Reduction of the cytosolic calcium activity in clonal insulin-releasing cells exposed to glucose

Abstract

The cytosolic Ca²⁺ activity of insulin-releasing clonal cells (RINmSF) was studied with the intracellular fluorescent indicator quin-2. When the extracellular Ca²⁺ concentration was 1 mM, the basal cytosolic Ca²⁺ activity was 101±5 nM. Depolarization with 25 mM K⁺ increased this Ca²⁺ activity to at least 318 nM, an effect completely reversed by the voltage-dependent channel blocker D-600. In the presence of K⁺ alone these channels appeared to have a half-life of 6.7±0.8 min. In contrast to the action of K⁺, exposure of the RINmSF cells to 4 mM glucose resulted in a reduction of the cytosolic Ca²⁺ activity . This effect was observed during K⁺ depolarization but was more pronounced under basal conditions when it amounted to 20%. The data provide the first direct evidence that glucose can decrease the cytosolic Ca²⁺ activity in β-cells. Unlike the case in normal β-cells the glucose effect on the voltage-dependent Ca²⁺ channels in the RINmSF cells is apparently not sufficient to overcome the intracellular buffering of Ca²⁺. A defective depolarization is therefore a probable cause of the failing insulin secretion of RINmSF cells exposed to glucose.