The inositol 1,4,5-trisphosphate receptor (InsP 3 R) is an intracellular Ca 2+ channel that plays a role in the regulation of insulin secretion. In rat isolated pancreatic islets the expression of types I, II and III InsP 3 R mRNA was identified by reverse transcriptase-polymerase chain reaction and confirmed by cDNA cloning and sequencing. The islet ratios of types I, II and III InsP 3 R mRNA to -actin mRNA were 0·08 0·02, 0·08 0·03 and 0·25 0·04 respectively. Types I, II and III InsP 3 R mRNA were also expressed in rat (RINm5F) and mouse ( HC9) pancreatic -cell lines, and rat cerebellum. Type III InsP 3 R mRNA was quantitatively the most abundant form in rat islets and RINm5F cells. In HC9 cells, types II and III InsP 3 R mRNA were expressed at similar levels, and in much greater abundance than type I mRNA. Type III was the least abundant InsP 3 R mRNA in cerebellum. Culture of HC9 cells for 5 days at 2·8 and 25 mM glucose, or RINm5F cells for 7 days at 5·5 and 20 mM glucose, resulted in significantly enhanced expression of type III, but not types I and II, InsP 3 R mRNA in the cells at the higher glucose concentrations. During short-term (0·5-2 h) incubations, HC9 cell type III InsP 3 R mRNA levels increased in response to glucose in a timeand concentration-dependent manner. Actinomycin D inhibited the glucose response. -Ketoisocaproic acid also stimulated HC9 cell type III InsP 3 R mRNA expression in a concentration-dependent manner, whereas 2-deoxyglucose and 3-Omethylglucose were without effect. The different levels of expression of mRNA for three InsP 3 R isoforms in islets and insulinoma cells, and the influence of glucose and -ketoisocaproic acid on the expression of type III mRNA, suggests that nutrient metabolism plays a role in the regulation of this gene and that the function of InsP 3 R subtypes may be unique with each playing a distinct role in -cell signal transduction and insulin secretion.

The Sp-isomer of 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole-3',5'-cyclic monophosphorothioate (Sp-5,6-DCl-cBIMPS) at micromolar concentrations was a more potent stimulus for insulin release than 8-bromo-cyclic (c) AMP in isolated pancreatic islets of the rat. Sp-5,6-DCl-cBIMPS increased basal secretion, and potentiated glucose-stimulated insulin release to levels similar to those evoked by glucagon. A ten-fold higher concentration of 8-bromo-cAMP was required to mimic the potentiating effects of Sp-5,6-DCl-cBIMPS. Neither 8-para-chlorophenylthio-cGMP, 8-bromo-cGMP, nor dibutyryl-cGMP affected insulin release. Thus, Sp-5,6-DCl-cBIMPS is a potent and specific stimulus for cAMP-mediated insulin release.