The effect of magnesium deficiency on glucose stimulated insulin secretion in rats

Weanling Sherman rats were pair-fed for 8 days on a control or a magnesium deficient diet containing 70.5% sucrose. After a 12-hour fast, the rats were injected intraperitoneally with glucose (250 mg/100 g body weight) and arterial blood was drawn at 0, 15, 30, 60, 90 minutes after injection. Before glucose loading, in magnesium deficient rats, plasma magnesium levels were significantly increased. The plasma triglyceride concentration was significantly higher in magnesium deficient rats compared to controls. After glucose loading, in the control group, the plasma insulin concentrations increased to 67.9 +/- 5.8 microU/ml at 15 minutes and returned to pretreatment levels by 30 minutes; in the magnesium-deficient rats, the plasma insulin levels were significantly lower at 15 minutes 32.9 +/- 5.6 microU/ml (P less than 0.01) and returned more slowly to the pre-challenge level. No significant differences were observed in plasma glucose levels between the two groups of rats.     

Dose comparison of ultrasonic transdermal insulin delivery to subcutaneous insulin injection

Prior studies have demonstrated the effectiveness of noninvasive transdermal insulin delivery using a cymbal transducer array. In this study the physiologic response to ultrasound mediated transdermal insulin delivery is compared to that of subcutaneously administered insulin. Anesthetized rats (350-550 g) were divided into four groups of four animals; one group representing ultrasound mediated insulin delivery and three representing subcutaneously administered insulin (0.15, 0.20, and 0.25 U/kg). The cymbal array was operated for 60 minutes at 20 kHz with 100 mW/cm2 spatial-peak temporal-peak intensity and a 20% duty cycle. The blood glucose level was determined at the beginning of the experiment and, following insulin administration, every 15 minutes for 90 minutes for both the ultrasound and injection groups. The change in blood glucose from baseline was compared between groups. When administered by subcutaneous injection at insulin doses of 0.15 and 0.20 U/kg, there was little change in the blood glucose levels over the 90 minute experiment. Following subcutaneous administration of insulin at a dose of 0.25 U/kg, blood glucose decreased by 190 +/- 96 mg/dl (mean +/- SD) at 90 minutes. The change in blood glucose following ultrasound mediated insulin delivery was -262 +/- 40 mg/dl at 90 minutes. As expected, the magnitude of change in blood glucose between the three injection groups was dependant on the dose of insulin administered. The change in blood glucose in the ultrasound group was greater than that observed in the injection groups suggesting that a higher effective dose of insulin was delivered.     

The determinants of insulin extraction in the isolated perfused rat liver.

The effect of hepatic blood flow and portal insulin concentration on insulin extraction during one passage through the isolated perfused rat liver was studied. The percentage of insulin extracted was constant over the physiological range of blood flows (4 to 28 ml/min). The total amount of insulin extracted increased as the input concentration was raised from 48 to 4860 microU/ml with the highest level of extraction being approximately 700 microU of insulin per gram of liver per minute. When square wave input pulses of 243 to 4860 microU/ml were presented, about 5% of this insulin was retained and then released by the liver for periods up to 15 minutes after the cessation of the input. The possible roles of glucose and glucagon as regulators of insulin extraction were studied. Glucose (300 mg/dl), as compared with no glucose, led to a significant reduction of insulin extraction (22% vs. 38%, p less than 0.001). Glucagon had no effect on insulin extraction in the presence of constant levels of glucose. It is concluded, therefore, that glucose may increase circulating insulin levels not only by its well known stimulation of insulin secretion by the pancreas, but also by inhibiting insulin extraction by the liver.     

Comparison of short-term diet and exercise on insulin action in individuals with abnormal glucose tolerance.

