Intraportal hyperinsulinemia decreases insulin-stimulated glucose uptake in the dog

Hyperinsulinemia and insulin resistance are commonly seen in obese and non-insulin-dependent diabetes mellitis (NIDDM) patients. While it is known that chronic exposure to severe hyperinsulinemia can lead to an insulin-resistant state and mild hyperinsulinemia for rather short durations (20 to 40 hours) and can also lead to insulin resistance, it is less clear whether mild hyperinsulinemia for a more prolonged duration can lead to insulin resistance. In the present study we determined the effects of chronic (28 days) exposure to mild hyperinsulinemia on insulin-stimulated glucose use. Chronic hyperinsulinemia was produced by an intraportal infusion of porcine insulin (425 microU/kg/min), which raised the basal peripheral insulin levels by approximately 50%. Insulin responsiveness was assessed using the euglycemic hyperinsulinemic clamp (2 mU/kg/min) in dogs before the induction of chronic hyperinsulinemia (day 0), after 28 days of hyperinsulinemia (day 28), and 28 days after discontinuation of the chronic insulin infusion (day 56). The amount of glucose (M) required to maintain euglycemia during the euglycemic hyperinsulinemic clamp was decreased (relative to day 0) 39% +/- 3% on day 28 and 18% +/- 3% on day 56 (P less than .05). In control animals that received a chronic infusion of saline for the 28-day period the glucose infusion rate (M) was not changed significantly (decreasing 2% +/- 5% and 5% +/- 10% on days 28 and 56, respectively). In conclusion insulin resistance can be produced by a mild hypersecretion of insulin and discontinuation of the chronic insulin infusion tends to reverse the resistance.     

Comparison of the priming effects of pulsatile and continuous insulin delivery on insulin action in man

Insulin is normally secreted in man in regular pulses every 5 to 15 minutes. Disordered pulsation has been demonstrated in several insulin-resistant states and it is unclear whether this represents a primary beta-cell defect contributing to impairment of peripheral insulin action or rather is a consequence of insulin resistance. Basal or near basal insulin administration by pulsatile infusion augments hypoglycemic effect and improves insulin-mediated glucose uptake compared with insulin by continuous infusion. To date no study has examined whether normal basal insulin pulsatility is required to preserve subsequent insulin sensitivity during hyperinsulinemia. We studied the effect of overnight pulsatile versus continuous basal insulin on a subsequent hyperinsulinemic euglycemic clamp. Nineteen normal volunteers (male:female ratio, 17:2; mean age +/- SEM, 26.1 +/- 2.3 years) were studied on 2 occasions each. Endogenous insulin secretion was inhibited by octreotide (0.43 microg kg(-1). h(-1)) and replaced overnight at 5.4 mU kg(-1). h(-1) either by continuous infusion or in 2-minute pulses every 13 minutes (n = 10) or every 7 minutes (n = 9). Glucagon was replaced at physiological concentration by continuous infusion (30 ng. kg(-1). h(-1)). Venous plasma glucose overnight was not significantly different between the pulsatile and continuous protocols. After discontinuing the overnight insulin infusion, insulin action was assessed during a hyperinsulinemic euglycemic clamp (1 mU kg(-1). h(-1)). Glucose infusion rates at steady-state during the hyperinsulinemic clamp were similar between continuous and both frequencies of pulsatile infusion (continuous 44.6 +/- 4.3 micromol. kg(-1). min(-1) v 13-minute pulsatile 41.7 +/- 5.9 micromol. kg(-1). min(-1), P =.27; continuous 34.6 +/- 2.5 micromol. kg(-1) min(-1) v 7-minute pulsatile 41.4 +/- 3.2 micromol. kg(-1). min(-1), P =.08). We conclude that overnight pulsatile compared with continuous insulin administration has no different effect on subsequent peripheral insulin-mediated glucose uptake. A priming effect cannot therefore explain the previously demonstrated association between endogenous insulin pulse frequency and peripheral insulin action.     

