Orville G. Kolterman

Receptor and Postreceptor Defects Contribute to the Insulin Resistance in Noninsulin-dependent Diabetes Mellitus

We have assessed the mechanisms involved in the pathogenesis of the insulin resistance associated with impaired glucose tolerance and Type II diabetes mellitus by exploring, by means of the euglycemic glucose-clamp technique, the in vivo dose-response relationship between serum insulin and the overall rate of glucose disposal in 14 control subjects; 8 subjects with impaired glucose tolerance, and 23 subjects with Type II diabetes. Each subject had at least three studies performed on separate days at insulin infusion rates of 40, 120, 240, 1,200, or 1,800 mU/M2 per min. In the subjects with impaired glucose tolerance, the dose-response curve was shifted to the right (half-maximally effective insulin level 240 vs. 135 microunits/ml for controls), but the maximal rate of glucose disposal remained normal. In patients with Type II diabetes mellitus, the dose-response curve was also shifted to the right, but in addition, there was a posal. This pattern was seen both in the 13 nonobese and the 10 obese diabetic subjects. Among these patients, an inverse linear relationship exists (r = -0.72) so that the higher the fasting glucose level, the lower the maximal glucose disposal rate. Basal rates of hepatic glucose output were 74 +/- 4, 82 +/- 7, 139 +/- 24, and 125 +/- 16 mg/M2 per min for the control subjects, subjects with impaired glucose tolerance, nonobese Type II diabetic subjects, and obese Type II diabetic subjects, respectively. Higher serum insulin levels were required to suppress hepatic glucose output in the subjects with impaired glucose tolerance and Type II diabetics, compared with controls, but hepatic glucose output could be totally suppressed in each study group. We conclude that the mechanisms of insulin resistance in patients with impaired glucose tolerance and in patients with Type II noninsulin-dependent diabetes are complex, and result from heterogeneous causes. (a) In the patients with the mildest disorders of carbohydrate homeostasis (patients with impaired glucose tolerance) the insulin resistance can be accounted for solely on the basis of decreased insulin receptors. (b) In patients with fasting hyperglycemia, insulin resistance is due to both decreased insulin receptors and postreceptor defect in the glucose mechanisms. (c) As the hyperglycemia worsens, the postreceptor defect in peripheral glucose disposal emerges and progressively increases. And (d) no postreceptor defect was detected in any of the patient groups when insulins ability to suppress hepatic glucose output was measured.

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

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

Mechanisms of Insulin Resistance in Aging

We have studied 17 elderly and 27 non-elderly, nonobese subjects (mean age 69+/-1 and 37+/-2 yr, respectively) to assess the mechanisms responsible for the abnormal carbohydrate tolerance associated with aging. Serum glucose and insulin levels were significantly elevated in the elderly subjects compared with the nonelderly subjects during a 75-g oral glucose tolerance test, suggesting an insulin resistant state. Peripheral insulin sensitivity was assessed in both groups using the euglycemic glucose clamp technique during an insulin infusion rate of 40 mU/m(2) per min. Similar steady-state serum insulin levels led to a peripheral glucose disposal rate of 151+/-17 mg/m(2) per min in the elderly compared with a value of 247+/-12 mg/m(2) per min in the nonelderly, thus documenting the presence of insulin resistance in the elderly subjects. Insulin binding to isolated adipocytes and monocytes was similar in the elderly and nonelderly groups (2.34+/-0.33 vs. 2.62+/-0.24% and 5.04+/-1.10 vs. 5.12+/-1.07%), respectively. Thus, insulin resistance in the presence of normal insulin binding suggests the presence of a postreceptor defect in insulin action. This was confirmed by performing additional euglycemic clamp studies using infusion rates of 15 and 1,200 mU/m(2) per min to assess the contours of the dose-response relationship. These studies revealed a 39 and 25% decrease in the glucose disposal rate in the elderly subjects, respectively. The results confirm the presence of a postreceptor defect as well as a rightward shift in the dose-response curve. Insulins ability to suppress hepatic glucose output was less in the elderly subjects during the 15 mU/m(2) per min insulin infusion (77+/-5 vs. 89+/-4% suppression), but hepatic glucose output was fully and equally suppressed in both groups during the 40 and 1,200 mU/m(2) per min infusion. Finally, a significant inverse relationship was observed between the degree of glucose intolerance in the individual elderly subjects, as reflected by the 2-h serum glucose level during the oral glucose tolerance test, and the degree of peripheral insulin resistance as assessed by the glucose disposal rate during the 40 mU/m(2) per min insulin infusion (r = 0.59, P < 0.01).We conclude that carbohydrate intolerance develops as part of the aging process. This carbohydrate intolerance appears to be the consequence of peripheral insulin resistance caused by a postreceptor defect in target tissue insulin action, which causes both a decrease in the maximal rate of peripheral glucose disposal and a rightward shift in the insulin action dose-response curve. In elderly subjects, the severity of the abnormality in carbohydrate tolerance is directly correlated to the degree of peripheral insulin resistance.

