Coupling of L-type voltage-sensitive calcium channels to P2X2 purinoceptors in PC-12 cells

Abstract

Extracellular ATP elevates cytosolic Ca2+ by activating P2X and P2Y purinoceptors and voltage-sensitive Ca2+ channels (VCCCs) in PC-12 cells, thereby facilitating catecholamine secretion. We investigated the mechanism by which ATP activates VSCCs. 2-Methylthioadenosine 5′-triphosphate (2-MeS-ATP) and UTP were used as preferential activators of P2X and P2Y, respectively. Nifedipine inhibited the ATP- and 2-MeS-ATP-evoked cytosolic Ca2+concentration increase and [3H]norepinephrine secretion, but not the UTP-evoked responses. Studies with Ca2+ channel blockers indicated that L-type VSCCs were activated after the P2X activation. Mn2+ entry profiles and studies with thapsigargin revealed that Ca2+ entry, rather than Ca2+ release, was sensitive to nifedipine. Although P2X2 and P2X4 receptor mRNAs were detected, studies with pyridoxal phosphate-6-azophenyl-2′,4′-disulfonic acid revealed that P2X2 was mainly coupled to the L-type VSCCs. The inhibitory effect of nifedipine did not occur in the absence of extracellular Na+, suggesting that Na+ influx, which induces depolarization, was essential for the P2X2-mediated activation of VSCCs. We report that depolarization induced by Na+ entry through the P2X2 purinoceptors effectively activates L-type VSCCs in PC-12 cells.