Ghrelin enhances glucose-induced insulin secretion in scheduled meal-fed sheep

The purpose of this study was to investigate the effects of physiologic levels of ghrelin on insulin secretion and insulin sensitivity (glucose disposal) in scheduled fed-sheep, using the hyperglycemic clamp and hyperinsulinemic euglycemic clamp respectively. Twelve castrated Suffolk rams (69.8 +/- 0.6 kg) were conditioned to be fed alfalfa hay cubes (2% of body weight) once a day. Three hours after the feeding, synthetic ovine ghrelin was intravenously administered to the animals at a rate of 0.025 and 0.05 mug/kg body weight (BW) per min for 3 h. Concomitantly, the hyperglycemic clamp or the hyperinsulinemic euglycemic clamp was carried out. In the hyperglycemic clamp, a target glucose concentration was clamped at 100 mg/100 ml above the initial level. In the hyperinsulinemic euglycemic clamp, insulin was intravenously administered to the animals for 3 h at a rate of 2 mU/kg BW per min. Basal glucose concentrations (44+/- 1 mg/dl) were maintained by variably infusing 100 mg/dl glucose solution. In both clamps, plasma ghrelin concentrations were dose-dependently elevated and maintained at a constant level within the physiologic range. Ghrelin infusions induced a significant (ANOVA; P < 0.01) increase in plasma GH concentrations. In the hyperglycemic clamp, plasma insulin levels were increased by glucose infusion and were significantly (P < 0.05) greater in ghrelin-infused animals. In the hyperinsulinemic euglycemic clamp, glucose infusion rate, an index of insulin sensitivity, was not affected by ghrelin infusion. In conclusion, the present study has demonstrated for the first time that ghrelin enhances glucose-induced insulin secretion in the ruminant animal.     

In vivo insulin sensitivity in adrenodemedullated rats

Insulin sensitivity and responsiveness were determined in adrenal demedullated rats (ADMX) with euglycemic insulin clamp technique. Adrenal medulla was extirpated bilaterally a week before the study. Catheters were placed at right atrium via right jugular vein for sampling blood and at inferior vena cava via femoral vein for the infusion of insulin and glucose solution. Insulin was infused at rates of 4.4, 8.8, 14.7, 29.3, 88.0, 293.0 mU/kg/min. Blood was collected every five min. during the clamp and glucose infusion rate was modulated to control the blood glucose concentrations at fasting levels. Glucose metabolism was calculated from the amount of glucose infused from 60th to 120th min. during the euglycemic clamp. The results obtained were as follows: 1. Glucose metabolisms of ADMX in each infusion rate of insulin, 4.4, 8.8, 14.7, 29.3, 88.0, 293.0 mU/kg/min were 5.2 +/- 0.5, 12.5 +/- 0.5, 17.6 +/- 1.2, 19.8 +/- 2.3, 29.0 +/- 1.5, and 25.2 +/- 1.9 mg/kg/min, respectively. 2. Glucose metabolisms of control group in each dose were 6.6 +/- 0.4, 9.0 +/- 0.9, 18.5 +/- 1.2, 23.4 +/- 2.4, 24.6 +/- 1.1, and 27.0 +/- 1.3 mg/kg/min, respectively. 3. Significant difference (p less than 0.01) in glucose metabolism between ADMX and control was observed at the insulin infusion rate of 8.8 mU/kg/min which might be equivalent to physiological hyperinsulinemia. 4. There were not any differences in insulin responsiveness between both groups. These results suggest that epinephrine regulates insulin sensitivity under physiological hyperinsulinemic condition via defects of insulin receptors.     

Insulin inhibits secretin-induced pancreatic bicarbonate output via cholinergic mechanisms

INTRODUCTION: Exogenous insulin inhibits secretin-stimulated pancreatic bicarbonate output via a dose-dependent mechanism; this effect is prevented by pancreatic denervation. AIMS: To investigate possible cholinergic mediation, we examined the effect of bethanechol on secretin-stimulated pancreatic secretion during euglycemic, hyperinsulinemic clamp. METHODOLOGY: In four dogs with chronic pancreatic fistulas, euglycemic, hyperinsulinemic clamp (1.25 mU/kg/min) was begun after a 30-minute basal period; computer-assisted glucose administration maintained euglycemia. Control studies were performed with volume-matched and rate-matched vehicle infusion. After 1 hour, secretin infusion was begun at a dosage of 16 ng/kg/h; the dose was doubled every 30 minutes. The studies were then repeated during background bethanechol infusion (90 microg/kg/h) begun 30 minutes after clamp initiation. Pancreatic juice was analyzed for bicarbonate and protein; serum samples were analyzed for glucose and insulin. RESULTS: Exocrine outputs and serum glucose and insulin levels did not differ in the basal period. Insulin levels were significantly elevated during the euglycemic, hyperinsulinemic clamp (62 microU/mL versus 12 microU/mL; p < 0.01); glucose levels did not differ. As before, secretin-induced bicarbonate output was inhibited by euglycemic, hyperinsulinemic clamp. The inhibitory effect of insulin was reversed by bethanechol. Despite the euglycemic, hyperinsulinemic clamp, secretin-induced bicarbonate output was potentiated by bethanechol at higher secretin doses (p < 0.05). CONCLUSION: These data confirm that cholinergic mechanisms mediate insulin's inhibition of secretin-induced pancreatic bicarbonate output.     

