Ca2+-induced Ca2+ release by activation of inositol 1,4,5-trisphosphate receptors in primary pancreatic β-cells

The effect of sarcoendoplasmic reticulum Ca2+-ATPase (SERCA) inhibition on the cytoplasmic Ca2+ concentration ([Ca2+]i) was studied in primary insulin-releasing pancreatic β-cells isolated from mice, rats and human subjects as well as in clonal rat insulinoma INS-1 cells. In Ca2+-deficient medium the individual primary β-cells reacted to the SERCA inhibitor cyclopiazonic acid (CPA) with a slow rise of [Ca2+]i followed by an explosive transient elevation. The [Ca2+]i transients were preferentially observed at low intracellular concentrations of the Ca2+ indicator fura-2 and were unaffected by pre-treatment with 100 μM ryanodine. Whereas 20 mM caffeine had no effect on basal [Ca2+]i or the slow rise in response to CPA, it completely prevented the CPA-induced [Ca2+]i transients as well as inositol 1,4,5-trisphosphate-mediated [Ca2+]i transients in response to carbachol. In striking contrast to the primary β-cells, caffeine readily mobilized intracellular Ca2+ in INS-1 cells under identical conditions, and such mobilization was prevented by ryanodine pre-treatment. The results indicate that leakage of Ca2+ from the endoplasmic reticulum after SERCA inhibition is feedback-accelerated by Ca2+-induced Ca2+ release (CICR). In primary pancreatic β-cells this CICR is due to activation of inositol 1,4,5-trisphosphate receptors. CICR by ryanodine receptor activation may be restricted to clonal β-cells.