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)
