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Am J Physiol Heart Circ Physiol 278: H41-H49, 2000;
0363-6135/00 $5.00
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Vol. 278, Issue 1, H41-H49, January 2000

Increased myometrial contracture frequency at 96-140 days accelerates fetal cardiovascular maturation

Norio Shinozuka1,2, Andrew Yen1, and Peter W. Nathanielsz1

1 Laboratory for Pregnancy and Newborn Research, Department of Biomedical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, New York 14853-6401; and 2 Department of Obstetrics and Gynecology, Faculty of Medicine, University of Tokyo, Nagano 113-8655, Japan

Fetal cardiovascular responses to an altered intrauterine environment of increased myometrial contractures induced by oxytocin (OT) pulses to the ewe over the final 50 days of gestation were studied in chronically instrumented sheep. Ewes received saline (Cntl) or long-term OT treatment (LTOT, 600 µU · kg-1 · min-1 in 5-min pulses every 20 min) from 96 days gestational age. Fetal baroreflex responses to sodium nitroprusside (SNP) and phenylephrine (PE) were studied at 133 days gestation. OT increased contractures in LTOT ewes. Fetal blood pressure (FBP) was higher, and fetal heart rate (FHR) and slope of daily change in FBP and FHR were lower in LTOT fetuses. Fetal SNP-induced hypotension resulted in a narrow R-R interval variation range in LTOT fetuses; Cntl fetuses showed early breakdown in compensation. Baroreflex response slope during PE-induced fetal hypertension was lower in LTOT than in Cntl fetuses. Although the cortisol-to-ACTH ratio was lower in LTOT fetuses, fetal plasma ACTH and cortisol changes were similar in control and LTOT fetuses. We hypothesize that contracture-induced alterations in the intrauterine environment accelerate fetal cardiovascular development through mild hypoxemia, repetitive fetal pituitary-adrenal stimulation, and/or physical stimulation.

fetus; cardiovascular response; baroreflex; oxytocin; uterine contractures


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