The binding and degradation of 125I-labelled insulin by rat kidney brush-border membranes

Insulin aspart: a review

Insulin aspart, an analogue of human insulin, which is approved for use in people with diabetes, is more rapidly absorbed and achieves higher plasma concentrations than human soluble insulin following subcutaneous injection. Hence, it has a faster and more effective glucose-lowering action, with superior control of postprandial hyperglycaemia compared with human soluble insulin. Its shorter duration of action makes interprandial and nocturnal hypoglycaemia less of a problem than with human soluble insulin. Insulin aspart is approved for use in continuous subcutaneous infusion and offers a valuable treatment option during pregnancy.

Clinical pharmacokinetics and pharmacodynamics of insulin aspart

Insulin aspart is a novel rapid-acting insulin analogue with improved subcutaneous absorption properties when compared with soluble human insulin. Pharmacokinetic studies show an absorption profile with a time to reach peak concentration (t(max)) about half that of human insulin, a peak plasma drug concentration (Cmax) approximately twice as high and shorter residence time. The potency and bioavailability of insulin aspart are similar to those of human insulin. The pharmacokinetics of insulin aspart have been studied in healthy Caucasian and Asian-Japanese volunteers, in patients with type 1 and 2 diabetes mellitus, and in children with diabetes, with both pre- and postprandial administration and during continuous subcutaneous insulin infusion (CSII). The pharmacokinetic profile was similar to that of another rapid-acting insulin analogue, insulin lispro, on the basis of published information for that agent. Pharmacodynamic studies show a smaller excursion of postprandial glucose with insulin aspart injected subcutaneously just before the meal compared with soluble human insulin injected 30 minutes before the meal in patients with type 1 diabetes mellitus, and an equivalent control in patients with type 2 diabetes displaying residual insulin production. In a treatment study, glucose excursions evaluated from 24-hour glucose profiles showed less variability with insulin aspart compared with human insulin. Adverse events, including hypoglycaemia-induced ventricular repolarisation and hypoglycaemic threshold and awareness, did not differ between insulin aspart and human insulin. The available data suggest that subcutaneous injections of insulin aspart just before meals better mimic the endogenous insulin profile in blood compared with human insulin, resulting in improved glucose control in a meal-related insulin regimen. This review summarises the clinical pharmacokinetics and pharmacodynamics of insulin aspart in relation to human insulin and insulin lispro.

Binding and degradation of 125I-insulin by isolated rat renal brush border membranes: evidence for low affinity, high capacity insulin recognition sites

The kidney plays a major role in the handling of circulating insulin in the blood, primarily via reuptake of filtered insulin at the luminal brush border membrane. 125I-insulin associated with rat renal brush border membrane vesicles (BBV) in a time- and temperature-dependent manner accompanied by degradation of the hormone to trichloroacetic acid (TCA)-soluble fragments. Both association and degradation of 125I-insulin were linearly proportional to membrane protein concentration with virtually all of the degradative activity being membrane associated. Insulin, proinsulin and desoctapeptide insulin all inhibited the association and degradation of 125I-insulin by BBV, but these processes were not appreciably affected by the insulin-like growth factors IGF-I and IGF-II or by cytochrome c and lysozyme, low molecular weight, filterable, proteins, which are known to be reabsorbed in the renal tubules by luminal endocytosis. When the interaction of 125I-insulin with BBV was studied at various medium osmolarities (300-1100 mosM) to alter intravesicular space, association of the ligand with the vesicles was unaffected, but degradation of the ligand by the vesicles decreased progressively with increasing medium osmolarity. Therefore, association of 125I-insulin to BBV represented binding of the ligand to the membrane surface and not uptake of the hormone or its degradation products into the vesicles. Attempts to crosslink 125I-insulin to a high-affinity insulin receptor using the bifunctional reagent disuccinimidyl suberate revealed only trace amounts of an 125I-insulin-receptor complex in brush border membrane vesicles in contrast to intact renal tubules where this complex was readily observed. Both binding and degradation of 125I-insulin by brush border membranes did not reach saturation even at concentrations of insulin approaching 10(-5) M. These results indicate the presence of low-affinity, high-capacity binding sites for 125I-insulin on renal brush border membranes which can clearly distinguish insulin from the insulin-like growth factors and other low molecular weight proteins and polypeptides, but which do not differentiate insulin from its analogues as do the biological receptors for the hormone. The properties and location of these binding sites make them attractive candidates for the sites at which insulin is reabsorbed in the renal tubule.

The effect of streptozotocin diabetes on insulin binding by isolated rat kidney tubules

Preparations of kidney tubules were isolated from rat kidney cortex and were demonstrated to possess specific binding sites for insulin. The binding was time-and temperature-dependent and the label was displaced by bovine insulin, A1-B29 dodecoyl insulin, proinsulin and insulin A- and B-chains in proportion to their relative activity. Cell-associated degradation was studied by incubating tubules in the presence of fatty-acid-free albumin. The tubules showed high insulin-degrading activity, which was dependent on temperature, time and cell concentration. The number and affinity of insulin receptors on tubules isolated from kidneys taken from streptozotocin-diabetic rats was not significantly different from tubules isolated from untreated control or insulin-treated diabetic rats. Diabetes did not alter the kinetics of insulin degradation by the tubules. This lack of response by the tubules to changes in the concentration of circulating insulin supports the hypothesis that the kidneys do not play an active role in modulating the rate of insulin removal from the circulation.