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.     

Regulation of myocardial amino acid balance in the conscious dog.

The effects in vivo of physiologic increases in insulin and amino acids on myocardial amino acid balance were evaluated in conscious dogs. Arterial and coronary sinus concentrations of amino acids and coronary blood flow were measured during a 30-min basal and a 100-min experimental period employing three protocols: euglycemic insulin clamp (plasma insulin equaled 70 +/- 11 microU/ml, n = 6); euglycemic insulin clamp during amino acid infusion (plasma insulin equaled 89 +/- 12 microU/ml, n = 6); and suppression of insulin with somatostatin during amino acid infusion (plasma insulin equaled 15 +/- 4 microU/ml, n = 6). Basally, only leucine and isoleucine were removed significantly by myocardium (net branched chain amino acid [BCAA] uptake equaled 0.5 +/- 0.2 mumol/min), while glycine, alanine, and glutamine were released. Glutamine demonstrated the highest net myocardial production (1.6 +/- 0.2 mumol/min). No net exchange was seen for valine, phenylalanine, tyrosine, cysteine, methionine, glutamate, asparagine, serine, threonine, taurine, and aspartate. In group I, hyperinsulinemia caused a decline of all plasma amino acids except alanine; alanine balance switched from release to an uptake of 0.6 +/- 0.4 mumol/min (P less than 0.05), while the myocardial balance of other amino acids was unchanged. In group II, amino acid concentrations rose, and were accompanied by a marked rise in myocardial BCAA uptake (0.4 +/- 0.1-2.6 +/- 0.3 mumol/min, P less than 0.001). Uptake of alanine was again stimulated (0.9 +/- 0.3 mumol/min, P less than 0.01), while glutamine production was unchanged (1.3 +/- 0.4 vs. 1.6 +/- 0.3 mumol/min). In group III, there was a 4-5-fold increase in the plasma concentration of the infused amino acids, accompanied by marked stimulation in uptake of only BCAA (6.8 +/- 0.7 mumol/min). Myocardial glutamine production was unchanged (1.9 +/- 0.4-1.3 +/- 0.7 mumol/min). Within the three experimental groups there were highly significant linear correlations between myocardial uptake and arterial concentration of leucine, isoleucine, valine, and total BCAA (r = 0.98, 0.98, 0.92, and 0.97, respectively); P less than 0.001 for each). In vivo, BCAA are the principal amino acids taken up by the myocardium basally and during amino acid infusion. Plasma BCAA concentration and not insulin determines the rate of myocardial BCAA uptake. Insulin stimulates myocardial alanine uptake. Neither insulin nor amino acid infusion alters myocardial glutamine release.     

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.     

Interaction between glucose and free fatty acid metabolism in human skeletal muscle.

The mechanism by which FFA metabolism inhibits intracellular insulin-mediated muscle glucose metabolism in normal humans is unknown. We used the leg balance technique with muscle biopsies to determine how experimental maintenance of FFA during hyperinsulinemia alters muscle glucose uptake, oxidation, glycolysis, storage, pyruvate dehydrogenase (PDH), or glycogen synthase (GS). 10 healthy volunteers had two euglycemic insulin clamp experiments. On one occasion, FFA were maintained by lipid emulsion infusion; on the other, FFA were allowed to fall. Leg FFA uptake was monitored with [9,10-3H]-palmitate. Maintenance of FFA during hyperinsulinemia decreased muscle glucose uptake (1.57 +/- 0.31 vs 2.44 +/- 0.39 mumol/min per 100 ml tissue, P < 0.01), leg respiratory quotient (0.86 +/- 0.02 vs 0.93 +/- 0.02, P < 0.05), contribution of glucose to leg oxygen consumption (53 +/- 6 vs 76 +/- 8%, P < 0.05), and PDH activity (0.328 +/- 0.053 vs 0.662 +/- 0.176 nmol/min per mg, P < 0.05). Leg lactate balance was increased. The greatest effect of FFA replacement was reduced muscle glucose storage (0.36 +/- 0.20 vs 1.24 +/- 0.25 mumol/min per 100 ml, P < 0.01), accompanied by decreased GS fractional velocity (0.129 +/- 0.26 vs 0.169 +/- 0.033, P < 0.01). These results confirm in human skeletal muscle the existence of competition between glucose and FFA as oxidative fuels, mediated by suppression of PDH. Maintenance of FFA levels during hyperinsulinemia most strikingly inhibited leg muscle glucose storage, accompanied by decreased GS activity.     

