Effects of hyperglycaemia and hyperinsulinaemia on plasma amino acid levels in obese subjects with normal glucose tolerance

OBJECTIVE: To investigate the effects of hyperglycaemia and hyperinsulinaemia on amino acid disposal in human obesity. DESIGN: Four sequential experimental conditions: (1) overnight fasting; (2) hyperglycaemia with hyperinsulinaemia (2 h hyperglycaemic clamp at 11 mmol/l); (3) hyperglycaemia with basal insulin (1 h hyperglycaemic clamp during somatostatin infusion), (4) hyperglycaemia with resuming hyperinsulinaemia (1 h hyperglycaemic clamp after somatostatin discontinuation). SUBJECTS: Seven non-obese and seven obese non-diabetic, normo-insulinaemic subjects. MEASUREMENTS: Glucose infused to maintain steady-state hyperglycaemia. Plasma insulin, glucagon, free fatty acid and amino acid concentrations in the last 20 min of the four experimental conditions. Net rates of plasma amino acid disappearance and appearance (micromol/l per hour), calculated as the slopes of the regression of amino acid concentration on time. RESULTS: The amount of glucose infused to maintain hyperglycaemia was reduced by nearly 50% in obese subjects. During hyperinsulinaemia, FFA suppression was lower in obese subjects. In all experimental conditions plasma amino acid levels were slightly, non-significantly higher in obese than in non-obese subjects. In both groups plasma amino acids decreased slightly with ongoing fasting, decreased remarkably during hyperglycaemia-hyperinsulinaemia, rose promptly when insulin concentration was suppressed by somatostatin infusion, and declined again after somatostatin discontinuation. Also the time-course of plasma branched-chain amino acids, which paralleled that of total amino acids, was similar in the two groups. The net rates of amino acid disappearance from plasma did not differ in obese and non-obese subjects both at fasting and during hyperglycaemia-hyperinsulinaemia. Also plasma amino acid appearance during hyperglycaemia with basal insulin was not different in the two groups. CONCLUSION: The net traffic of amino acids to and from plasma in relation to insulin drive and prevailing glucose is not impaired in obese subjects with normal glucose tolerance, in spite of a decreased insulin sensitivity of glucose and lipid metabolism.     

Fasting hyperglycemia normalizes oxidative and nonoxidative pathways of insulin-stimulated glucose metabolism in noninsulin-dependent diabetes mellitus

The present studies were undertaken to determine whether fasting hyperglycemia can compensate for decreased insulin-stimulated glucose disposal, oxidation, and storage in noninsulin-dependent diabetes mellitus (NIDDM) as well as to determine whether hyperglycemia normalizes insulin-stimulated skeletal muscle glycogen synthase and pyruvate dehydrogenase (PDH) activities. To accomplish this, we used the glucose clamp technique with isotopic determination of glucose disposal and indirect calorimetry for measuring the pathways of glucose metabolism, and vastus lateralis muscle biopsies to determine the effects of insulin on glycogen synthase and PDH activities. Nine patients with NIDDM and eight matched non-diabetic subjects were infused with insulin (40 mU/m2.min) while plasma glucose was maintained at the prevailing fasting concentration. During insulin infusion, rates of glucose disposal, storage, and oxidation were the same in the two groups. Insulin infusion significantly activated glycogen synthase fractional velocity to the same extent in NIDDM (0.210 +/- 0.056 vs. 0.332 +/- 0.079) and controls (0.192 +/- 0.036 vs. 0.294 +/- 0.050). Insulin infusion increased PDH fractional velocity in controls (from 0.281 +/- 0.022 to 0.404 +/- 0.038), but not in NIDDM (from 0.356 +/- 0.043 to 0.436 +/- 0.060), although the activity of PDH during insulin infusion did not differ between the groups. We conclude that prevailing fasting hyperglycemia normalizes the nonoxidative and oxidative pathways of insulin-stimulated glucose in metabolism in NIDDM and may act as a homeostatic mechanism to normalize muscle glucose metabolism.     

