FAILURE OF MAGNESIUM-SULFATE INFUSION TO INHIBIT UTERINE ACTIVITY IN PREGNANT SHEEP

OBJECTIVES: Our purpose was to determine the effect of magnesium sulfa te infusion on nonlabor uterine contractures and corticotropin-induced preterm uterine contractions in pregnant sheep. STUDY DESIGN: Fetal a nd maternal vascular catheters and uterine electromyographic electrode s were surgically placed in 15 pregnant sheep between 118 and 125 days ' gestation. After 3 to 5 days of recovery, magnesium sulfate was infu sed 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 elec tromyographic activity, amniotic pressure, fetal heart rate, blood pre ssure, and tracheal pressure were made. Maternal and fetal magnesium, calcium, albumin concentrations, and blood gases were determined befor e 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 befo re the bolus) did not change with the maternal infusion. In ewes not i n labor, uterine contractures occurred 3.7 +/- 0.7 times per 2 hours b efore and did not change significantly with the infusion of magnesium sulfate. During corticotropin-induced preterm labor uterine contractio ns were present 13 +/- 3.2 times per hour before infusions and were un changed by infusion of magnesium sulfate to the ewes. CONCLUSIONS: Mag nesium sulfate infusion in pregnant sheep has no effect on either nonl abor uterine contractures or on corticotropin-induced preterm uterine contractions.