The effects of a 10-day low-calorie diet (LCD; n = 8) or exercise training (ET; n = 8) on insulin secretion and action were compared in obese men (n = 9) and women (n = 7), aged 53 +/- 1 yr, with abnormal glucose tolerance by using a hyperglycemic clamp with superimposed arginine infusion and a high-fat drink. Body mass (LCD, 115 +/- 5 vs. 110 +/- 5 kg; ET, 111 +/- 7 vs. 109 +/- 7 kg; P < 0. 01) and fasting plasma glucose (LCD, 115 +/- 10 vs. 99 +/- 4 mg/dl; ET, 112 +/- 4 vs. 101 +/- 5 mg/dl, P < 0.01) and insulin (LCD, 23.9 +/- 5.6 vs. 15.2 +/- 3.9 microU/ml; ET, 17.6 +/- 1.9 vs. 13.9 +/- 2. 4 microU/ml; P < 0.05) decreased in both groups. There was a 40% reduction in plasma insulin during hyperglycemia (0-45 min) after LCD (peak: 118 +/- 18 vs. 71 +/- 14 microU/ml; P < 0.05) and ET (69 +/- 14 vs. 41 +/- 7 microU/ml; P < 0.05) and trends for reductions during arginine infusion and a high-fat drink. The 56% increase in glucose uptake after ET (4.95 +/- 0.90 vs. 7.74 +/- 0.82 mg. min-1. kg fat-free mass-1; P < 0.01) was significantly (P < 0.01) greater than the 19% increase (5.72 +/- 1.12 vs. 6.80 +/- 0.94 mg. min-1. kg fat-free mass-1; P = not significant) that occurred after LCD. The marked increase in glucose disposal after ET, despite lower insulin levels, suggests that short-term exercise is more effective than diet in enhancing insulin action in individuals with abnormal glucose tolerance.     

Alterations in liver, muscle, and adipose tissue insulin sensitivity in men with HIV infection and dyslipidemia

Dyslipidemia is common in patients with HIV infection. In this study, a two-stage euglycemic hyperinsulinemic clamp, with infusion of stable isotopically labeled tracers, was used to evaluate insulin action in skeletal muscle, liver, and adipose tissue in HIV-infected men with dyslipidemia (HIV-DL; plasma triglyceride >250 mg/dl and HDL <45 mg/dl; n=12), HIV-infected men without dyslipidemia (HIV w/o DL; n=12), and healthy men (n=6). Basal rates of glucose production (glucose R(a)), glucose disposal (glucose R(d)), and lipolysis (palmitate R(a)) were similar between groups. The relative suppression of glucose R(a) (63+/- 4, 77+/- 2, and 78+/- 3%, P=0.008) and palmitate R(a) (49+/-4, 63+/-3, and 68+/-3%, P=0.005) during ow-dose insulin infusion (plasma insulin approximately 30 microU/ml), and the relative stimulation of glucose R(d) (214+/-21, 390+/-25, and 393+/-46%, P=0.001) during high-dose insulin infusion (plasma insulin approximately 75 microU/ml) were lower in HIV-DL than in HIV w/o DL and healthy volunteers, respectively. Suppression of basal glucose R(a) correlated with plasma adiponectin (r=0.44, P=0.02) and inversely with plasma IL-6 (r=-0.49, P<0.001). Stimulation of glucose R(d) correlated directly with adiponectin (r=0.48, P<0.01) and inversely with IL-6 (r=-0.49, P=0.02). We conclude that dyslipidemia in HIV-infected men is indicative of multiorgan insulin resistance, and circulating adipokines may be important in the pathogenesis of impaired insulin action.     

Visfatin levels do not change after the oral glucose tolerance test and after a dexamethasone-induced increase in insulin resistance in humans