Glucose-free fatty acid cycle operates in human heart and skeletal muscle in vivo.

Positron emission tomography permits noninvasive measurement of regional glucose uptake in vivo in humans. We employed this technique to determine the effect of FFA on glucose uptake in leg, arm, and heart muscles. Six normal men were studied twice under euglycemic hyperinsulinemic (serum insulin approximately 500 pmol/liter) conditions, once during elevation of serum FFA by infusions of heparin and Intralipid (serum FFA 2.0 +/- 0.4 mmol/liter), and once during infusion of saline (serum FFA 0.1 +/- 0.01 mmol/liter). Regional glucose uptake rates were measured using positron emission tomography-derived 18F-fluoro-2-deoxy-D-glucose kinetics and the three-compartment model described by Sokoloff (Sokoloff, L., M. Reivich, C. Kennedy, M. C. Des Rosiers, C. S. Patlak, K. D. Pettigrew, O. Sakurada, and M. Shinohara. 1977. J. Neurochem. 28: 897-916). Elevation of plasma FFA decreased whole body glucose uptake by 31 +/- 2% (1,960 +/- 130 vs. 2,860 +/- 250 mumol/min, P less than 0.01, FFA vs. saline study). This decrease was due to inhibition of glucose uptake in the heart by 30 +/- 8% (150 +/- 33 vs. 200 +/- 28 mumol/min, P less than 0.02), and in skeletal muscles; both when measured in femoral (1,594 +/- 261 vs. 2,272 +/- 328 mumol/min, 25 +/- 13%) and arm muscles (1,617 +/- 411 to 2,305 +/- 517 mumol/min, P less than 0.02, 31 +/- 6%). Whole body glucose uptake correlated with glucose uptake in femoral (r = 0.75, P less than 0.005), and arm muscles (r = 0.69, P less than 0.05) but not with glucose uptake in the heart (r = 0.04, NS). These data demonstrate that the glucose-FFA cycle operates in vivo in both heart and skeletal muscles in humans.     

Normalization of blood glucose in diabetic rats with phlorizin treatment reverses insulin-resistant glucose transport in adipose cells without restoring glucose transporter gene expression.

Insulin secretion and insulin sensitivity were evaluated in eight clinically stable cirrhotic patients and in 12 controls. OGTT was normal in cirrhotics but plasma insulin response was increased approximately twofold compared with controls. Subjects received a three-step (0.1, 0.5, 1.0 mU/kg.min) euglycemic insulin clamp with indirect calorimetry, [6-3H]-glucose, and [1-14C]-palmitate. During the two highest insulin infusion steps glucose uptake was impaired (3.33 +/- 0.31 vs. 5.06 +/- 0.40 mg/kg.min, P less than 0.01, and 6.09 +/- 0.50 vs. 7.95 +/- 0.52 mg/kg.min, P less than 0.01). Stimulation of glucose oxidation by insulin was normal; in contrast, nonoxidative glucose disposal (i.e., glycogen synthesis) was markedly reduced. Fasting (r = -0.553, P less than 0.01) and glucose-stimulated (r = -0.592, P less than 0.01) plasma insulin concentration correlated inversely with the severity of insulin resistance. Basal hepatic glucose production was normal in cirrhotics and suppressed normally with insulin. In postabsorptive state, plasma FFA conc (933 +/- 42 vs. 711 +/- 44 mumol/liter, P less than 0.01) and FFA turnover (9.08 +/- 1.20 vs. 6.03 +/- 0.53 mumol/kg.min, P less than 0.01) were elevated in cirrhotics despite basal hyperinsulinemia; basal FFA oxidation was similar in cirrhotic and control subjects. With low-dose insulin infusion, plasma FFA oxidation and turnover failed to suppress normally in cirrhotics. During the two higher insulin infusion steps, all parameters of FFA metabolism suppressed normally. In summary, stable cirrhotic patients with normal glucose tolerance exhibit marked insulin resistance secondary to the impaired nonoxidative glucose disposal. Our results suggest that chronic hyperinsulinism may be responsible for the insulin resistance observed in cirrhosis.     