Insulin-mediated hypokalemia and paralysis in familial hypokalemic periodic paralysis

To elucidate a potential role for insulin-mediated extra-renal potassium disposal in the clinical syndrome of hypokalemic periodic paralysis, an obese affected man was studied using the euglycemic insulin clamp, which, in normal and obese subjects, produces predictable, insulin dose-dependent declines in plasma potassium levels. During a 20 mU/m2/minute euglycemic clamp (insulin level, 88 microU/ml) procedure, while the patient with hypokalemic periodic paralysis demonstrated severe resistance to insulin-mediated glucose uptake (glucose uptake 50 percent of that of normal control subjects, n = 17), his plasma potassium declined to a degree similar to that seen in normal subjects. During a subsequent higher dose, 200 mU/m2/minute insulin infusion (insulin level, 914 microU/ml), plasma potassium declined to 2.5 meq/liter, a value significantly below that seen in normal (n = 19) (3.3 +/- 0.1 meq/liter) and obese (n = 6) (3.2 +/- 0.1 meq/liter) subjects. During this study, paralysis began in the patient's hand and forearm at the potassium nadir and lasted three hours, despite restoration of normokalemia 30 minutes after paralysis began. Glucose disposal rates during this high-dose insulin infusion were one-half that seen in lean control subjects (n = 19) and similar to those in obese control subjects. If these findings are representative of hypokalemic periodic paralysis and can be generalized to larger numbers of patients, they indicate several new features of this syndrome. The ability of insulin to induce hypokalemia is enhanced in this syndrome even in the presence of marked coexistent obesity-related resistance to the action of insulin to promote glucose utilization. Enhanced sensitivity of potassium uptake systems to activation by insulin (and other factors) may be a central feature of this syndrome. Additionally, paralytic hypokalemia can be induced during a euglycemic insulin clamp procedure, which could be utilized as a diagnostic test for this syndrome.     

Glucose and insulin suppression of plasma free fatty acids in obese subjects with normal glucose tolerance or mild, newly diagnosed type 2 (non-insulin-dependent) diabetes

The in vivo suppressive effect of glucose and insulin on plasma free fatty acid concentrations was investigated in obese subjects with (n = 6) and without (n = 6) Type 2 (non-insulin-dependent) diabetes mellitus during a 4h-hyperglycemic glucose clamp (about 11.2 mmol/l). Somatostatin was infused (250 micrograms/h) during the third h of glucose clamp to inhibit glucose-stimulated insulin secretion. Plasma insulin values were similar in the two groups at fasting and all throughout the study (F = 0.04; p = NS, two way analysis of variance), while the amount of glucose metabolized during the clamp was lower in diabetic subjects. Plasma free fatty acid concentrations, which were similar in the two groups at fasting, decreased during hyperglycemia and glucose-induced hyperinsulinemia (0-120 min; 180-240 min), and rose during hyperglycemia and somatostatin-inhibited insulin secretion (120-180 min). However, plasma free fatty acid concentrations were significantly higher in diabetic subjects all along the study period both in absolute terms (F = 11.4; p less than 0.0001) and when individual data were recalculated as percent of fasting value (F = 13.3; p less than 0.0001). Our data suggest that suppressibility of fasting plasma free fatty acids is lower in obese Type 2 diabetes in comparison with obese non-diabetic subjects.     

Dose-response relationship of insulin to glucose fluxes in the awake and unrestrained mouse.

The purpose of the study was to use the hyperinsulinemic-euglycemic clamp technique to generate insulin dose-response curves for insulin suppression of endogenous glucose output (EGO) and stimulation of the glucose disposal rate (GDR) in conscious unstressed mice. Five groups of male ICR (Institute for Cancer Research) mice were studied (N = 43). The animals underwent surgery for implantation of a jugular vein catheter 2 to 3 days before the clamp and were fasted 6 hours before the study. Each group was clamped at a different insulin infusion rate of 0, 2.5, 10, or 20 mU/kg/min. 3H-3-glucose was infused for measurement of the glucose turnover rate (rate of appearance [Ra]). Blood samples were collected by milking a severed tail-tip. EGO was calculated as the difference between the Ra and glucose infusion rate (GIR), and the glucose clearance rate (GCR) as the GDR divided by the plasma glucose concentration. From the curves generated, half-maximal EGO and GCR were obtained at a plasma insulin concentration of 20 to 30 microU/mL, which was achieved at an insulin infusion rate of about 4 to 5 mU/kg/min. Maximal suppression of EGO and stimulation of the GCR occurred at an insulin infusion rate of 10 mU/kg/min. The establishment of normative curves for insulin-stimulated glucose metabolism in conscious mice facilitates the evaluation of glucose metabolism in a variety of mouse models of insulin resistance.     