INTRODUCTION, MATERIAL AND METHODS: Visfatin is a cytokine, mainly expressed in visceral fat, that exerts insulin-mimicking effects in rodents through activation of an insulin receptor, although the binding-site is distinct from that of insulin. However, the mechanisms that regulate visfatin synthesis are still not fully understood. In particular, it is not clear whether short-term glucose-induced hyperglycaemia and hyperinsulinaemia as well as a glucocorticoid-induced increase in insulin resistance are reflected in appreciable alterations in serum visfatin levels in humans. In order to investigate this we measured serum visfatin, glucose and insulin concentrations during a 75.0 gram oral glucose tolerance test (OGTT) [Study 1], as well as before and after oral administration of dexamethasone [Study 2]. Study 1 included 17 subjects (2 males), aged 35.7 +/- 15.6 (mean +/- SD) years of BMI 35.2 +/- 9.3 kg/m(2). Blood samples were taken before (0 minutes) and at 60 and 120 minutes after glucose administration. Study 2 included 20 subjects (4 males, 5 subjects with type 2 diabetes), aged 42.1 +/- 17.2 years of BMI 36.7 +/- 8.38 kg/m(2) who underwent screening for Cushing's disease/syndrome. Dexamethasone was administered at a dose of 0.5 mg every 6 hours for 48 hours. Fasting serum concentrations of visfatin, glucose and insulin were assessed before (D0) and after 48 hours of dexamethasone administration (D2). Insulin resistance was assessed according to the HOMA method in non-diabetic individuals (n = 15). RESULTS: In Study 1 two subjects were found to have impaired glucose tolerance and one subject was found to have diabetes mellitus. Glucose administration resulted in a highly significant increase in insulin (from 11.4 +/- 7.2 microU/mL at 0 min to 98.9 +/- 68.6 microU/mL at 60 min and 72.6 +/- 45.1 microU/mL at 120 minute of OGTT, p < 0.001 for 60 and 120 minutes in comparison to baseline). However, there was no change in serum visfatin concentrations (84.6 +/- 11.6 ng/mL at 0 minutes, 82.6 +/- 12.7 ng/mL at 60 minutes and 81.1 +/- 14.5 ng/mL at 120 minutes of OGTT, p = ns). All subjects in Study 2 achieved suppression of cortisol concentrations below 50 nmo/l. Dexamethasone administration resulted in an increase in fasting insulin (from 11.5 +/- 6.9 to 16.9 +/- 7.6 microU/mL; p = 0.011) and an increase in HOMA (from 2.73 +/- 1.74 to 4.02 +/- 2.27; p = 0.015), albeit without a significant change in serum visfatin concentrations (61.1 +/- 19.8 vs. 68.3 +/- 19.4 ng/mL, p = ns). In neither Study 1 nor Study 2 was there any significant correlation between serum visfatin and age, BMI or HOMA. CONCLUSIONS: There is a striking difference between the marked rise in insulin concentrations and the lack of change in visfatin concentrations during the oral glucose tolerance test. This implies that it is highly unlikely that visfatin is involved in the short-term regulation of glucose homeostasis in human subjects. Dexamethasone administration (4 mg/48 hours) induces an increase in insulin resistance, although without significant change in serum visfatin concentrations. Therefore in contrast to the in vitro data, short term glucocorticoid administration does not result in appreciable changes in serum levels of this adipocytokine. Furthermore, the results of our study do not support the notion that glucocorticoid-induced insulin resistance is likely to be related to changes in serum concentrations of visfatin.     

High-dose oral vitamin C partially replenishes vitamin C levels in patients with Type 2 diabetes and low vitamin C levels but does not improve endothelial dysfunction or insulin resistance

