The MCR and placental permeability to oxytocin were determined in chronically catheterized pregnant sheep. Simultaneous maternal and fetal plasma oxytocin (OT) concentrations were measured by RIA before and during continuous infusion of synthetic OT to steady state conditions. Baseline fetal plasma OT concentrations were significantly higher than simultaneously collected maternal concentrations (1.6 ± 0.13 vs. 1.1 ± 0.13 μU/ml, respectively; P < 0.01). Mean fetal OT MCRs were 12.0 ± 1.35 and 12.1 ± 1.09 ml/kg · min at OT infusion rates of 64 and 640 μU/kg · min. Mean maternal MCRs were 12.1 ± 2.64 and 12.4 ± 1.38 ml/kg · min at OT infusion rates of 80 and 800 μU/kg · min. Uterine contractions were induced by maternal OT infusion of 800 μU/kg · min but not by lower infusion rates; no uterine contractions were induced by fetal OT infusion. OT did not appear to cross the placenta in either direction under the present study conditions.
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