Effect of long chain triglyceride infusion on glucose metabolism in man

The effect of long chain triglyceride infusions (Intralipid 20%, 1 ml/min) on total body glucose uptake, glucose oxidation and glucose storage was examined in 25 healthy young volunteers by employing the euglycemic insulin clamp technique in combination with indirect calorimetry. Insulin was infused at three different rates (0.5, 2 and 4 mU/kg min) to achieve steady state hyperinsulinemic plateaus of 60 ± 4, 170 ± 10 and 420 ± 15 μU/ml. Prior to Intralipid infusion, the mean basal plasma free fatty acid concentration of all subjects was 385 ± 8 μmole/l. Following 90 min Intralipid infusion, the mean plasma free fatty acid level was increased to 760 ± 20 μmole/l (p < 0.001). At each insulin dose level, hyperlipidemia caused a significant reduction in total glucose uptake (5.9–3.5, 9.9–7.1, 11.1–8.8 mg/kg min, all p < 0.001. The decrease in total body glucose uptake was reflected by a decrease in both total glucose oxidation (2.4–1.6, 3.4–2.2, 3.7–2.8 mg/kg min, all p < 0.001) and glucose storage (3.6–1.9, 6.5–4.9, 7.4–5.9 mg/kg min, all p < 0.001). Basal glucose oxidation (1.3 ± 0.1 mg/kg min) fell by about 30% following 90 min of Intralipid infusion (0.9 ± 0.1 mg/kg min). Six additional subjects were studied with a lower infusion rate of Intralipid (0.5 ml/min). In these studies, insulin was infused at two different doses (0.5 and 2 mU/kg min) to achieve steady state plasma levels of 62 ± 2 and 171 ± 4 μU/ml. Intralipid caused again a significant reduction in total body glucose uptake during both the low (5.9 to 4.5 mg/kg min, p < 0.001) and the high (9.9–8.7 mg/kg min, p < 0.01) insulin clamp studies. This decrease in total glucose uptake was again the combined effect of an inhibition of both glucose storate (p < 0.05) and glucose oxidation (p < 0.001). In both high and low dose Intralipid infusion protocols, a strong inverse correlation was noted between the plasma free fatty acid concentration during the insulin clamp study and total body glucose uptake (r = 0.92, p < 0.001), glucose oxidation (r = 0.95, p < 0.001), and glucose storage (r = 0.90, p < 0.01). These results indicate that the inhibitory effect of free fatty acids on glucose utilization involves the biochemical pathways regulating both glucose oxidation and glycogen synthesis.     

Mechanism of insulin resistance associated with liver cirrhosis.

