The effect of insulin treatment on insulin secretion and insulin action in type II diabetes mellitus

Pathogenesis of type 2 diabetes mellitus: an interpretation of current data

The available evidence concerning the pathogenesis of type 2 diabetes mellitus suggests that although insulin antagonism is common to many patient groups (i.e., obese, older), overt NIDDM will only develop in a subset who may have a genetic susceptibility for inadequate beta-cell reserves. Since the role of a receptor defect in causing type 2 diabetes is doubtful, the insulin antagonism in these patients is due to a post-receptor defect.

Chronic hyperglycemia is associated with impaired glucose influence on insulin secretion. A study in normal rats using chronic in vivo glucose infusions

We have proposed that chronic hyperglycemia alters the ability of glucose to modulate insulin secretion, and have now examined the effects of different levels of hyperglycemia on B cell function in normal rats using chronic glucose infusions. Rats weighing 220-300 g were infused with 0.45% NaCl or 20, 30, 35, or 50% glucose at 2 ml/h for 48 h, which raised the plasma glucose by 18 mg/dl in the 30% rats, 37 mg/dl in the 35% rats, and 224 mg/dl in the 50% group. Insulin secretion was then examined using the in vitro isolated perfused pancreas. Glucose-induced insulin secretion remained intact in the normoglycemic 20% glucose rats and it was potentiated in the mildly hyperglycemic 30% glucose rats. However, with even greater hyperglycemia in the 35% glucose group the insulin response to a high glucose perfusate was severely blunted, and it was totally lost in the most hyperglycemic 50% glucose rats. In a second protocol that examined glucose potentiation of arginine-stimulated insulin release, a similar impairment in the ability of glucose to modulate the insulin response to arginine was found with increasing levels of chronic hyperglycemia. On the other hand, the ability of a high glucose concentration to inhibit arginine-stimulated glucagon release was preserved in all glucose-infused rats, but the glucagon levels attained in response to the arginine at 2.8 mM glucose were much less in the 50% glucose rats than in all the other groups. These data clearly show that after 48 h of marked hyperglycemia, glucose influence upon insulin secretion in the rat is severely impaired. This model provides a relatively easy and reproducible method to study the effects of long-term hyperglycemia on B cell function.

Reduced incretin effect in type 2 (non-insulin-dependent) diabetes

Integrated incremental immunoreactive insulin and connecting peptide responses to an oral glucose load of 50 g and an "isoglycaemic" intravenous glucose infusion, respectively, were measured in 14 Type 2 (non-insulin-dependent) diabetic patients and 8 age- and weight-matched metabolically healthy control subjects. Differences between responses to oral and intravenous glucose administration are attributed to factors other than glucose itself (incretin effect). Despite higher glucose increases, immunoreactive insulin and connecting peptide responses after oral glucose were delayed in diabetic patients. Integrated responses were not significantly different between both groups. However, during "isoglycaemic" intravenous infusion, insulin and connecting peptide responses were greater in diabetic patients than in control subjects as a consequence of the higher glycaemic stimulus. The contribution of incretin factors to total insulin responses was 72.8 +/- 6.9% (100% = response to oral load) in control subjects and 36.0 +/- 8.8% in diabetic patients (p less than or equal to 0.05). The contribution to connecting peptide responses was 58.4 +/- 7.6% in control subjects and 7.6 +/- 14.5% (p less than or equal to 0.05) in diabetic patients. Ratios of integrated insulin to connecting peptide responses suggest a reduced (hepatic) insulin extraction in control subjects after oral as compared to intravenous glucose. This was not the case in diabetic patients. Immunoreactive gastric inhibitory polypeptide responses were not different between control subjects and diabetic patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Rationale for glycemic control

Whether long-term glycemic control will prevent the chronic vascular complications of diabetes mellitus remains unknown. Microangiopathy and accelerated macroangiopathy are prevalent in both type I, or insulin-dependent diabetes mellitus, and type II, or non-insulin-dependent diabetes mellitus. Microangiopathy is predominantly responsible for the excessive morbidity and mortality in type I diabetic patients, whereas accelerated macroangiopathy directly relates to the excessive morbidity and mortality in type II diabetic patients. Institution of euglycemia for short periods will reverse preclinical, functional, renal, and retinal abnormalities, but will not reverse clinical nephropathy and retinopathy. Intensive insulin therapy, although it increases the risk of hypoglycemic encephalopathy, seems rational for type I diabetic patients without vascular complications who can recognize and respond normally to hypoglycemia. In patients with type II diabetes, sulfonylurea therapy, which is associated with fewer adverse reactions than intensive insulin therapy, may lower the risk of atherosclerosis development by correcting hyperglycemia and associated lipid abnormalities.

Fat cells: model system to investigate molecular mechanism(s) of sulfonylurea-potentiated glucose transport

The cellular mechanism of action of sulfonylureas may vary depending upon the exact nature of the drug, the tissue or cell type in question, and the status of the subject from which it was obtained. In adipocytes from patients with type II non-insulin-dependent diabetes, there is increasing evidence indicating that sulfonylureas ameliorate a post-receptor defect in insulin action by potentiating the insulin-stimulated glucose transport normally seen in these cells. Studies undertaken to elucidate the molecular mechanism of this potentiation investigated the effects of a 48-hour incubation with glyburide (2 micrograms/ml) on the recruitment of glucose carriers from microsomal storage pools to the plasma membrane. With the use of cytochalasin B, a potent competitive inhibitor of glucose transport, glucose-sensitive cytochalasin B binding was studied in basal and insulin-stimulated adipocytes from control and sulfonylurea-treated tissue. The data indicated that sulfonylurea treatment did not affect the total glucose-sensitive cytochalasin B binding capacity of adipocyte membranes. It did, however, increase the insulin-induced recruitment of the glucose carrier from the microsome to the plasma membrane by 27 to 31 percent. This suggests that the molecular mechanism of sulfonylurea-enhanced insulin-stimulated glucose transport is the recruitment of glucose transporters from an intracellular microsomal storage pool to the plasma membrane.