The Effect of Insulin Treatment on Insulin Secretion and Insulin Action in Type II Diabetes Mellitus

We have studied the effects of 3 wk of continuous subcutaneous insulin infusion (CSII) on endogenous insulin secretion and action in a group of 14 type II diabetic subjects with a mean (±SEM) fasting glucose level of 286 ± 1 7 mg/dl. Normal basal and postprandial glucose levels were achieved during insulin therapy at the expense of marked peripheral hyperinsulinemia. During the week of posttreatment evaluation, the subjects maintained a mean fasting glucose level of 155 ± 11 mg/dl off insulin therapy, indicating a persistent improvement in carbohydrate homeostasis. Adipocyte insulin binding and in vivo insulin doseresponse curves for glucose disposal using the euglycemic clamp technique were measured before and after therapy to assess the effect on receptor and postreceptor insulin action. Adipocyte insulin binding did not change. The insulin dose-response curve for overall glucose disposal remained right-shifted compared with age-matched controls, but the mean maximal glucose disposal rate increased by 74% from 160 ± 14 to 278 ± 18 mg/m2min (P < 0.0005). The effect of insulin treatment on basal hepatic glucose output was also assessed; the mean rate was initially elevated at 159 ± 8 mg/m2/min but fell to 90 ± 5 mg/m2/min in the posttreatment period (P < 0.001), a value similar to that in control subjects. Endogenous insulin secretion was assessed in detail and found to be improved after exogenous insulin therapy. Mean 24-h integrated serum insulin and C-peptide concentrations were increased from 21,377 ± 2766 to 35,584 ± 4549 μU/ml/ min (P < 0.01) and from 1653 ± 215 to 2112 ± 188 pmol/ml/min (P < 0.05), respectively, despite lower glycemia. Second-phase insulin response to an intravenous (i.v.) glucose challenge was enhanced from 170 ± 53 to 1022 ± 376 μU/ml/min (P < 0.025), although first-phase response remained minimal. Finally, the mean insulin and C-peptide responses to an i.v. glucagon pulse were unchanged in the posttreatment period, but when glucose levels were increased by exogenous glucose infusion to approximate the levels observed before therapy and the glucagon pulse repeated, responses were markedly enhanced. Simple and multivariate correlation analysis showed that only measures of basal hepatic glucose output and the magnitude of the postbinding defect in the untreated state could be related to the respective fasting glucose levels in individual subjects. We conclude that after 3 wk of intensive insulin therapy, diabetic subjects maintain lower glucose values concomitant with: (1) partial reversal of the postbinding defect in peripheral insulin action, (2) near-normalization of basal hepatic glucose output, and (3) enhanced insulin secretory responses. First-phase insulin response remained minimal and may be a marker for the diabetic state. Correlation analysis could only implicate basal hepatic glucose output and the postbinding defect in the untreated state as direct determinants of the fasting glucose level.

Role of Hyperglucagonemia in Maintenance of Increased Rates of Hepatic Glucose Output in Type II Diabetics

Elevated rates of basal hepatic glucose output (bHGO) are significantly correlated with the fasting serum glucose (FSG) level in subjects with non-insulin-dependent diabetes mellitus (NIDDM). This observation suggests that bHGO is a major determinant of the severity of the diabetic state in these subjects. In addition, basal glucagon levels (bGL) are higher in these diabetics than in control subjects, despite the concurrent basal hyperglycemia and hyperinsulinemia, two factors known to suppress glucagon secretion. Although bGL is responsible for sustaining two-thirds of bHGO in normal humans, its role in sustaining elevated rates of bHGO in NIDDM has not been previously defined. To this end, we have studied 13 normal and 10 NIDDM subjects; mean FSG levels were 90 ± 2 and 262 ± 21 mg/dl, respectively (P <.001). The mean fasting serum insulin and glucagon levels were higher in the diabetics than in the controls: 17 ± 2 vs. 9 ± 1 μU/ml (P < .01) and 208 ± 37 vs. 104 ± 15 pg/ ml (P < .01), respectively. On separate days, HGO was assessed isotopically (with 3-[3H]glucose) in the basal state and during infusion of somatostatin (SRIF) (600 μg/h) alone and in conjunction with replacement infusions of glucose and insulin. The results demonstrate that 1) bHGO is higher in diabetics than in controls (145 ± 12 vs. 89 ± 3 mg m2 ml−1, P < .01); 2) during infusion of SRIF alone, HGO was suppressed by 25% (P < .05) and 34% (P < .05) of the basal value in controls and diabetics, respectively; 3) when the studies were repeated with glucose levels held constant at or near the FSG level by the glucose-clamp technique, the pattern and degree of HGO suppression was similar to that obtained by infusion of SRIF alone; 4) during isolated glucagon deficiency (SRIF + insulin, 5 mU m−2 min−1 with serum glucose maintained at basal level), HGO was suppressed by 71 ± 8% of the basal value in controls (P < .001) and by 58 ± 7% in diabetics (P < .001); and 5) when isolated glucagon deficiency with similar hyperglycemia was created in control subjects, HGO was suppressed by 87% of the basal value. We conclude that 1) elevated glucagon levels contribute significantly to the elevated rates of bHGO in NIDDM subjects, 2) basal glucagon levels sustain ∼71 % of basal HGO in control and 58% in NIDDM subjects, and 3) the excess glucagon effect is largely responsible for the apparent hepatic insulin resistance seen in NIDDM subjects.