Insulin-induced glucose metabolism was investigated in 26 patients with biopsy-proven liver cirrhosis and 10 control subjects. Two glucose clamp protocols together with continuous indirect calorimetry were performed to examine whether reduced rates of glucose oxidation and/or nonoxidative glucose metabolism explain insulin resistance in liver cirrhosis. Using a 4-hour, two-step protocol (0-2 hours, plasma glucose 5.2 mmol/L, plasma insulin 92 mU/L to test the half-maximum response; 2-4 hours, hyperglycemia 10.0 mmol/L, plasma insulin 442 mU/L to test the maximum cellular glucose disposal) liver cirrhosis reduced glucose disposal to 45% and 60% of control values, respectively. Simultaneously, insulin-induced increases in glucose oxidation, plasma lactate levels, and lipogenesis were normal, whereas nonoxidative glucose metabolism was reduced (-82% and -47% of controls, respectively). To determine whether reduced nonoxidative glucose metabolism was caused by reduced glucose disposal, glucose disposal was "matched" to normal values in a subgroup of cirrhotic patients. Nonoxidative glucose metabolism values were normal, but plasma lactate concentrations disproportionally increased (+96%) after "matching" glucose disposal. Insulin resistance was independent of the etiology of the cirrhosis, the biochemical parameters of parenchymal cell damage and liver function, and the clinical and nutritional state of the patients. It is concluded that liver cirrhosis impairs insulin sensitivity and maximum cellular glucose disposal. Reduced glucose disposal is caused by defective glucose storage. Insulin resistance is independent of the etiology of liver cirrhosis and of the clinical and nutritional state of the patient.     

Insulin secretion and cellular glucose metabolism after prolonged low-grade intralipid infusion in young men

We examined the simultaneous effects of a 24-h low-grade Intralipid infusion on peripheral glucose disposal, intracellular glucose partitioning and insulin secretion rates in twenty young men, by 2-step hyperinsulinemic euglycemic clamp [low insulin clamp (LI), 10 mU/m(2) x min; high insulin clamp (HI), 40 mU/m(2) x min], 3-(3)H-glucose, indirect calorimetry, and iv glucose tolerance test. Free fatty acid concentrations were similar during basal steady state but 3.7- to 13-fold higher during clamps. P-glucagon increased and the insulin/glucagon ratio decreased at both LI and HI during Intralipid infusion. At LI, glucose oxidation decreased by 10%, whereas glucose disposal, glycolytic flux, glucose storage, and glucose production were not significantly altered. At HI, glucose disposal, and glucose oxidation decreased by 12% and 24%, respectively, during Intralipid infusion. Glycolytic flux, glucose storage, and glucose production were unchanged. Insulin secretion rates increased in response to Intralipid infusion, but disposition indices (DI = insulin action.insulin secretion) were unchanged. In conclusion, a 24-h low-grade Intralipid infusion caused insulin resistance in the oxidative (but not in the nonoxidative) glucose metabolism in young healthy men. Moreover, insulin hypersecretion perfectly countered the free-fatty acid-induced insulin resistance. Future studies are needed to determine the role of a prolonged moderate lipid load in subjects at increased risk of developing diabetes.     

Pulsatile insulin secretion in elderly patients with diabetes

Insulin pulsation is impaired in type 2 diabetes. GLP-1 increases pulsatile insulin secretion in these patients. We conducted these studies with the hypothesis that GLP-1 would enhance pulsatile insulin secretion and alter glucose metabolism in elderly patients with type 2 diabetes. Experiments were conducted in nine patients (age: 72+/-5 years; BMI: 27+/-3kg/m(2); diabetes duration: 7+/-3 years; HbA(1c): 6.6+/-0.9%). Subjects underwent three glucose clamp studies. The first was a euglycemic clamp to determine individual insulin clearance. In the second, GLP-1 was infused from 0-240min (0.75pM/kg/min) and glucose was maintained at fasting levels. The third was similar except that octreotide (30ng/kg/min) was infused with GLP-1 to suppress pulsatile insulin. Insulin and glucose were given to match levels during the second study. 3-(3)H-glucose was infused to allow calculation of hepatic glucose production and glucose disposal rates. There was no significant difference in measurements of pulsatile insulin secretion or hepatic glucose production and glucose disposal rates between the studies. Because there was no difference in pulsatile insulin between experiments, we could not test the effect of pulsatile insulin on glucose metabolism. Further studies are required to determine the impact of insulin pulses on glucose metabolism.     

