ROLE OF THE SODIUM-CALCIUM EXCHANGE MECHANISM AND THE EFFECT OF MAGNESIUM ON SODIUM-FREE AND HIGH-POTASSIUM CONTRACTURES IN PREGNANT HUMAN MYOMETRIUM

OBJECTIVE: Periodic uterine relaxation in labor relieves the fetus fro m hypoxia related to sustained contractions. A reduction in intracellu lar calcium is essential for relaxation. Therefore we aimed to clarify the extrusion mechanisms of intracellular calcium ions in pregnant hu man myometrium. STUDY DESIGN: Isometric contraction of small muscle bu ndles obtained from human myometrium at term was recorded. High-potass ium and sodium-free solutions could induce stable contractures that re vealed initial phasic contractions followed by tonic contractions. The effects of external calcium, external magnesium, nifedipine, and calc ium adenosine triphosphatase inhibitors on both contractures were then examined. RESULTS: Because maximum tonic contraction was induced by 4 8 mmol/L potassium, this concentration was used. Both contractures dep ended on external calcium concentrations. Nifedipine, 10(-7) mol/L inh ibited both initial phasic contractions and the tonic phase of the hig h-potassium contracture; however, it could not suppress the tonic phas e of the sodium-free contracture. Calcium adenosine triphosphatase inh ibitors, cyclopiazonic acid and lauryl sulfate, potentiated the tensio ns of tonic phases in both contractures; the effect of lauryl sulfate was stronger than that of cyclopiazonic acid. When external magnesium was gradually increased (from 0 to 9.6 mmol/L), both phasic contractio ns and the tonic phase of the high-potassium contracture transiently i ncreased, followed by a reduction of tension, whereas the tonic phase of the sodium-free contracture was markedly inhibited by magnesium in a dose-dependent manner. CONCLUSION: The presence of the sodium-calciu m exchange mechanism and the specific inhibitory effect of magnesium w ere indicated in human myometrium during pregnancy. These mechanisms m ight prevent the long tonic contractions, to protect the fetus from hy poxia during pregnancy and parturition.