Changes of pH affect calcium currents but not outward potassium currents in rat myometrial cells

Spontaneous contraction of uterine smooth muscle is enhanced by alkalinization and depressed by acidification. We have investigated the ionic currents responsible for this in single myometrial cells. Intracellular acidification (20 mM butyrate) at constant external pH depressed the magnitude of the calcium current to 58+/-6% of control, but had little effect on outward currents. Similar but slower effects were also observed when the extracellular pH was lowered to 6.9 (56+/-9% of control). Correspondingly, when the intracellular or extracellular pH was elevated (20 mM NH4Cl or pH 7.9 respectively) the calcium current magnitude increased (165+/-15% in NH4Cl; 136+/-2% at pH 7.9) and there was, again, no effect on the outward currents. These observations are consistent with the effects of pH on spontaneous contractile activity being due to an effect on the membrane calcium current.