THE WHOLE-CELL CA-2+ 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. © 1990 The Physiological Society