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 M) elicited a transient inward current and increased [Ca²⁺]_i. These effects of ATP were inhibited by ,-methylene adenosine 5-triphosphate (AMPCPP, 10 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 M and 100 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.