Glucose transport in human skeletal muscle. The in vivo response to insulin

Transmembrane glucose transport plays a key role in determining insulin sensitivity. We have measured in vivo WBGU, FGU, and K(in) and K(out) of 3-O-methyl-D-glucose in forearm skeletal muscle by combining the euglycemic clamp technique, the forearm-balance technique, and a novel dual-tracer (1-[3H]-L-glucose and 3-O-[14C]-methyl-D-glucose) technique for measuring in vivo transmembrane transport. Twenty-seven healthy, lean subjects were studied. During saline infusion, insulin concentration, FGU (n = 6), K(in), and K(out) (n = 4) were similar to baseline. During SRIF-induced hypoinsulinemia (insulin < 15 pM, n = 4) WBGU was close to 0, and FGU, K(in), and K(out) were unchanged from basal (insulin = 48 pM) values. During insulin clamps at plasma insulin levels of approximately 180 (n = 4), approximately 420 (n = 5), approximately 3000 (n = 4), and approximately 9500 pM (n = 4), WBGU was 14.2 +/- 1.3, 34.2 +/- 4.1 (P < 0.05 vs. previous step), 55.8 +/- 1.8 (P < 0.05 vs. previous step), and 56.1 +/- 6.3 mumol.min-1.kg-1 of body weight (NS vs. previous step), respectively. Graded hyperinsulinemia concomitantly increased FGU from a basal value of 4.7 +/- 0.5 mumol.min-1.kg-1 up to 10.9 +/- 2.3 (P < 0.05 vs. basal value), 26.6 +/- 4.5 (P < 0.05 vs. previous step), 54.8 +/- 4.3 (P < 0.05 vs. previous step), and 61.1 +/- 10.8 mumol.min-1.kg-1 of forearm tissues (NS vs. previous step), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Total body fat content and fat topography are associated differently with in vivo glucose metabolism in nonobese and obese nondiabetic women

In this study, total body fat content and fat topography were related to glucose metabolism in the basal and insulin-stimulated states in 18 nonobese and 18 obese premenopausal nondiabetic women. All subjects received a euglycemic insulin (20 mU.min-1.m2) clamp study in combination with [3-3H]-D-glucose infusion and indirect calorimetry to quantitate total body glucose uptake, glucose oxidation, and nonoxidative glucose disposal. Total body fat content was determined with tritiated water, whereas body fat distribution was estimated from the WHR, the STR, and the VSR (measured by magnetic resonance imaging). In the postabsorptive state, total body glucose utilization, glucose oxidation, and nonoxidative glucose disposal rates were similar in nonobese and obese women, whereas during the insulin clamp all three metabolic parameters were reduced significantly in the obese group. In nonobese women, total body fat content was related inversely to both total and nonoxidative glucose disposal during the insulin clamp, whereas no relationship was found between glucose metabolism (total, oxidative, and nonoxidative) and WHR, STR, or VSR. In contrast, in obese women, no relationship was observed between total body fat content and any measure of insulin-mediated glucose metabolism. However, both WHR and VSR were related inversely to total, oxidative, and nonoxidative glucose disposal rates during the insulin clamp. These results suggest that total body fat content and body fat topography are associated differently with insulin-mediated glucose metabolism in nonobese and obese women. In the nonobese women, total body fat mass appears to be a primary determinant of tissue sensitivity to insulin, whereas in obese women, body fat topography exerts a more dominant effect.

Effect of insulin on oxidative and nonoxidative pathways of free fatty acid metabolism in human obesity

