Effects of amino acids on membrane potential and 86Rb+ fluxes in pancreatic beta-cells

Abstract

The membrane potential of beta-cells was studied with microelectrodes in mouse islets and their potassium permeability was evaluated by measuring 86Rb+ fluxes in rat islets. In the absence of glucose, L-leucine, its metabolite ketoisocaproate, and its nonmetabolized analogue 2-aminonorbornane-2-carboxylic acid (BCH) depolarized beta-cells and triggered bursts of electrical activity like glucose. The effect of leucine was weak, but was potentiated by a low concentration of glucose or by theophylline; the effect of ketoisocaproate was stronger and faster than that of an equimolar concentration of glucose. Arginine alone produced only a fast depolarization of beta-cells, insufficient to trigger electrical activity. Leucine and arginine potentiated the activity induced by glucose. In a glucose-free medium, alanine only slightly depolarized beta-cells, whereas isoleucine and phenylalanine had no effect. Leucine, ketoisocaproate, and BCH reversibly decreased 86Rb+ efflux from islets perifused in the absence of glucose and increased 86Rb+ uptake. By contrast, both in the absence or presence of glucose, arginine increased 86Rb+ efflux and decreased 86Rb+ uptake. It is proposed that leucine, ketoisocaproate, and BCH, as glucose, depolarize beta-cells by decreasing their potassium permeability, whereas arginine acts differently. The appearance of bursts of electrical activity with secretagogues unrelated to glucose suggests that they reflect an intrinsic property of the beta-cell membrane.

Nutrient Regulation of the Enteroinsular Axis And Insulin Secretion

“…Recent detailed studies using leucine, arginine, lysine and alanine have demonstrated three basic mechanisms which appear to underlie the effects of amino acids on insulin release (Hellman et al 1971;Henquin & Meissner, 1981Charles & Henquin, 1983;Henquin, 1987). Leucine is transported into the p-cells where it is efficiently metabolized, giving rise to qualitatively similar changes in membrane potential and ionic fluxes as described for glucose.…”

“…Whereas glucose and other nutrients (e.g. certain amino acids) are thought to stimulate insulin secretion primarily by increasing cytoplasmic Ca2+ (Henquin, 1987), neural and hormonal entero-insular stimuli appear to amplify insulin secretion through direct intracellular activation of adenylate cyclase or phospholipase C (Hellman, 1986;Turk et al 1987).…”

“…In view of the crucial role of glucose in modulating the insulinotropic potency of other stimulators, but mainly because of its direct stimulatory effect, its mechanism of action on the secretory behaviour of pancreatic ,&cells has been extensively investigated (Wollheim & Sharp, 198 1 ;Malaisse, 1983 ;Hellman, 1986;Henquin, 1987 ;Petersen & Findley, 1987).…”

“…In the presence of a continuing glucose stimulus, the repolarization is followed by successive cycles of depolarization-repolarization, with corresponding episodes of Ca2+ influx which are coupled to extrusion of preformed insulin from the p-granules into the extracellular space and ultimately the circulation. At very-high glucose concentrations (> 16 mmol/l) the pcell membrane remains depolarized for longer periods, with continuous spike activity as Ca2+ enters the cell and insulin discharge is sustained (Hellman, 1986;Henquin, 1987;Petersen & Findlay, 1987).…”

“…In contrast to leucine, arginine and lysine, alanine is only a weak stimulator of insulin release and is poorly metabolized by p-cells. It increases cytoplasmic Ca2+ by virtue of interference with Na+ influx (Henquin, 1987).…”