Mechanism of insulin resistance in human liver cirrhosis. Evidence of a combined receptor and postreceptor defect.

Insulin resistance in liver cirrhosis may depend on either reduced sensitivity (receptor defect) and/or reduced response to insulin (postreceptor defect). To clarify the mechanism of such resistance, a [3H]glucose infusion (0.2 microCi/min) was performed for 120 min before and during a euglycemic clamp at approximately 100, 1,000, and 10,000 microU/ml steady state plasma insulin concentration in 18 compensated cirrhotics with portal hypertension and impaired glucose tolerance, and 18 healthy volunteers with no family history of diabetes, matched for sex, age, and weight. Mean fasting plasma insulin (29.2 +/- 3.4 SEM vs. 14.8 +/- 1.1 microU/ml) was significantly higher (P less than 0.001) in cirrhotics, while fasting plasma glucose was much the same in the two groups. Glucose use (milligrams per kilogram per minute) was significantly lower in cirrhotics at all three steady state plasma insulin levels: 3.04 +/- 0.34 vs. 7.72 +/- 0.61 (P less than 0.001) at approximately 100; 6.05 +/- 1.07 vs. 11.45 +/- 1.24 (P less than 0.001) at approximately 1,000; and 11.69 +/- 0.69 vs. 14.13 +/- 0.74 (P less than 0.05) at approximately 10,000 microU/ml. Mean plasma C-peptide was significantly higher in cirrhotics both basally and during the steady states (P less than 0.001); it was completely suppressed at approximately 10,000 microU/ml in controls and only 57.5% of the baseline in cirrhotics. Endogenous glucose production (milligrams per kilogram per minute) was much the same in the two groups in the fasting state and almost entirely suppressed in the controls (0.10 +/- 0.05 vs. 0.48 +/- 0.11, P less than 0.001) at approximately 100 microU/ml; at approximately 1,000 microU/ml a residual glucose production, 0.07 +/- 0.05, was observed in the cirrhotics only. In addition, insulin binding and 3-ortho-methyl-glucose transport were studied in vitro in six cirrhotics and six controls. Insulin binding to circulating monocytes and isolated adipocytes was significantly lower (P less than 0.025) in cirrhotics in all insulin concentration studies. Glucose transport values on isolated adipocytes were significantly lower in cirrhotics both basally (P less than 0.001) and at maximal insulin concentration (P less than 0.05). These results suggest that insulin resistance in human cirrhosis is more dependent on depressed peripheral glucose use than on increased endogenous glucose production, and that a combined receptor and postreceptor defect in insulin action on target cells seems to be present.     

Thermic effect of glucose in man. Obligatory and facultative thermogenesis.

The contribution of the sympathetic nervous system to the thermic effect of intravenously infused glucose and insulin was studied in 10 healthy young men before and after beta-adrenergic receptor blockade with propranolol during conditions of normoglycemia (90 mg/dl) at two levels of hyperinsulinemia (approximately 90 microU/ml and approximately 620 microU/ml). During steady state conditions of glucose uptake (0.515 +/- 0.046 and 0.754 +/- 0.056 g/min), significant increases were observed in energy expenditure (0.10 +/- 0.02 kcal/min, P less than 0.001, and 0.21 +/- 0.02 kcal/min, P less than 0.01, respectively). Similarly, glucose oxidation increased from 0.100 +/- 0.015 to 0.266 +/- 0.022 g/min (P less than 0.001) at approximately microU/ml insulin and from 0.082 +/- 0.013 to 0.295 +/- 0.018 g/min (P less than 0.001) at approximately 620 microU/ml insulin. Concomitantly, the rate of nonoxidative glucose disposal or "glucose storage" was 0.249 +/- 0.033 and 0.459 +/- 0.048 g/min, respectively. At this time the thermic effect of infused glucose/insulin was 5.3 +/- 0.9 and 7.5 +/- 0.7%, and the energy cost of "glucose storage" was 0.50 +/- 0.16 kcal/g and 0.47 +/- 0.04 kcal/g at the two different levels of glucose uptake. After beta-adrenergic receptor blockade with propranolol, glucose uptake, oxidation, and "storage" were unchanged in both studies, but significant decreases in energy expenditure were observed (1.41 +/- 0.06-1.36 +/- 0.05 kcal/min, P less than 0.01 at approximately 90 microU/ml insulin, and 1.52 +/- 0.07-1.43 +/- 0.05 kcal/min, P less than 0.005 at approximately 620 microU/ml insulin) causing significant falls in both the estimated thermic effect of infused glucose/insulin and the energy cost of "glucose storage". Regression analysis of the results from both studies indicated a mean energy cost for "glucose storage" of 0.36 kcal/g (r = 0.74, P less than 0.001), which fell significantly (P less than 0.005) to 0.21 kcal/g (r = 0.49, P less than 0.05) during beta-adrenergic receptor blockade with propranolol. The latter is in close agreement with that calculated on theoretical grounds for the metabolic cost of glucose storage as glycogen, i.e., obligatory thermogenesis. It is concluded that beta-adrenergically mediated sympathetic nervous activity is responsible for almost the entire rise in energy expenditure in excess of the obligatory requirements for processing and storing glucose during conditions of normoglycemia and hyperinsulinemia in healthy man, and that the energy cost of "glucose storage" is not different at normal (approximately 90 microU/ml) and supraphysiological (approximately 620 microU/ml) plasma insulin concentrations.     