The Acute and Chronic Effects of Sulfonylurea Therapy in Type II Diabetic Subjects

Although sulfonylurea agents have been used in the clinical management of type II diabetes (non-insulin-dependent diabetes mellitus, NIDDM) for over two decades, the mechanisms responsible for their hypoglycemie action remain controversial. We have quantitated glycemie control, endogenous insulin secretion in response to mixed meals, adipocyte insulin binding, insulin-mediated peripheral glucose disposal, and basal hepatic glucose output in 17 type II diabetic subjects before and after 3 mo of therapy with the second-generation, sulfonylurea compound glyburide in an attempt to identify the factors responsible for the clinical response to the drug. In addition, 9 subjects were treated for an additional 15 mo to see if the response to the drug changed with time. The mean fasting serum glucose level fell from an initial value of 264 ± 17 mg/dl to 178 ± 16 mg/dl after 3 mo of drug therapy. Endogenous insulin secretion increased in all subjects, but the increase was most marked in those subjects who continued to exhibit fasting hyperglycemie (fasting serum glucose > 175 mg/dl) after 3 mo of therapy. Adipocyte insulin binding was unchanged after 3 mo of therapy, while the maximal rate of peripheral glucose disposal was increased by 23%, indicating enhancement of peripheral insulin action at a postreceptor site(s). Basal hepatic glucose output showed a significant correlation with the fasting serum glucose level both before and after therapy (r = 0.86, P < 0.001) and fell from 141 ±12 mg/m2/min before therapy to 107 ± 11 mg/m2/min after 3 mo of therapy. A significant correlation was also found between the decrease in the fasting glucose level and both the reduction in basal hepatic glucose output (r = 0.81, P < 0.001) and the enhancement of postreceptor function in peripheral tissues (r = 0.68, P < 0.005). After 18 mo of therapy, those subjects exhibiting an initial good response to the drug demonstrated a slight decrease in endogenous insulin secretion compared with the levels seen at 3 mo, adipocyte insulin binding had increased to the normal range, postreceptor function was further enhanced, and basal hepatic glucose output remained unchanged from the levels observed after 3 mo of therapy. We conclude that (1) glyburide therapy increases endogenous insulin secretion, increases adipocyte insulin binding after 18, but not 3, mo of therapy, enhances peripheral insulin action by acting primarily at a post-receptor site, and reduces basal hepatic glucose output; (2) the increase in postreceptor function and the reduction of basal hepatic glucose output appear to be the crucial determinants of the clinical response to the sulfonylurea agent; and (3) the response pattern to sulfonylurea compounds in terms of these various parameters can vary as a function of the duration of treatment.

Amylin replacement with pramlintide as an adjunct to insulin therapy improves long-term glycaemic and weight control in Type 1 diabetes mellitus: a 1-year, randomized controlled trial

Aims  The autoimmune‐mediated destruction of pancreatic β‐cells in Type 1 diabetes mellitus renders patients deficient in two glucoregulatory peptide hormones, insulin and amylin. With insulin replacement alone, most patients do not achieve glycaemic goals. We aimed to determine the long‐term efficacy and safety of adjunctive therapy with pramlintide, a synthetic human amylin analogue, in patients with Type 1 diabetes.