The dose-response relationship between the plasma insulin concentration and oxidative and nonoxidative pathways of free fatty acid (FFA) metabolism was examined in 11 obese and 7 lean subjects using a stepwise insulin clamp technique in combination with indirect calorimetry and infusion of [1-14C]palmitate. The fasting plasma FFA concentration was elevated in obese subjects (793 +/- 43 vs. 642 +/- 39 mumol/l; P less than 0.01) and was associated with an increased basal rate of plasma FFA turnover, FFA oxidation, and nonoxidative FFA disposal, i.e., reesterification (all P less than 0.01). Suppression of plasma FFA turnover by physiological increments in plasma insulin was impaired in obese compared with lean subjects. However, plasma FFA turnover expressed per kilogram fat mass was normally suppressed by insulin in obese subjects. Although insulin suppressed plasma FFA oxidation to the same extent in lean and obese subjects, inhibition of total lipid oxidation by insulin was impaired in the obese group. Obese subjects had an enhanced basal rate of nonoxidative FFA disposal, which was suppressed less by physiological increments in plasma insulin compared with lean controls. Therefore, we conclude that 1) lipolysis in uncomplicated obesity is normally sensitive to insulin; the enhanced FFA flux is simply a consequence of the increased fat mass. 2) Nonoxidative FFA disposal expressed per lean body mass is enhanced in obese subjects and correlates with the increase in plasma FFA concentration and fat mass. 3) Enhanced oxidation of intracellular lipids contributes to the enhanced rate of total lipid oxidation in obese subjects.

Pathogenesis of NIDDM. A balanced overview

Non-insulin-dependent diabetes mellitus (NIDDM) results from an imbalance between insulin sensitivity and insulin secretion. Both longitudinal and cross-sectional studies have demonstrated that the earliest detectable abnormality in NIDDM is an impairment in the body's ability to respond to insulin. Because the pancreas is able to appropriately augment its secretion of insulin to offset the insulin resistance, glucose tolerance remains normal. With time, however, the beta-cell fails to maintain its high rate of insulin secretion and the relative insulinopenia (i.e., relative to the degree of insulin resistance) leads to the development of impaired glucose tolerance and eventually overt diabetes mellitus. The cause of pancreatic "exhaustion" remains unknown but may be related to the effect of glucose toxicity in a genetically predisposed beta-cell. Information concerning the loss of first-phase insulin secretion, altered pulsatility of insulin release, and enhanced proinsulin-insulin secretory ratio is discussed as it pertains to altered beta-cell function in NIDDM. Insulin resistance in NIDDM involves both hepatic and peripheral, muscle, tissues. In the postabsorptive state hepatic glucose output is normal or increased, despite the presence of fasting hyperinsulinemia, whereas the efficiency of tissue glucose uptake is reduced. In response to both endogenously secreted or exogenously administered insulin, hepatic glucose production fails to suppress normally and muscle glucose uptake is diminished. The accelerated rate of hepatic glucose output is due entirely to augmented gluconeogenesis. In muscle many cellular defects in insulin action have been described including impaired insulin-receptor tyrosine kinase activity, diminished glucose transport, and reduced glycogen synthase and pyruvate dehydrogenase. The abnormalities account for disturbances in the two major intracellular pathways of glucose disposal, glycogen synthesis, and glucose oxidation. In the earliest stages of NIDDM, the major defect involves the inability of insulin to promote glucose uptake and storage as glycogen. Other potential mechanisms that have been put forward to explain the insulin resistance, include increased lipid oxidation, altered skeletal muscle capillary density/fiber type/blood flow, impaired insulin transport across the vascular endothelium, increased amylin, calcitonin gene-related peptide levels, and glucose toxicity.

Glucose metabolism in obesity and type 2 diabetes

The aim of this review is to provide a comprehensive summary of the subject of glucose metabolism in normal and obese subjects, and in those with type 2 diabetes. The following topics are discussed: Glucose and insulin metabolism, including characterization of the role of various organs in maintaining glucose homeostasis in the basal state and after food. The action of insulin and the concept of insulin resistance, its main characteristics as revealed by studies of glucose metabolism in liver and skeletal muscle, the two organs primarily involved. Changes in insulin secretion in obese subjects and in diabetics are reviewed. Finally, the development of diabetes as a consequence of increasing insulin resistance in the obese diabetic subject is discussed. The principal features--insulin resistance, elevated free fatty acid levels and glucotoxicity--are emphasized. A possible explanation of how obesity leads to diabetes, emphasizing the role played by upper body fat distribution, is provided.