Kinetics of glucose metabolism in relation to insulin concentrations in patients with alcoholic cirrhosis and in healthy persons

To characterize the insulin resistance in alcoholic cirrhosis we determined in vivo insulin-glucose disposal dose-response relationships in 6 patients with alcoholic cirrhosis of varying severity and in 6 control subjects, using the glucose-insulin clamp technique. Each subject was infused sequentially with insulin at rates of 0.5, 1.0, 2.0, and 10 mU/min X kg, each rate for 2 h. Euglycemia was maintained by a continuous servo-adjusted glucose infusion. The amount of glucose infused during the last 40 min of each 2-h period, corrected for accumulation in the glucose space, reflects overall net glucose metabolism. The dose-response curves demonstrated saturation kinetics. Lineweaver-Burk plots were consistently convex, rejecting a simple Michaelis-Menten relationship, but were linear when accommodated to allosterism with two active sites. The calculated affinity constant (i.e., the concentration of insulin leading to half-maximum glucose metabolism) of patients with cirrhosis was higher than that of normal controls (104 +/- 30 vs. 32 +/- 3 mU/L, mean +/- SD). In addition the maximum rate of glucose metabolism was significantly lower than that of normals (53 +/- 9 vs. 72 +/- 16 mumol/min X kg). We conclude that, in patients with alcoholic cirrhosis, insulin resistance is caused both by a marked decrease in sensitivity to insulin and a decreased maximum effect of insulin, indicating a combined receptor-postreceptor defect as the underlying cause.     

Effect of acute hyperglycemia on plasma triglyceride concentration and triglyceride secretion rate in non-fasted rats

The effect of an intravenous infusion of glucose on plasma triglyceride (TG) concentration in fed rats was determined in order to partially elucidate the mechanism of diabetes-induced hypertriglyceridemia. Glucose infused at 8 mg/kg per min caused the plasma TG concentration to be elevated significantly when compared to controls infused with saline alone. In rats which were euglycemic (clamped, insulin infused at 2.5 mU/kg per min), plasma TG concentration remained constant throughout the glucose infusion period (8 mg/kg per min). Hyperglycemic rats infused with insulin (2.5 mU/kg per min) as well as with glucose (16 mg/kg per min) were also hypertriglyceridemic. Infusion of insulin alone did not change the concentration of plasma TG over a 150 min period. Glucose was also infused (8 mg/kg per min) with somatostatin (1 micrograms/kg per min) to block endogenous production of insulin. Somatostatin infusion did not suppress glucose-induced hypertriglyceridemia. For all treatments, the net change in TG concentration was found to positively correlate with the net change in plasma glucose concentration at 150 min after the infusions (r = 0.83, P less than 0.001). The higher TG concentration in the glucose infused, hyperglycemic clamp and glucose plus somatostatin groups reflected an increased rate of TG secretion, in the presence of a lower concentration of plasma free fatty acids. These results suggest that in a non-fasted state, acute hyperglycemia increases plasma TG by stimulating hepatic TG secretion, in a manner which is independent of either plasma insulin or free fatty acids levels.     

Comparison of porcine and semisynthetic human insulins using euglycemic clamp-derived glucose-insulin dose-response curves in insulin-dependent diabetes

In order to compare the biologic effectiveness of porcine and semisynthetic human insulins, a euglycemic clamp method was used in eight insulin-dependent diabetic subjects. Each subject was tested for each insulin on separate days. In order to derive glucose-insulin dose-response curves for both insulins, sequential but constant infusion rates of 0.2, 0.5, 1.0, and 2.0 mU/kg/min were performed. Plasma glucose levels attained during the euglycemic clamp were 96 +/- 3 mg/dL. At each insulin infusion rate, the steady-state glucose infusion rate required to maintain euglycemia was measured. At each increment of insulin infused, steady-state glucose infusion rates for porcine insulin were 1.12 +/- 0.22, 1.90 +/- 0.59, 4.28 +/- 0.61, and 9.37 +/- 0.66 mg/kg/min compared with 1.27 +/- 0.42, 2.38 +/- 0.20, 4.25 +/- 0.43, and 8.87 +/- 0.67 mg/kg/min for semisynthetic human insulin. By ANOVA, no significant difference was noted between the two insulins. Because insulin infusion rates may not result in predictable circulating free insulin levels in subjects who have circulating insulin antibodies, free insulin levels were determined. When steady-state glucose infusion rates were compared with free insulin levels achieved at the four insulin infusion rates, dose-response curves for both porcine and semisynthetic human insulins were virtually identical. These data suggest that semisynthetic human insulin has equivalent biologic effects on overall glucose metabolism compared with porcine insulin in insulin-dependent diabetes.     

