Modulation by cytosolic pH of calcium and rubidium fluxes in rat pancreatic islets

Abstract

Cytosolic pH (pH i ) of pancreatic islet cells was assessed using the fluorescent dye 2'7'-biscarboxyethyl-5'(6')-carboxyfluorescein (BCECF). pH i was rapidly lowered by addition of the sodium salt of a weak acid or by treatment with amiloride. In the latter case, no recovery of pH i occurred. NH 4Cl produced a rise in pH i . Stimulation of islet cells with glyceraldehyde produced a sustained fall in pH i , whereas glucose and α-ketoisocaproate caused a small, gradual rise in pH i . Intracellular acidification, particularly with amiloride, resulted in an immediate potentiation of glucose-induced insulin secretion from perifused islets. In the case of weak acid treatment, subsequent removal of the weak acid produced a paradoxical stimulation of insulin release which was not observed upon removal of amiloride. NH 4Cl produced a transient stimulation followed by a reduction in the rate of glucose-induced insulin secretion. A reduction in pH i , either in response to weak acid or amiloride treatment, was associated with a diminution in the rate of efflux of 86Rb + and of 45Ca 2+. Removal of weak acid produced a marked “rebound” stimulation of 86Rb + and 45Ca 2+ efflux. Treatment of islets with NH 4Cl, either in the presence or absence of glucose or Ca 2+, resulted in a marked stimulation of efflux of 86Rb + and 86Ca 2+. The stimulatory effect of NH 4Cl on 45Ca 2+ efflux was markedly impaired in the absence of Na +. It is concluded that pH i can influence the secretory activity of pancreatic islets, possibly via effects on potassium permeability and sodium-calcium exchange across the plasma membrane, resulting in altered mobilisation of calcium in the islet cell. However, it is unlikely that glucose or other nutrient stimuli activate islets solely via an effect on pH i .