Effect of Oxytocin Receptor Blockade on Rat Myometrial Responsiveness to Prostaglandin F2α

Abstract In the present study we have shown that the genetic expression of prostaglandin (PG)F2α receptor (R) and cyclooxygenase (COX)-2 increases in laboring rat myometrium. This finding was associated with a relatively weak contractile in vitro response (Emax) of isolated uterine strips when challenged with PGF2α. Five days postpartum PGF2α-R mRNA values exceeded those during labor while COX-2 mRNA was reduced to preparturient values. Maximal contractility of isolated strips stimulated with PGF2α at this time was enhanced and EC50 decreased. Oxytocin treatment of estrogen-primed nonpregnant rats down-regulated uterine contractile responsiveness to PGF2α, leaving mRNA values for this receptor unchanged, whereas oxytocin receptor blockade with atosiban (an oxytocin receptor antagonist) left Emax unaltered. In contrast, atosiban treatment of pregnant rats resulted in a 2.5-fold increase in Emax and a considerably reduced EC50 during labor when compared to untreated delivering rats. The increased contractil...