Desensitization of insulin secretory response to imidazolines, tolbutamide, and quinine: II. Electrophysiological and fluorimetric studies

Abstract

Prolonged in vitro exposure (18 h) of pancreatic islets to insulin secretagogues that block ATP-dependent K + channels (K ATP channels), such as sulfonylureas, imidazolines, and quinine, induced a desensitization of insulin secretion (Rustenbeck et al., pages 1685–1694, this issue). To elucidate the underlying mechanisms, K ATP channel activity, plasma membrane potential and the cytosolic Ca 2+ concentration ([Ca 2+] i) were measured in mouse single B-cells. In B-cells desensitized by phentolamine or quinine (100 μM each) K ATP channel activity was virtually absent and could not be elicited by diazoxide. Desensitization by alinidine (100 μM) induced a marked reduction of K ATP channel activity, which could be reversed by diazoxide, whereas exposure to idazoxan (100 μM) or tolbutamide (500 μM) had no lasting effect on K ATP channel activity. Correspondingly, phentolamine-, alinidine-, and quinine-desensitized B-cells were markedly depolarized, whereas B-cells that had been exposed to tolbutamide or idazoxan had an unchanged resting membrane potential. The increase in [Ca 2+] i normally elicited by phentolamine and alinidine was suppressed after desensitization by these compounds, whereas the [Ca 2+] i increase by re-exposure to quinine was markedly reduced and that by tolbutamide only minimally affected as compared with control-cultured B-cells. The increase in [Ca 2+] i elicited by a K + depolarization was diminished in secretagogue-pretreated B-cells, the extent depending on the secretagogue. This effect was closely correlated with the degree of depolarization after pretreatment with the respective secretagogue. In conclusion, the apparently uniform desensitization of secretion by K ATP channel blockers is due to different effects at two stages located distally in the stimulus-secretion coupling: either at the stage of [Ca 2+] i regulation, where the increase is depressed as a consequence of a persistent depolarization (e.g. in the case of phentolamine or alinidine) and/or at the stage of exocytosis, which responds only weakly to substantial increases in [Ca 2+] i (in the case of tolbutamide).