Effects of intravesicular H⁺ and extracellular H⁺ and Zn²⁺ on insulin secretion in pancreatic beta cells

The effects of extracellular Zn²⁺ and pH and intravesicular pH on insulin and 5-hydroxytryptamine (5-HT) secretion from pancreatic beta cells were investigated. Insulin and 5-HT secretion from single cells was detected by amperometry as a series of current spikes corresponding to detection of multimolecular packets secreted by exocytosis. Spike width was used as a measure of the kinetics of clearance from the cell and the area of spikes as a measure of amount released. Changes in extracellular pH from 6.9 to 7.9 caused insulin spikes to become narrower with no change in area, whereas the same treatments had no effect on 5-HT secretion. Treatment of cells with Bafilomycin A₁ or N-ethylmaleimide, both of which are expected to increase intravesicular pH by inhibiting V-type H⁺-ATPase, had no effect on 5-HT secretion but caused insulin spikes to become more narrow. These results indicate that exposure to high pH, whether intravesicular or extracellular, accelerates release of insulin during exocytosis without affecting the amount of insulin released. Increasing extracellular Zn²⁺ concentration from 0 to 25 μM increased the width and decreased the area of insulin spikes without affecting 5-HT secretion. Zn²⁺ effects were likely exerted through a common-ion effect on Zn²⁺-insulin dissociation. It was concluded that intravesicular storage conditions and extracellular ions can affect free insulin concentration in the vicinity of beta cells during secretion.