0005). The effect of insulin treatment on basal hepatic glucose output was also assessed; the mean rate was initially elevated at 159 +/- 8 mg/m2/min but fell to 90 +/- 5 mg/m2/min in the posttreatment period (P less than 0.001), a value similar to that in control subjects. Endogenous insulin secretion was assessed in detail and found to be improved after exogenous insulin therapy. Mean 24-h integrated serum insulin and C-peptide concentrations were increased from 21,377 +/- 2766 to 35,584 +/- 4549 microU/ml/min (P less than 0.01) and from 1653 +/- 215 to 2112 +/- 188 pmol/ml/min (P less than 0.05), respectively, despite lower glycemia. Second-phase insulin response to an intravenous (i.v.) glucose challenge was enhanced from 170 +/- 53 to 1022 +/- 376 microU/ml/min (P less than 0.025), although first-phase response remained minimal. Finally, the mean insulin and C-peptide responses to an i.v. glucagon pulse were unchanged in the posttreatment period, but when glucose levels were increased by exogenous glucose infusion to approximate the levels observed before therapy and the glucagon pulse repeated, responses were markedly enhanced. Simple and multivariate correlation analysis showed that only measures of basal hepatic glucose output and the magnitude of the postbinding defect in the untreated state could be related to the respective fasting glucose levels in individual subjects.(ABSTRACT TRUNCATED AT 400 WORDS)

Adipose Tissue↗
PubMed1985

Insulin resistance in non-insulin dependent (type II) and insulin dependent (type I) diabetes mellitus.

Insulin resistance is a characteristic feature of non-insulin dependent diabetes mellitus (NIDDM) due to target tissue defects in insulin action. Abnormalities of cellular insulin action can be divided into receptor and post-receptor defects. Patients with impaired glucose tolerance are insulin resistant due to decreased insulin receptors resulting in decreased insulin sensitivity and rightward shifted in vivo dose response curves. Patients with NIDDM are insulin resistant due to a combination of receptor and post-receptor defects. The greater the severity of the diabetes (greater fasting hyperglycemia) the greater the post-receptor defect, and in those patients with more significant fasting hyperglycemia the post-receptor defect is the predominant abnormality leading to the insulin resistant state. At least one of the abnormalities underlying this post-receptor defect involves a decrease in glucose transport system activity in freshly isolated adipocytes. This defect in glucose transport, is not expressed in cultured fibro-blasts, indicating that the abnormality in glucose disposal seen in vivo and in glucose transport seen in freshly isolated cells is an acquired phenomenon. Consistent with this, the post-receptor defect is partially reversible by insulin therapy, which leads to a 50-70% reversal of the reduced rates of in vivo glucose disposal and in vitro glucose transport. Insulin resistance also exists in poorly controlled IDDM patients, due to a postreceptor defect in insulin action. This insulin resistance is not present in well controlled IDDM patients, and is completely reversible when poorly controlled patients are treated with intensive insulin therapy. Insulin is produced in the pancreatic beta cell as the primary biosynthetic product preproinsulin. This peptide is rapidly converted to proinsulin (MW approximately 9000). Proinsulin is converted to insulin (MW approximately 6000) plus C-peptide in the secretory granule with a small amount (approximately 5 percent) of the proinsulin remaining unconverted. After a brief time in the peripheral circulation (half-life six to 10 minutes), insulin interacts with target tissues to exert its biologic effects. One of insulin's major biologic effects is the promotion of overall glucose metabolism, and abnormalities of this aspect of insulin action can lead to a number of important clinical and pathophysiologic states including Type II diabetes, also known as non-insulin-dependent diabetes mellitus (NIDDM). Since insulin travels from the beta cell through the circulation to the target tissues, abnormalities at any of these loci can influence the ultimate action of the hormone. These abnormalities, all

Diabetes Mellitus↗