The Effect of Magnesium on Calcium Uptake and Contractility in the Human Myometrium

High concentrations of magnesium (12 and 24 mM) in the extracellular medium markedly inhibited both spontaneous activity and K+-induced contracture in strips of nonpregnant human myometrium. Net calcium influx measured by the uptake of 45Ca2+ in the myometrium was considerably decreased by high concentrations of magnesium. This was true for both resting (unstimulated) and K+-stimulated uptake of 45Ca2. While calcium uptake in K+-stimulated tissues exposed to 24 mM magnesium was significantly lower than in those exposed to 12 mM, no difference was found in the unstimulated tissues. These data indicate that the tocolytic action of magnesium most probably results from the inhibition of calcium entry into myometrial cells.