Extracellular calcium influx activates adenylate cyclase 1 and potentiates insulin secretion in MIN6 cells

Intracellular cAMP and Ca2+ are important second messengers that regulate insulin secretion in pancreatic beta-cells; however, the molecular mechanism underlying their mutual interaction for exocytosis is not fully understood. In the present study, we investigated the interplay between intracellular cAMP and Ca2+ concentrations ([cAMP](i) and [Ca2+], respectively) in the pancreatic beta-cell line MIN6 using total internal reflection fluorescence microscopy. For measuring [cAMP](i), we developed a genetically encoded yellow fluorescent biosensor for cAMP [Flamindo (fluorescent cAMP indicator)], which changes fluorescence intensity with cAMP binding. Application of high-KCl or glucose to MIN6 cells induced the elevation of [cAMP](i) and exocytosis. Furthermore, application of an L-type Ca2+ channel agonist or ionomycin to induce extracellular Ca2+ influx evoked the elevation of [cAMP] whereas application of carbachol or thapsigargin, which mobilize Ca2+ from internal stores, did not evoke the elevation of [cAMP](i). We performed RT (reverse transcription)-PCR analysis and found that Ca2+-sensitive Adcy1 (adenylate cyclase 1) was expressed in MIN6 cells. Knockdown of endogenous ADCY1 by small interference RNA significantly suppressed glucose-induced exocytosis and the elevation of both [cAMP](i) and [Ca2+](i). Taken together, the findings of the present study demonstrate that ADCY1 plays an important role in the control of pancreatic beta-cell cAMP homoeostasis and insulin secretion.