Effect of extracellular phosphate on Ca²⁺ and K⁺ fluxes in pancreatic islets

Abstract

The idea that a lowering in cytosolic Ca²⁺ concentration may cause a decrease m K⁺ conductance in the pancreatic B-cell was tested by investigating the effect of a high extracellular phosphate concentration on ⁴⁵Ca and ⁸⁶Rb efflux from prelabelled rat pancreatic islets. Whether in the absence or presence of glucose, 20 mM phosphate tended to decrease ⁴⁵Ca efflux. This effect was not suppressed in the absence of extracellular Ca²⁺, at least in glucose-deprived islets, suggesting that it may reflect a fall in cytosolic Ca²⁺ concentration. The administration of phosphate failed, however, to decrease ⁸⁶Rb efflux from the islets. In the presence of extracellular Ca²⁺, 20 mM phosphate also failed to stimulate insulin release from islets perifused at low glucose concentration and inhibited insulin release stimulated by a high glucose concentration. These data indicate that the sequestration of Ca²⁺ in intracellular organelles and concomitant decrease in cytosolic Ca²⁺ concentration, as presumably provoked by a rise in extracellular phosphate concentration, is not sufficient to simulate the effect of glucose on K⁺ conductance.