Insulin-mediated reduction of whole body protein breakdown. Dose-response effects on leucine metabolism in postabsorptive men.

In vivo effects of insulin on plasma leucine and alanine kinetics were determined in healthy postabsorptive young men (n = 5) employing 360-min primed, constant infusions of L-[1-13C]leucine and L-[15N]alanine during separate single rate euglycemic insulin infusions. Serum insulin concentrations of 16.4 +/- 0.8, 29.1 +/- 2.7, 75.3 +/- 5.0, and 2,407 +/- 56 microU/ml were achieved. Changes in plasma 3-methyl-histidine (3-MeHis) were obtained as an independent qualitative indicator of insulin-mediated reduction in proteolysis. Hepatic glucose output was evaluated at the lowest insulin level using D-[6,6-2H2]glucose. The data demonstrate a dose-response effect of insulin to reduce leucine flux, from basal values of 77 +/- 1 to 70 +/- 2, 64 +/- 3, 57 +/- 3, and 52 +/- 4 mumol(kg X h)-1 at the 16, 29, 75, and 2,407 microU/ml insulin levels, respectively (P less than 0.01). A parallel, progressive reduction in 3-MeHis from 5.8 +/- 0.3 to 4.3 +/- 0.3 microM was revealed. Leucine oxidation estimated from the 13C-enrichment of expired CO2 and plasma leucine (12 +/- 1 mumol[kg X h]-1) and from the 13C-enrichment of CO2 and plasma alpha-ketoisocaproate (19 +/- 2 mumol[kg X h]-1) increased at the 16 microU/ml insulin level to 16 +/- 1 and 24 +/- 2 mumol(kg X h)-1, respectively (P less than 0.05 for each), but did not increase at higher insulin levels. Alanine flux (206 +/- 13 mumol(kg X h)-1) did not increase during the clamp, but alanine de novo synthesis increased in all studies from basal rates of 150 +/- 13 to 168 +/- 23, 185 +/- 21, 213 +/- 29, and 187 +/- 15 mumol(kg X h)-1 at 16, 29, 75, and 2,407 microU/ml insulin levels, respectively (P less than 0.05). These data indicate the presence of insulin-dependent suppression of leucine entry into the plasma compartment in man secondary to a reduction in proteolysis and the stimulation of alanine synthesis during euglycemic hyperinsulinemia.     

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.     

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.     

Insulin action on heart and skeletal muscle glucose uptake in essential hypertension.

Essential hypertension is characterized by skeletal muscle insulin resistance but it is unknown whether insulin resistance also affects heart glucose uptake. We quantitated whole body (euglycemic insulin clamp) and heart and skeletal muscle (positron emission tomography and 18F-fluoro-2-deoxy-D-glucose) glucose uptake rates in 10 mild essential hypertensive (age 33 +/- 1 yr, body mass index 23.7 +/- 0.8 kg/m2, blood pressure 146 +/- 3/97 +/- 3 mmHg, VO2max 37 +/- 3 ml/kg per min) and 14 normal subjects (29 +/- 2 yr, 22.5 +/- 0.5 kg/m2, 118 +/- 4/69 +/- 3 mmHg, 43 +/- 2 ml/kg per min). Left ventricular mass was similar in the hypertensive (155 +/- 15 g) and the normotensive (164 +/- 13 g) subjects. In the hypertensives, both whole body (28 +/- 3 vs 44 +/- 3 mumol/kg per min, P < 0.01) and femoral (64 +/- 11 vs 94 +/- 8 mumol/kg muscle per min, P < 0.05) glucose uptake rates were decreased compared to the controls. In contrast, heart glucose uptake was 33% increased in the hypertensives (939 +/- 51 vs 707 +/- 46 mumol/kg muscle per min, P < 0.005), and correlated with systolic blood pressure (r = 0.66, P < 0.001) and the minute work index (r = 0.48, P < 0.05). We conclude that insulin-stimulated glucose uptake is decreased in skeletal muscle but increased in proportion to cardiac work in essential hypertension. The increase in heart glucose uptake in mild essential hypertensives with a normal left ventricular mass may reflect increased oxygen consumption and represent an early signal which precedes the development of left ventricular hypertrophy.     