Feedback inhibition of insulin secretion is altered in cirrhosis

Hyperinsulinemia in human cirrhosis is generally considered an expression of reduced hepatic insulin degradation. To determine whether hyperinsulinemia may also depend on an altered feedback inhibition of insulin secretion, we performed euglycemic hyperinsulinemic clamp studies, infusing 40, 372, or 1280 mU/m2 X min biosynthetic human insulin in 30 compensated cirrhotic patients with portal hypertension and impaired glucose tolerance and 25 normal subjects, matched for age, sex, and weight. Mean fasting plasma insulin was significantly higher in cirrhotic patients [26.1 +/- 2.3 vs. 12.4 +/- 0.6 (+/- SE) microU/ml; P less than 0.001], while fasting plasma glucose levels were similar in the 2 groups. The mean plasma C-peptide level was significantly higher in cirrhotic patients, both basally (2.7 +/- 0.1 vs. 1.7 +/- 0.1 ng/ml; P less than 0.001) and during the clamp studies. Suppression of C-peptide at 120 min of the clamp was significantly less in cirrhotic patients (37 +/- 7% vs. 79 +/- 4%, 52 +/- 9% vs. approximately 100%, and 54 +/- 4% vs. approximately 100% during the 40, 372, and 1280 mU/m2 X min insulin infusions, respectively). The fasting C-peptide to insulin molar ratio was significantly lower in cirrhotic patients (5.4 +/- 0.3 vs. 6.4 +/- 0.3; P less than 0.005). The MCR of insulin at the three steady states was not significantly different between the 2 groups, whereas the basal systemic delivery rate of insulin was significantly higher in cirrhotic patients (14.7 +/- 1.7 vs. 6.5 +/- 0.4 mU/m2 X min; P less than 0.001). These results suggest that reduced feedback inhibition of insulin secretion may contribute to the hyperinsulinemia associated with cirrhosis.     

Similar effects of pulsatile and constant intravenous insulin delivery

Effects of constant and pulsatile i.v. insulin delivery were examined in seven healthy subjects by means of euglycemic clamp technique. Each subject received constant insulin infusion (0.175 mU/kg.min) or insulin pulses at 12-min intervals (2.1 mU/kg) in randomized order for 8-h periods (08.00-16.00 h). Endogenous secretion of insulin was inhibited by concomitant administration of somatostatin (300 micrograms/h). Serum insulin concentrations during constant infusion (12 +/- 1 microU/ml) did not differ from basal values (11 +/- 1 microU/ml). Pulsatile insulin delivery resulted in oscillations of mean concentrations between values of about 10 and 20 microU/ml. Mean blood glucose concentrations during experiments were kept at 80 +/- 1 mg/dl, irrespective of the mode of insulin administration. Moreover, dextrose requirements for maintenance of these glucose concentrations did not differ over the hole periods of examination. We conclude that effects of constant and pulsatile delivery of basal amounts of insulin are not different. This at least applies to peripheral, short-term insulin administration in somatostatin-treated normal man, during an euglycemic clamp.     

The role of endocrine counterregulation for estimating insulin sensitivity from intravenous glucose tolerance tests

CONTEXT: During insulin-modified frequently sampled iv glucose tolerance tests (IM-FSIGT), which allow assessment of insulin action, plasma glucose can markedly decrease. OBJECTIVE: This study aimed to assess the counterregulatory impact of the insulin-induced fall of glucose on minimal model-derived indices of insulin sensitivity (S(I)) and glucose effectiveness. PARTICIPANTS: Thirteen nondiabetic volunteers (seven males, six females, aged 26 +/- 1 yr, body mass index 22.1 +/- 0.7 kg/m(2)) were studied. DESIGN: All participants were studied in random order during IM-FSIGT (0.3 g/kg glucose; 0.03 U/kg insulin at 20 min) and during identical conditions but with a variable glucose infusion preventing a decrease of plasma glucose concentration below euglycemia (IM-FSIGT-CLAMP). Five participants additionally underwent euglycemic-hyperinsulinemic (1 mU.kg(-1).min(-1)) clamp tests. RESULTS: Plasma glucose declined during IM-FSIGT to its nadir of 50 +/- 3 mg/dl at 60 min in parallel to a rise (P < 0.05 vs. basal) of plasma glucagon, cortisol, epinephrine, and GH. Glucose infusion rates of 4.6 +/- 0.5 mg.kg(-1).min(-1) between 30 and 180 min during IM-FSIGT-CLAMP prevented the decline of plasma glucose and the hypoglycemia counterregulatory hormone response. S(I) was approximately 68% lower during IM-FSIGT (3.40 +/- 0.36 vs. IM-FSIGT-CLAMP: 10.71 +/- 1.06 10(-4).min(-1) per microU/ml, P < 0.0001), whereas glucose effectiveness did not differ between both protocols (0.024 +/- 0.002 vs. 0.021 +/- 0.003 min(-1), P = NS). Compared with the euglycemic hyperinsulinemic clamp test, S(I) expressed in identical units from IM-FSIGT was approximately 66% (P < 0.001) lower but did not differ between the euglycemic hyperinsulinemic clamp test and the IM-FSIGT-CLAMP (P = NS). CONCLUSIONS: The transient fall of plasma glucose during IM-FSIGT results in lower estimates of S(I), which can be explained by hormonal response to hypoglycemia.     

