EFFECTS OF ATP AND UTP ON [CA-2+]I, MEMBRANE-POTENTIAL AND FORCE IN ISOLATED RAT SMALL ARTERIES

We have investigated excitation-contraction coupling mechanisms associated with the activation of purinoceptors and putative pyrimidinoceptors by assessing the effects of ATP and UTP on cytoplasmic Ca2+ activity ([Ca2+]i), membrane potential (E(m)) and force in rat mesenteric small arteries. UTP induced a sustained concentration-dependent contractions, closely associated with concentration-dependent increases in [Ca2+]i. Superfusion with 0.1 mM UTP caused a sustained depolarisation of 12 +/- 1 mV (SE, n = 8). In Ca2+-free medium, the increase in [Ca2+]i and the contraction obtained with UTP (1 mM) were both transient and were inhibited by prior exposure to noradrenaline (NA). In vessels depolarised with KCl, UTP caused no change in E(m), but a sustained increase in force and a transient increase in [Ca2+]i were induced, leading to an increased force/[Ca2+]i ratio. Similar effects on [Ca2+]i, E(m) and force were observed with ATP; but the effect of ATP on force was transient, whereas the effect on [Ca2+]i and E(m) declined only slowly. There was no cross-tachyphylaxis between the responses to ATP and UTP: in the presence of 1 mM of either, the other drug induced contractions in low concentrations, as if they acted through distinct receptors. The results suggest that both UTP and probably ATP release intracellular Ca2+, possibly from the stores emptied by NA. The sustained response to UTP appears to be due to an influx of extracellular Ca2+. UTP but not ATP was found to enhance the force-generating effect of [Ca2+]i.