Michael S. Greenfield

Assessment of Insulin Resistance with the Insulin Suppression Test and the Euglycemic Clamp

Insulin resistance was quantified with two different methods in 30 subjects with varying degrees of glucose tolerance. One method, the insulin suppression test, is performed by continuously infusing epinephrine, propranolol, insulin, and glucose. Epinephrine and propranolol suppress endogenous insulin release, and steady-state plasma levels of exogenous insulin and glucose are reached in all individuals. Because the steady-state insulin level is the same in all subjects, the height of the steady-state plasma glucose level provides a direct estimate of insulin resistance. The other method, the euglycemic clamp technique, produces a steady-state level of exogenous hyperinsulinemia by means of a primed and continuous insulin infusion. Glucose is also infused at a rate sufficient to prevent an insulin-induced fall in glucose concentration, and the amount of glucose required to maintain the basal plasma glucose level provides the estimate of insulin resistance. The results indicated that estimates of insulin resistance generated by the two methods were highly correlated (r = 0.93). Furthermore, both methods of assessing insulin resistance indicated that the greater the degree of glucose intolerance, the more severe the insulin resistance. These results serve to further emphasize the importance of insulin resistance in the pathogenesis of hyperglycemia in type II diabetes.

Relationship Between Insulin Resistance, Insulin Secretion, Very Low Density Lipoprotein Kinetics, and Plasma Triglyceride Levels in Normotriglyceridemic Man

We have previously postulated that resistance to insulin-mediated glucose uptake was the basic metabolic abnormality in patients with endogenous hypertriglyceridemia. In this situation, glucose tolerance would tend to deteriorate, and could only be maintained by the increased secretion of insulin. Although the ensuing hyperinsulinemia might prevent the development of glucose intolerance, we suggested that it would also lead to increased hepatic very low density (VLDL) triglyceride (TG) synthesis and secretion. In the current study we have quantified these four metabolic variables in 16 nonobese human subjects with plasma TG concentrations less than 175 mg/dl. The results demonstrate the following degree of correlation: insulin resistance (Formula: see text) insulin response to food (Formula: see text) VLDL-TG secretion rate (Formula: see text) plasma TG concentration. These data indicate that nonobese subjects with normal TG levels have the same relationship between degree of insulin sensitivity, insulin response to food, VLDL-TG secretion, and TG concentration previously described in patients with endogenous hypertriglyceridemia.

Diabetic Hypertriglyceridemia: Evidence for Three Clinical Syndromes

Based on studies of both man and rat, it is proposed that at least three distinct syndromes of hypertriglyceridemia occur as the result of abnormalities of carbohydrate metabolism. We believe that these syndromes are the predictable results of the changes in plasma insulin and/or free fatty acid (FFA) concentration that occur in patients with impaired glucose tolerance (IGT), non-insulin-dependent diabetes mellitus (NIDDM), and insulin-dependent diabetes mellitus (IDDM). In patients with IGT the basic defect is postulated to be loss of normal insulin sensitivity, leading to compensatory hyperinsulinism, increased very low density lipoprotein (VLDL)-triglyceride (TG) secretion, and hypertriglyceridemia. In contrast, patients with NIDDM have ambient insulin concentrations that are “normal” in absolute terms, and in these subjects we believe that the elevated circulating FFA concentrations increase hepatic VLDL-TG secretion, which, in turn, is responsible for the hypertriglyceridemia. However, elevated FFA levels do not stimulate hepatic VLDL-TG secretion in insulin-deficient patients with IDDM, presumably because the livers of such individuals are not capable of responding to the increased FFA flux. In these subjects, the development of hypertriglyceridemia, when it does occur, is secondary to the existence of a profound defect in the removal of VLDL-TG from plasma. These generalizations are consistent with results in both man and rat, and provide models that account for the development of hypertriglyceridemia in the three major clinical categories of abnormal carbohydrate metabolism.

Enhanced Glucose Utilization During Prolonged Glucose Clamp Studies

Glucose clamp studies were performed to see if glucose utilization increased during extended periods of moderate hyperinsulinemia (80–100 μu/ml). Modifications of the standard euglycemic clamp were performed, and the amount of glucose metabolized (M) and the metabolic clearance rate (MCR) of glucose were calculated for each 10-min interval during the study. In seven subjects, the clamp was performed for 180 min, with the glucose infusion rate fixed from 120 to 180 min at the rate necessary to maintain basal plasma glucose concentration from 80 to 120 min. Under these conditions, mean plasma glucose concentration fell 18% during the last 60 min of the study, indicating increased glucose utilization. Twenty-four subjects, nonobese and obese normals and type II diabetics, had 3-h clamp studies performed. We documented an average increase of M by 21% and MCR by 28% in the third hour as compared with the second hour. Five-hour clamp studies in nine subjects and 8-h studies in two subjects indicated a continued rise in glucose MCR, until a plateau was reached at 4–6 h after initiation of hyperinsulinemia (mean = 88 μU/ml). This plateau was approximately 85% above the MCR of the last 30 min of a standard glucose clamp. Measurement of glucose turnover with 3H-3-glucose suggested that increasing suppression of hepatic glucose production was not responsible for the increases in glucose utilization noted. Insulin binding to erythrocytes was determined in 13 subjects before and after 180 min of hyperinsulinemia, and showed decreased total specific insulin binding. In a two-component high- and low-affinity model of insulin binding, the observed decrease in total insulin binding was primarily caused by a decrease in the high-affinity receptor component. These results suggested that glucose utilization continuously increased during a 5-h period of constant hyperinsulinemia, and this was due possibly to enhanced insulin sensitivity. This change took place in spite of a concomitant decrease in number of insulin receptors.