No effect of simvastatin treatment on insulin sensitivity in patients with primary hypercholesterolemia

OBJECTIVE: Statins, in addition to cholesterol lowering, have nonlipid effects on formation and progression of atheromatous plaque. Insulin resistance and hyperinsulinemia may have detrimental influences on the arterial wall. Statins (may also) inhibit insulin signal transferring in vascular smooth cell cultures. However, their effect on insulin sensitivity remains controversial. Therefore, we decided to investigate the effect of simvastatin on insulin sensitivity in hypercholesterolemic patients. PATIENTS AND METHODS: Eighteen patients with primary hypercholesterolemia were divided into simvastatin group (n = 9; 4 females, 5 males; BMI 30.6 +/- 4 kg/m2; mean ages 57 +/- 6 years) and placebo group (n = 9; 4 females, 5 males; BMI 28 +/- 2.9 kg/m2; mean ages 49 +/- 10 years). Simvastatin (20 mg/day) or placebo were given for 2 months. Total and HDL cholesterol were measured and LDL cholesterol was calculated by Friedewald formula. Insulin sensitivity was determined by using euglycemic hyperinsulinemic clamp technique [40 microU/m2/min insulin infusion rate; glucose disposal rate (M)= mg/kg/min] before and after treatment. RESULTS: Plasma levels of total, LDL and HDL cholesterol decreased significantly in simvastatin group after treatment (p = 0.000, p = 0.000, and p = 0.048, respectively). Plasma levels of total cholesterol decreased significantly (p = 0.032), whereas LDL and HDL levels remained unchanged in placebo group. M value (mg/kg/min) decreased insignificantly in simvastatin group (4.32 +/- 1.57 vs. 3.71 +/- 1.91) and increased in placebo group (3.55 +/- 1.91 vs. 3.95 +/- 0.95). CONCLUSION: Short-term simvastatin treatment did not affect insulin sensitivity determined by "gold standard" euglycemic hyperinsulinemic clamp method in hypercholesterolemic patients in this research. Further studies with simvastatin using higher doses and longer duration should be performed.     

Peripheral and hepatic insulin antagonism in hyperthyroidism

Eight hyperthyroid and eight normal subjects underwent 2-h oral glucose tolerance tests (OGTT) and euglycemic clamp studies to assess the presence of peripheral and hepatic insulin antagonism in hyperthyroidism. Although the mean total glucose area during the OGTT was similar in the hyperthyroid patients and normal subjects [16.4 +/- 0.8 (+/- SE) vs. 15.8 +/- 0.7 mmol/L.h], the mean insulin area was significantly elevated in the hyperthyroid group (1413 +/- 136 vs. 1004 +/- 122 pmol/L.h; P less than 0.05). Basal hepatic glucose production was measured during the second hour of a primed [3-3H]glucose infusion. A two-insulin dose euglycemic clamp study with [3-3H]glucose and somatostatin (500 micrograms/h) was carried out during the next 6 h. The insulin infusion rate was 0.05 mU/kg.min during the third, fourth, and fifth hours and 0.60 mU/kg.min during the sixth, seventh, and eighth hours. Hepatic glucose production and glucose utilization were measured during the final 0.5 h of each clamp period. Serum C-peptide concentrations were measured in the initial sample and in the last sample of each clamp period. The mean equilibrium serum insulin concentrations were similar in both groups during the final 0.5 h of the low (90 +/- 8 vs. 79 +/- 6 pmol/L) and high (367 +/- 11 vs. 367 +/- 15 pmol/L) insulin infusion rates. Basal serum C-peptide levels were significantly increased in the hyperthyroid patients (596 +/- 17 vs. 487 +/- 43 pmol/L; P less than 0.05) but were suppressed equally in both groups at the end of both clamp periods. The MCRs of insulin were similar in the hyperthyroid and normal subjects during the low (6.7 +/- 1.1 vs. 5.6 +/- 0.5 mL/kg.min) and high (11.9 +/- 0.4 vs. 12.1 +/- 0.5 mL/kg.mm) insulin infusion rates. Glucose production was significantly increased in the hyperthyroid patients during the basal state (17.6 +/- 0.9 vs. 11.5 +/- 0.5 mumol/kg.min; P less than 0.001) and remained elevated during the final 0.5 h of the low (12.1 +/- 1.1 vs. 5.9 +/- 1.7; P less than 0.01) and high (3.2 +/- 1.2 vs. 0.5 +/- 0.3; P less than 0.05) insulin infusion rates. Peripheral insulin action, assessed by Bergman's sensitivity index, was significantly decreased in the hyperthyroid patients (7.4 +/- 2.2 vs. 15.6 +/- 2.1 L/kg min-1/pmol/L; P less than 0.02). In conclusion, hyperthyroidism is characterized by 1) hyperinsulinemia after oral glucose loading, 2) increased basal hepatic glucose production, 3) impairment of insulin-mediated suppression of hepatic glucose production, and 4) antagonism to insulin-stimulated peripheral glucose utilization.     