Insulin resistance is a prominent feature of patients with pancreatogenic diabetes

To compare in vivo insulin action in patients with diabetes secondary to pancreatic diseases (n = 9) to that in type I diabetic patients (n = 13) and in normal subjects (n = 8), we measured insulin-mediated glucose disposal by the euglycemic insulin clamp technique. Five of the nine patients with pancreatogenic diabetes had undergone total pancreatectomy. Similar plasma glucose (approximately 4.8 mmol/l) and insulin (approximately 70 mU/l) levels were maintained in all groups. The rate of glucose metabolism in the pancreatogenic diabetic patients (3.77 +/- 0.55 mg/kg/min) was 47% lower (P less than 0.001) than in normal subjects (7.05 +/- 0.57 mg/kg/min) and 21% lower (P less than 0.05) than in type I diabetic patients (5.54 +/- 0.39 mg/kg/min). The rates of glucose uptake were similarly reduced in totally pancreatectomized patients and in those with pancreatogenic diabetes due to other causes. During hyperinsulinemia induced by the clamp, glucose production (measured using 3-3H-glucose infusion) was completely suppressed in both the pancreatogenic diabetic patients and the normal subjects indicating that the impairment of in vivo insulin action was localized to the peripheral tissues. However, basal glucose production was elevated in the pancreatogenic diabetic patients (2.75 mg/kg/min, P less than 0.001) compared to the normal subjects (1.79 +/- 0.07 mg/kg/min). Glucose production rates were comparable in the totally pancreatectomized patients and in the other patients with pancreatogenic diabetes. The fasting plasma insulin level was, however, lower in the totally pancreatectomized (3.2 +/- 1.6 mU/L, P less than 0.05) than the other pancreatogenic (11.5 +/- 3.7 mU/L) diabetic patients. To examine the mechanisms of peripheral insulin resistance in the pancreatogenic diabetic patients, insulin binding and action were measured in isolated adipocytes. The pancreatogenic diabetic patients displayed normal insulin binding as well as normal rates of glucose transport and oxidation in adipocytes. In conclusion, patients with pancreatogenic diabetes demonstrated marked insulin resistance. Thus, impaired regulation of glucose production is a more likely explanation for the special clinical features of pancreatogenic diabetes than enhanced glucose utilization.     

Differential effects of insulin and hyperglycemia on intracellular glucose disposition in humans

Insulin stimulates both glucose oxidation and nonoxidative glucose disposal (glycogen and lipid synthesis, anaerobic glycolysis) in vivo. The influence of hyperglycemia per se on these two major pathways of intracellular glucose disposition has not been established. Whole-body glucose oxidation (by continuous indirect calorimetry) and total glucose turnover (by the glucose clamp technique) were measured in six healthy volunteers under four different experimental conditions: (protocol A) insulin was infused at a rate of 1 mU/min/kg while euglycemia (92 +/- 1 mg/100 mL) was maintained by an exogenous glucose infusion (8.05 +/- 0.94 mg/min/kg over three hours); (protocol B) the insulin infusion was halved but the same glucose infusion was given, thereby raising plasma glucose levels to a plateau of 144 +/- 14 mg/100 mL over the third hour; (protocol C) the insulin infusion was further reduced to 0.25 mU/min/kg, but the glucose infusion rate was left unchanged, whereby plasma glucose plateaued at 275 +/- 21 mg/100 mL; and (protocol D) the insulin infusion rate was 0.5 mU/min/kg), but the glucose infusion was adjusted (5.03 +/- 0.69 mg/min/kg) to maintain euglycemia. In all protocols, somatostatin was used to block endogenous insulin response. Under euglycemic conditions (protocols A and D), the presence of higher plasma insulin levels (80 +/- 6 v 39 +/- 5 microU/mL) caused the expected stimulation of both glucose oxidation (4.08 +/- 0.29 v 3.27 +/- 0.36 mg/min/kg) and nonoxidative glucose uptake (4.84 +/- 0.67 v 2.96 +/- 0.77 mg min/kg).(ABSTRACT TRUNCATED AT 250 WORDS)     

The relationship between first-phase insulin secretion and glucose metabolism.

