Modification by magnesium of the excitatory effect of oxytocin in electrical and mechanical activities of pregnant human myometrium.

The modification by magnesium of the excitatory effect of oxytocin (10(-5)-10(-2) U/mL) on electrical and mechanical activities of pregnant human myometrium was examined. The excitatory effect of oxytocin was enhanced by external magnesium, and the dose-response curve between oxytocin and relative tension in the presence of 118 mM potassium in tiny muscle strips shifted to the left with increases in magnesium from 0 to 2.4 mM. Oxytocin potentiates spontaneous contractions by enhancing the plateau part of action potentials, and the plateau potential induced in 2.4-mM magnesium was larger than that in magnesium-free solution. In potassium contracture experiments, the muscle contraction was potentiated in accordance with the concentration of preloaded magnesium when 10(-3) U/mL oxytocin was added at the tonic phase. These results suggest that magnesium might primarily potentiate the excitatory effect of oxytocin in electrical and mechanical activities of pregnant human myometrium at superficial sites of the plasma membrane, allowing the possibility of its intracellular action.