Insulin binding and degradation by muscles from streptozotocin-diabetic rats

Insulin degradation: progress and potential

Insulin degradation is a regulated process that plays a role in controlling insulin action by removing and inactivating the hormone. Abnormalities in insulin clearance and degradation are present in various pathological conditions including type 2 diabetes and obesity and may be important in producing clinical problems. The uptake, processing, and degradation of insulin by cells is a complex process with multiple intracellular pathways. Most evidence supports IDE as the primary degradative mechanism, but other systems (PDI, lysosomes, and other enzymes) undoubtedly contribute to insulin metabolism. Recent studies support a multifunctional role for IDE, as an intracellular binding, regulatory, and degradative protein. IDE increases proteasome and steroid hormone receptor activity, and this activation is reversed by insulin. This raises the possibility of a direct intracellular interaction of insulin with IDE that could modulate protein and fat metabolism. The recent findings would place intracellular insulin-IDE interaction into the insulin signal transduction pathway for mediating the intermediate effects of insulin on fat and protein turnover.

Insulin and forearm vasodilation in hypertension-prone men

Insulin-stimulated peripheral glucose uptake and insulin-mediated forearm vasodilation were investigated in 38 normotensive men with a family history of hypertension (relatives) compared with 27 age- and body mass index-matched normotensive men with no family history of hypertension (control subjects). The euglycemic hyperinsulinemic clamp technique was used to measure peripheral glucose uptake (insulin sensitivity index) and the metabolic clearance rate of insulin. Intra-arterial blood pressure and forearm blood flow were determined simultaneously, and forearm vascular resistance was calculated. The insulin sensitivity index was lower in relatives than in control subjects. The metabolic clearance rate of insulin was reduced and did not correlate to glucose disposal in the relatives as opposed to the control subjects. Forearm blood flow increased and forearm vascular resistance decreased to a similar extent in the two groups during the clamp. The vasodilator response was positively correlated to glucose disposal only in the relatives. In conclusion, impaired insulin-stimulated peripheral glucose uptake in normotensive sons from hypertensive families was accompanied by retained insulin-mediated forearm vasodilation. Thus, skeletal muscle blood flow supply does not seem to be the major determinant for glucose disposal. On the other hand, the positive correlation between glucose disposal and decrease in forearm vascular resistance in the relatives suggests that insulin-mediated vasodilation may be a limiting factor for peripheral glucose uptake in insulin-resistant individuals.

Acute exercise, epinephrine, and diabetes enhance insulin binding to skeletal muscle

We measured insulin binding to crude membranes from rat skeletal muscle, with binding expressed relative to the sarcolemmal marker, cholesterol ester (CE). The amount of CE in the sarcolemma remained constant after streptozotocin-induced diabetes and acute exercise (swam for 90 min). Soleus (predominantly slow-twitch fibers) had higher insulin binding capacity than extensor digitorum longus (predominantly fast twitch). Both diabetes and acute exercise enhanced insulin binding. The shape of the enhanced insulin binding curve differed, however, between diabetes and acute exercise. Diabetes elicited a uniform increase in binding across the insulin concentrations measured (0.04-166 nM); acute exercise elicited the largest increase at the lower concentrations, suggesting different mechanisms cause the enhanced binding. Addition of 13.1 nM epinephrine to the perfusate in a rat hindlimb preparation increased insulin binding in a pattern similar to acute exercise. In contrast, muscular contraction stimulated by the sciatic nerve (1 Hz) or reduction of perfusate insulin from 100 to 40 pM, two additional correlates of acute exercise, had no effect. The increased insulin binding after acute exercise, therefore, appears to be mediated through elevated levels of catecholamines and not upregulation of the insulin receptor.

Reversal of insulin resistance in type 1 diabetes following initiation of insulin treatment

The biological action and pharmacokinetics of insulin were assessed in nine type 1 (insulin-dependent) diabetic patients before and after 3 months conventional insulin treatment, and in seven age and weight-matched non-diabetic controls, by means of the euglycaemic insulin clamp technique. The mean (+/- S.E.) metabolic clearance rate of insulin, when infused at 1 mU/kg/min, was similar in untreated and treated diabetic patients and in controls (22.7 +/- 2.0, 19.3 +/- 3.8, and 22.9 +/- 3.3 ml/kg/min) but, when infused at 6 mU/kg/min, was greater (p less than 0.01 and less than 0.01) in untreated patients (18.0 +/- 2.5 ml/kg/min) than in treated patients (11.5 +/- 1.4 ml/kg/min) and controls (12.7 +/- 1.3 ml/kg/min). Insulin-mediated glucose disposal was reduced (p less than 0.01 and less than 0.01) at insulin infusion rates 1 and 6 mU/kg/min in untreated patients (18.5 +/- 1.9 and 33.8 +/- 4.5 mumol/kg/min) when compared with controls (35.8 +/- 3.4 and 62.0 +/- 4.7 mumol/kg/min) and was improved (p less than 0.01 and less than 0.01) following insulin treatment (36.1 +/- 4.6 and 64.8 +/- 4.2 mumol/kg/min). Daily insulin requirement fell by 33% following 3 months insulin treatment with improvement in mean HbA1 from 16.3 +/- 0.7 to 8.2 +/- 0.4%, but without significant increase in endogenous insulin secretion. The 'honeymoon phenomenon', which has traditionally been attributed exclusively to resurrection of endogenous insulin release, may also be related to normalization of insulin action following institution of insulin treatment.

