Inactivation of the voltage‐dependent Ca2+ channel current in smooth muscle cells isolated from the guinea‐pig detrusor.

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Abstract
1. Whole-cell voltage clamp techniques were applied to single smooth muscle cells enzymatically dissociated from guinea-pig urinary bladder. The inactivation and recovery of voltage-dependent Ca2+ channel currents were examined by manipulating the membrane potential over a wide range and by changing the extracellular divalent cation concentrations. 2. After exposing the cells to conditioning potentials (-100 to +80 mV in 20 mV increments), the degree of inactivation was estimated by stepping to a 0 mV test potential. In the presence of 2.5 mM Ca2+, the inactivation of the current was U-shaped with respect to the conditioning potential, with maximum inactivation at 0 mV. The maximal inactivation was 60 and 90% after conditioning durations of 0.8 and 5 s, respectively. The U-shaped curve is characteristic of Ca(2+)-dependent inactivation. When conditioning potentials of +80 mV with either duration were applied, the inward current at the test potential and the subsequent tail current on returning to the holding potential were larger than in control conditions (when the conditioning potential = the holding potential, -60 mV). 3. A U-shaped inactivation curve was also observed in the presence of 2.5 mM Ba2+. The inactivation was maximal with a conditioning potential of about -20 mV, and the inactivation was smaller than seen with Ca2+ entry. 4. Paired-pulse protocols were applied to examine the voltage dependence of recovery of the Ca2+ inward current. After the inward current had been inactivated during a 100 ms depolarization at 0 mV, it took 700 ms at -60 mV for nearly complete recovery of the current. Recovery was also observed at +80 mV. When the potential of the paired pulses was increased to +20 mV, less recovery was seen when the interpulse potential was at +80 mV. When a longer (3 s) depolarization was applied, the peak amplitude of the inward current took much longer to recover, and had not completely recovered after 4 s at either of the interpulse potentials, although recovery was greater with an interpulse potential of -60 mV than with one of +80 mV. Similar recoveries were observed in the presence of Ba2+. 5. During a long depolarization (8 s, 0 mV), the effects of rapid changes in the extracellular solution were examined. Partial recovery of the inward current occurred after a period in which Ca2+ was replaced with Mg2+. This recovery was not observed in the presence of Ba2+.(ABSTRACT TRUNCATED AT 400 WORDS)

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