Mechanisms of [Ca2+]i elevation following P2X receptor activation in the guinea-pig small mesenteric artery myocytes.

BACKGROUND There is growing evidence suggesting involvement of L-type voltage-gated Ca2+ channels (VGCCs) in purinergic signaling mechanisms. However, detailed interplay between VGCCs and P2X receptors in intracellular Ca2+ mobilization is not well understood. This study examined relative contribution of the Ca2+ entry mechanisms and induced by this entry Ca2+ release from the intracellular stores engaged by activation of P2X receptors in smooth muscle cells (SMCs) from the guinea-pig small mesenteric arteries. METHODS P2X receptors were stimulated by the brief local application of αβ-meATP and changes in [Ca2+]i were monitored in fluo-3 loaded SMCs using fast x-y confocal Ca2+ imaging. The effects of the block of L-type VGCCs and/or depletion of the intracellular Ca2+ stores on αβ-meATP-induced [Ca2+]i transients were analyzed. RESULTS Our analysis revealed that Ca2+ entry via L-type VGCCs is augmented by the Ca2+-induced Ca2+ release significantly more than Ca2+ entry via P2X receptors, even though net Ca2+ influxes provided by the two mechanisms are not significantly different. CONCLUSIONS Thus, arterial SMCs upon P2X receptor activation employ an effective mechanism of the Ca2+ signal amplification, the major component of which is the Ca2+ release from the SR activated by Ca2+ influx via L-type VGCCs. This signaling pathway is engaged by depolarization of the myocyte membrane resulting from activation of P2X receptors, which, being Ca2+ permeable, per se form less effective Ca2+ signaling pathway. This study, therefore, rescales potential targets for therapeutic intervention in purinergic control of vascular tone.