Protein kinase A increases the binding affinity and the Ca2+ release activity of the inositol 1,4,5-trisphosphate receptor type 3 in RINm5F cells

Background information. In endocrine cells, IP3R (inositol 1,4,5-trisphosphate receptor), a ligand-gated Ca2+ channel, plays an important role in the control of intracellular Ca2+ concentration. There are three subtypes Of IP3R that are distributed differentially among cell types. RINm5F cells express almost exclusively the IP3R-3 subtype. The purpose of the present study was to investigate the effect of PKA (protein kinase A) on the activity of IP3R-3 in RINm5F cells. Results. We show that immunoprecipitated IP3R-3 is a good substrate for PKA. Using a back-phosphorylation approach, we show that endogenous PKA phosphorylates IP3R-3 in intact RINm5F cells. [H-3]IP3 (inositol 1,4,5-trisphosphate) binding affinity and IP3-induced Ca2+ release activity were enhanced in permeabilized cells that were pre-treated with forskolin or PKA. The PKA-induced enhancement of IP3R-3 activity was also observed in intact RINm5F cells stimulated with carbachol and epidermal growth factor, two agonists that use different receptor types to activate phospholipase C. Conclusion. The results of the present study reveal a converging step where the CAMP and the Ca2+ signalling systems act co-operatively in endocrine cell responses to external stimuli.