The role of free fatty acid metabolism in the pathogenesis of insulin resistance in obesity and noninsulin-dependent diabetes mellitus

To investigate the mechanisms of insulin resistance in obesity and noninsulin-dependent diabetes mellitus (NIDDM), we examined oxidative and nonoxidative pathways of free fatty acid (FFA) and glucose metabolism in 14 lean and 17 obese (with normal oral glucose tolerance) nondiabetic subjects and in 8 lean and 8 obese subjects with NIDDM. FFA and glucose metabolism were measured using the sequential insulin clamp technique in combination with indirect calorimetry and infusion of [3-3H]glucose and [1-14C]palmitate. Obesity was characterized by enlarged fat mass, which correlated positively with the plasma FFA concentration (r = 0.62; P less than 0.01). FFA metabolism was less sensitive to insulin in obese than in lean nondiabetic subjects, but this defect could be overcome by increasing the plasma insulin concentration. NIDDM patients showed normal sensitivity to the inhibitory action of insulin on FFA metabolism; however, maximal suppression by insulin was impaired. The combination of obesity and NIDDM was associated with a further enhancement of reesterification of FFA than observed in either condition alone. In both obesity and NIDDM, the dose-response curve for suppression of hepatic glucose production by insulin was impaired. While obesity was primarily characterized by reduced sensitivity to the stimulatory action of insulin on oxidative and nonoxidative pathways of glucose metabolism, resistance to the effect of insulin on glucose metabolism in NIDDM was characterized by a reduced maximal response. The combination of obesity and NIDDM further impaired the sensitivity of liver glucose output and glucose oxidation to insulin. The hypothesis is advanced that in uncomplicated obesity, increased availability and oxidation of FFA leads, by the FFA/glucose cycle, to the impairment in glucose utilization. In NIDDM, on the other hand, the defect in glucose utilization is primary, and the enhanced rate of FFA oxidation may represent a compensatory phenomenon.

Role of free fatty acids and insulin in determining free fatty acid and lipid oxidation in man

Plasma FFA oxidation (measured by infusion of 14C-palmitate) and net lipid oxidation (indirect calorimetry) are both inhibited by insulin. The present study was designed to examine whether these insulin-mediated effects on lipid metabolism resulted from a decline in circulating FFA levels or from a direct action of the hormone on FFA/lipid oxidation. Nine subjects participated in two euglycemic insulin clamps, performed with and without heparin. During each insulin clamp study insulin was infused at two rates, 4 and 20 mU/m2.min for 120 min. The studies were performed with indirect calorimetry and 3-3H-glucose and 14C-palmitate infusion. During the control study plasma FFA fell from 610 +/- 46 to 232 +/- 42 to 154 +/- 27 mumol/liter, respectively. When heparin was infused basal plasma FFA concentration remained constant. During the control study, FFA/lipid oxidation rates decreased in parallel with the fall in the plasma FFA concentration. During the insulin/heparin study, plasma 14C-FFA oxidation remained unchanged while net lipid oxidation decreased. In conclusion, when the plasma FFA concentration is maintained unchanged by heparin infusion, insulin has no direct effect on FFA turnover and disposal. These results thus suggest that plasma FFA oxidation is primarily determined by the plasma FFA concentration, while net lipid oxidation is regulated by both the plasma FFA and the insulin level.

Effect of sulphonylurea on glucose-stimulated insulin secretion in healthy and non-insulin dependent diabetic subjects: a dose-response study

The effect of a rapid-acting sulphonylurea, glipizide, on the dose-response relationship between the beta-cell response (insulin and C-peptide secretion) and the ambient plasma glucose concentration was examined in 12 healthy and 6 non-insulin-dependent diabetic subjects. The subjects participated in two sets of experiments which were performed in random order: (A) four hyperglycaemic clamp studies, during which the plasma glucose concentration was raised for 120 min by 1 (only in healthy subjects), 3, 7, and 17 mmol/l; and (B) the same four hyperglycaemic clamp studies preceded by ingestion of 5 mg glipizide. All subjects participated in a further study, in which glipizide was ingested and the plasma glucose concentration was maintained at the basal level. In control subjects in the absence of glipizide, the first-phase plasma insulin response (0-10 min) increased progressively with increasing plasma glucose concentration up to 10 mmol/l, above which it tended to plateau. Glipizide augmented the first-phase insulin response without changing the slope of the regression line relating plasma insulin to glucose concentrations. The second-phase plasma insulin response (20-120 min) increased linearly with increasing hyperglycaemia (r = 0.997). Glipizide alone increased the plasma insulin response by 180 pmol/l. A similar increase in plasma insulin response following glipizide was observed at each hyperglycaemic step, indicating that glipizide did not affect the sensitivity of the beta-cell to glucose. First-phase insulin secretion was reduced in the type 2 (non-insulin-dependent) diabetic patients, and was not influenced by glipizide.(ABSTRACT TRUNCATED AT 250 WORDS)