Endothelial dysfunction is a hallmark of Type 2 diabetes related to hyperglycemia and oxidative stress. Nitric oxide-dependent vasodilator actions of insulin may augment glucose disposal. Thus endothelial dysfunction may worsen insulin resistance. Intra-arterial administration of vitamin C improves endothelial dysfunction in diabetes. In the present study, we investigated effects of high-dose oral vitamin C to alter endothelial dysfunction and insulin resistance in Type 2 diabetes. Plasma vitamin C levels in 109 diabetic subjects were lower than healthy (36 +/- 2 microM) levels. Thirty-two diabetic subjects with low plasma vitamin C (<40 microM) were subsequently enrolled in a randomized, double-blind, placebo-controlled study of vitamin C (800 mg/day for 4 wk). Insulin sensitivity (determined by glucose clamp) and forearm blood flow in response to ACh, sodium nitroprusside (SNP), or insulin (determined by plethysmography) were assessed before and after 4 wk of treatment. In the placebo group (n = 17 subjects), plasma vitamin C (22 +/- 3 microM), fasting glucose (159 +/- 12 mg/dl), insulin (19 +/- 7 microU/ml), and SI(Clamp) [2.06 +/- 0.29 x 10(-4) dl x kg(-1) x min(-1)/(microU/ml)] did not change significantly after placebo treatment. In the vitamin C group (n = 15 subjects), basal plasma vitamin C (23 +/- 2 microM) increased to 48 +/- 6 microM (P < 0.01) after treatment, but this was significantly less than that expected for healthy subjects (>80 microM). No significant changes in fasting glucose (156 +/- 11 mg/dl), insulin (14 +/- 2 microU/ml), SI(Clamp) [2.71 +/- 0.46 x 10(-4) dl x kg(-1) x min(-1)/(microU/ml)], or forearm blood flow in response to ACh, SNP, or insulin were observed after vitamin C treatment. We conclude that high-dose oral vitamin C therapy, resulting in incomplete replenishment of vitamin C levels, is ineffective at improving endothelial dysfunction and insulin resistance in Type 2 diabetes.     

Glucose tolerance and insulinemia in patients with hepatic cirrhosis and portal hypertension treated by portacaval anastomosis]

Development of diabetes mellitus is a common complication of side to side porta-caval anastomosis (PCA). Five patients with liver cirrhosis and portal hypertension have been studied with intravehous (IVGTT, 0,5 g/Kg B.W.) and oral (OGTT, 1 g/Kg B.W.) glucose tolerance tests before and three weeks after PCA. Fasting plasma glucose was 84 +/- 7 before and 87 +/- 3 mg/dl after PCA. Fasting IRI increased from 17 +/- 3 to 31 +/- 6 microU/ml. The pattern of plasma glucose and IRI response to IVGTT did not change after PCA. Plasma glucose resonse to OGTT after PCA showed only an earlier rise at 60 instead of 90 minutes, whereas IRI resonse (area under the insulin curve) was significantly enhanced (from 12.4 to 19.8 U/l, p < 0.05). These data suggest a role of gut polipeptides in determining hyperinsulinemia and insulin resistence in PCA patients.     

Nonessential amino acids are not necessary to stimulate net muscle protein synthesis in healthy volunteers

The present study was performed to test the hypothesis that orally administered essential amino acids, in combination with carbohydrate, will stimulate net muscle protein synthesis in resting human muscle in vivo. Four volunteers ingested 500 mL of a solution containing 13.4 g of essential amino acids and 35 g sucrose (EAA). Blood samples were taken from femoral arterial and venous catheters over a 2-hour period following the ingestion of EAA to measure arteriovenous concentrations of amino acids across the muscle. Two muscle biopsies were taken during the study, one before administration of the drink and one approximately 2 hours after consumption of EAA. Serum insulin increased from normal physiologic levels at baseline (9.2 +/- 0.8 microU/mL) and peaked (48 +/- 7.1 microU/mL) 30 minutes after EAA ingestion. Arterial essential amino acid concentrations increased approximately 100 to 400% above basal levels between 10 and 30 minutes following drink ingestion. Net nitrogen (N) balance changed from negative (-495 +/- 128 nmol/mL) prior to consumption of EAA to a peak positive value (416 +/- 140 nmol/mL) within 10 minutes of ingestion of the drink. EAA resulted in an estimated positive net N uptake of 307.3 mg N above basal levels over the 2-hour period. Muscle amino acid concentrations were similar prior to and 2 hours following ingestion of EAA. We conclude that ingestion of a solution composed of carbohydrates to stimulate insulin release and a small amount of essential amino acids to increase amino acid availability for protein synthesis is an effective stimulator of muscle protein anabolism.     

