Extracellular ATP increases [Ca2+](i) in primarily cultured pig coronary smooth muscle cells via a P2Y purinoceptor subtype

In primarily cultured pig coronary smooth muscle cells, extracellular adeno sine triphosphate (ATP; 10(-9) to 10(-3) M) dose-dependently increases intr acellular calcium ([Ca2+](i)). The [Ca2+](i) transients measured by fura-2 fluorescence consist of peak and plateau phases with [Ca2+](i) values of 19 1.84 +/- 5.67 nM (n = 10) and 91.67 +/- 1.89 nM, respectively. In Ca2+-free solution, the peak phases persisted, but there was a loss of the plateau r esponse, indicating an initial ATP-stimulated intracellular Ca2+ release an d a subsequent transarcolemmal Ca2+ entry. Various agonists have been used to characterize the P2 purinoceptor subtype involved in the ATP-induced Ca2 + transients. The rank order of potency was uridine triphosphate (UTP) > AT P >> 2-meSATP > beta,gamma-meATP = alpha,beta-meATP = adenosine = 0. To exa mine the refilling of ATP-sensitive stores, four repetitive 60-s ATP respon ses were produced throughout with a 5-min recovery period in between, Now t he ATP peaks gradually declined in Ca2+-free solution, indicating the empty ing of the stores. If, however, Ca2+ entry was allowed in the "refilling pe riod" (i.e., between the ATP pulses), the Ca2+ peaks could be maintained or restored, respectively. The data suggest that the ATP-dependent [Ca2+](i) transients may be mediated via a UTP > ATP-activated P2Y purinoceptor subty pe, mediating both an intracellular Ca2+ release and a transarcolemmal Ca2 influx. The refilling of Ca2+ stores may occur through the unstimulated me mbrane after agonist stimulation. A putative pathway may be a "capacitative " Ca2+ entry induced on depletion of intracellular Ca2+ stores.