Role of different subtypes of P2 purinoceptor on cytosolic Ca2+ levels in rat aortic smooth muscle.

The role of different subtypes of P2 purinoceptors on cytosolic Ca2+ level ([Ca2+]i) was examined in vascular smooth muscle of rat aorta. alpha beta-Methylene-ATP (P2X agonist), 2-methylthio-ATP (P2Y agonist), UTP and ATP gamma S (P2U agonists), and ATP (nonselective P2 agonist) induced a transient increase followed by a small sustained increase in [Ca2+]i in a concentration dependent manner. Among these agonists, alpha beta-methylene-ATP was the most potent. In the absence of extracellular Ca2+ (with 0.5 mM EGTA), ATP, UTP and ATP gamma S induced a transient increase in [Ca2+]i whereas alpha beta-methylene-ATP and 2-methylthio-ATP were ineffective. ATP gamma S showed the highest potency in Ca(2+)-free solution. After desensitization of P2X purinoceptor, ATP, UTP and ATP gamma S induced a rapid increase in [Ca2+]i followed by a sustained increase while alpha beta-methylene-ATP and 2-methylthio-ATP were ineffective. These results suggest that Ca2+ release from the intracellular Ca2+ store is mediated by P2U purinoceptor whereas Ca2+ influx is mediated by both P2X and P2U purinoceptors in the rat aortic smooth muscle.