Interplay of Ca²⁺ and cAMP signaling in the insulin-secreting MIN6 β-cell line

Ca²⁺ and cAMP are important second messengers that regulate multiple cellular processes. Although previous studies have suggested direct interactions between Ca²⁺ and cAMP signaling pathways, the underlying mechanisms remain unresolved. In particular, direct evidence for Ca ²⁺-regulated cAMP production in living cells is incomplete. Genetically encoded fluorescence resonance energy transfer-based biosensors have made possible real-time imaging of spatial and temporal gradients of intracellular cAMP concentration in single living cells. Here, we used confocal microscopy, fluorescence resonance energy transfer, and insulin-secreting MIN6 cells expressing Epac1-camps, a biosynthetic unimolecular cAMP indicator, to better understand the role of intracellular Ca²⁺ in cAMP production. We report that depolarization with high external K⁺, tolbutamide, or glucose caused a rapid increase in cAMP that was dependent on extracellular Ca²⁺ and inhibited by nitrendipine, a Ca²⁺ channel blocker, or 2′,5′-dideoxyadenosine, a P-site antagonist of transmembrane adenylate cyclases. Stimulation of MIN6 cells with glucose in the presence of tetraethylammonium chloride generated concomitant Ca²⁺ and cAMP oscillations that were abolished in the absence of extracellular Ca²⁺ and blocked by 2′,5′-dideoxyadenosine or 3-isobutyl-1-methylxanthine, an inhibitor of phosphodiesterase. Simultaneous measurements of Ca²⁺ and cAMP concentrations with Fura-2 and Epac1-camps, respectively, revealed a close temporal and causal interrelationship between the increases in cytoplasmic Ca²⁺ and cAMP levels following membrane depolarization. These findings indicate highly coordinated interplay between Ca²⁺ and cAMP signaling in electrically excitable endocrine cells and suggest that Ca²⁺-dependent cAMP oscillations are derived from an increase in adenylate cyclase activity and periodic activation and inactivation of cAMP-hydrolyzing phosphodiesterase.