Insulin Treatment Reverses the Insulin Resistance of Type II Diabetes Mellitus

Type II diabetic subjects are both insulin-deficient and insulin-resistant. Recent studies suggest that the insulin resistance is due to a combined receptor and postreceptor defect with the postreceptor defect being the predominant lesion. In the present study, we examined the effects of exogenous insulin therapy upon these defects in insulin action in six untreated type II diabetic subjects. Glycemic control and adipocyte insulin binding were measured and in vivo insulin dose-response curves for overall glucose disposal and suppression of hepatic glucose output were constructed before treatment. Following these initial studies, the diabetic subjects were treated with twice-daily injections of regular and NPH purified pork insulin for 14 days and the pretreatment studies repeated. Glycemic control was significantly improved by this treatment regimen. The mean fasting serum glucose level (±SE) fell from 287 ± 20 to 125 ± 13 mg/dl, the mean glycosylated hemoglobin level (± SE) decreased from 14.2 ± 1.1% to 8.3 ± 0.5%, and the mean 24-h urinary glucose excretion (±SE) declined from 65.6 ± 40.3 to 0.6 ± 0. 1 g/24 h. Adipocyte insulin binding did not change significantly during the treatment period. In contrast, the 14-day period of insulin treatment produced a 72% increase (P < 0.005) in the maximal rate of insulin-stimulated glucose disposal, 321 ± 32 mg/M2/min compared with 187 ± 32 mg/M2/min before treatment, indicating that the postreceptor defect in insulin action was significantly ameliorated by insulin treatment. The dose-response curve for insulin-mediated suppression of hepatic glucose output was rightshifted, consistent with the decrease in insulin binding, with no decrease in the maximal effect before treatment and not significantly changed following insulin treatment. In conclusion, the postreceptor defect in insulin-stimulated glucose disposal is largely ameliorated by exogenous insulin treatment, suggesting that this defect in insulin action is an acquired abnormality which is secondary to some aspect of the insulin-deficient state.

Effects of insulin infusion on human skeletal muscle pyruvate dehydrogenase, phosphofructokinase, and glycogen synthase. Evidence for their role in oxidative and nonoxidative glucose metabolism.

To determine whether activation by insulin of glycogen synthase (GS), phosphofructokinase (PFK), or pyruvate dehydrogenase (PDH) in skeletal muscle regulates intracellular glucose metabolism, subjects were studied basally and during euglycemic insulin infusions of 12, 30, and 240 mU/m2 X min. Glucose disposal, oxidative and nonoxidative glucose metabolism were determined. GS, PFK, and PDH were assayed in skeletal muscle under each condition. Glucose disposal rates were 2.37 +/- 0.11, 3.15 +/- 0.19, 6.71 +/- 0.44, and 11.7 +/- 1.73 mg/kg X min; glucose oxidation rates were 1.96 +/- 0.18, 2.81 +/- 0.28, 4.43 +/- 0.32, and 5.22 +/- 0.52. Nonoxidative glucose metabolism was 0.39 +/- 0.13, 0.34 +/- 0.26, 2.28 +/- 0.40, and 6.52 +/- 1.21 mg/kg X min. Both the proportion of active GS and the proportion of active PDH were increased by hyperinsulinemia. PFK activity was unaffected. Activation of GS was correlated with nonoxidative glucose metabolism, while activation of PDH was correlated with glucose oxidation. Sensitivity to insulin of GS was similar to that of nonoxidative glucose metabolism, while the sensitivity to insulin of PDH was similar to that of glucose oxidation. Therefore, the activation of these enzymes in muscle may regulate nonoxidative and oxidative glucose metabolism.

Adjunctive Therapy with the Amylin Analogue Pramlintide Leads to a Combined Improvement in Glycemic and Weight Control in Insulin-Treated Subjects with Type 2 Diabetes

The objective of this study was to assess the effect of mealtime amylin replacement with pramlintide on long-term glycemic and weight control in subjects with type 2 diabetes. This 52-week, randomized, placebo-controlled, multicenter, double-blind, dose-ranging study in 538 insulin-treated subjects with type 2 diabetes compared the efficacy and safety of 30-, 75-, or 150-microg doses of pramlintide, a synthetic analogue of the beta-cell hormone amylin, to placebo when injected subcutaneously three times daily (TID) with major meals. Pramlintide therapy led to a mean reduction in HbA1c of 0.9% and 1.0% from baseline to week 13 in the 75- and 150-microg dose groups, which was significant compared to placebo (p = 0.0004 and p = 0.0002, respectively). In the 150-microg dose group, there was a mean reduction in HbA1c of 0.6% from baseline to week 52 (p = 0.0068 compared to placebo). The greater reduction in HbA1c with pramlintide was achieved without increases in insulin use or severe hypoglycemia, and was accompanied by a significant (p < 0.05) reduction in body weight in all dose groups compared to placebo. Three times the proportion of subjects in the 150-microg pramlintide group compared to the placebo group achieved a concomitant reduction in both HbA1c and body weight from baseline to week 52 (48% versus 16%). The most common adverse event reported with pramlintide treatment was nausea, which was mild to moderate and dissipated early in treatment. The results from this study support the safety and efficacy of pramlintide administered three times a day with major meals, in conjunction with insulin therapy, for improving long-term glycemic and weight control in subjects with type 2 diabetes.