Effect of acute physiological elevations of insulin on circulating androgen levels in nonobese women

Extreme pharmacological elevation of the circulating insulin level acutely lowers dehydroepiandrosterone sulfate (DHEAS) levels. To assess whether more physiological elevations in plasma insulin (due to exogenous infusion or endogenous secretion) would have similar effects, we examined the levels of DHEAS, androstenedione, testosterone, and free testosterone before and after euglycemic hyperinsulinemic and hyperglycemic hyperinsulinemic clamp studies. Studies were performed in women within 20% of ideal body weight after an overnight fast. Androgen levels were measured before and at the conclusion of studies in which either insulin was infused exogenously at 1 mU/kg.min or endogenous insulin secretion was stimulated for 2 h by elevation of the plasma glucose concentration by 125 mg/dL above basal levels by an exogenous glucose infusion. Basal plasma DHEAS (6.2 +/- 0.5 mumol/L) declined to 5.2 +/- 0.4 mumol/L (P less than 0.001) during the euglycemic insulin clamp, without any significant change in testosterone, free testosterone, or androstenedione. During the hyperglycemic clamp, DHEAS fell from 6.7 +/- 0.5 to 5.1 +/- 0.4 mumol/L (P less than 0.001) in response to endogenous hyperinsulinemia; plasma testosterone, free testosterone, and androstenedione did not change significantly. There was no correlation between the elevation in plasma insulin concentration and the fall in DHEAS during either the euglycemic or hyperglycemic clamps. However, the magnitude of fall of DHEAS was directly correlated with the initial DHEAS level in both the euglycemic (r = 0.51; P less than 0.05) and hyperglycemic (r = 0.75; P less than 0.01) studies. This association of hyperinsulinemia with a reduction of circulating levels of DHEAS, but not other C-19 steroids (e.g. testosterone and androstenedione) may reflect differential mechanisms by which DHEAS levels are regulated and suggests that insulin either inhibits its biosynthesis and/or secretion, or enhances its MCR.     

Characterization of the late posthypoglycemic insulin resistance in insulin-dependent diabetes mellitus

The insulin effect (6.5 to 7.5 hours) following hypoglycemia was studied with the euglycemic clamp technique in eight patients with insulin-dependent diabeteses mellitus (IDDM). The results were compared with a control study with the same insulin infusion, but where hypoglycemia was prevented by a glucose infusion. Glucose production (Ra) and utilization (Rd) were evaluated with D-(3-3H) glucose infusion. Hypoglycemia (glucose nadir, 1.5 +/- 0.1 mmol/L) caused a marked increase in cortisol and growth hormone, whereas the release of adrenaline and, in particular, glucagon was low. The plasma free insulin levels were similar in the studies, including during the clamp periods. The glucose infusion rates (GIR) were significantly lower after the hypoglycemia as compared with the control study (control, 2.4 +/- 0.3; hypoglycemia, 1.5 +/- 0.3 mg/kg x min; P less than .05). Thus, hypoglycemia induces prolonged insulin resistance. The posthypoglycemic insulin resistance during a moderate hyperinsulinemic (approximately 30 mU/L) clamp was mainly due to a decreased insulin effect on glucose utilization (control, 2.9 +/- 0.2; hypoglycemia, 2.2 +/- 0.2 mg/kg x min; P less than .02), whereas the insulin effect on glucose production was not significantly different after hypoglycemia.     