Effect of Sulfonylurea Treatment on In Vivo Insulin Secretion and Action in Patients With Non-insulin-dependent Diabetes Mellitus

The effect of glipizide treatment on diabetic control and on in vivo insulin secretion and action was studied in 20 patients with non-insulin-dependent diabetes mellitus (NIDDM). Patients were examined before and after a minimum of 3 mo treatment. Mean (± SEM) fasting plasma glucose level fell from 264 ± 12 mg/dl to 172 ± 10 mg/dl (P < 0.001) after glipizide treatment, and this was associated with a fall in total plasma glucose response to a test meal of approximately 35﹪. Mean (±SEM) fasting plasma insulin levels increased slightly from 15 ± 2 μU/ml to 18 ± 2 μU/ml following sulfonylurea treatment, and the total plasma insulin response to the test meal increased by 63﹪. However, there was no correlation (r = −0.20) between the increase in plasma insulin response and the fall in plasma glucose levels that occurred as the result of sulfonylurea therapy. Glipizide treatment also led to enhanced in vivo insulin action, whether measured by the insulin clamp technique (P < 0.001) or the insulin suppression test (P < 0.02). Furthermore, in this instance there was a significant correlation (r = 0.69, P < 0.001) between the enhanced insulin action and the improvement on diabetes control. Thus, chronic therapy with glipizide, a new sulfonylurea agent, led to increased in vivo insulin secretion and insulin action. These results lend direct support to the assumption that sulfonylurea compounds have a substantial extrapancreatic effect on glucose homeostasis, and suggest that this effect contributes to the therapeutic efficacy of these drugs.

Effect of a high carbohydrate diet on insulin binding to adipocytes and on insulin action in vivo in man.

We studied the effects of short-term (5 days) and long-term (2 wk) high carbohydrate (75%) feedings on insulin binding to isolated adipocytes and insulin sensitivity in vivo in normal subjects. Ingestion of the high carbohydrate diet led to daylong hyperinsulinemia in both short- and long-term groups. Insulin binding to isolated adipocytes was decreased in both groups; in the short-term groups this decrease in insulin binding was caused by a decrease in the receptor affinity, whereas in the long-term group it was caused by a decrease in receptor number. On the other hand, despite this decrease in insulin binding, total in vivo insulin sensitivity was markedly improved in both groups. In conclusion, (1) the short-term adaptive response of the insulin receptor is a decrease in binding affinity whereas the long-term response is a decrease in receptor number, (2) sustained and chronic hyperinsulinemia can lead to a decrease in the number of cellular insulin receptors, (3) high carbohydrate diets lead to a general increase in insulins ability to promote glucose removal from plasma, and (4) the paradox of enhanced insulin sensitivity in the face of decreased insulin binding can be explained if high carbohydrate diets also lead to an increase in the activity of steps in glucose metabolism distal to the insulin receptor.

Mechanism of hypertriglyceridaemia in diabetic patients with fasting hyperglycaemia.

SummarySeveral aspects of lipid metabolism were studied to define the mechanism of hypertriglyceridaemia in insulin-independent diabetic patients with fasting hyperglycaemia. Patients with insulin-independent diabetes were more obese (p<0.001) and had a significantly (p<0.001) higher mean (± SEM) fasting plasma triglyceride concentration (387 ±66 mg/dl) than did either insulin-dependent diabetics (133±11 mg/dl) or normal (73±1 mg/dl) subjects. Very low density lipoprotein secretion rate was also significantly (p<0.01 — <0.001) higher in patients with insulin-independent diabetes (14.65±1.37 mg/kg/h) as compared to 7.64±0.60 mg · kg/h and 9.86±0.75 mg/kg/h in normal subjects and patients with insulin-dependent diabetes, respectively. However, the relationship between plasma triglyceride concentration and very low density lipoprotein-triglyceride secretion was similar in diabetics and in normals. The diabetic groups had equivalent degrees of fasting and postprandial hyperglycaemia, and comparable elevations of fasting plasma nonesterified free fatty acid levels (insulin-independent = 0.72±0.07 mmol/L, insulin-dependent = 0.63±0.08 mmol/L). Postprandial plasma insulin concentrations, however, reached normal levels in insulin-independent diabetics and were higher (p<0.001) than in insulin-dependent diabetics. Thus, hypertriglyceridaemia in insulin-independent diabetics with fasting hyperglycaemia was associated with increased hepatic very low density lipoprotein-triglyceride secretion, and normal plasma insulin levels. The lower triglyceride levels in the insulin-dependent diabetics is assumed to be due to their relative hypoinsulinaemia.

