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 adenosine triphosphate (ATP; 10 -9 to 10 -3 M) dose-dependently increases intracellular calcium ([Ca 2+ ] i ). The [Ca 2+ ] i transients measured by fura-2 fluorescence consist of peak and plateau phases with [Ca 2+ ] i values of 191.84 ± 5.67 nM (n = 10) and 91.67 ± 1.89 nM, respectively. In Ca 2+ -free solution, the peak phases persisted, but there was a loss of the plateau response, indicating an initial ATP-stimulated intracellular Ca 2+ release and a subsequent transarcolemmal Ca 2+ entry. Various agonists have been used to characterize the P2 purinoceptor subtype involved in the ATP-induced Ca 2+ transients. The rank order of potency was uridine triphosphate (UTP) > ATP > 2-meSATP > β,γ-meATP = α,β-meATP = adenosine = 0. To examine the refilling of ATP-sensitive stores, four repetitive 60-s ATP responses were produced throughout with a 5-min recovery period in between. Now the ATP peaks gradually declined in Ca 2+ -free solution, indicating the emptying of the stores. If, however, Ca 2+ entry was allowed in the refilling period (i.e., between the ATP pulses), the Ca 2+ peaks could be maintained or restored, respectively. The data suggest that the ATP-dependent [Ca 2+ ] i transients may be mediated via a UTP > ATP-activated P2Y purinoceptor subtype, mediating both an intracellular Ca 2+ release and a transarcolemmal Ca 2+ influx. The refilling of Ca 2+ stores may occur through the unstimulated membrane after agonist stimulation. A putative pathway may be a capacitative Ca 2+ entry induced on depletion of intracellular Ca 2+ stores.