Counterregulatory deficits occur within 24 h of a single hypoglycemic episode in conscious, unrestrained, chronically cannulated mice

Hypoglycemia-induced counterregulatory failure is a dangerous complication of insulin use in diabetes mellitus. Controlled hypoglycemia studies in gene knockout models, which require the use of mice, would aid in identifying causes of defective counterregulation. Because stress can influence counterregulatory hormones and glucose homeostasis, we developed glucose clamps with remote blood sampling in conscious, unrestrained mice. Male C57BL/6 mice implanted with indwelling carotid artery and jugular vein catheters were subjected to 2 h of hyperinsulinemic glucose clamps 24 h apart, with a 6-h fast before each clamp. On day 1, blood glucose was maintained (euglycemia, 178 +/- 4 mg/dl) or decreased to 62 +/- 1 mg/dl (hypoglycemia) by insulin (20 mU x kg(-1) x min(-1)) and variable glucose infusion. Donor blood was continuously infused to replace blood sample volume. Baseline plasma epinephrine (32 +/- 8 pg/ml), corticosterone (16.1 +/- 1.8 microg/dl), and glucagon (35 +/- 3 pg/ml) were unchanged during euglycemia but increased significantly during hypoglycemia, with a glycemic threshold of approximately 80 mg/dl. On day 2, all mice underwent a hypoglycemic clamp (blood glucose, 64 +/- 1 mg/dl). Compared with mice that were euglycemic on day 1, previously hypoglycemic mice had significantly higher glucose requirements and significantly lower plasma glucagon and corticosterone (n = 6/group) on day 2. Epinephrine tended to decrease, although not significantly, in repeatedly hypoglycemic mice. Pre- and post-clamp insulin levels were similar between groups. We conclude that counterregulatory responses to acute and repeated hypoglycemia in unrestrained, chronically cannulated mice reproduce aspects of counterregulation in humans, and that repeated hypoglycemia in mice is a useful model of counterregulatory failure.     

Metabolic and performance effects of raisins versus sports gel as pre-exercise feedings in cyclists

Research suggests that pre-exercise sources of dietary carbohydrate with varying glycemic indexes may differentially affect metabolism and endurance. This study was designed to examine potential differences in metabolism and cycling performance after consumption of moderate glycemic raisins vs. a high glycemic commercial sports gel. Eight endurance-trained male (n = 4) and female (n = 4) cyclists 30 +/- 5 years of age completed 2 trials in random order. Subjects were fed 1 g carbohydrate per kilogram body weight from either raisins or sports gel 45 minutes prior to exercise on a cycle ergometer at 70% V(.-)O2max. After 45 minutes of submaximal exercise, subjects completed a 15-minute performance trial. Blood was collected prior to the exercise bout, as well as after the 45th minute of exercise, to determine serum concentrations of glucose, insulin, lactate, free fatty acids (FFAs), triglycerides, and beta-hydroxybutyrate. Performance was not different (p > 0.05) between the raisin (189.5 +/- 69.9 kJ) and gel (188.0 +/- 64.8 kJ) trials. Prior to exercise, serum concentrations of glucose and other fuel substrates did not differ between trials; however, insulin was higher (p < 0.05) for the gel (110.0 +/- 70.4 microU x ml(-1)) vs. raisin trial (61.4 +/- 37.4 microU x ml(-1)). After 45 minutes of exercise, insulin decreased to 14.2 +/- 6.2 microU x ml(-1) and 13.3 +/- 18.9 microU x ml(-1) for gel and raisin trials, respectively. The FFA concentration increased (+0.2 +/- 0.1 mmol x L(-1)) significantly (p < 0.05) during the raisin trial. Overall, minor differences in metabolism and no difference in performance were detected between the trials. Raisins appear to be a cost-effective source of carbohydrate for pre-exercise feeding in comparison to sports gel for short-term exercise bouts.     

