Magnesium sulfate-induced relaxation of uterine arteries from pregnant and nonpregnant patients.

The effects of magnesium sulfate at different concentrations were investigated in isolated rings of uterine arteries from pregnant and nonpregnant patients. Addition of magnesium sulfate (0.5 to 9.6 mmol/L) to the suffusion medium containing the normal concentration of 1.2 mmol/L magnesium sulfate produced concentration-dependent relaxation of norepinephrine (1 mumol/L)-induced or potassium chloride (35 mmol/L)-induced contractions. Magnesium sulfate was about three times more potent in causing inhibition of potassium chloride-induced contractions in uterine arteries from pregnant patients than in those from nonpregnant patients. The concentrations that inhibited 50% of the contraction induced by potassium chloride were 0.6 +/- 0.2 mmol/L (n = 4) in the arteries from pregnant patients and 1.6 +/- 0.2 mmol/L (n = 4) in the arteries from nonpregnant patients. At high concentrations (4.8 and 9.6 mmol/L), magnesium sulfate also was more potent in inhibiting norepinephrine-induced contractions in arteries from pregnant women than in arteries from nonpregnant women. The magnesium sulfate-induced relaxation was almost completely inhibited by calcium chloride (2 mmol/L) added to the suffusion medium containing the normal concentration of 2.5 mmol/L calcium chloride but was not affected by indomethacin (5 mumol/L), methylene blue (10 mumol/L), or removal of the endothelium. The results show that magnesium sulfate, at therapeutic blood concentrations, acts as a potent dilator of human uterine arteries, especially those from pregnant patients. The results are consistent with the view that magnesium sulfate may facilitate uteroplacental perfusion.