A new alpha-glucosidase inhibitor (Bay-m-1099) reduces insulin requirements with meals in insulin-dependent diabetes mellitus

Retardation of meal carbohydrate absorption by inhibition of starch degradation improves glucose tolerance in normal and diabetic humans. To determine the effects of Bay-m-1099, a new alpha-glucosidase inhibitor, on insulin requirements and prandial glucose tolerance in patients with insulin-dependent diabetes mellitus (IDDM), plasma glucose, triglyceride, and free insulin concentrations were measured after ingestion of a standard breakfast, lunch, and dinner in nine patients with IDDM in a single-blind, randomized, crossover design. A 20% reduction in insulin was given 30 minutes before the meals when the subjects received Bay-m-1099 (50 mg). This resulted in the AUC for plasma insulin to be significantly less with Bay-m-1099 (AUC, 8.2 +/- 1.3 vs. 12.8 +/- 1.6 microU/ml/min with placebo; P less than 0.01). Despite this reduction in plasma insulin levels, postprandial plasma glucose concentrations were reduced for the breakfast (73 +/- 15 vs. 112 +/- 14 mg/dl/min with placebo; P less than 0.01) and dinner (23 +/- 8 vs. 4 +/- 1 mg/dl/min with placebo; P less than 0.05) meal with Bay-m-1099. Bay-m-1099 did not affect postprandial plasma triglycerides and was well tolerated, the major side effect being flatulence (4/9) and mild diarrhea (4/9). We conclude that inhibition of intestinal alpha-glucosidases by Bay-m-1099 in IDDM reduces meal insulin requirements by at least 20% and that such an agent could be useful in the management of diabetes mellitus by reducing hyperinsulinemia.     

Effect of fatty acids on glucose production and utilization in man.

Since the initial proposal of the glucose fatty acid cycle, considerable controversy has arisen concerning its physiologic significance in vivo. In the present study, we examined the effect of acute, physiologic elevations of FFA concentrations on glucose production and uptake in normal subjects under three controlled experimental conditions. In group A, plasma insulin levels were raised and maintained at approximately 100 microU/ml above base line by an insulin infusion, while holding plasma glucose at the fasting level by a variable glucose infusion. In group B, plasma glucose concentration was raised by 125 mg/100 ml and plasma insulin was clamped at approximately 50 microU/ml by a combined infusion of somatostatin and insulin. In group C, plasma glucose was raised by 200 mg/100 ml above the fasting level, while insulin secretion was inhibited with somatostatin and peripheral glucagon levels were replaced with a glucagon infusion (1 ng/min X kg). Each protocol was repeated in the same subject in combination with a lipid-heparin infusion designed to raise plasma FFA levels by 1.5-2.0 mumol/ml. With euglycemic hyperinsulinemia (study A), lipid infusion caused a significant inhibition of total glucose uptake (6.3 +/- 1.3 vs. 7.4 +/- 0.6 mg/min X kg, P less than 0.02). Endogenous glucose production (estimated by the [3-3H]glucose technique) was completely suppressed both with and without lipid infusion. With hyperglycemic hyperinsulinemia (study B), lipid infusion also induced a marked impairment in glucose utilization (6.2 +/- 1.1 vs. 9.8 +/- 1.9 mg/min X kg, P less than 0.05); endogenous glucose production was again completely inhibited despite the increase in FFA concentrations. Under both conditions (A and B), the percentage inhibition of glucose uptake by FFA was positively correlated with the total rate of glucose uptake (r = 0.69, P less than 0.01). In contrast, when hyperglycemia was associated with relative insulinopenia and hyperglucagonemia (study C), thus simulating a diabetic state, lipid infusion had no effect on glucose uptake (2.9 +/- 0.2 vs. 2.6 +/- 0.2 mg/min X kg) but markedly stimulated endogenous glucose production (1.4 +/- 0.5 vs. 0.5 +/- 0.4 mg/min X kg, P less than 0.005). Under the same conditions as study C, a glycerol infusion producing plasma glycerol levels similar to those achieved with lipid-heparin, enhanced endogenous glucose production (1.5 +/- 0.5 vs. 0.7 +/- 0.6 mg/min X kg, P less than 0.05). We conclude that, in the well-insulinized state raised FFA levels effectively compete with glucose for uptake by peripheral tissues, regardless of the presence of hyperglycemia. When insulin is deficient, on the other hand, elevated rates of lipolysis may contribute to hyperglycemia not by competition for fuel utilization, but through an enhancement of endogenous glucose output.     

Hyperinsulinemia produces both sympathetic neural activation and vasodilation in normal humans.