Insulin antibody does not cause insulin resistance during glucose clamping in rats.

Although it has often been stated that insulin antibodies cause insulin resistance, this concept is still controversial. The effect of insulin antibody GP30, commonly used in insulin radioimmunoassay, on insulin action was investigated in Wistar rats in vivo by the euglycemic glucose clamp technique. As a preliminary experiment, the equilibrium time required for insulin antibody to bind with endogenous insulin was examined. One hundred microliters/kg insulin antibody took 60 min or more to attain equilibrium, but 10 microliters/kg insulin antibody almost immediately equilibrated with endogenous insulin. During a 60-min glucose clamp study, 2 mU/kg/min porcine insulin was infused with 100 microliters/kg insulin antibody. At steady state, during the last 20-min period, the mean glucose infusion rate was 2.10 +/- 0.85 mg/kg/min (n = 5, mean +/- SD), significantly lower than the 5.77 +/- 1.61 mg/kg/min of the control, indicating insulin resistance before equilibrium was reached. However, the glucose infusion rates during the clamp with 10 microliters/kg insulin antibody and 100 microliters/kg insulin antibody infused 75 min before the insulin were 6.10 +/- 1.44 and 7.12 +/- 1.19 mg/kg/min, respectively, no different from the control. In these instances, free insulin levels measured by radioimmunoassay using the polyethyleneglycol method were 43.8 +/- 20.4 and 15.4 +/- 6.1 microU/ml, respectively, lower than the control (77.0 +/- 16.1 microM/ml).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of voluntary wheel-running on insulin sensitivity and responsiveness in high-fat-fed rats.

The effect of voluntary wheel-running on insulin resistance was studied in high-fat-fed rats. A sequential hyperinsulinemic euglycemic clamp procedure was employed (insulin infusion rates: 3 and 30 mU/kg BW/min) in 14 high-fat-fed rats and 7 chow-fed rats under the awake condition. The high-fat-fed rats were further divided into a sedentary (n=7) and a voluntary wheel-running (n=7) groups. Blood glucose was clamped at the fasting level in each rat. Plasma insulin levels during the 3- and 30-mU/kg BW/min insulin infusions were 40-50 and 450-550 microU/ml, respectively. At both 3 and 30 mU/kg BW/min insulin infusions, high-fat-feeding showed a significant decrease in glucose infusion rate (GIR), compared with the chow-fed rats. However, decreased GIRs were restored by the 4-wk wheel-running and reached similar levels as the chow-fed rats. Therefore, it could be concluded that voluntary wheel-running prevents insulin resistance induced by high-fat feeding.     

Effects of hyperinsulinemia and hyperglycemia on insulin receptor function and glycogen synthase activation in skeletal muscle of normal man

Insulin receptor function, glycogen synthase activity, and activation by phosphatases were studied in biopsies of human skeletal muscle under conditions of hyperglycemia and/or hyperinsulinemia for 150 minutes. Twenty-one healthy volunteers underwent either (A) a hyperinsulinemic, euglycemic clamp (serum insulin, 160.0 +/- 7.7 mU/L; plasma glucose, 4.9 +/- 0.1 mmol/L; n = 9), (B) a hyperglycemic clamp during normoinsulinemia (serum insulin, 18.1 +/- 3.3 mU/L; plasma glucose, 12.9 +/- 0.2 mmol/L; n = 6), or (C) a combined hyperinsulinemic, hyperglycemic clamp (serum insulin, 158.3 +/- 15.0 mU/L; plasma glucose, 11.4 +/- 0.8 mmol/L; n = 6). During all studies, the endogenous insulin secretion was inhibited with somatostatin. Insulin binding and kinase activity of insulin receptors solubilized from vastus lateralis muscle biopsies were unaffected by hyperglycemia and/or hyperinsulinemia. Hyperinsulinemia activated the muscle glycogen synthase with a decrease in the half-maximal activation constant (A0.5) for glucose-6-phosphate (G6P) from 0.53 +/- 0.04 to 0.21 +/- 0.02 mmol/L (study A, P less than .02) and from 0.53 +/- 0.06 to 0.19 +/- 0.05 mmol/L (study C, P less than .03). In addition, the rate of glycogen synthase activation by phosphatases increased from 0.078 +/- 0.017 to 0.134 +/- 0.029 U/min/mg protein (study A, P less than .03) and from 0.082 +/- 0.013 to 0.145 +/- 0.033 U/min/mg protein (study C, P = .05). Hyperglycemia during normoinsulinemia did not affect A0.5 or phosphatase activity. In conclusion, (1) hyperinsulinemia for 2 1/2 hours increases glycogen synthase activity and activation by phosphatases independently on the glycemia; and (2) insulin receptor binding and basal and insulin-stimulated receptor kinase activity are not modified during short-term hyperinsulinemia and/or hyperglycemia.     