Transient glucose intolerance during attacks of thyrotoxic periodic paralysis

In a 19-year-old Japanese male (case 1) with thyrotoxic periodic paralysis (TPP), an increase of plasma glucose concentration together with abnormally high levels of serum immunoreactive insulin (IRI) was observed preceding a spontaneous attack of paralysis. Therefore, the plasma glucose, glucagon, epinephrine, norepinephrine, serum IRI, growth hormone and cortisol levels, and the erythrocyte insulin receptors were measured in case 1 and a 40-year-old Japanese male (case 2) with TPP during attacks of paralysis induced by prolonged glucose loading. In case 1, the serum IRI concentration was elevated to the extraordinarily high level of 655.0 microU/ml at the beginning of paralysis, and at that time, the plasma glucose concentration was 147 mg/dl. However, when paralysis was not induced by a similar glucose loading during methimazole treatment, the serum IRI and plasma glucose levels at the corresponding time after glucose loading were 20.9 microU/ml and 87 mg/dl, respectively. Furthermore, the affinity of the erythrocyte insulin receptors was decreased during the attack. In case 2, plasma glucose and serum IRI concentrations were increased in accordance with the initiation of paralysis although the blood levels of hormones counteracting insulin were not significantly changed. These findings suggest that there is something interacting with the normal action of the insulin in the early phase of paralysis.     

Comparison of local and systemic effects of insulin on myocardial glucose extraction in ischemic heart disease

Physiological increases in circulating insulin level significantly increase myocardial glucose uptake in vivo. To what extent this represents a direct insulin action on the heart or results indirectly from reduction in circulating concentrations of free fatty acids (FFA) is uncertain. To examine this, we measured myocardial glucose, lactate, and FFA extraction in 10 fasting men (ages 49-76 yr) with stable coronary artery disease during sequential intracoronary (10 mU/min, coronary plasma insulin = 140 +/- 20 microU/ml) and intravenous (100 mU/min, systemic plasma insulin = 168 +/- 26 microU/ml) insulin infusion. Basally, hearts extracted 2 +/- 2% of arterial glucose and extracted 27 +/- 6% of FFA. Coronary insulin infusion increased glucose extraction to 5 +/- 3% (P < 0.01 vs. basal) without changing plasma FFA or heart FFA extraction. Conversion to intravenous infusion lowered plasma FFA by approximately 50% and heart FFA extraction by approximately 75%, increasing heart glucose extraction still further to 8 +/- 3% (P < 0. 01 vs. intracoronary). This suggests the increase in myocardial glucose extraction observed in response to an increment in systemic insulin concentration is mediated equally by a reduction in circulating FFA and by direct insulin action on the heart itself. Coronary insulin infusion increased myocardial lactate extraction as well (from 20 +/- 10% to 29 +/- 9%, P < 0.05), suggesting the local action may include stimulation of a metabolic step distal to glucose transport and glycolysis.     

Insulin secretion and glucose uptake in hypomagnesemic sheep fed a low magnesium, high potassium diet

Hyperglycemic and euglycemic clamp experiments were conducted to evaluate insulin secretion and glucose uptake in the hypomagnesemic sheep fed a low magnesium (Mg), high potassium (K) diet. Five mature sheep were fed a semipurified diet containing 0.24% Mg and 0.56% K (control diet) and five were fed 0.04% Mg and 3.78% K (low Mg/high K diet) for at least 2 weeks. In the hyperglycemic clamp experiment, plasma glucose concentrations were raised and maintained at a hyperglycemic steady-state (approximately 130 mg/100 ml) by variable rates of glucose infusion during the experimental period (120 minutes). The insulin response in the sheep fed the low Mg/high K diet (31.0 microU/ml) were significantly (P < 0.01) lower than those (111.7 microU/ml) of the sheep fed the control diet. In the euglycemic clamp experiment, insulin was infused at rates of 5, 10, 15, or 20 mU/kg/min, each followed by variable rates of glucose infusion to maintain a euglycemic steady-state (basal fasting levels). Hypomagnesemic sheep fed the low Mg/high K diet had significantly (P < 0.01) lower mean glucose disposal (3.72 mg/kg/min) across the insulin infusion rates compared with those of the sheep fed the control diet (5.37 mg/kg/min). These results suggest that glucose-induced insulin secretion and insulin-induced glucose uptake would be depressed in hypomagnesemic sheep and are caused by feeding the low Mg/high K diet.     