A possible pathogenetic link between absence of first-phase insulin secretion and development of impaired glucose metabolism has been suggested by the results of several cross-sectional studies. First-phase insulin secretion measured during a +7 mmol/l hyperglycemic glucose clamp correlated with total glucose disposal during the clamp (r = 0.65, p < 0.001, N = 59). To examine whether restoration of first-phase insulin secretion improves peripheral glucose uptake in subjects with impaired glucose utilization, seven insulin-resistant subjects (age 54 (38-62) years: BMI 29.3 (21.7-35.8); fasting plasma glucose 5.5 (4.8-7.2) mmol/l; fasting insulin 57 (37-105) pmol/l with impaired first-phase (148 (29-587) vs controls 485 (326-1086) pmol/l x 10 min; p < 0.05) and normal second-phase (1604 (777-4480) vs controls (1799 (763-2771) pmol/l x 110 min) insulin secretion were restudied. The impaired first-phase insulin secretion was restored by an iv insulin bolus at the start of the hyperglycemic clamp. Substrate oxidation rates and hepatic glucose production were determined by indirect calorimetry and [3-3H]glucose infusion. Total glucose uptake was impaired in the insulin-resistant subjects with impaired first-phase insulin secretion compared to controls (18.8 (13.2-22.2) vs 34.8 (24.3-62.1) mumol.kg-1 x min-1; p < 0.01). Restoration of first-phase insulin secretion (1467 (746-2440) pmol/l x 10 min) did not affect glucose uptake (20.2 (9.9-23.8) mumol.kg-1.min-1), with no difference in oxidative and non-oxidative glucose metabolism between the experiments. Second-phase insulin secretion was similar during both experiments.(ABSTRACT TRUNCATED AT 250 WORDS)     

In vivo insulin sensitivity in adrenodemedullated rats

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

The dawn phenomenon does not occur in normal elderly subjects

To determine whether the dawn phenomenon occurs in normal elderly subjects and thus contributes to the progressive mild fasting hyperglycemia of aging, we examined the effect of physiological insulin levels on glucose disposal and hepatic glucose production (HGO) between 0530 and 0800 h, and 0930 and 1200 h. Paired euglycemic insulin clamp studies (8 mU/m2 X min) were performed on healthy old subjects (n = 5), employing [3H]glucose methodology to measure glucose production and disposal rates. Basal plasma insulin, GH, glucagon, and cortisol levels, and HGO and glucose disposal rates were similar before each study. Steady state plasma insulin values were slightly, but not significantly, lower during the dawn study [dawn: 20.3 +/- 1.1 (SE); control: 23.5 +/- 2.1 microU/ml, P = 0.08]. Insulin clearance rates were higher during the dawn study (dawn: 523 +/- 16; control: 430 +/- 19 ml/m2 X min, P less than 0.01). Maximum glucose disposal rates (dawn: 3.10 +/- 0.24; control: 3.03 +/- 0.23 mg/kg X min) and minimum HGO levels (dawn: 0.83 +/- 0.09; control: 0.62 +/- 0.03 mg/kg X min) were not significantly different in each part of the study. There was a significant decrease in plasma GH during the dawn (P less than 0.01, analysis of variance) but not the control studies. There was no difference in cortisol levels during the euglycemic clamp between the dawn and control studies. The mean decrement in glucagon during the insulin infusion was similar in each part of the study. We conclude that the dawn phenomenon does not occur in healthy elderly subjects despite an increase in insulin clearance during the dawn period.     