Cardiac and skeletal muscle insulin resistance in patients with coronary heart disease. A study with positron emission tomography.

Patients with coronary artery disease or heart failure have been shown to be insulin resistant. Whether in these patients heart muscle participates in the insulin resistance, and whether reduced blood flow is a mechanism for such resistance is not known. We measured heart and skeletal muscle blood flow and glucose uptake during euglycemic hyperinsulinemia (insulin clamp) in 15 male patients with angiographically proven coronary artery disease and chronic regional wall motion abnormalities. Six age- and weight-matched healthy subjects served as controls. Regional glucose uptake was measured by positron emission tomography using [18F]2-fluoro-2-deoxy-D-glucose (FDG), blood flow was measured by the H2(15)O method. Myocardial glucose utilization was measured in regions with normal perfusion and wall motion as assessed by radionuclide ventriculography. Whole-body glucose uptake was 37+/-4 micromol x min(-1) x kg(-1) in controls and 14+/-2 mciromol x min(-1) x kg(-1) in patients (P = 0.001). Myocardial blood flow (1.09+/-0.06 vs. 0.97+/-0.04 ml x min(-1) x g(-1), controls vs. patients) and skeletal muscle (arm) blood flow (0.046+/-0.012 vs. 0.043+/-0.006 ml x min(-1) x g(-1)) were similar in the two groups (P = NS for both). In contrast, in patients both myocardial (0.38+/-0.03 vs. 0.70+/-0.03 micromol x min(-1) x g(-1), P = 0.0005) and muscle glucose uptake (0.026+/-0.004 vs. 0.056+/-0.006 micromol x min(-1) x g(-1), P = 0.005) were markedly reduced in comparison with controls. In the whole dataset, a direct relationship existed between insulin-stimulated glucose uptake in heart and skeletal muscle. Patients with a history of myocardial infarction and a low ejection fraction are insulin resistant. This insulin resistance affects both the myocardium and skeletal muscle and is independent of blood flow.     

Effect of insulin on the distribution and disposition of glucose in man.

Understanding the influence of insulin on glucose turnover is the key to interpreting a great number of metabolic situations. Little is known, however, about insulin's effect on the distribution and exchange of glucose in body pools. We developed a physiological compartmental model to describe the kinetics of plasma glucose in normal man in the basal state and under steady-state conditions of euglycemic hyperinsulinemia. A bolus of [3-3H]glucose was rapidly injected into a peripheral vein in six healthy volunteers, and the time-course of plasma radioactivity was monitored at very short time intervals for 150 min. A 1-mU/min kg insulin clamp was then started, thereby raising plasma insulin levels to a high physiological plateau (approximately 100 microU/ml). After 90 min of stable euglycemic hyperinsulinemia, a second bolus of [3-3H]glucose was given, and plasma radioactivity was again sampled frequently for 90 min more while the clamp was continued. Three exponential components were clearly identified in the plasma disappearance curves of tracer glucose of each subject studied, both before and after insulin. Based on stringent statistical criteria, the data in the basal state were fitted to a three-compartment model. The compartment of initial distribution was identical to the plasma pool (40 +/- 3 mg/kg); the other two compartments had similar size (91 +/- 12 and 96 +/- 9 mg/kg), but the former was in rapid exchange with plasma (at an average rate of 1.09 +/- 0.15 min-1), whereas the latter exchanged 10 times more slowly (0.12 +/- 0.01 min-1). The basal rate of glucose turnover averaged 2.15 +/- 0.12 mg/min kg, and the total distribution volume of glucose in the postabsorptive state was 26 +/- 1% of body weight. In view of current physiological information, it was assumed that the more rapidly exchanging pool represented the insulin-independent tissues of the body, while the slowly exchanging pool was assimilated to the insulin-dependent tissues. Insulin-independent glucose uptake was estimated (from published data) at 75% of basal glucose uptake, and was constrained not to change with euglycemic hyperinsulinemia. When the kinetic data obtained during insulin administration were fitted to this model, neither the size nor the exchange rates of the plasma or the rapid pool were appreciably changed. In contrast, the slow pool was markedly expanded (from 96 +/- 9 to 190 +/- 30 mg/kg, P less than 0.02) at the same time as total glucose disposal rose fourfold above basal (to 7.96 +/- 0.85 mg/min kg, P less than 0.001). Furthermore, a significant direct correlation was found to exist between the change in size of the slow pool and the insulin-stimulated rate of total glucose turnover (r=0.92, P<0.01). We conclude that hyperinsulinemia, independent of hyperglycemia, markedly increases the exchangeable mass of glucose in the body, presumably reflecting the accumulation of free, intracellular glucose in insulin-dependent tissues.     

