MECHANISM OF THE ATP-INDUCED RISE IN CYTOSOLIC CA2+ IN FRESHLY ISOLATED SMOOTH-MUSCLE CELLS FROM HUMAN SAPHENOUS-VEIN

Effects of exogenous adenosine 5'-triphosphate (ATP) were studied by measurements of intracellular Ca2+ concentration ([Ca2+](i)) and membrane currents in myocytes freshly isolated from the human saphenous vein. At a holding potential of -60 mV, ATP (10 mu M) elicited a transient inward current and increased [Ca2+](i). These effects of ATP were inhibited by alpha,beta-methylene adenosine 5'-triphosphate (AMP-CPP, 10 mu M). The ATP-gated current corresponded to a non-selective cation conductance allowing Ca2+ entry. The ATP-induced [Ca2+](i) rise was abolished in Ca2+-free solution and was reduced to 30.1 +/- 5.5% (n = 14) of the control response when ATP was applied immediately after caffeine, and to 23.7 +/- 3.8% (n = 11) in the presence of thapsigargin. The Ca2+-induced Ca2+ release blocker tetracaine inhibited the rise in [Ca2+](i) induced by both caffeine and ATP, with apparent inhibitory constants of 70 mu M and 100 mu M, respectively. Of the ATP-induced increase in [Ca2+](i) 29.3 +/- 3.9% (n = 8) was tetracaine resistant. It is concluded that the effects of ATP in human saphenous vein myocytes are only mediated by activation of P-2x receptor channels. The ATP-induced [Ca2+](i) rise is due to both Ca2+ entry and Ca2+ release activated by Ca2+ ions that enter the cell through P-2x receptor channels.