The calcium channel current of pregnant rat single myometrial cells in short‐term primary culture.

1. The passive and active electrical properties of pregnant rat single myometrial cells in short‐term primary culture were analysed using a single‐electrode voltage or current clamp. 2. Action potentials and membrane currents were recorded in the presence of tetraethylammonium chloride and 4‐aminopyridine (10 mM each) and with Cs+ solution (4 M) in the microelectrode. 3. The voltage dependence, the action of Ca2+ antagonists and the effects of Sr2+ or Ba2+ substitution were studied. The peak Ca2+ current density was in the range 15‐20 microA/cm2 in 10 mM‐Ca2+ solution. 4. According to both measurement of the reversal potential of Ca2+ channel currents and comparison of the inward currents after correction for changing surface charge, the relative selectivity sequence of the Ca2+ channel for divalent cations was Ca2+ greater than Sr2+ = Ba2+. 5. The decay of Ca2+ channel current during a maintained depolarization was slowed when external Ca2+ was replaced by Sr2+ or Ba2+. The decay reflected an inactivation of Ca2+ channel conductance, as assessed by the decreased amplitude of inward tail currents following progressively longer depolarizations and the stable value of the reversal potential when Ca2+ channel current was increased during conditioning pulses. 6. Voltage‐dependent inactivation was illustrated by inactivation of outward Ca2+ channel current due to K+ and/or Cs+ efflux with external Ba2+ or in the absence of any permeant divalent cation. 7. The relationship between inactivation and the intracellular Ca2+ concentration was assessed by a double‐pulse method. Conditioning pulses that produced maximal Ca2+ current induced maximal inactivation; with stronger depolarizations, inactivation decreased but was not completely prevented at the expected Ca2+ reversal potential. Increasing the amount of Ca2+ entering the cell during the pre‐pulse reduced both amplitude and kinetics of test Ca2+ currents. These results were not observed with Ba2+ as the charge carrier. 8. Ca2+ channel current inactivation was best fitted by a two‐exponential function. The fast time constant of inactivation was larger in Ba2+ solution than in Ca2+ solution but both time constants showed little variation with membrane potential. The slow time constants of inactivation were steeply voltage dependent.(ABSTRACT TRUNCATED AT 400 WORDS)