ATP‐activated channels gate calcium entry in single smooth muscle cells dissociated from rabbit ear artery.

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Abstract
1. A combination of the techniques of microspectrofluorimetry and whole‐cell patch clamp was used to investigate changes in cytoplasmic Ca2+ concentration (Cai2+) in single arterial smooth muscle cells on external application of ATP. 2. ATP applied to cells held under voltage clamp at ‐‐60 mV evoked an inward current and an associated rise in Cai2+. In the absence of extracellular Ca2+. ATP‐activated inward currents were observed but there was no rise in Cai2+. 3. Pre‐treatment of cells with noradrenaline or caffeine did not prevent the rise in Cai2+ on subsequent application of ATP. 4. The ATP‐activated rise in Cai2+ was voltage dependent as outward currents evoked by ATP at positive membrane potentials were not associated with a change in Cai2+. 5. At ‐‐60 mV, the rise in Cai2+ due to ATP application was dependent on the magnitude of the ATP current response, such that Cai2+ increased by about 0.5 nM/pC charge transferred through ATP‐gated channels. 6. The results suggest that ATP‐gated channels in these cells admit sufficient Ca2+ in a physiological Ca2+ gradient to significantly elevate Cai2+. About 10% of the ATP‐gated current may be carried by Ca2+ ions. Thus the ATP‐activated channels have a dual excitatory function: depolarization due to Na+ entry promotes action potential discharge and voltage‐gated Ca2+ entry, and also direct entry of Ca2+ through the ATP‐activated channels.