Insulin resistance and vasodilation in essential hypertension. Studies with adenosine.

Insulin-mediated vasodilation has been proposed as a determinant of in vivo insulin sensitivity. We tested whether sustained vasodilation with adenosine could overcome the muscle insulin resistance present in mildly overweight patients with essential hypertension. Using the forearm technique, we measured the response to a 40-min local intraarterial infusion of adenosine given under fasting conditions (n = 6) or superimposed on a euglycemic insulin clamp (n = 8). In the fasting state, adenosine-induced vasodilation (forearm blood flow from 2.6 +/- 0.6 to 6.0 +/- 1.2 ml min-1dl-1, P < 0.001) was associated with a 45% rise in muscle oxygen consumption (5.9 +/- 1.0 vs 8.6 +/- 1.7 mumol min-1dl-1, P < 0.05), and a doubling of forearm glucose uptake (0.47 +/- 0.15 to 1.01 +/- 0.28 mumol min-1dl-1, P < 0.05). The latter effect remained significant also when expressed as a ratio to concomitant oxygen balance (0.08 +/- 0.03 vs 0.13 +/- 0.04 mumol mumol-1, P < 0.05), whereas for all other metabolites (lactate, pyruvate, FFA, glycerol, citrate, and beta-hydroxybutyrate) this ratio remained unchanged. During euglycemic hyperinsulinemia, whole-body glucose disposal was stimulated (to 19 +/- 3 mumol min-1kg-1), but forearm blood flow did not increase significantly above baseline (2.9 +/- 0.2 vs 3.1 +/- 0.2 ml min-1dl-1, P = NS). Forearm oxygen balance increased (by 30%, P < 0.05) and forearm glucose uptake rose fourfold (from 0.5 to 2.3 mumol min-1dl-1, P < 0.05). Superimposing an adenosine infusion into one forearm resulted in a 100% increase in blood flow (from 2.9 +/- 0.2 to 6.1 +/- 0.9 ml min-1dl-1, P < 0.001); there was, however, no further stimulation of oxygen or glucose uptake compared with the control forearm. During the clamp, the ratio of glucose to oxygen uptake was similar in the control and in the infused forearms (0.27 +/- 0.11 and 0.23 +/- 0.09, respectively), and was not altered by adenosine (0.31 +/- 0.9 and 0.29 +/- 0.10). We conclude that in insulin-re15-76sistant patients with hypertension, adenosine-induced vasodilation recruits oxidative muscle tissues and exerts a modest, direct metabolic effect to promote muscle glucose uptake in the fasting state. Despite these effects, however, adenosine does not overcome muscle insulin resistance.     

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.     

An in vivo and in vitro study of the mechanism of prednisone-induced insulin resistance in healthy subjects.

