Differences in the in vitro sensitivity of ovine myometrium and mesometrium to oxytocin and prostaglandins E₂ and F(2α)

We compared the in vitro response to oxytocin, prostaglandin (PG)E₂, and PGF(2α) of myometrium and mesometrium from six ovariectomized ewes and 53 ewes at 106-145 days gestational age (dGA), including 14 ewes in spontaneous or beta-methasone-induced 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 oh 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.