Hyperinsulinemia may contribute to hypertension by increasing sympathetic activity and vascular resistance. We sought to determine if insulin increases central sympathetic neural outflow and vascular resistance in humans. We recorded muscle sympathetic nerve activity (MSNA; microneurography, peroneal nerve), forearm blood flow (plethysmography), heart rate, and blood pressure in 14 normotensive males during 1-h infusions of low (38 mU/m2/min) and high (76 mU/m2/min) doses of insulin while holding blood glucose constant. Plasma insulin rose from 8 +/- 1 microU/ml during control, to 72 +/- 8 and 144 +/- 13 microU/ml during the low and high insulin doses, respectively, and fell to 15 +/- 6 microU/ml 1 h after insulin infusion was stopped. MSNA, which averaged 21.5 +/- 1.5 bursts/min in control, increased significantly (P less than 0.001) during both the low and high doses of insulin (+/- 5.4 and +/- 9.3 bursts/min, respectively) and further increased during 1-h recovery (+15.2 bursts/min). Plasma norepinephrine levels (119 +/- 19 pg/ml during control) rose during both low (258 +/- 25; P less than 0.02) and high (285 +/- 95; P less than 0.01) doses of insulin and recovery (316 +/- 23; P less than 0.01). Plasma epinephrine levels did not change during insulin infusion. Despite the increased MSNA and plasma norepinephrine, there were significant (P less than 0.001) increases in forearm blood flow and decreases in forearm vascular resistance during both doses of insulin. Systolic pressure did not change significantly during infusion of insulin and diastolic pressure fell approximately 4-5 mmHg (P less than 0.01). This study suggests that acute increases in plasma insulin within the physiological range elevate sympathetic neural outflow but produce forearm vasodilation and do not elevate arterial pressure in normal humans.     

Defective suppression by insulin of leucine-carbon appearance and oxidation in type 1, insulin-dependent diabetes mellitus. Evidence for insulin resistance involving glucose and amino acid metabolism.

To determine whether a resistance to insulin in type 1, insulin-dependent diabetes mellitus (IDDM) is extended to both glucose and amino acid metabolism, six normal subjects and five patients with IDDM, maintained in euglycemia with intravenous insulin administration, were infused with L-[4,5-3H]leucine (Leu) and [1-14C]alpha ketoisocaproate (KIC). Steady-state rates of leucine-carbon appearance derived from protein breakdown (Leu + KIC Ra) and KIC (approximately leucine) oxidation were determined at basal and during sequential euglycemic, hyperinsulinemic (approximately 40, approximately 90 and approximately 1,300 microU/ml) clamps. In the euglycemic postabsorptive diabetic patients, despite basal hyperinsulinemia (24 +/- 6 microU/ml vs. 9 +/- 1 microU/ml in normals, P less than 0.05), Leu + KIC Ra (2.90 +/- 0.18 mumol/kg X min), and KIC oxidation (0.22 +/- 0.03 mumol/kg X min) were similar to normal values (Leu + KIC Ra = 2.74 +/- 0.25 mumol/kg X min) (oxidation = 0.20 +/- 0.02 mumol/kg X min). During stepwise hyperinsulinemia, Leu + KIC Ra in normals decreased to 2.08 +/- 0.19, to 2.00 +/- 0.17, and to 1.81 +/- 0.16 mumol/kg X min, but only to 2.77 +/- 0.16, to 2.63 +/- 0.16, and to 2.39 +/- 0.08 mumol/kg X min in the diabetic patients (P less than 0.05 or less vs. normals at each clamp step). KIC oxidation decreased in normal subjects to a larger extent than in the diabetic subjects. Glucose disposal was reduced at all insulin levels in the patients. In summary, in IDDM: (a) Peripheral hyperinsulinemia is required to normalize both fasting leucine metabolism and blood glucose concentrations. (b) At euglycemic hyperinsulinemic clamps, lower glucose disposal rates and a defective suppression of leucine-carbon appearance and oxidation were observed. We conclude that in type 1 diabetes a resistance to the metabolic effects of insulin on both glucose and amino acid metabolism is present.     

Regulation by insulin of myocardial glucose and fatty acid metabolism in the conscious dog.

In vivo small doses of insulin inhibit lipolysis, lower plasma FFA, and stimulate glucose disposal. Lowering of plasma FFA, either in the absence of a change in insulin or during combined hyperglycemia and hyperinsulinemia, promotes glucose uptake by heart muscle in vivo. In the isolated perfused heart, large doses of insulin directly stimulate heart glucose uptake. To assess the effect of physiological elevations of plasma insulin upon myocardial glucose and FFA uptake in vivo independent of changes in plasma substrate concentration, we measured arterial and coronary sinus concentrations of glucose, lactate, and FFA, and coronary blood flow in conscious dogs during a 30 min basal and a 2 h experimental period employing three protocols: (a) euglycemic hyperinsulinemia (insulin clamp, n = 5), (b) euglycemic hyperinsulinemia with FFA replacement (n = 5), (c) hyperglycemic euinsulinemia (hyperglycemic clamp with somatostatin, n = 5). In group 1, hyperinsulinemia (insulin = 73 +/- 13 microU/ml) stimulated heart glucose uptake (7.3 +/- 4.4 vs. 28.2 +/- 2.8 mumol/min, P less than 0.002), lowered plasma FFA levels by 80% (P less than 0.05), and decreased heart FFA uptake (28.4 +/- 4 vs. 1.5 +/- 0.9, P less than 0.01). When the fall in plasma FFA was prevented by FFA infusion (group 2), hyperinsulinemia (86 +/- 10 microU/ml) provoked a lesser (P less than 0.05) stimulation of glucose uptake (delta = 8.2 +/- 4.2 mumol/min) than in group 1, and there was no significant change in FFA uptake (25.3 +/- 16 vs. 16.5 +/- 4). Hyperglycemia (plasma glucose = 186 +/- 8 mg/100 ml) during somatostatin infusion resulted in only a small rise in plasma insulin (delta = 12 +/- 7 microU/ml), and although plasma FFA tended to decline, heart glucose uptake did not rise significantly (delta = 5.5 +/- 3.2 mumol/min, P = NS). There was no significant change in coronary blood flow during any of the three study protocols. We conclude that, in the dog, insulin at physiologic concentrations: (a) stimulates heart glucose uptake, both directly and by suppressing the plasma FFA concentration, and (b) does not alter coronary blood flow. Hyperglycemia per se has little effect on heart glucose uptake.     

