The effects of ATP and α,β-methylene-ATP on cytosolic Ca2+ level and force in rat isolated aorta

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Abstract
1 The effects of a non-selective P2-receptor agonist ATP and a selective P2x-receptor agonist α,β-methylene-ATP on intracellular free Ca2+ level ([Ca2+]i) and force were examined in rat isolated aorta without endothelium. 2 Both ATP (1–1000 μm) and α,β-methylene-ATP (0.1–100 μm) induced transient increase followed by small sustained increase in [Ca2+]i in a concentration-dependent manner. Compared with the force induced by a high concentration of KCl, the force induced by α,β-methylene-ATP was smaller and that induced by ATP was much smaller at a given [Ca2+]i. 3 An L-type Ca2+ channel blocker, verapamil (10 μm), completely inhibited the high K+-stimulated [Ca2+]i and force. Verapamil partially inhibited the transient and sustained increases in [Ca2+]i induced by 10 μm α,β-methylene-ATP and the sustained increase but not the transient increase induced by 1 mm ATP. 4 In the absence of extracellular Ca2+ (with 0.5 mm EGTA) 1 mm ATP caused transient increase in [Ca2+]i while 10 μm α,β-methylene-ATP was ineffective 5 ATP, but not α,β-methylene-ATP, increased the tissue adenosine 3′:5′-cyclic monophosphate (cyclic AMP) level. 6 These data suggest that ATP and α,β-methylene-ATP increase [Ca2+]i by an activation of both L-type and non-L-type Ca2+ channels. In addition, ATP, but not α,β-methylene-ATP, increases [Ca2+]i by a release of Ca2+ from an intracellular Ca2+ store. Possible reasons are discussed as to why the increase in [Ca2+]i due to ATP and α,β-methylene-ATP resulted in only a small contraction.