Effect of Acarbose on Carbohydrate and Lipid Metabolism in NIDDM Patients Poorly Controlled by Sulfonylureas

The ability of acarbose to lower plasma glucose concentration was studied in 12 patients with noninsulin-dependent diabetes mellitus (NIDDM) who were poorly controlled by diet plus sulfonylurea drugs. Patients were studied before and 3 mo after the addition of acarbose to their treatment program, and a significant improvement in glycemic control was noted. Although the decrease in fasting plasma glucose concentration was modest (12.0 ± 0.8 to 10.8 ± 0.3 mM), average postprandial plasma glucose concentration decreased by 3.4 mM. When acarbose therapy was discontinued in 5 patients, plasma glucose levels rapidly returned toward pretreatment levels. In addition to the improvement in glycemia, acarbose treatment also led to a significant reduction in HbA1c (7.4 ± 0.2 to 6.4 ± 0.2%, P < 0.01) and triglyceride (2.4 ± 0.1 to 2.1 ± 0.1 mM, P < 0.01) concentrations. Neither the plasma insulin response to meals nor insulin-stimulated glucose uptake improved with acarbose therapy, consistent with the view that acarbose improves glycemic control by delaying glucose absorption. Considerable individual variation was noted in the response to acarbose, and the results in 4 patients were dramatic, with striking reductions in both fasting and postprandial glucose concentrations. The addition of acarbose to patients with NIDDM not well controlled by sulfonylureas appears to have significant clinical benefit.

Quantification of Insulin Secretion and In Vivo Insulin Action in Nonobese and Moderately Obese Individuals with Normal Glucose Tolerance

Insulin secretion and in vivo insulin action were quantified in nonobese and moderately obese subjects (approximately 35% above desirable body weight) with normal glucose tolerance. Insulin secretion was estimated by determining plasma insulin responses to a 75-g oral challenge, and in vivo insulin-stimulated glucose uptake by the euglycemic clamp technique. Plasma glucose levels of the two groups were identical during the glucose tolerance test, butthe plasma insulin response was significantly greater (P < 0.01) in the bese subjects. However, insulin-stimulated glucose utilization by the two groups was equal during the euglycemic clamp studies. These results were supported by the fact that degree of obesity correlated significantly with insulin response (r = 0.61, P < 0.005), but not with insulin-stimulated glucose utilization (r = −0.25, P > 0.2). Thus, indirect evidence that moderately obese subjects were more insulin-resistant based on measurement of plasma insulin responsewas not supported by direct quantification of insulin action. One explanation for these findings is that the height of the plasma insulin response bears no relationship to loss of in vivo insulin action, but that seems unlikely in view of the fact that there was a significant correlation (r = −0.52, P < 0.01) between these two variables in the group as a whole. Therefore, it appears that the hyperinsulinemia seen in obese individuals may not be a simple function of insulin resistance, and that the ability of insulin to stimulate glucose utilization is not significantly impaired in moderately obese subjects with normal glucose tolerance. Alternatively, the degree of impairment in insulin action seen in these individuals is insufficient to be detected by the euglycemic clamp technique.

Effect of Variations in Basal Plasma Glucose Concentration on Glucose Utilization (M) and Metabolic Clearance (MCR) Rates During Insulin Clamp Studies in Patients with Non-insulin-dependent Diabetes Mellitus

Two insulin clamp studies were performed at different steady-state plasma glucose concentrations in 13 patients with non-insulin-dependent diabetes mellitus (NIDDM). Steady-state plasma insulin concentrations were comparable, with mean ± SEM levels of 94 ± 3 and 95 ± 4 μU/ml being achieved during the two studies. Glucose utilization rate (M) varied directly with plasma glucose concentration in each subject. Thus, the mean ± SEM value of M was 4.92 ± 0.73 mg/kg/min when patients were studied at a mean ± SEM plasma glucose concentration of 226 ± 15 mg/dl, and M was 2.71 mg/kg/min when the same subjects were studied at a glucose concentration of 118 ± 6 mg/dl. In contrast, the values for glucose metabolic clearance rate (MCR), which were 2.35 ± 0.50 and 2.49 ± 0.47 ml/kg/min, respectively, during the two studies, did not vary significantly with plasma glucose concentration. These data indicate that the glucose metabolic clearance rate (MCR), but not glucose utilization rate (M), can be used to compare in vivo insulin action when insulin clamp studies are performed in subjects with different basal plasma glucose concentrations.