Defective glucose counterregulation after subcutaneous insulin in noninsulin-dependent diabetes mellitus. Paradoxical suppression of glucose utilization and lack of compensatory increase in glucose production, roles of insulin resistance, abnormal neuroendocrine responses, and islet paracrine interactions.

To characterize glucose counterregulatory mechanisms in patients with noninsulin-dependent diabetes mellitus (NIDDM) and to test the hypothesis that the increase in glucagon secretion during hypoglycemia occurs primarily via a paracrine islet A-B cell interaction, we examined the effects of a subcutaneously injected therapeutic dose of insulin (0.15 U/kg) on plasma glucose kinetics, rates of glucose production and utilization, and their relationships to changes in the circulating concentrations of neuroendocrine glucoregulatory factors (glucagon, epinephrine, norepinephrine, growth hormone, and cortisol), as well as to changes in endogenous insulin secretion in 13 nonobese NIDDM patients with no clinical evidence of autonomic neuropathy. Compared with 11 age-weight matched nondiabetic volunteers in whom euglycemia was restored primarily by a compensatory increase in glucose production, in the diabetics there was no compensatory increase in glucose production (basal 2.08 +/- 0.04----1.79 +/- 0.07 mg/kg per min at 21/2 h in diabetics vs. basal 2.06 +/- 0.04----2.32 +/- 0.11 mg/kg per min at 21/2 h in nondiabetics, P less than 0.01) despite the fact that plasma insulin concentrations were similar in both groups (peak values 22 +/- 2 vs. 23 +/- 2 microU/ml in diabetics and nondiabetics, respectively). This abnormality in glucose production was nearly completely compensated for by a paradoxical decrease in glucose utilization after injection of insulin (basal 2.11 +/- 0.03----1.86 +/- 0.06 mg/kg per min at 21/2 h in diabetics vs. basal 2.08 +/- 0.04----2.39 +/- 0.11 mg/kg per min at 21/2 h nondiabetics, P less than 0.01), which could not be accounted for by differences in plasma glucose concentrations; the net result was a modest prolongation of hypoglycemia. Plasma glucagon (area under the curve [AUC] above base line, 12 +/- 3 vs. 23 +/- 3 mg/ml X 12 h in nondiabetics, P less than 0.05), cortisol (AUC 2.2 +/- 0.5 vs. 4.0 +/- 0.7 mg/dl X 12 h in nondiabetics, P less than 0.05), and growth hormone (AUC 1.6 +/- 0.4 vs. 2.9 +/- 0.4 micrograms/ml X 12 h in nondiabetics, P less than 0.05) responses in the diabetics were decreased 50% while their plasma norepinephrine responses (AUC 49 +/- 12 vs. 21 +/- 5 ng/ml X 12 h in nondiabetics, P less than 0.05) were increased twofold (P less than 0.05) and their plasma epinephrine responses were similar to those of the nondiabetics (AUC 106 +/- 17 vs. 112 +/- 10 ng/ml X 12 h in nondiabetics). In both groups of subjects, increases in plasma glucagon were inversely correlated with plasma glucose concentrations (r = -0.80 in both groups, P less than 0.01) and suppression of endogenous insulin secretion (r = -0.57 in nondiabe     

Role of parathyroid hormone in the glucose intolerance of chronic renal failure.

Evidence has accumulated suggesting that the state of secondary hyperparathyroidism and the elevated blood levels of parathyroid hormone (PTH) in uremia participate in the genesis of many uremic manifestations. The present study examined the role of PTH in glucose intolerance of chronic renal failure (CRF). Intravenous glucose tolerance tests (IVGTT) and euglycemic and hyperglycemic clamp studies were performed in dogs with CRF with (NPX) and without parathyroid glands (NPX-PTX). There were no significant differences among the plasma concentrations of electrolytes, degree of CRF, and its duration. The serum levels of PTH were elevated in NPX and undetectable in NPX-PTX. The NPX dogs displayed glucose intolerance after CRF and blood glucose concentrations during IVGTT were significantly (P less than 0.01) higher than corresponding values before CRF. In contrast, blood glucose levels after IVGTT in NPX-PTX before and after CRF were not different. K-g rate fell after CRF from 2.86 +/- 0.48 to 1.23 +/- 0.18%/min (P less than 0.01) in NPX but remained unchanged in NPX-PTX (from 2.41 +/- 0.43 to 2.86 +/- 0.86%/min) dogs. Blood insulin levels after IVGTT in NPX-PTX were more than twice higher than in NPX animals (P less than 0.01) and for any given level of blood glucose concentration, the insulin levels were higher in NPX-PTX than NPX dogs. Clamp studies showed that the total amount of glucose utilized was significantly lower (P less than 0.025) in NPX (6.64 +/- 1.13 mg/kg X min) than in NPX-PTX (10.74 +/- 1.1 mg/kg X min) dogs. The early, late, and total insulin responses were significantly (P less than 0.025) greater in the NPX-PTX than NPX animals. The values for the total response were 143 +/- 28 vs. 71 +/- 10 microU/ml, P less than 0.01. There was no significant difference in the ratio of glucose metabolized to the total insulin response, a measure of tissue sensitivity to insulin, between the two groups. The glucose metabolized to total insulin response ratio in NPX (5.12 +/- 0.76 mg/kg X min per microU/ml) and NPX-PTX (5.18 +/- 0.57 mg/kg X min per microU/ml) dogs was not different but significantly (P less than 0.01) lower than in normal animals (9.98 +/- 1.26 mg/kg X min per microU/ml). The metabolic clearance rate of insulin was significantly (P less than 0.02) reduced in both NPX (12.1 +/- 0.7 ml/kg X min) and NPX-PTX (12.1 +/- 0.9 ml/kg X min) dogs, as compared with normal animals (17.4 +/- 1.8 ml/kg X min). The basal hepatic glucose production was similar in both groups of animals and nor different from normal dogs; both the time course and the magnitude of suppression of hepatic glucose production by insulin were similar in both in groups. There were no differences in the binding affinity, binding sites concentration, and binding capacity of monocytes to insulin among NPX, NPX-PTX, and normal dogs. The data show that (a) glucose intolerance does not develop with CRF in the absence of PTH, (b) PTH does not affect metabolic clearance of insulin or tissue resistance to insulin in CRF, and (c) the normalization of metabolism in CRF in the absence of PTH is due to increased insulin secretion. The results indicate that excess PTH in CRF interferes with the ability of the beta-cells to augment insulin secretion appropriately in response to the insulin-resistant state.     

