Maturation of spontaneous and agonist-induced uterine contractions in the peripartum mouse uterus

This study tested the hypothesis that the uterus achieves maximum contracti le capabilities before the onset of labor. Basal and agonist-stimulated con tractions 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); c ontraction frequency was greater in peripartum groups than in virgin contro ls (similar to 4.6 versus 2.8/200 sec). Peak amplitude was nearly 9-fold hi gher 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 ampl itude were achieved in response to 10(-6) to 10(-8) M oxytocin or arginine vasopressin (OTmax or AVP(max)). Frequency of contractions in response to O Tmax peaked on Day 18 and then declined. Contraction amplitude increased 5- fold on Day 15, declined on the day of birth (equivalent to nonpregnant lev el), then rebounded to peak on postpartum Day 1. AVP(max) similarly increas ed 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 t erm, the data suggest that peripartum increases in contraction amplitude ch aracterize the transition to the powerful synchronous contractions of partu rition.