ATP-activated channels gate calcium entry in single smooth muscle cells dissociated from rabbit ear artery.
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.
Benham
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CD
Department of Pharmacology, Smith Kline and French Research Ltd., Welwyn, Herts.
D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000032:analysis*; D015220:Calcium Channels / Q000187:drug effects*; D003600:Cytosol / Q000032:analysis*; D004431:Ear, External / Q000098:blood supply; D066298:In Vitro Techniques; D009131:Muscle, Smooth, Vascular / Q000032:analysis / Q000166:cytology / Q000187:drug effects*; D011817:Rabbits
D015220:Calcium Channels; D000255:Adenosine Triphosphate; D002118:Calcium