A case of insulin resistance associated with acanthosis nigricans

We described here a 12-year-old male patient with the syndrome of insulin resistance and acanthosis nigricans type A. Insulin levels at fasting state and after glucose loading were 149 +/- 63 microU/ml (mean +/- S.D.) and over 1,000 microU/ml respectively, while the fasting level of blood glucose was 77.7 +/- 8.9 mg/ml (mean +/- S.D.). A marked resistance to exogenous insulin was observed. Circulating levels of insulin antagonists such as growth hormone, cortisol and glucagon were within the normal range. Proinsulin was less than 5% of the radioimmunoassayable insulin. No insulin antibody or antireceptor antibody was detected. Insulin binding to mononuclear cells was decreased to about 50% of the controls. Analysis of membrane receptors demonstrated the normal average affinity, dissociation kinetics and negative cooperativity with a decreased number of receptors. After two days fasting, plasma IRI levels decreased to 27 microU/ml, while insulin binding kinetics were not affected; which suggests that the receptor decrease was not secondary to hyperinsulinemia. These findings indicate that the decreased number of receptors was one of the causes for insulin resistance in this patient.     

Effects of insulin on peripheral and splanchnic glucose metabolism in noninsulin-dependent (type II) diabetes mellitus.

The mechanism(s) and site(s) of the insulin resistance were examined in nine normal-weight noninsulin-dependent diabetic (NIDD) subjects. The euglycemic insulin clamp technique (insulin concentration approximately 100 microU/ml) was employed in combination with hepatic and femoral venous catheterization and measurement of endogenous glucose production using infusion of tritiated glucose. Total body glucose metabolism in the NIDD subjects (4.37 +/- 0.45 mg/kg per min) was 38% (P less than 0.01) lower than in controls (7.04 +/- 0.63 mg/kg per min). Quantitatively, the most important site of the insulin resistance was found to be in peripheral tissues. Leg glucose uptake in the diabetic group was reduced by 45% as compared with that in controls (6.0 +/- 0.2 vs. 11.0 +/- 0.1 mg/kg leg wt per min; P less than 0.01). A strong positive correlation was observed between leg and total body glucose uptake (r = 0.70, P less than 0.001). Assuming that muscle is the primary leg tissue responsible for glucose uptake, it could be estimated that 90 and 87% of the infused glucose was disposed of by peripheral tissues in the control and NIDD subjects, respectively. Net splanchnic glucose balance during insulin stimulation was slightly more positive in the control than in the diabetic subjects (0.31 +/- 0.10 vs. 0.05 +/- 0.19 mg/kg per min; P less than 0.07). The difference (0.26 mg/kg per min) in net splanchnic glucose balance in NIDD represented only 10% of the reduction (2.67 mg/kg per min) in total body glucose uptake in the NIDD group and thus contributed very little to the insulin resistance. The results emphasize the importance of the peripheral tissues in the disposal of infused glucose and indicate that muscle is the most important site of the insulin resistance in NIDD.     

Influence of a 60-hour fast on insulin-mediated splanchnic and peripheral glucose metabolism in humans.