Sensitivity and responsiveness of glucose output to insulin in isolated perfused liver from dexamethasone-treated rats

To elucidate insulin action on hepatic glucose output (glycogenolysis) in the state exposed to an excess glucocorticoid, the fed rat liver was isolated and cyclically perfused with a medium containing 5 mM glucose and various concentrations of insulin. The rat was subcutaneously injected with 1 mg/kg of dexamethasone (Dex) for 7 days. Dex-treated rats showed marked increases of serum insulin and plasma glucose level compared with those in control rats. Hepatic glycogen contents in Dex group were markedly increased compared with those in control (115 +/- 5 and 28 +/- 4 mg/g, respectively). Insulin extraction rate in the perfused liver was not different between control and Dex group. Perfusate glucose level after 60 min perfusion was much higher in the Dex-treated rat liver than that of the control at 0 microU/ml insulin (34.5 +/- 2.5 vs 23.0 +/- 2.0 mM, P less than 0.01), and reduced to the nadir level (19.0 +/- 3.0 and 13.0 +/- 1.5 mM, respectively) at 100 microU/ml insulin in both groups, i.e., the decreasing rate in perfusate glucose level was not different between Dex and control group (43% and 44%, respectively). These results suggest that Dex-treatment augments hepatic glucose output, but does not affect the sensitivity and responsiveness of that to insulin.     

Insulin action and secretion in endurance-trained and untrained humans

To evaluate insulin sensitivity and responsiveness, a two-stage hyperinsulinemic euglycemic clamp procedure (insulin infusions of 40 and 400 mU.m-2.min-1) was performed on 11 endurance-trained and 11 untrained volunteers. A 3-h hyperglycemic clamp procedure (plasma glucose approximately 180 mg/dl) was used to study the insulin response to a fixed glycemic stimulus in 15 trained and 12 untrained subjects. During the 40-mU.m-2.min-1 insulin infusion, the glucose disposal rate was 10.2 +/- 0.5 mg.kg fat-free mass (FFM)-1.min-1 in the trained group compared with 8.0 +/- 0.6 mg.kg FFM-1.min-1 in the untrained group (P less than 0.01). In contrast, there was no significant difference in maximally stimulated glucose disposal: 17.7 +/- 0.6 in the trained vs. 16.7 +/- 0.7 mg.kg FFM-1.min-1 in the untrained group. During the hyperglycemic clamp procedure, the incremental area for plasma insulin was lower in the trained subjects for both early (0-10 min: 140 +/- 18 vs. 223 +/- 23 microU.ml-1.min; P less than 0.005) and late (10-180 min: 4,582 +/- 689 vs. 8,895 +/- 1,316 microU.ml-1.min; P less than 0.005) insulin secretory phases. These data demonstrate that 1) the improved insulin action in healthy trained subjects is due to increased sensitivity to insulin, with no change in responsiveness to insulin, and 2) trained subjects have a smaller plasma insulin response to an identical glucose stimulus than untrained individuals.     

Infection impairs insulin-dependent hepatic glucose uptake during total parenteral nutrition

Total parenteral nutrition (TPN) markedly augments net hepatic glucose uptake (NHGU) and hepatic glycolysis in the presence of mild hyperglycemia and hyperinsulinemia. This increase is impaired by an infection. We determined whether the adaptation to TPN alters the responsiveness of the liver to insulin and whether infection impairs that response. Chronically catheterized dogs received TPN for 5 days. On day 3 of TPN, either a nonlethal hypermetabolic infection was induced (INF, n = 5) or a sham surgery was performed (SHAM, n = 5). Forty-two hours after clot implantation, somatostatin and glucagon (34 +/- 3 vs. 84 +/- 11 pg/ml in artery, SHAM vs. INF) were infused, and a three-step (120 min each) isoglycemic (approximately 120 mg/dl) hyperinsulinemic (approximately 12, 25, and 50 microU/ml) clamp was performed to simulate levels seen in normal, infected, and exogenous insulin treatment states. In SHAM, NHGU (3.5 +/- 0.2 to 4.2 +/- 0.4 to 4.6 +/- 0.5 mg x kg(-1) x min(-1)) modestly increased. In INF, NHGU was consistently lower at each insulin step (1.1 +/- 0.5 to 2.6 +/- 0.5 to 2.8 +/- 0.7 mg x kg(-1) x min(-1)). Although NHGU increased from the first to the second step in INF, it did not increase further with the highest dose of insulin. Despite increases in NHGU, net hepatic lactate release did not increase in SHAM and fell in INF. In summary, in the TPN-adapted state, liver glucose uptake is unresponsive to increases in insulin above the basal level. Although the infection-induced increase in insulin sustains NHGU, further increments in insulin enhance neither NHGU nor glycolysis.     

