Ca2+ stores in insulin-secreting cells: lack of effect of cADP ribose.

Ca2+ stores were examined in several insulin secreting cell types by measuring uptake and release of Ca2+ by permeabilised cells. In pancreatic islet cells or INS-1 cells, < 20% of the ATP-dependent, thapsigargin-sensitive Ca2+ pool could be released by saturating concentrations of inositol (1,4,5)P3 (InsP3). InsP3 released > 60% of the thapsigargin-sensitive Ca2+ pool in RINm5F cells. The total Ca2+ content of the thapsigargin-sensitive pool was similar in each of these cell types. Neither cADP ribose (cADPR; 1 microM) nor caffeine (10 mM) caused significant Ca2+ release from any of the permeabilised insulin-secreting cell preparations. ATP elicited similar increases in intracellular Ca2+ concentration ([Ca2+]i) in single, living INS-1 and RINm5F cells, and similar fold increases in InsP3 levels in cell populations. The Ca2+ ATPase inhibitor thapsigargin, added after ATP, caused smaller [Ca2+]i increases in RINm5F than in INS-1 cells. This is consistent with the presence of a smaller InsP3-sensitive Ca2+ pool in living INS-1 cells. The data indicate that InsP3 receptors are present in only a small subfraction of the Ca2+ ATPase-containing Ca2+ stores in INS-1 and pancreatic beta-cells, and that cADP ribose/caffeine-sensitive Ca(2+)-induced Ca2+ release channels may be entirely absent from this endocrine cell type.