A brief period of starvation (2-3) depletes the hepatic glycogen stores but results in only a limited reduction of the muscle glycogen depots. In this situation insulin resistance contributes to the glucose intolerance, but it is not known which tissue or tissues are responsible for the decreased insulin sensitivity. The present study was therefore undertaken to examine the influence of a 60-h fast on insulin sensitivity in splanchnic and peripheral tissues in normal humans. Euglycemic (95 mg/dl) 1-mU insulin and hyperglycemic (215-225 mg/dl) glucose clamp studies were conducted for 2 h in overnight (12 h) and prolonged (60 h) fasted nonobese subjects. Splanchnic exchange of glucose and gluconeogenic precursors was measured using the hepatic vein catheter technique. During the euglycemic clamp, insulin infusion resulted in similar steady state insulin levels in 60-h and 12-h fasted subjects (73 +/- 7 vs. 74 +/- 5 microU/ml). Total glucose disposal was reduced by 45% after 60 h of fasting (4.0 +/- 0.3 vs. 7.6 +/- 1.1 mg/kg per min, P less than 0.05) and the splanchnic glucose balance reverted from a net release in the basal state (12 h fast, -1.7 +/- 0.2, and 60-h fast, -0.9 +/- 0.1 mg/kg per min, P less than 0.01) to a net uptake during the clamps that was similar after 60 h and 12 h of fasting (0.6 +/- 0.1 vs. 0.6 +/- 0.2 mg/kg per min). During the hyperglycemic clamp, insulin levels rose rapidly in all subjects. In the 12-h fasted group this rise was followed by a further gradual one, reaching significantly higher values than in 60-h fasted subjects during the second hour (67 +/- 15 vs. 25 +/- 2 microU/ml, P less than 0.05). Total glucose disposal was lower, though not significantly so, after the 60-h fast (2.6 +/- 0.4 vs. 5.4 +/- 1.3 mg/kg per min, 0.05 less than P less than 0.10), and as with the euglycemic clamp, the splanchnic glucose balance was altered from a basal net release to a net uptake during the clamp (1.3 +/- 0.2 vs. 1.1 +/- 0.2 mg/kg per min). After an overnight fast, splanchnic lactate uptake fell and the arterial lactate concentration rose in response to both hyperglycemia and hyperinsulinemia, whereas these variables were unchanged in the 60-h fasted subjects during both types of clamp studies.     

Fenfluramine increases insulin action in patients with NIDDM

These studies examined the effect of fenfluramine on insulin action and insulin secretion in healthy subjects and patients with non-insulin-dependent diabetes mellitus (NIDDM). In the first study, a double-blind crossover design was used in healthy subjects to compare the effect of short-term fenfluramine therapy (60 mg orally for 3 days) with placebo. Insulin secretion and whole-body insulin sensitivity (determined by frequently sampled intravenous glucose tolerance tests with analysis by the minimal-model method) were unchanged by fenfluramine. In the second study, involving patients with NIDDM inadequately controlled on submaximal to maximal doses of oral hypoglycemic agents, a double-blind crossover strategy was used to compare baseline studies (conducted after a run-in period) with fenfluramine (60 mg orally) or placebo for 4 wk. There was a significant fall in fasting blood glucose after therapy with fenfluramine compared with the baseline study period (13.0 +/- 1.2 vs. 8.4 +/- 0.89 mM, mean +/- SE, P less than .01) with no significant fall in fasting serum insulin (20 +/- 2 vs. 24 +/- 3 microU/ml) or C-peptide (1.3 +/- 0.2 vs. 1.3 +/- 0.1 nM). During euglycemic-hyperinsulinemic (1 mU.kg-1.min-1) clamp studies there was a significant increase in insulin action from 12.7 +/- 2.3 to 17.3 +/- 1.8 min-1.10(3) microU.ml-1 (P less than .05), although clamp insulin levels were lower after fenfluramine treatment (136 +/- 14 vs. 96 +/- 9 microU/ml, P less than .02), reflecting an enhanced metabolic clearance rate for insulin (12.7 +/- 1.5 vs. 20.1 +/- 2.1 ml.kg-1.min-1, P less than .025).(ABSTRACT TRUNCATED AT 250 WORDS)     

Insulin resistance in uremia.

Tissue sensitivity to insulin was examined with the euglycemic insulin clamp technique in 17 chronically uremic and 36 control subjects. The plasma insulin concentration was raised by approximately 100 microU/ml and the plasma glucose concentration was maintained at the basal level with a variable glucose infusion. Under these steady-state conditions of euglycemia, the glucose infusion rate is a measure of the amount of glucose taken up by the entire body. In uremic subjects insulin-mediated glucose metabolism was reduced by 47% compared with controls (3.71 +/- 0.20 vs. 7.38 +/- 0.26 mg/kg . min; P less than 0.001). Basal hepatic glucose production (measured with [3H]-3-glucose) was normal in uremic subjects (2.17 +/- 0.04 mg/kg . min) and suppressed normally by 94 +/- 2% following insulin administration. In six uremic and six control subjects, net splanchnic glucose balance was also measured directly by the hepatic venous catheterization technique. In the postabsorptive state splanchnic glucose production was similar in uremics (1.57 +/- 0.03 mg/kg . min) and controls (1.79 +/- 0.20 mg/kg . min). After 90 min of sustained hyperinsulinemia, splanchnic glucose balance reverted to a net uptake which was similar in uremics (0.42 +/- 0.11 mg/kg . min) and controls (0.53 +/- 0.12 mg/kg . min). In contrast, glucose uptake by the leg was reduced by 60% in the uremic group (21 +/- 1 vs. 52 +/- 8 mumol/min . kg of leg wt; P less than 0.005) and this decrease closely paralleled the decrease in total glucose metabolism by the entire body. These results indicate that: (a) suppression of hepatic glucose production by physiologic hyperinsulinemia is not impaired by uremia, (b) insulin-mediated glucose uptake by the liver is normal in uremic subjects, and (c) tissue insensitivity to insulin is the primary cause of insulin resistance in uremia.     

