Failure of magnesium sulfate infusion to inhibit uterine activity in pregnant sheep

OBJECTIVES: Our purpose was to determine the effect of magnesium sulfate infusion on nonlabor uterine contractures and corticotropin-induced preterm uterine contractions in pregnant sheep. STUDY DESIGN: Fetal and maternal vascular catheters and uterine electromyographic electrodes were surgically placed in 15 pregnant sheep between 118 and 125 daysʹ gestation. After 3 to 5 days of recovery, magnesium sulfate was infused into 7 ewes with a 0.11 gm/kg bolus over 20 minutes, followed by 0.08 gm/kg/hr. In 8 animals labor was induced with use of an intrafetal corticotropin infusion, after which 4 ewes received magnesium sulfate and 4 received saline solution. Continuous recordings of uterine electromyographic activity, amniotic pressure, fetal heart rate, blood pressure, and tracheal pressure were made. Maternal and fetal magnesium, calcium, albumin concentrations, and blood gases were determined before and during the infusion. RESULTS: Maternal magnesium concentrations increased from an average of 0.94 ± 0.03 mmol/L to 2.73 ± 0.1 mmol/L at the end of the bolus, remaining elevated (2.44 ± 0.17 mmol/L) for 8 hours. Fetal magnesium concentrations (0.89 ± 0.03 mmol/L before the bolus) did not change with the maternal infusion. In ewes not in labor, uterine contractures occurred 3.7 ± 0.7 times per 2 hours before and did not change significantly with the infusion of magnesium sulfate. During corticotropin-induced preterm labor uterine contractions were present 13 ± 3.2 times per hour before infusions and were unchanged by infusion of magnesium sulfate to the ewes. CONCLUSIONS: Magnesium sulfate infusion in pregnant sheep has no effect on either nonlabor uterine contractures or on corticotropin-induced preterm uterine contractions. (Am J Obstet Gynecol 1997;177:185-9.)