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Maturation of Spontaneous and Agonist-Induced Uterine Contractions in the Peripartum Mouse Uterus¹

^a Center for Perinatal Biology, Loma Linda University School of Medicine, Loma Linda, California 92350 ^b Division of Endocrinology and Metabolism, University of Virginia Health Sciences Center, Charlottesville, Virginia 22908

This study tested the hypothesis that the uterus achieves maximum contractile capabilities before the onset of labor. Basal and agonist-stimulated contractions were assessed in uterine strips on Day 15 or 18 of pregnancy, the day of parturition, or 1 day postpartum (n = 4–13 per group). Spontaneous contractions were evident in all groups (n = 4–13 per gestational group); contraction frequency was greater in peripartum groups than in virgin controls (~4.6 versus 2.8/200 sec). Peak amplitude was nearly 9-fold higher on Days 15 and 18 and over 30-fold higher in the postpartum and 1 day postpartum groups than in nonpregnant mice. Maximum frequency and peak amplitude were achieved in response to 10^-6 to 10^-8 M oxytocin or arginine vasopressin (OT_max or AVP_max). Frequency of contractions in response to OT_max peaked on Day 18 and then declined. Contraction amplitude increased 5-fold on Day 15, declined on the day of birth (equivalent to nonpregnant level), then rebounded to peak on postpartum Day 1. AVP_max similarly increased frequency and amplitude of contractions, except that maximum contraction amplitude occurred postpartum. Thus, an endogenous oscillator, residing in the uterus, sustains high basal and agonist-induced contraction frequency during pregnancy. Although acceleration of this pacemaker occurred before term, the data suggest that peripartum increases in contraction amplitude characterize the transition to the powerful synchronous contractions of parturition.

¹ Supported in part by a Basic Science Research Grant from the Dean of the School of Medicine.

² Correspondence. FAX: 909 824 4029; syellon{at}som.llu.edu

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