Effects of D-Chiroinositol Added to a Meal on Plasma Glucose and Insulin in Hyperinsulinemic Rhesus Monkeys

We have previously demonstrated that D-chiroinositol, administered intravenously to insulin-resistant monkeys, increases the rate of disappearance of plasma glucose and insulin. The purpose of the present study was to determine whether orally administered D-chiroinositol might also similarly improve the postprandial plasma glucose profile of hyperinsulinemic insulin-resistant monkeys. A complete liquid diet meal (15 ml/kg body weight) was ingested by each of six monkeys on two occasions separated by 10 days, with conditions identical except D-chiroinositol (500 mg/kg body weight) was added to the second meal. At 110 minutes following each meal, the monkeys were anesthetized and blood samples obtained at 120, 150, 180, 210, 240, 270 and 300 minutes. Plasma glucose and insulin concentrations were determined. The mean plasma glucose concentration (120 ? 300 minutes) was significantly lower after the meal containing D-chiroinositol compared to the control meal (7.1 ± 1.2 vs. 7.8 ± 1.2 mM) (p<0.05). Plasma insulin concentrations tended to be lower after the meal containing D-chiroinositol compared to the control meal (3930 ± 1068 vs. 4518 ± 1200 pM) (p<0.15, ns). We conclude that in hyperinsulinemic monkeys, D-chiroinositol added to a meal lowers postprandial plasma glucose without an increase in plasma insulin, and therefore may be a useful agent for reducing meal-induced hyperglycemia without inducing hyperinsulinemia.     

Stimulation of insulin secretion by beta-endorphins (1-27 & 1-31)

Synthetic human beta-endorphin potentiates insulin secretion by the isolated perfused rat pancreas when glucose is present in the perfusate at concentrations of either 125 or 200 mg/dl, whereas it fails to exert any effect on insulin secretion in the presence of a substimulatory concentration of 100 mg/dl. Similar potentiation of insulin secretion occurred in response to the 1-27 fragment (beta-endorphin1-27) of beta-endorphin. This transient potentiation lasts only 3 to 4 minutes, whereupon secretion returns toward control levels. Thus beta-endorphin produces only a transient spike-like secretory profile similar to the first phase of glucose-induced insulin secretion and it fails to produce any chronic insulin secretory response comparable to the second phase of insulin secretion. The insulinotropic effect of beta-endorphins occurred at concentrations varying from 0.1 to 5.0 ug/ml. These data suggest that beta-endorphin and beta-endorphin1-27 potentiate insulin secretion via a common beta cell opioid receptor, and that beta-endorphin may exert a paracrine control of insulin secretion. However, any such regulation appears to be via short-term alterations in the secretory process per se.     

Combined effects of glucose and oxygen concentrations upon the ultrastructure of cultured islet cells and insulin accumulation in culture media

Rat pancreatic islets cultured for 6 days at 100 or 500 mg/dl glucose and 20 or 7% O2 were examined electron-microscopically, and insulin accumulation in the culture media was assayed immunologically. In the islets cultured at 500 mg/dl glucose and 20% O2, B cells exhibited hypertrophy of granular endoplasmic reticulum and Golgi apparatus, an abundance of free ribosomes, degranulation and the margination of secretory granules. In islets cultured at 500 mg/dl glucose and 7% O2, B cells exhibited dilatation of endoplasmic reticulum cisternae and dominance of Golgi vesicles in addition to the above-mentioned changes. These changes, together with the correlated data on insulin accumulation, are discussed with special reference to the effects of glucose and oxygen upon the synthesis and release of insulin in B cells.