The whole-cell Ca2+ channel current in single smooth muscle cells of the guinea-pig ureter.
1. Calcium channel currents were recorded in single, enzymatically isolated smooth muscle cells of the guinea-pig ureter using a single-electrode whole-cell voltage clamp technique. Calcium and barium currents through voltage-activated Ca2+ channels were recorded in cells dialysed with Cs(+)- or Na(+)-containing saline which suppressed K+ currents. 2. Inward currents in Ca2+ (1.5-7.5 mM) or Ba2+ (1.5-7.5 mM) were recorded at potentials positive to -50 to -30 mV. Inward currents were maximal at 0 mV in 1.5 mM-Ca2+ and at +10 mV in 7.5 mM-Ba2+. Current flow through Ca2+ channels in Cs(+)-filled cells (in 1.5 mM-Ca2+ or 7.5 mM-Ba2+) changed from inward to outward at potentials positive to +70 mV. In Na(+)-filled cells this reversal potential was between +50 and +60 mV. 3. Replacing Ca2+ or Ba2+ with Co2+ (1.5 mM) blocked all inward current flow through these Ca2+ channels; outward currents at potentials positive to +40 mV, however, were increased. Cadmium (100 microM) and nifedipine (0.1-10 microM) reduced both inward and outward current flow. 4. Calcium channel activation showed a sigmoidal relationship with membrane potential; the potential of half-maximal activation was -8.4 mV in 1.5 mM-Ca2+ and -10.8 mV in 7.5 mM-Ba2+. The maximum membrane conductance to Ca2+ (in 1.5 mM-Ca2+) was 2.57 nS/cell or approximately 0.05 mS/cm2. 5. Evidence for a voltage-dependent inactivation mechanism included (a) the time-dependent relaxation of the outward currents at potentials positive to the reversal potential and (b) a steady-state inactivation (f infinity (V] vs. membrane potential relationship (in 7.5 mM-Ba2+) which ranged between -80 and 0 mV, with a half-maximal availability at -40.5 mV. 6. The voltage dependencies of the inward current elicited from -80 and -30 mV were similar, suggesting that depolarization activated only L-type Ca2+ channels. 7. It was concluded that the processes controlling the time course of the Ca2+ current in single ureteral cells bathed in physiological concentrations of Ca2+ were mostly voltage-dependent.
Lang
R J
RJ
Department of Physiology, Monash University, Clayton, Victoria, Australia.
D000818:Animals; D001464:Barium / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D003035:Cobalt / Q000494:pharmacology; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007700:Kinetics; D008564:Membrane Potentials; D009130:Muscle, Smooth / Q000166:cytology / Q000378:metabolism*; D009543:Nifedipine / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism; D014513:Ureter / Q000166:cytology / Q000378:metabolism
D015220:Calcium Channels; D001464:Barium; D003035:Cobalt; D012964:Sodium; D009543:Nifedipine; D002118:Calcium