MAGNESIUM-SULFATE INHIBITS OXYTOCIN-INDUCED CALCIUM MOBILIZATION IN HUMAN PUERPERAL MYOMETRIAL CELLS - POSSIBLE INVOLVEMENT OF INTRACELLULAR FREE MAGNESIUM CONCENTRATION

OBJECTIVE: Our purpose was to elucidate the mechanisms of the tocolytic action of Mg2+ on the human myometrium. STUDY DESIGN: The effects of extracellular Mg2+ on oxytocin-induced increases in the intracellular free Ca2+ concentration in human puerperal myometrial cells were examined by means of indo-1-AM. The changes in the intracellular free Mg2+ concentration under various conditions were also measured with Mg2+-fura-2-AM. RESULTS: The increase in intracellular free Ca2+ concentration induced by oxytocin was reduced to 26% of that in the normal solution 20 minutes after replacement of the normal solution with an extracellular Mg2+ Solution, 10 mmol/L. When extracellular Ca2+ was removed, the increase in intracellular free Ca2+ concentration was not suppressed even 20 minutes after the replacement. A solution of extracellular Mg2+ concentration, 10 mmol/L, raised the intracellular free Mg2+ concentration gradually, by approximately 150% in 20 minutes, concomitant with the suppression of the response to oxytocin in the intracellular free Ca2+ concentration. CONCLUSION: High intracellular free Mg2+ concentration, caused by high extracellular Mg2+, is essential for suppression of oxytocin-induced Ca2+ influx across the cell membrane; this presumably results in inhibition of uterine contractions.