The effects of euglycemic hyperinsulinemia and amino acid infusion on regional and whole body glucose disposal in man

We investigated the effects of amino acid infusion on regional and whole body glucose metabolism in 16 normal volunteers, age 32 to 70 years. Ten subjects underwent 140-minute euglycemic insulin infusions at the rate of 1 mU/kg.min with concomitant 10% amino acid infusion. Six volunteers who underwent identical euglycemic insulin infusions without amino acid infusion served as controls. Whole body glucose disposal was estimated by the rate of exogenous glucose infusion required to maintain euglycemia, and peripheral glucose balance was evaluated by the forearm balance technique. In four subjects from each group, a primed, continuous infusion of [3-3H]glucose was used to quantify endogenous glucose production (EGP). Comparable states of hyperinsulinemia were achieved with insulin concentrations (microU/mL) of 101 +/- 7 observed in the group with amino acid infusion and 95 +/- 14 in the control group. Whole body glucose utilization was significantly lower (P less than .001) in the subjects receiving amino acid infusion (5.0 +/- 0.4 mg/kg.min) compared with the control group (8.7 +/- 0.8 mg/kg.min). Forearm glucose disposal was markedly reduced (P less than .05) in the group receiving amino acid infusion (1,385 +/- 330 nmol/100 g.min) compared with controls (2,980 +/- 460 nmol/100 g.min). Under comparable conditions of euglycemia and hyperinsulinemia, virtually complete suppression of EGP was observed in both groups. We conclude that infusion of amino acids with insulin under euglycemic conditions reduces whole body glucose utilization primarily by reducing peripheral glucose disposal.     

Study on insulin resistance in rats treated with estrogen and progesterone--assessment with the euglycemic glucose clamp technic

To clarify the mechanism of insulin resistance in pregnancy, we have used the euglycemic glucose clamp technique in estradiol(E) treatment(n = 6), progesterone(P) treatment (n = 28), and Control(n = 29) female rats. E(10 micrograms/day) and P(10 mg/day) were injected subcutaneously into female rats for 14 days, to increase E and P concentrations to pregnant levels. Glucose production and glucose utilization were measured by using [3-3H]-glucose. The results were as follows, 1) Glucose production was almost suppressed at hyperinsulinemia(11,000 microU/ml) both Control and P treatment rats. Then at hyperinsulinemia, glucose utilization rate was almost equal to glucose infusion rate. 2) In P treatment rats glucose utilization was significantly lower (p less than 0.05) than in Control rats at hyperinsulinemia (11,000 microU/ml). 3) In P treatment rats glucose infusion rate was significantly lower than in Control rats at plasma insulin concentrations of 1,000 microU/ml(p less than 0.02), and 11,000 microU/ml(p less than 0.01), and lower than in E treatment rats at plasma insulin concentrations of 11,000 microU/ml(p less than 0.05). 4) In a dose-response curve for the effects of four different concentrations of insulin on glucose infusion rate, the insulin resistance induced by progesterone is characterized by a decreased responsiveness to insulin. The results suggest that progesterone may play an important role in inducing insulin resistance in pregnancy.     

Glucose intolerance and insulin resistance in cirrhosis are normalized after liver transplantation.

Cirrhosis is often associated with insulin resistance and glucose intolerance. We evaluated if these alterations are restored by liver transplantation (LT). Glucose tolerance (oral glucose tolerance test [OGTT]), peripheral insulin sensitivity (euglycemic insulin clamp technique), glucose oxidation (indirect calorimetry), nonoxidative glucose disposal, and insulin secretion (hyperglycemic clamp technique) were measured in 6 patients (Group 1) before and 6 months after LT, in 12 patients (Group 2) who underwent LT 6 to 30 months previously, and in 6 healthy individuals (controls). In Group 1, glucose tolerance and insulin sensitivity (3.24 +/- 0.37 mg/kg/min) were normalized after LT (8.6 +/- 0.77 mg/kg/min; P <.0001; P = not significant vs. controls). The improved insulin-mediated glucose uptake was the result of a normalization of nonoxidative glucose disposal. Fasting insulin and C-peptide decreased from 24.6 +/- 3.3 microU/mL and 4.37 +/- 0.46 ng/dL, respectively, to 12.7 +/- 1.9 microU/mL and 2.46 +/- 0.5 ng/dL (controls: 10.0 +/- 3 microU/mL and 1.45 +/- 0.34 ng/dL). The glucose-induced increase of insulin concentration, which was higher before LT, showed a significant reduction, although the first phase of beta-cell secretion remained significantly higher compared with that of controls. All these findings were also confirmed in Group 2. The present data indicate that LT normalizes glucose tolerance and insulin sensitivity in cirrhotic patients through an improvement of both hepatic glucose clearance and the peripheral glucose disposal. The latter effect may be the result of the correction of chronic hyperinsulinemia. An increased first-phase beta-cell insulin secretion in response to high glucose levels persists, suggesting that a memory of previous insulin resistance is maintained.     

