Differences in the in vitro sensitivity of ovine myometrium and mesometrium to oxytocin and prostaglandins E2 and F2α

We compared the in vitro response to oxytocin, prostaglandin (PC)E-2, and PCF2 alpha of myometrium and mesometrium from six ovariectomized ewes and 53 ewes at 106-145 days gestational age (dCA), including 14 ewes in spontaneous or betamethasone-induced labor. Myometrial baseline activity increased from 217 +/- 27 mN/cm(2) of cross-sectional area (mean +/- SEM) in ovariectomized ewes to a plateau of 696 +/- 39 mN/cm(2) 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 dCA. The pD(2) (negative logarithm of the EC50) values for oxytocin were similar in both tissues and did not change with gestation. During pregnancy, the myometrial pD(2) of both PGs was one order of magnitude higher than the mesometrial pD(2). 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 dCA. The greater sensitivity of the myometrium than the mesometrium to PGs supports a major role for intrauterine paracrine factors in regulating myometrial contractility.