Prednisone-induced insulin resistance may depend on either reduced sensitivity (receptor defect) or reduced response to insulin (postreceptor defect). To clarify the mechanism of prednisone-induced insulin resistance, a [3H]glucose infusion (1 microCi/min) was performed for 120 min before and during a euglycemic clamp repeated at approximately 100, approximately 1,000, and approximately 10,000 microU/ml steady state plasma insulin concentration in 10 healthy, normal weight subjects, aged 35 +/- 7 yr. Each test was repeated after 7-d administration of placebo or prednisone (15 plus 15 mg/d per subject), in a randomized sequence with an interval of 1 mo between the two tests. Mean fasting blood glucose (89.5 +/- 2.1 vs. 83.7 +/- 1.9 mg/dl) and mean fasting plasma insulin values (17.8 +/- 1.2 vs. 14.3 +/- 0.8 microU/ml) were significantly higher (P less than 0.01) after prednisone. The insulin sensitivity index (glucose metabolic clearance rate in ml/kg per min) was significantly lower (P less than 0.001) after prednisone at all three steady state plasma insulin levels: 2.8 +/- 0.3 vs. 7.4 +/- 1.1 at approximately 100 microU/ml; 6.0 +/- 0.5 vs. 12.2 +/- 1.1 at approximately 1,000 microU/ml; 7.4 +/- 0.6 vs. 14.4 +/- 0.5 at approximately 10,000 microU/ml. Fasting glucose production (in mg/kg per min) was significantly higher after prednisone: 3.7 +/- 0.2 vs. 2.9 +/- 0.2, P less than 0.001. Suppression of glucose production at steady state plasma insulin level of approximately 100 microU/ml was less after prednisone (1.01 +/- 0.35 vs. 0.14 +/- 0.13, NS), and total at approximately 1,000 and approximately 10,000 microU/ml after both prednisone and placebo. The metabolic kinetic parameters of insulin after prednisone were not significantly different from those after placebo. In addition, insulin binding and 3-ortho-methyl-glucose transport were studied in vitro on fat cells from 16 normal-weight surgical candidates aged 40 +/- 8 yr (10 treated with placebo and 6 with prednisone as above). No significant difference was observed with regard to specific insulin binding (tested with 1 ng/ml hormone only), whereas significant transport differences were noted at the basal level (0.40 +/- 0.10 vs. 0.54 +/- 0.12 pmol/10(5) cells, P less than 0.05), and at increasing concentrations up to the maximum stimulation values (5 ng/ml): 0.59 +/- 0.04 vs. 0.92 +/- 0.12 pmol/10(5) cells, P less than 0.005. These results suggest that (a) administration of an anti-inflammatory dose of prednisone for 7 d induces insulin resistance in man; (b) this is more dependent on depressed peripheral glucose utilization than on increased endogenous production; (c) total insulin binding on isolated adipocytes is not significantly affected; (d) insulin resistance is primarily the outcome of postreceptor defect (impaired glucose transport).     

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.     

Characterization of cellular defects of insulin action in type 2 (non-insulin-dependent) diabetes mellitus.

Seven non-insulin-dependent diabetes mellitus (NIDDM) patients participated in three clamp studies performed with [3-3H]- and [U-14C]glucose and indirect calorimetry: study I, euglycemic (5.2 +/- 0.1 mM) insulin (269 +/- 39 pM) clamp; study II, hyperglycemic (14.9 +/- 1.2 mM) insulin (259 +/- 19 pM) clamp; study III, euglycemic (5.5 +/- 0.3 mM) hyperinsulinemic (1650 +/- 529 pM) clamp. Seven control subjects received a euglycemic (5.1 +/- 0.2 mM) insulin (258 +/- 24 pM) clamp. Glycolysis and glucose oxidation were quantitated from the rate of appearance of 3H2O and 14CO2; glycogen synthesis was calculated as the difference between body glucose disposal and glycolysis. In study I, glucose uptake was decreased by 54% in NIDDM vs. controls. Glycolysis, glycogen synthesis, and glucose oxidation were reduced in NIDDM patients (P < 0.05-0.001). Nonoxidative glycolysis and lipid oxidation were higher. In studies II and III, glucose uptake in NIDDM was equal to controls (40.7 +/- 2.1 and 40.7 +/- 1.7 mumol/min.kg fat-free mass, respectively). In study II, glycolysis, but not glucose oxidation, was normal (P < 0.01 vs. controls). Nonoxidative glycolysis remained higher (P < 0.05). Glycogen deposition increased (P < 0.05 vs. study I), and lipid oxidation remained higher (P < 0.01). In study III, hyperinsulinemia normalized glycogen formation, glycolysis, and lipid oxidation but did not normalize the elevated nonoxidative glycolysis or the decreased glucose oxidation. Lipid oxidation and glycolysis (r = -0.65; P < 0.01), and glucose oxidation (r = -0.75; P < 0.01) were inversely correlated. In conclusion, in NIDDM: (a) insulin resistance involves glycolysis, glycogen synthesis, and glucose oxidation; (b) hyperglycemia and hyperinsulinemia can normalize total body glucose uptake; (c) marked hyperinsulinemia normalizes glycogen synthesis and total flux through glycolysis, but does not restore a normal distribution between oxidation and nonoxidative glycolysis; (d) hyperglycemia cannot overcome the defects in glucose oxidation and nonoxidative glycolysis; (e) lipid oxidation is elevated and is suppressed only with hyperinsulinemia.     