Dose-dependent effect of insulin on plasma free fatty acid turnover and oxidation in humans

Methodology for measuring plasma free fatty acid (FFA) turnover/oxidation with [1-14C]palmitate was tested in normal subjects. In study 1, two different approaches (720-min tracer infusion without prime vs. 150-min infusion with NaH14CO3 prime) to achieve steady-state conditions of 14CO2 yielded equivalent rates of plasma FFA turnover/oxidation. In study 2, during staircase NaH14CO3 infusion, calculated rates of 14CO2 appearance agreed closely with NaH14CO3 infusion rates. In study 3, 300-min euglycemic insulin clamp documented that full biological effect of insulin on plasma FFA turnover/oxidation was established within 60-120 min. In study 4, plasma insulin concentration was raised to 14 +/- 2, 23 +/- 2, 38 +/- 2, 72 +/- 5, and 215 +/- 10 microU/ml. A dose-dependent insulin suppression of plasma FFA turnover/oxidation was observed. Plasma FFA concentration correlated positively with plasma FFA turnover/oxidation in basal and insulinized states. Total lipid oxidation (indirect calorimetry) was significantly higher than plasma FFA oxidation in the basal state, suggesting that intracellular lipid stores contributed to whole body lipid oxidation. Hepatic glucose production and total glucose disposal showed the expected dose-dependent suppression and stimulation, respectively, by insulin. In conclusion, insulin regulation of plasma FFA turnover/oxidation is maximally manifest at low physiological plasma insulin concentrations, and in the basal state a significant contribution to whole body lipid oxidation originates from lipid pool(s) that are different from plasma FFA.

Obesity and insulin resistance in humans: a dose-response study

Insulin-mediated glucose metabolism (euglycemic insulin clamp at plasma insulin concentration of 100 microU/mL) and glucose-stimulated insulin secretion (hyperglycemic clamp) were examined in 42 obese subjects (ideal body weight [IBW], 158 +/- 4%) with normal glucose tolerance and in 36 normal weight (IBW, 102% +/- 1%) age-matched controls. In 10 obese and eight control subjects, insulin was infused at six rates to increase plasma insulin concentration by approximately 10, 20, 40, 80, 2,000, and 20,000 microU/mL. Throughout the physiologic range of plasma insulin concentrations, both the increase in total body glucose uptake and the suppression of hepatic glucose production (HGP) were significantly impaired in the obese group (P less than .001 to .01). At the two highest plasma insulin concentrations, inhibition of HGP and the stimulation of glucose disposal were similar in both the obese and control groups. Insulin secretion during the hyperglycemic (+/- 125 mg/dL) clamp was twofold greater in obese subjects than in controls (P less than .01) and was inversely related to the rate of glucose uptake during the insulin clamp (r = -.438, P less than .05), but was still unable to normalize glucose disposal (P less than .05). In conclusion, our results indicate that insulin resistance is a common accompaniment of obesity and can be overcome at supraphysiological insulin concentrations. Both in the basal state and following a hyperglycemic stimulus obese people display hyperinsulinemia, which correlates with the degree of insulin resistance. However, endogenous hyperinsulinemia fails to fully compensate for the insulin resistance.

