Effect of extracellular ATP on cytosolic Ca2+ concentration in rat pulmonary artery myocytes

Changes in cytosolic free Ca2+ concentration ([Ca2+](i)) induced by ATP and other P-2 purinoceptor agonists were investigated using indo 1 microspectrofluorimetry in single smooth muscle cells of the rat pulmonary artery. ATP (100 mu M, 30 s) induced 3-4 cyclic rises in [Ca2+](i) of decreasing amplitude. The first peak reached 743 +/- 24 nM from the resting value of 103 +/- 5 nM (n = 86). In similar to 50% of the cells, the ATP-induced [Ca2+](i) oscillations were accompanied by a small but maintained rise in [Ca2+](i). In a series of 10 cells, the amplitude of this rise averaged 41 +/- 9 nM. The small rise 1) was also induced by 2-methylthio-ATP (2-MeS-ATP) and alpha,beta-methylene-ATP (alpha,beta-MeATP), 2) was insensitive to thapsigargin (TG, 1 mu M), and 3) was abolished by the removal of external Ca2+. ATP-induced [Ca2+](i) oscillations 1) were not abolished in the absence of external Ca2+, 2) were suppressed by treatment of the cells with TG (1 mu M), and 3) were mimicked by UTP but not by 2-MeS-ATP or alpha,beta-MeATP. Both the number of cells that responded by [Ca2+](i) oscillations and the maximal amplitude of the response depended on the agonist (ATP or UTP) concentration. The ATP-induced [Ca2+](i) oscillations were not modified by tetracaine (500 mu M) but were inhibited by forskolin (1 mu M) and by phorbol 12,13-dibutyrate (PDB, 0.03 mu M) The effect of PDB was reversed by the protein kinase C antagonist calphostin C (0.01 mu M). These results suggest that the ATP-induced [Ca2+](i) rise is mediated by the activation of P-2x and P-2u purinoceptors. Ca2+ entry through the P-2x receptor channels produces a small and maintained [Ca2+](i) rise. [Ca2+](i) rise. Stimulation of P-2u purinoceptor induces [Ca2+](i) oscillations due to cyclic Ca2+ release from intracellular stores through inositol trisphosphate receptor channels.