Starvation enhances the ability of insulin to inhibit its own secretion

To examine whether decreased insulin secretion during starvation is related to a change in the ability of insulin to inhibit its own secretion, plasma C-peptide was measured after plasma insulin levels were acutely raised by intravenous (IV) insulin infusion in a dose of 40 and 80 mU/M2/min in obese subjects before and after a 72 hour fast. Plasma glucose concentration was maintained +/- 4% of basal levels by a variable glucose infusion. During the 80 mU infusion, at plasma insulin levels of 200 microU/mL, plasma C-peptide fell by 0.17 pmol/mL in the fed state. In the fasted state, despite basal levels that were 36% lower, C-peptide decreased by 0.21 pmol/mL. Highly significant increases in percent suppression after fasting were noted during both 40 mU and 80 mU studies. The plasma C-peptide response was related to the insulin infusion dose in both the fed and fasted state. In contrast, alpha cell suppression by insulin, as determined by plasma glucagon levels, was not altered by fasting. It is concluded that enhanced inhibitory influences of insulin on the beta cell during starvation may be a physiologically important mechanism for diminished insulin secretion during the transition from the fed to the fasting state.     

Glucose storage is a major determinant of in vivo "insulin resistance" in subjects with normal glucose tolerance

In vivo "resistance" to the action of insulin on glucose uptake is commonly found in obesity and is characteristic of noninsulin-dependent diabetes mellitus in obese subjects. To investigate the relationship among glucose uptake, glucose oxidation, and nonoxidative glucose disposal (storage) in subjects with normal glucose tolerance, we studied 25 caucasians and 79 southwestern American Indians, including lean and obese subjects in both groups. The euglycemic clamp technique with simultaneous indirect calorimetry was used to determine rates of glucose uptake and glucose oxidation. These studies were performed at two rates of insulin infusion (40 and 400 mU/m2 X min), with resulting mean plasma insulin concentrations of 113 and 1839 microU/ml, respectively. At the lower insulin infusion rate, there was no glucose storage in subjects with a glucose uptake rate of about 2.2 mg/kg fat free mass X min. In contrast, glucose storage accounted for over 45% of the glucose disposal in subjects with glucose uptake rates over 7.0 mg/kg fat free mass X min studied at similar insulin concentrations. At the high insulin infusion rate, over 70% of the difference in glucose uptake between subjects with a low or high capacity for glucose disposal was due to glucose storage. These studies demonstrated that in normal subjects at both physiological and maximally stimulating plasma insulin concentrations, glucose storage is a major factor in distinguishing between those with low or high rates of insulin-mediated glucose disposal. Since glucose storage may be a specifically activated process, we hypothesize that failure to activate glucose storage is a major defect causing in vivo insulin resistance in subjects with normal glucose tolerance.     

Effect of increased plasma non-esterified fatty acids (NEFAs) on arginine-stimulated insulin secretion in obese humans

Aims/hypothesis: We have shown previously that the increase of plasma non-esterified fatty acids for 48 h results in decreased glucose-stimulated insulin secretion in lean and non-diabetic obese subjects. It is currently not known if a prolonged increase in non-esterified fatty acids also impairs the insulin secretory response to non-glucose secretagogues. Methods: Heparin and intralipid (to increase plasma non-esterified fatty acid concentrations by about two- to fourfold) or normal saline was infused intravenously for 48 h in 14 non-diabetic obese subjects. On the third day in both studies, insulin, C-peptide, proinsulin, and insulin secretion rate were assessed in response to an intravenous arginine infusion at fasting glucose concentration and a second arginine infusion after a 60-min 11 mmol/l hyperglycaemic clamp. Results: There were no significant differences detected in acute (5 min) or total (90 min) arginine-stimulated C-peptide or insulin secretion response in the heparin-intralipid study compared with the control group at fasting glucose or during hyperglycaemia. Conclusion/interpretation: We have shown that a prolonged increase in plasma NEFA does not blunt arginine-stimulated insulin secretion or plasma insulin concentrations in non-diabetic obese subjects. These findings suggest that the previously demonstrated NEFA-induced impairment in insulin secretory response to glucose cannot be generalized for non-glucose secretagogues. [Diabetologia (2001) 44: 1989–1997] Received: 27 November 2001 and in revised form: 2 August 2001