Synergistic interaction between exercise and insulin on peripheral glucose uptake.

The interaction of exercise and insulin on glucose metabolism was examined in 10 healthy volunteers. Four study protocols were used: study 1: plasma insulin was raised by approximately 100 microunits/ml while plasma glucose was maintained at basal levels for 2 h (insulin clamp). Study 2: subjects performed 30 min of bicycle exercise at 40% of VO2 max. Study 3: an insulin clamp was performed as per study 1. Following 60 min of sustained hyperinsulinemia, however, subjects exercised for 30 min as per study 2. Study 4: subjects were studied as per study 3 except that catheters were inserted into the femoral artery and vein to quantitate leg glucose uptake. During the 60-90 min period of hyperinsulinemia (study 1), glucose uptake averaged 8.73 +/- 0.10 mg/kg per min. With exercise alone (study 2), the increment in peripheral glucose uptake was 1.43 +/- 0.30 mg/kg per min. When hyperinsulinemia and exercise were combined (study 3), glucose uptake averaged 15.06 +/- 0.98 mg/kg per min (P less than 0.01) and this was significantly (P less than 0.001) greater than the sum of glucose uptake when exercise and the insulin clamp were performed separately. The magnitude of rise in glucose uptake correlated closely with the increase in leg blood flow (r = 0.935, P less than 0.001), suggesting that the synergism is the result of increased blood flow and increased capillary surface area to exercising muscle. More than 85% of total body glucose metabolism during studies 1 and 3 was accounted for by skeletal muscle uptake. These results demonstrate that (a) insulin and exercise act synergistically to enhance glucose disposal in man, and (b) muscle is the primary tissue responsible for the increase in glucose metabolism following hyperinsulinemia and exercise.     

Relationships between insulin secretion, insulin action, and fasting plasma glucose concentration in nondiabetic and noninsulin-dependent diabetic subjects

The relationships between insulin secretion, insulin action, and fasting plasma glucose concentration (FPG) were examined in 34 southwest American Indians (19 nondiabetics, 15 noninsulin-dependent diabetics) who had a broad range of FPG (88-310 mg/100 ml). Fasting, glucose-stimulated, and meal-stimulated plasma insulin concentrations were negatively correlated with FPG in diabetics but not in nondiabetics. In contrast, fasting and glucose-stimulated plasma C-peptide concentrations did not decrease with increasing FPG in either group and 24-h urinary C-peptide excretion during a diet of mixed composition was positively correlated with FPG for all subjects (r = 0.36, P less than 0.05). Fasting free fatty acid (FFA) was correlated with FPG in nondiabetics (r = 0.49, P less than 0.05) and diabetics (r = 0.77, P less than 0.001). Fasting FFA was also correlated with the isotopically determined endogenous glucose production rate in the diabetics (r = 0.54, P less than 0.05). Endogenous glucose production was strongly correlated with FPG in the diabetics (r = 0.90, P less than 0.0001), but not in the nondiabetics. Indirect calorimetry showed that FPG was also negatively correlated with basal glucose oxidation rates (r = -0.61, P less than 0.001), but positively with lipid oxidation (r = 0.74, P less than 0.001) in the diabetics. Insulin action was measured as total insulin-mediated glucose disposal, glucose oxidation, and storage rates, using the euglycemic clamp with simultaneous indirect calorimetry at plasma insulin concentrations of 135 +/- 5 and 1738 +/- 59 microU/ml. These parameters of insulin action were significantly, negatively correlated with FPG in the nondiabetics at both insulin concentrations, but not in the diabetics although all the diabetics had markedly decreased insulin action. We conclude that decreased insulin action is present in the noninsulin-dependent diabetics in this population and marked hyperglycemia occurs with the addition of decreased peripheral insulin availability. Decreased peripheral insulin availability leads to increased FFA concentrations and lipid oxidation rates (and probably also increased concentrations of gluconeogenic precursors) that together stimulate gluconeogenesis, hepatic glucose production, and progressive hyperglycemia.     

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.