Effect of insulin on motilin release in man

The effect of insulin on motilin release was investigated by use of the euglycemic glucose clamp technique. By use of this technique plasma glucose concentration was maintained constant at 80-90 mg/100 ml, and plasma insulin immunoreactivity (IRI) was increased from 15 +/- 6 microU/ml to 171 +/- 22 microU/ml in 10 min, and remained at this level for 2 hr. Plasma motilin like immunoreactivity (MLI) concentration decreased within 10 min from 199 +/- 36 pg/ml to 120 +/- 28 pg/ml and remained low during the course of study. A significant negative correlation between MLI and IRI concentrations (r = -0.72, p less than 0.01) was observed. The present results indicate that the suppressive effect of insulin on motilin release is a direct action of insulin and is not mediated by glucose.     

Mechanisms of insulin resistance in human obesity: evidence for receptor and postreceptor defects.

To assess the mechanisms of the insulin resistance in human obesity, we have determined, using a modification of the euglycemic glucose clamp technique, the shape of the in vivo insulin-glucose disposal dose-response curves in 7 control and 13 obese human subjects. Each subject had at least three euglycemic studies performed at insulin infusion rates of 15, 40, 120, 240, or 1,200 mU/M2/min. The glucose disposal rate was decreased in all obese subjects compared with controls (101 +/- 16 vs. 186 +/- 16 mg/M2/min) during the 40 mU/M2/min insulin infusion. The mean dose-response curve for the obese subjects was displaced to the right, i.e., the half-maximally effective insulin concentration was 270 +/- 27 microU/ml for the obese compared with 130 +/- 10 microU/ml for controls. In nine of the obese subjects, the dose-response curves were shifted to the right, and maximal glucose disposal rates (at a maximally effective insulin concentration) were markedly decreased, indicating both a receptor and a postreceptor defect. On the other hand, four obese patients had right-shifted dose-response curves but reached normal maximal glucose disposal rates, consistent with decreased insulin receptors as the only abnormality. When the individual data were analyzed, it was found that the lease hyperinsulinemic, least insulin-resistant patients displayed only the receptor defect, whereas those with the greatest hyperinsulinemia exhibited the largest post-receptor defect, suggesting a continuous spectrum of defects as one advances from mild to severe insulin resistance. When insulin's ability to suppress hepatic glucose output was assessed, hyperinsulinemia produced total suppresssion in all subjects. The dose-response curve for the obese subjects was shifted to the right, indicating a defect in insulin receptors. Insulin binding to isolated adipocytes obtained from the obese subjects was decreased, and a highly significant inverse linear relationship was demonstrated between insulin binding and the serum insulin concentration required for halfmaximal stimulation of glucose disposal. In conclusion: (a) decreased cellular insulin receptors contribute to the insulin resistance associated with human obesity in all subjects; (b) in the least hyperinsulinemic, insulin-resistant patients, decreased insulin receptors are the sole defect, whereas in the more hyperinsulinemic, insulin-resistant patients, the insulin resistance is the result of a combination of receptor and postreceptor abnormalities; (c) all obese patients were insensitive to insulin's suppressive effects on hepatic glucose output; this was entirely the result of decreased insulin receptors; no postreceptor defect in this insulin effect was demonstrated.     

Insulin transport across capillaries is rate limiting for insulin action in dogs.

This study examined the relationship between transcapillary insulin transport and insulin action in vivo. During euglycemic clamps (n = 7) in normal conscious dogs we simultaneously measured plasma and thoracic duct lymph insulin and glucose utilization (Rd). Clamps consisted of an activation phase with constant insulin infusion (0.6 mU/kg per min) and a deactivation phase. [14C]Inulin was infused as a passively transported control substance. While [14C]inulin reached an equilibrium between plasma and lymph, steady-state (ss) plasma insulin was higher than lymph (P less than 0.05) and the ratio of 3:2 was maintained during basal, activation, and deactivation phases: 18 +/- 2 vs. 12 +/- 1, 51 +/- 2 vs. 32 +/- 1, and 18 +/- 3 vs. 13 +/- 1 microU/ml. In addition, it took longer for lymph insulin to reach ss than plasma insulin during activation and deactivation: 11 +/- 2 vs. 31 +/- 5 and 8 +/- 2 vs. 32 +/- 6 min (P less than 0.02). Rd increased from 2.6 +/- 0.1 to a ss of 6.6 +/- 0.4 mg/kg per min within 50 +/- 8 min. There was a remarkable similarity in the dynamics of insulin in lymph and Rd: the time to reach ss for Rd was not different from lymph insulin (P greater than 0.1), and the relative increases of the two measurements were similar, 164 +/- 45% and 189 +/- 29% (P greater than 0.05). While there was only a modest correlation (r = 0.78, P less than 0.01) between Rd and plasma insulin, the dynamic changes of lymph insulin and Rd showed a strong correlation (r = 0.95, P less than 0.01). The intimate relationship between lymph insulin and Rd suggests that the transcapillary insulin transport is primarily responsible for the delay in Rd. Thus, transcapillary transport may be rate limiting for insulin action, and if altered, it could be an important component of insulin resistance in obesity and diabetes mellitus.     

Role of the glucosamine pathway in fat-induced insulin resistance.