In vivo insulin action in type 1 (insulin-dependent) diabetic pregnant women as assessed by the insulin clamp technique

To determine the influence of pregnancy on insulin sensitivity in patients with type 1 diabetes mellitus in more detail, a hyperinsulinemic euglycemic clamp study was performed in six pregnant type 1 diabetic women and eight nonpregnant women with type 1 diabetes mellitus. All of the pregnant women were studied three times: in early pregnancy (mean, week 13), late pregnancy (mean, week 34), and within a week after delivery. Insulin was infused in a constant rate of 1.0 mU/kg X min, which resulted in steady state serum free insulin levels (I) of 44 +/- 3 (+/- SEM), 56.6 +/- 6, and 55 +/- 8 microU/ml in the pregnant diabetic women and 52 +/- 4 microU/ml in the nonpregnant women. Mean glucose disposal (M) was 5.6 +/- 0.3 mg/kg X min early in pregnancy and 3.4 +/- 0.5 mg/kg X min late in pregnancy (P less than 0.02). However, in the early postpartum period, M was again higher (7.2 +/- 0.7 mg/kg X min; P less than 0.02) and similar to values in early pregnancy and nonpregnant diabetic women (7.2 +/- 0.6 mg/kg X min). When tissue sensitivity to insulin was expressed as the M to I ratio, similar results were obtained (nonpregnant women, early stage of gestation, and postpartum vs. late stage of gestation: 0.13 +/- 0.01, 0.13 +/- 0.01, and 0.15 +/- 0.03 mg/kg X min per microU/ml vs. 0.06 +/- 0.1 mg/kg X min per microU/ml; P less than 0.03 in all). There tended to be an inverse relationship between serum levels of human placental lactogen and the M to I ratio during pregnancy (r = -0.74; P = 0.09). However, we found no association between changes in the impairment of insulin action and serum estradiol, progesterone, or cortisol levels. In conclusion, pregnant type 1 diabetic women have insulin resistance in peripheral tissues in the late stage of gestation. Insulin sensitivity returns to values found in nonpregnant diabetic women within the first week after delivery.     

The effect of CP 68,722, a thiozolidinedione derivative, on insulin sensitivity in lean and obese Zucker rats

The effect of a new drug (CP 68,722, Pfizer) on parameters of insulin sensitivity in an established insulin-resistant animal model was examined. Rates of hepatic glucose production (HGP) and peripheral glucose uptake in obese Zucker (fa/fa) rats treated with a 10-day course of the medication using a two-step (2 and 10 mU/kg/min) euglycemic hyperinsulinemic clamp technique were measured. In addition, changes in substrate concentrations after drug treatment were examined. Basal HGP rates were similar in the lean versus the obese animals (37 +/- 3 v 39 +/- 3 mumol/kg/min); however, the obese animals had impaired insulin-induced suppression of HGP at both 2 mU/kg/min (36 +/- 3 v 23 +/- 4 mumol/kg/min) and 10 mU/kg/min (18 +/- 5 v 2 +/- 1 mumol/kg/min). Insulin stimulation of glucose disposal was also defective in the obese animals (37 +/- 2 v 88 +/- 7 mumol/kg/min at 2 mU/kg/min and 98 +/- 9 v 219 +/- 18 mumol/kg/min at 10 mU/kg/min). In addition, obese animals had elevated free fatty acid (FFA) and ketone levels, both of which were resistant to insulin-induced suppression. After drug treatment, few alterations in glucose or lipid metabolism were found in the lean animals. In the obese animals, insulin suppression of HGP was normalized during the higher insulin infusion rate (0 v 18 +/- 5 mumol/kg/min at 10 mU/kg/min), and peripheral glucose disposal was enhanced at both steps of the insulin clamp (54 +/- 4 v 37 +/- 2 mumol/kg/min at 2 mU/kg/min and 134 +/- 12 v 98 +/- 9 mumol/kg/min at 10 mU/kg/min).(ABSTRACT TRUNCATED AT 250 WORDS)