Glucose and free fatty acid metabolism in non-insulin-dependent diabetes mellitus. Evidence for multiple sites of insulin resistance

The effect of graded, physiologic hyperinsulinemia (+5, +15, +30, +70, +200 microU/ml) on oxidative and nonoxidative pathways of glucose and FFA metabolism was examined in nine lean non-insulin dependent diabetic patients (NIDDM) and in eight age- and weight-matched control subjects. Glucose and FFA metabolism were assessed using stepwise insulin clamp in combination with indirect calorimetry and infusion of [3H]3-glucose/[14C]palmitate. The basal rate of hepatic glucose production (HGP) was higher in NIDDM than in control subjects, and suppression of HGP by insulin was impaired at all but the highest insulin concentration. Glucose disposal was reduced in the NIDD patients at the three highest plasma insulin concentrations, and this was accounted for by defects in both glucose oxidation and nonoxidative glucose metabolism. In NIDDs, suppression of plasma FFA by insulin was impaired at all five insulin steps. This was associated with impaired suppression by insulin of plasma FFA turnover, FFA oxidation (measured by [14C]palmitate) and nonoxidative FFA disposal (an estimate of reesterification of FFA). FFA oxidation and net lipid oxidation (measured by indirect calorimetry) correlated positively with the rate of HGP in the basal state and during the insulin clamp. In conclusion, our findings demonstrate that insulin resistance is a general characteristic of glucose and FFA metabolism in NIDDM, and involves both oxidative and nonoxidative pathways. The data also demonstrate that FFA/lipid and glucose metabolism are interrelated in NIDDM, and suggest that an increased rate of FFA/lipid oxidation may contribute to the impaired suppression of HGP and diminished stimulation of glucose oxidation by insulin in these patients.

Time dependence of the interaction between lipid and glucose in humans

The time-dependent effect of Intralipid infusion on glucose metabolism was examined in seven healthy young subjects who participated in the following three experimental protocols: study I, a 4-h euglycemic insulin clamp (0-240 min) with [3-3H]glucose and indirect calorimetry; study II, a 4-h insulin clamp with Intralipid infusion started at time 0; and study III, a 4-h insulin clamp with Intralipid infusion started at 120 min. When Intralipid infusion was begun at the start of the insulin clamp, the increase in insulin-mediated glucose oxidation was completely inhibited, and the rise in nonoxidative glucose disposal was diminished by 22%. When Intralipid infusion was begun 120 min after the start of the insulin clamp, no inhibitory effect on either glucose oxidation or nonoxidative glucose disposal was observed. The change in lipid oxidation was closely and inversely correlated with the change in glucose oxidation (r = -0.826, P less than 0.001) during studies I-III; no correlation between the change in lipid oxidation and nonoxidative glucose disposal was observed. These results indicate that, in healthy subjects, the metabolic competition between lipid and glucose is very time dependent. Furthermore, mitochondrial oxidative processes are more sensitive and are affected earlier than the cytosolic metabolic pathways, i.e., nonoxidative glucose disposal.

Glucose and free fatty acid metabolism in non-insulin-dependent diabetes mellitus. Evidence for multiple sites of insulin resistance

The effect of graded, physiologic hyperinsulinemia (+5, +15, +30, +70, +200 microU/ml) on oxidative and nonoxidative pathways of glucose and FFA metabolism was examined in nine lean non-insulin dependent diabetic patients (NIDDM) and in eight age- and weight-matched control subjects. Glucose and FFA metabolism were assessed using stepwise insulin clamp in combination with indirect calorimetry and infusion of [3H]3-glucose/[14C]palmitate. The basal rate of hepatic glucose production (HGP) was higher in NIDDM than in control subjects, and suppression of HGP by insulin was impaired at all but the highest insulin concentration. Glucose disposal was reduced in the NIDD patients at the three highest plasma insulin concentrations, and this was accounted for by defects in both glucose oxidation and nonoxidative glucose metabolism. In NIDDs, suppression of plasma FFA by insulin was impaired at all five insulin steps. This was associated with impaired suppression by insulin of plasma FFA turnover, FFA oxidation (measured by [14C]palmitate) and nonoxidative FFA disposal (an estimate of reesterification of FFA). FFA oxidation and net lipid oxidation (measured by indirect calorimetry) correlated positively with the rate of HGP in the basal state and during the insulin clamp. In conclusion, our findings demonstrate that insulin resistance is a general characteristic of glucose and FFA metabolism in NIDDM, and involves both oxidative and nonoxidative pathways. The data also demonstrate that FFA/lipid and glucose metabolism are interrelated in NIDDM, and suggest that an increased rate of FFA/lipid oxidation may contribute to the impaired suppression of HGP and diminished stimulation of glucose oxidation by insulin in these patients.