Differences in the in vitro sensitivity of ovine myometrium and mesometrium to oxytocin and prostaglandins E2 and F2(alpha)

We compared the in vitro response to oxytocin, prostaglandin (PG)E2, and PGF2(alpha) of myometrium and mesometrium from six ovariectomized ewes and 53 ewes at 106-145 days gestational age (dGA), including 14 ewes in spontaneous or betamethasoneinduced labor. Myometrial baseline activity increased from 217+/-27 mN/cm2 of cross-sectional area (mean +/-SEM) in ovariectomized ewes to a plateau of 696+/-39 mN/cm2 at 126- 135 dGA. No gestation-related changes were observed in mesometrial baseline activity. Myometrial, but not mesometrial, maximum tension in response to agonists increased with gestation to a plateau at 126-135 dGA. The pD2 (negative logarithm of the EC50) values for oxytocin were similar in both tissues and did not change with gestation. During pregnancy, the myometrial pD2 of both PGs was one order of magnitude higher than the mesometrial pD2. The results indicate an increase in myometrial uterotonic receptor-mediated activity that precedes labor with no increase at labor, suggesting that in sheep, activation of the basic mechanisms responsible for strength of myometrial activity at labor occurs by 135 dGA. The greater sensitivity of the myometrium than the mesometrium to PGs supports a major role for intrauterine paracrine factors in regulating myometrial contractility.