OXYTOCIN-INDUCED PROSTAGLANDIN-F2-ALPHA RELEASE AND ENDOMETRIAL OXYTOCIN RECEPTOR CONCENTRATIONS THROUGHOUT PREGNANCY IN EWES

Endometrial oxytocin receptor concentrations and oxytocin-induced plasma concentrations of 13,14,dihydro-15-keto prostaglandin F-2 alpha were investigated on days 14 and 17 of the oestrous cycle and on days 14, 17, 25, 65, 85 and 145 of gestation in ewes. Total 13,14,dihydro-15-keto prostaglandin F-2 alpha release in response to a bolus injection of oxytocin was significantly (P < 0.05) higher at luteolysis (day 17 of the oestrous cycle) than at any other stage of the oestrous cycle or in early gestation. On days 65, 85 and 145 of gestation, total prostaglandin release was significantly (P < 0.05) increased compared with earlier in gestation. Maximum concentrations of 13,14,dihydro-15-keto prostaglandin F-2 alpha in response to oxytocin followed a similar pattern. Oxytocin receptor concentrations reflected total oxytocin-induced 13, 14,dihydro-15-keto prostaglandin F-2 alpha release, with increased oxytocin receptor concentrations occurring on day 17 of the oestrous cycle, compared with those observed on day 14 of the oestrous cycle and on days 14, 17 and 25 of gestation. By day 65 of gestation, oxytocin receptor concentrations were again increased. However, on days 85 and 145 of gestation, oxytocin receptor concentrations had decreased to concentrations similar to those observed in early gestation. These results indicate that oxytocin-induced 13,14,dihydro-15-keto prostaglandin F-2 alpha release during early gestation is minimal despite the presence of endometrial oxytocin receptors. In mid-gestation, ocytocin-stimulated 13,14,dihydro-15-keto prostaglandin F-2 alpha release is increased with a concomitant increase in uterine oxytocin receptor concentrations. However, by the later stages of gestation, oxytocin receptor concentrations, but not oxytocin-stimulated 13,14,dihydro-15-keto prostaglandin F-2 alpha release, were again decreased. Thus, there appears to be a dissociation between the response to oxytocin and endometrial oxytocin concentrations at specific stages of the reproductive cycle in sheep.