Modulation of Ca2+ oscillations by phosphorylation of Ins(1,4,5)P3 receptors

Activation of InsP₃Rs (InsP₃ receptors) represents the major mechanism underlying intracellular calcium release in non-excitable cells such as hepatocytes and exocrine cells from the pancreas and salivary glands. Modulation of calcium release through InsP₃Rs is therefore a major route whereby the temporal and spatial characteristics of calcium waves and oscillations can potentially be ‘shaped’. In this study, the functional consequences of phosphoregulation of InsP₃Rs were investigated. Pancreatic and parotid acinar cells express all three types of InsP₃R in differing abundance, and all are potential substrates for phosphoregulation. PKA (protein kinase A)-mediated phosphorylation of InsP₃Rs in pancreatic acinar cells resulted in slowed kinetics of calcium release following photo-release of InsP₃. In contrast, activation of PKA in parotid cells resulted in a marked potentiation of calcium release. In pancreatic acinar cells the predominant InsP₃R isoform phosphorylated was the type 3 receptor, while the type 2 receptor was markedly phosphorylated in parotid acinar cells. In order to further decipher the effects of phosphorylation on individual InsP₃R subtypes, DT-40 cell lines expressing homotetramers of a single isoform of InsP₃R were utilized. These data demonstrate that phosphoregulation of InsP₃Rs results in subtype-specific effects and may play a role in the specificity of calcium signals by ‘shaping’ the spatio-temporal profile of the response.