Insulin secretion from isolated rat islets induced by the novel hypoglycemic agent A-4166, a derivative of D-phenylalanine

Chick Pancreatic B Islets as an Alternative In Vitro Model for Screening Insulin Secretagogues

Previously, we reported a simple technique to isolate functional B islets from chick pancreata with retention of their insulin secretory ability in response to glucose challenge. To test the hypothesis that chick B islets are equally good candidates as mammalian islets for screening hypoglycemics and insulin secretagogues, we compared the structural and functional status of chick B islets with those of normal and diabetic mouse islets. Pancreata from chick, normal (nondiabetic) mice, and diabetic mice were collected, fixed, and processed for histological analysis using Gomori stain to distinguish A and B cells from islets. Similarly isolated islets from these animals were treated with different concentrations of tolbutamide, a known insulin secretagogue, and glucose to study insulin release. Histological analysis of pancreata from chicks and normal mice revealed intact B cells, whereas those from diabetic mice were destroyed. The insulin secretory response of chick B islets against the tolbutamide and glucose challenge was comparable to that of normal mouse islets. However, diabetic mouse islets did not respond to glucose challenge, indicating impaired functionality. We have identified a critical window that lies within 5 to 6 d posthatching for isolating chick B islets showing maximum glucose responsiveness and insulin secretion. The previous reports on chicken pancreatic islets involve the use of 4- to 6-wk-old chicks in which islets were found to be nonresponsive to glucose and, hence, could not be used for testing insulin secretory activity. However, our data on B islets from 5- to 6-d-old chick pancreata is highly promising, as islets are responsive to insulin secretagogues. The present data thus indicates that chick B islets can be used as an alternative in vitro model for screening insulin secretagogue and hypoglycemics.

Interaction of nateglinide with K(ATP) channel in beta-cells underlies its unique insulinotropic action

Nateglinide is a novel insulinotropic agent for the treatment of type 2 diabetes. It is a D-phenylalanine derivative, chemically distinct from repaglinide and sulphonylureas (glyburide or glimepiride). Although each agent is known to stimulate insulin release via the signaling cascade initiated by closure of ATP-dependent K+ (K(ATP)) channels in pancreatic beta-cells, the pharmacological effect of nateglinide is reportedly fast-acting, short-lasting, sensitive to ambient glucose and more resistant to metabolic inhibition. The aim of the present study was to elucidate the molecular mechanism(s) underlying the distinct properties of the insulinotropic action of nateglinide. By using the patch-clamp methods, we comparatively characterized the potency and kinetics of the effect of these agents on K(ATP) channels in rat beta-cells at normal vs. elevated glucose and under physiological condition vs. experimentally induced metabolic inhibition. Our results demonstrated that the mode of the action of nateglinide on K(ATP) current was unique in (a) glucose dependency; (b) increased potency and efficacy under ATP depletion and uncoupling of mitochondrial oxidative phosphorylation than physiological condition; (c) substantially more rapid onset and offset kinetics. The data provide mechanistic rationale for the unique in vivo and ex vivo activity profile of nateglinide and may contribute to reduced hypoglycemic potential associated with excessive insulin secretion.