Insulin resistance: receptor and post-binding defects in human obesity and non-insulin-dependent diabetes mellitus

Insulin resistance is a prominent feature of three clinical conditions: obesity, impaired glucose tolerance, and non-insulin-dependent (type II) diabetes mellitus. Numerous studies over the past 15 years have provided a better understanding, from both a clinical and cellular standpoint, of the pathophysiology of these insulin-resistant states as well as of insulin action. In addition, it has recently been recognized that correction of glucose intolerance leads to an improvement in insulin secretion and a reduction in insulin resistance. Examination of the most recent data suggests that the basis for insulin resistance in these common clinical disorders is often multifactorial. In uncomplicated obesity, the cellular alterations responsible for insulin resistance appear to be at the level of the hepatic insulin receptor and in post-binding processes in peripheral target tissues. In type II diabetes, a post-binding defect(s) in peripheral tissues appears to be the primary lesion. In humans, many of the factors that mediate the changes leading to insulin resistance are still unknown and are the object of current investigations.

Glucose regulation in non-insulin-dependent diabetes mellitus. Interaction between pancreatic islets and the liver

The degree of fasting hyperglycemia in patients with non-insulin-dependent diabetes mellitus is dependent on the rate of hepatic glucose production. The basal rate of hepatic glucose production is increased in patients with non-insulin-dependent diabetes mellitus, and there is a positive correlation between hepatic glucose production and fasting glucose levels. Diminished secretion of insulin, impaired hepatic sensitivity to insulin's effects, or a combination of these factors could contribute to the elevated hepatic glucose production in patients with non-insulin-dependent diabetes mellitus. The relationship between insulin secretion and hepatic glucose production is regulated by a closed feedback loop operating between glucose levels and pancreatic beta cells. Although fasting insulin levels are usually comparable between patients with non-insulin-dependent diabetes mellitus and normal subjects, insulin secretion is markedly impaired in non-insulin-dependent diabetes mellitus in relation to the degree of hyperglycemia present. In fact, the degree of fasting hyperglycemia in a given patient with non-insulin-dependent diabetes mellitus is closely related to the degree of impaired pancreatic beta-cell responsiveness to glucose. Such findings suggest that impaired insulin secretion leads to increased hepatic glucose production, which raises the plasma glucose level. The resulting hyperglycemia helps to maintain relatively normal basal insulin output. Chronic sulfonylurea drug therapy of patients with non-insulin-dependent diabetes mellitus enhances pancreatic islet sensitivity to glucose, leading to increased insulin secretion, suppression of hepatic glucose production, and a decline in the steady-state fasting glucose level.

Glycemic control with an artificial pancreas improves insulin responses to both oral and glucose in nonobese noninsulin-dependent diabetic subjects

Insulin secretory responses to both oral and intravenous glucose were investigated in 12 nonobese noninsulin-dependent diabetic subjects before and after strict metabolic control of blood glucose levels without weight loss. Glycemic control was achieved by applying an artificial pancreas to all diabetics for 2 or 3 days, which led to restoration of normal fasting blood glucose levels and to significant reduction of fasting plasma insulin (p less than 0.01) and C-peptide (p less than 0.05) levels. Initially, the insulin response to oral glucose was weak and delayed, but increased significantly after treatment (p less than 0.01), although none of the diabetic subjects achieved completely normal glucose tolerance. The i.v. glucose tolerance test (0.33 g/kg) revealed that all diabetics lacked acute insulin response in the basal state with low glucose disappearance rates (0.37 +/- 0.07 %/min). After 48h of normoglycemia, these figures did not change significantly, although the insulinogenic index (insulin area/glucose area) was significantly increased (p less than 0.05). A marked increase in both phases of insulin secretion was evident when a larger intravenous glucose pulse (0.66 g/kg) was used in some diabetics in order to raise the blood glucose concentrations of the post-treatment test to those of the pre-treatment test. In absolute terms, the insulin responses of the post-treatment tests were not significantly different from those of sex-, age- and weight-matched control subjects, but were significantly lower if related to the corresponding plasma glucose responses (insulinogenic index lower than that of controls). These studies in nonobese noninsulin-dependent diabetic subjects indicate that glycemic control with an artificial pancreas improves insulin response to glucose, suggesting that chronic hyperglycemia may stress the impaired B-cell secretory capacity of diabetes.

Subcellular translocation of glucose transporters: role in insulin action and its perturbation in altered metabolic states

In this article we have described the hypothesis that insulin stimulates glucose transport through glucose transporter translocation from an intracellular pool to the plasma membrane. In addition, we have shown that changes in the numbers and subcellular distributions of glucose transporters correlate with alterations in insulin-stimulated glucose transport activity in several experimental models of insulin resistance and hyperresponsiveness. However, in experiments with counterregulatory hormones and with hyperresponsive states induced by nutritional repletion following deprivation, changes in insulin responsiveness cannot be fully explained by such alterations in the numbers and/or subcellular distribution of glucose transporters. Thus, evidence has been presented for changes in glucose transporter intrinsic activity that both inhibit and augment insulin-stimulated glucose transport rates. Finally, we have discussed data suggesting that the translocation process is applicable to human tissue and that significant changes in adipose cell glucose transport activity have been correlated with total glucose disposal in various metabolic states in humans. Determining the physiologic factors involved in modulating these events at the cellular level is an important area for further investigation.