Ca²⁺-induced Ca²⁺ release in insulin-secreting cells

The sulphydryl reagent thimerosal (50 μM) released Ca²⁺ from a non-mitochondrial intracellular Ca²⁺ pool in a dose-dependent manner in permeabilized insulin-secreting RINm5F cells. This release was reversed after addition of the reducing agent dithiothreitol, Ca²⁺ was released from an Ins(1,4,5)P₃-insensitive pool, since release was observed even after depletion of the Ins(1,4,5)P₃-sensitive pool by a supramaximal dose of Ins(2,4,5)P₃ or thapsigargin. The Ins(1,4,5)P₃-sensitive pool remained essentially unaltered by thimerosal. Thimerosal-induced Ca²⁺ release was potentiated by caffeine. These findings suggest the existence of Ca²⁺-induced Ca²⁺ release also in insulin-secreting cells.