A novel ryanodine receptor expressed in pancreatic islets by alternative splicing from type 2 ryanodine receptor gene

Cyclic ADP-ribose (cADPR), a potent Ca(2+) mobilizing intracellular messenger synthesized by CD38, regulates the opening of ryanodine receptors (RyRs). Increases in intracellular Ca(2+) concentrations in pancreatic islets, resulting from Ca(2+) mobilization from RyRs as well as Ca(2+) influx from extracellular sources, are important in insulin secretion by glucose. In the present study, by screening a rat islet cDNA library, we isolated a novel RyR cDNA (the islet-type RyR), which is generated from the RyR2 gene by alternative splicing of exons 4 and 75. When the expression vectors for the islet-type and the authentic RyRs were transfected into HEK293 cells, the islet-type RyR2 as well as the authentic one showed high affinity [(3)H]ryanodine binding. Intracellular Ca(2+) release in the islet-type RyR2-transfected cells was enhanced in the presence of cADPR but not in the authentic RyR2-transfected cells. The islet-type RyR2 mRNA was expressed in a variety of tissues such as in pancreatic islets, cerebrum, and cerebellum, whereas the authentic RyR2 mRNA was predominantly expressed in heart and aorta. These results suggest that the islet-type RyR2 may be an intracellular target for cADPR signaling.