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.

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