To examine whether the hexosamine biosynthetic pathway might play a role in fat-induced insulin resistance, we monitored the effects of prolonged elevations in FFA availability both on skeletal muscle levels of UDP-N-acetyl-hexosamines and on peripheral glucose disposal during 7-h euglycemic-hyperinsulinemic (approximately 500 microU/ml) clamp studies. When the insulin-induced decrease in the plasma FFA levels (to approximately 0.3 mM) was prevented by infusion of a lipid emulsion in 15 conscious rats (plasma FFA approximately 1.4 mM), glucose uptake (5-7 h = 32.5+/-1.7 vs 0-2 h = 45.2+/-2.8 mg/kg per min; P < 0.01) and glycogen synthesis (P < 0.01) were markedly decreased. During lipid infusion, muscle UDP-N-acetyl-glucosamine (UDP-GlcNAc) increased by twofold (to 53.4+/-1.1 at 3 h and to 55.5+/-1.1 nmol/gram at 7 h vs 20.4+/-1.7 at 0 h, P < 0.01) while glucose-6-phosphate (Glc-6-P) levels were increased at 3 h (475+/-49 nmol/gram) and decreased at 7 h (133+/-7 vs 337+/-28 nmol/gram at 0 h, P < 0.01). To discern whether such an increase in the skeletal muscle UDP-GlcNAc concentration could account for the development of insulin resistance, we generated similar increases in muscle UDP-GlcNAc using three alternate experimental approaches. Euglycemic clamps were performed after prolonged hyperglycemia (18 mM, n = 10), or increased availability of either glucosamine (3 micromol/kg per min; n = 10) or uridine (30 micromol/kg per min; n = 4). These conditions all resulted in very similar increases in the skeletal muscle UDP-GlcNAc (to approximately 55 nmol/gram) and markedly impaired glucose uptake and glycogen synthesis. Thus, fat-induced insulin resistance is associated with: (a) decreased skeletal muscle Glc-6-P levels indicating defective transport/phosphorylation of glucose; (b) marked accumulation of the endproducts of the hexosamine biosynthetic pathway preceding the onset of insulin resistance. Most important, the same degree of insulin resistance can be reproduced in the absence of increased FFA availability by a similar increase in skeletal muscle UDP-N-acetyl-hexosamines. In conclusion, our results support the hypothesis that increased FFA availability induces skeletal muscle insulin resistance by increasing the flux of fructose-6-phosphate into the hexosamine pathway.     

Evidence that the brain of the conscious dog is insulin sensitive.

The aim of this study was to determine whether a selective increase in the level of insulin in the blood perfusing the brain is a determinant of the counterregulatory response to hypoglycemia. Experiments were carried out on 15 conscious 18-h-fasted dogs. Insulin was infused (2 mU/kg per min) in separate, randomized studies into a peripheral vein (n = 7) or both carotid and vertebral arteries (n = 8). This resulted in equivalent systemic insulinemia (84 +/- 6 vs. 86 +/- 6 microU/ml) but differing insulin levels in the head (84 +/- 6 vs. 195 +/- 5 microU/ml, respectively). Glucose was infused during peripheral insulin infusion to maintain the glucose level (56 +/- 2 mg/dl) at a value similar to that seen during head insulin infusion (58 +/- 2 mg/dl). Despite equivalent peripheral insulin levels and similar hypoglycemia; steady state plasma epinephrine (792 +/- 198 vs. 2394 +/- 312 pg/ml), norepinephrine (404 +/- 33 vs. 778 +/- 93 pg/ml), cortisol (6.8 +/- 1.8 vs. 9.8 +/- 1.6 micrograms/dl) and pancreatic polypeptide (722 +/- 273 vs. 1061 +/- 255 pg/ml) levels were all increased to a greater extent during head insulin infusion (P < 0.05). Hepatic glucose production, measured with [3-3H]glucose, rose from 2.6 +/- 0.2 to 4.3 +/- 0.4 mg/kg per min (P < 0.01) in response to head insulin infusion but remained unchanged (2.6 +/- 0.5 mg/kg per min) during peripheral insulin infusion. Similarly, gluconeogenesis, lipolysis, and ketogenesis were increased twofold (P < 0.001) during head compared with peripheral insulin infusion. Cardiovascular parameters were also significantly higher (P < 0.05) during head compared with peripheral insulin infusion. We conclude that during hypoglycemia in the conscious dog (a) the brain is directly responsive to physiologic elevations of insulin and (b) the response includes a profound stimulation of the autonomic nervous system with accompanying metabolic and cardiovascular changes.     

Improvement in glucose tolerance after 1 wk of exercise in patients with mild NIDDM

We investigated the effects of 1 wk of intense exercise on glucose tolerance in 10 men with abnormal glucose tolerance [7 had mild non-insulin-dependent diabetes mellitus (NIDDM), and 3 had impaired glucose tolerance]. The 7 days of exercise did not result in significant changes in body weight or maximal oxygen uptake. Plasma glucose concentration at 120 min averaged 227 +/- 23 mg/dl in an oral glucose tolerance test (OGTT) before and 170 +/- 18 mg/dl after the 7 days of exercise (P less than .001). There was a 36% reduction in the area under the glucose tolerance curve. Plasma insulin concentration at 120 min of the OGTT averaged 172 +/- 27 microU/ml before and 106 +/- 13 microU/ml after 7 days of exercise (P less than .001); the area under the insulin curve was decreased by 32%. In contrast to the response to 7 days of exercise, one bout of exercise did not result in an improvement in glucose tolerance. These results provide evidence that regularly performed, vigorous exercise can be effective in decreasing insulin resistance and improving glucose tolerance within 7 days in some patients with mild NIDDM.