Glycogen repletion in different skeletal muscles from diabetic rats

It was hypothesized that chronically untreated streptozotocin-diabetic rats may compensate for the detrimental effect of insulin deficiency on glycogen restoration following muscular work. Glycogen concentration ([GLY]) was reduced by in situ stimulation of the sciatic nerve of anesthetized normal (N) and diabetic (D) Sprague-Dawley albino rats. Glycogen repletion occurred most rapidly within 30 min after stimulation. By 2 h all N muscles returned to basal [GLY]. D muscles repleted glycogen at a rate 45-75% of normal and not all of the D muscle returned to basal [GLY] by 8 h poststimulation. Neither the partial reduction in diabetic hyperglycemia by using phlorizin nor the acute further lowering of diabetic hypoinsulinemia by using insulin antibody affected glycogen restoration in D muscles. Acute insulin replenishment resulted in restoration of normal [GLY] in D muscles within 2 h poststimulation. These findings are not consistent with the proposed hypothesis and indicate insulin is necessary to have normal rates of glycogen restoration following muscular activity.

Insulin degrading enzyme activity and insulin binding of erythrocytes in normal subjects and Type 2 (non-insulin-dependent) diabetic patients

Specific insulin degrading enzyme activity of erythrocytes was determined in relation to erythrocyte insulin binding in 16 healthy subjects, 14 Type 1 (insulin-dependent) and various groups of Type 2 (non-insulin-dependent) diabetic patients (n = 39). Degrading activity was increased in Type 2 diabetic patients on sulphonylureas, as well as in a subgroup with good metabolic control (p less than 0.001) and in patients with secondary failure to oral therapy (p less than 0.02); degrading activity returned to normal in the latter patients after 1 week of insulin treatment. Highest degrading activity was found in insulin-treated, yet insulin-insensitive patients (daily insulin dose greater than 80 U). Degrading activity was significantly correlated in healthy subjects both with circulating insulin concentrations and maximal specific insulin binding. In contrast, in Type 2 diabetic subjects, degrading activity was inversely correlated with serum insulin with no apparent association with maximal specific insulin binding except in those patients given 1 week of insulin treatment. High erythrocyte insulin degrading enzyme activity might be a common feature in the insulin-insensitive Type 2 diabetic patient and might occur subsequent to some aspect of insulin deficiency at the tissue level.

Increased clearance and degradation of [3H]insulin in streptozotocin diabetic rats

The role of the insulin-receptor compartment in the pharmacokinetics of intravenously injected insulin in rats was studied. Since streptozotocin-diabetes in rats results in increased insulin binding to tissues in vitro, insulin pharmacokinetics in streptozotocin-diabetic rats were compared to controls, using semisynthetic [(3)H]insulin as the tracer. The initial distribution volume for [(3)H]insulin was elevated by 60% in diabetic rats. By contrast, no difference in initial distribution volume for [(14)C]inulin was observed, and the absolute values were lower than those found for [(3)H]insulin. The metabolic clearance rate of [(3)H]insulin was elevated by 44% in diabetic rats. That these differences were the result of increased binding of insulin to a specific receptor compartment in diabetic rats was shown by three additional experiments. The first involved receptor saturation by injection of 10 U native insulin 2 min before the tracer injection, resulting in identical [(3)H]insulin disappearance rates in the two groups of rats. The second consisted of displacing [(3)H]insulin from receptors by injecting 10 U unlabeled insulin 6 min after the tracer injection. Displacement of intact [(3)H]insulin from receptors and subsequent reappearance in the circulation occurred in both control and diabetic animals; however, such displacement was 25% greater in the diabetic rats. Finally, treatment of diabetic rats with insulin for 8 d normalized [(3)H]insulin clearance even though the tracer was injected at a time when the animals were again hyperglycemic and hypoinsulinemic. This suggests that down-regulation of insulin receptors had occurred during insulin therapy. These results confirm that a specific compartment for insulin exists (the insulin-receptor compartment) and that this compartment plays an important role in insulin clearance.