Cyclic ADP-ribose and inositol 1,4,5-trisphosphate as alternate second messengers for intracellular Ca2+ mobilization in normal and diabetic β-cells

Intracellular Ca2+ mobilization occurs in a variety of cellular processes and is mediated by two major systems, the inositol 1,4,5-trisphosphate (IP3) and cyclic ADP-ribose (cADPR) systems, cADPR has been proposed to be a second messenger for insulin secretion induced by glucose in pancreatic beta-cells (Takasawa, S., Nata, K., Yonekura, H., and Okamoto, H. (1993) Science 259, 370-373). Here we show that the cADPR signal system for insulin secretion is replaced by the IP3 system in diabetic beta-cells such as ob/ob mouse islets and RINm5F cells. We measured the cADPR content in these beta-cells by radioimmunoassay and found that the increase of the cADPR content by glucose did not occur in ob/ob mouse islets and RINm5F cells, whereas the increased cADPR level by glucose was observed in normal rat and mouse islets, Microsomes of these diabetic beta-cells released Ca2+ in response to IP3 but not to cADPR. In the diabetic beta-cells, CD38 (ADP-ribosyl cyclase/cADPR hydrolase) and type 2 ryanodine receptor mRNAs were scarcely detected and, in contrast, an increased expression of IP3 receptor mRNAs was observed, The diabetic beta-cells secreted insulin rather by carbamylcholine than by glucose.