Distribution and metabolism of intravenously injected tritiated insulin in rats

Effect of receptor up-regulation on insulin pharmacokinetics in streptozotocin-treated diabetic rats

The present study investigated the mechanism by which the disposition of insulin is altered in streptozotocin (STZ)-treated diabetic rats as compared with 48-hr-fasted normal (control) rats. It was shown by an indocyanine green infusion method that the hepatic plasma flow rate (QH) in diabetic rats (1.64 ml/min/g liver) is significantly higher than that in control rats (0.982 ml/min/g liver). The portal injection technique revealed that the unidirectional clearance (CLon), which represents the binding of A14-125I-insulin to surface receptors in the liver, is significantly elevated in diabetic rats, suggesting an increase in the surface receptor number (RT), i.e., up-regulation in the liver. In both control and diabetic rats, the total-body clearance (CLtot) and steady-state volume of distribution (Vdss) of labeled insulin decreased significantly with a simultaneous injection of unlabeled insulin (8 U/kg), confirming that the disposition of insulin is affected largely by specific, saturable receptor-mediated processes. The CLtot and Vdss increased significantly in diabetic rats, while nonspecific portions of these parameters were not changed. From the increases in CLtot (80%) and QH (67%) in diabetic rats, a pharmacokinetic analysis has revealed a 40% increase in the hepatic intrinsic clearance (CLint,sp) of A14-125I-insulin via a specific mechanism in diabetic rats. In conclusion, we have provided in vivo evidence for a small increase in CLint,sp of insulin in STZ-diabetic rats compared with control rats, which may be caused by an increase in the surface receptor number in the livers of diabetic rats.

Intermediate peptides of insulin degradation in liver and cultured hepatocytes of rats

1. Bestatin, a microbial aminopeptidase inhibitor, induced accumulation of low-molecular weight intermediate peptides of insulin degradation in liver of rats in vivo and in primary cultured rat hepatocytes. However, bestatin did not affect the association and internalization of the hormone into hepatic cells. 2. Results of the HPLC analyses showed that the intermediate peptides of insulin degradation are small ones and specifically accumulate only in the presence of bestatin. 3. The above results, together with those employing other protease inhibitors, show that cytosolic bestatin-sensitive protease(s), trypsin-like protease(s) and thiol protease(s) play an important role in the intracellular degradation process of insulin.

Regulation of peripheral insulin/glucagon levels by rat liver

The concentrations of insulin and glucagon were measured in the portal and hepatic vein, the abdominal aorta and caval vein in the rat during a normal 24-h feeding cycle. Portal insulin levels showed little diurnal variation while hepatovenous and peripheral values were clearly increased during the eating phase. Conversely, portal glucagon levels were maximal during the fasting period while hepatovenous and peripheral concentrations showed little diurnal variation. The removal of insulin and glucagon by the liver was not constant, but independently regulated. During meals the liver increased the high portal insulin/glucagon ratio further to an even higher peripheral ratio favouring glucose utilization, e.g. by muscle and adipose tissue. During a short fast the liver decreased the low portal insulin/glucagon ratio further to an even lower peripheral ratio leading to glucose saving, e.g. by muscle and adipose tissue in favour of the brain and erythrocytes. The results indicate that the liver has an important role in the regulation of peripheral insulin/glucagon levels.

Physiological significance of altered insulin metabolism in the conscious rat during lactation

Uptake of radioactively labelled insulin by the mammary gland of the rat increased 12-fold in lactation compared with non-lactating controls. This uptake was decreased by the presence of unlabelled insulin, indicating that it occurred via insulin receptors. The plasma half-life of insulin is decreased in lactation from 9.4 min to 4.8 min, and the metabolic clearance rate for insulin increased from 7.26 to 13.03 ml/kg body wt. per min. The basal insulin and glucose concentrations in the plasma were decreased in lactation. Infusion of insulin at a dose which led to a small physiological rise in plasma insulin concentration increased lipogenic rates in the mammary gland by 100% without causing marked hypoglycaemia. It is concluded that the lactating mammary gland is a highly insulin-sensitive tissue and that the lower plasma insulin during lactation occurs primarily as a result of this sensitivity increasing extraction of glucose by the gland and thus producing a decrease in the plasma glucose concentration. It is suggested that a secondary result of the fall in plasma insulin concentration is the preferential direction of substrates (glucose and non-esterified fatty acids) towards the lactating mammary gland and away from adipose tissue and the liver.

Role of the kidneys in elimination of glucagon, insulin, secretin and somatostatin in the rat

Immunoreactive plasma glucagon and secretin in the rat was elevated 48 hours after nephrectomy and ureteral ligation. Since kidneys obstructed by ureteral ligation were unable to remove glucagon and secretin from the blood, renal handling of glucagon and secretin must include glomerular filtration. Insulin and somatostatin levels were significantly elevated 48 hours after nephrectomy, but not after ureteral ligation, indicating partial uptake from peritubular capillaries.

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.

Degradation of pork insulin and biosynthetic human insulin in vitro and in vivo: a comparative study

The clearance and the degradation of native pork insulin and human insulin prepared by recombinant DNA techniques (biosynthetic human insulin) were compared. Following intravenous injection in rats, biosynthetic human insulin was cleaved slightly more rapidly than pork insulin. Also in vitro, monoiodinated biosynthetic human insulin was degraded slightly more rapidly than pork insulin by the enzyme insulin protease. The apparent Km for pork insulin was 4.0 x 10-8 M and for the human product was 2.7 x 10-8 M. These results thus provide evidence for the integrity of the biosynthetic product.

Importance of preabsorptive insulin release on oral glucose tolerance: studies in pancreatic islet transplanted rats

The role of preabsorptive (cephalic phase) insulin release in oral glucose tolerance was investigated using diabetic rats treated by intraportal transplantation of isogenic islets. This early neurally mediated phase of insulin release has been shown to be absent in such rats. When the body weight of transplanted rats was normalised, glucose tolerance tests (GTTs) were performed in the unstressed state using permanent cardiac catheters. Transplanted rats had a normalised intravenous GTT, whereas, as we have shown previously, their oral GTT remained clearly pathological. During both tests peripheral insulin levels were decreased compared with controls. While during intravenous GTT the onset of insulin release occurred as early in transplanted rats as in controls, during oral GTT there was a clear delay, probably because of the absence of the cephalic phase. Re-establishment of normal preabsorptive insulin levels was attempted by a small intravenous insulin injection during this period. This resulted in a transient increase in peripheral insulin levels, which, at two minutes after glucose ingestion, gave values similar to those found in controls at that time. This small insulin injection caused a marked improvement of the oral GTT which was most evident after exogenous insulin had disappeared from the blood. While the injection did not affect the 60 minute incremental insulin area, the glucose area was decreased by 50%, to a value not significantly different from that of control rats. The cephalic phase of insulin release appears, therefore, to be one important factor in the control of glycaemia during food intake. Its absence plays a major role in the pathological oral glucose tolerance of diabetic rats treated by intraportal islet transplantation.