EXTERNAL ALKALINIZATION DECREASES INTRACELLULAR CA++ AND SPONTANEOUS CONTRACTIONS IN PREGNANT RAT MYOMETRIUM

OBJECTIVES: As plasma pH rises during pregnancy, the effect of raising external pH on spontaneous contractions in pregnant rat myometrium wa s investigated to test the hypothesis that elevated external pH depres ses contraction. STUDY DESIGN: Strips of longitudinal myometrium were loaded with SNARF (seminaphthorhodafluor) or Indo-1 for simultaneous i ntracellular pH or Ca++ and force measurements. Results were obtained from a minimum of five animals in each group, and significant differen ces were tested for by paired Student t tests. RESULTS: Raising the ex ternal pH significantly reduced spontaneous force and calcium transien t in the pregnant uterus. Raising the external pH led to a slow rise i n intracellular pH, but this could not account for the functional effe ct. K+ rubidium 86-labeled efflux rates were unaffected by external pH , suggesting no hyperpolarization. The Ca++ channel agonist Bay K8644 (5 mu mol/L) restored contractions abolished by raised external pH. CO NCLUSIONS: Raised external pH reduces spontaneous contractions in the pregnant rat uterus, probably by an external effect Ca++ entry. This e ffect may contribute to uterine quiescence before term.