Tocolytic agents act on calcium channel current in single smooth muscle cells of pregnant rat uterus

Effects of Ca2+ channel blockers and clinically important tocolytic agents, Mg2+ and beta-agonists, were examined on the Ca2+ channel current recorded from freshly isolated single pregnant rat myometrial cells using the whole-cell voltage clamp method. Nifedipine inhibited the Ca2+ channel currents (Ba2+ current) dose-dependently. Inhibition by nifedipine was greater at higher (more positive) holding potentials and higher command potentials. According to the modulated receptor hypothesis, Kd values for resting state and inactivated state of channel were calculated to be 150-300 and 1.2-6.8 nM, respectively (at a command potential of -20 mV). Mg2+ applied in the bath dose-dependently inhibited the Ca2+ current recorded with 2 mM Ca2+ (Ki = 12 mM, at a command potential of -10 mV). Inhibition by Mg2+ was weaker at the higher command potentials. Voltage dependency observed for the Mg2+ block may be due to a shift of the activation curve (Mg2+ neutralizes the outer surface charge) and/or due to the location of the binding site for Mg2+ in the Ca2+ channel electric field. In contrast, high dose of isoproterenol (10 microM) did not produce significant change in the Ca2+ current. In summary, nifedipine and Mg2+ inhibited the Ca2+ channel, with opposite voltage dependencies, whereas isoproterenol had no effect on the Ca2+ current. Thus, beta-agonist may relax the uterine muscle by mechanisms other than inhibition of the Ca2+ channels.