Oxytocin modulates the pulsatile secretion of prostaglandin F2αin initiated luteolysis in cattle

Abstract

Subluteolytic doses of prostaglandin F2αanalogue (oestrophan) given i.m. and oxytocin (OT) antagonist (CAP) and noradrenaline (NA) infused into the abdominal aorta were used to test the importance of lutealOTin pulsatile secretion of prostaglandin F2α(PGF) during luteolysis in heifers (n = 17). In experiment 1, heifers were pre-infused for 30 minutes with saline on either day 17 of the oestrous cycle (group 1; n = 4) or on day 18 of the oestrous cycle (group 2; n = 3), and with CAP (8 mg per animal) on day 17 of the oestrous cycle (group 3; n = 4). Next, heifers were injected with oestrophan (30 μg per animal). Injection of oestrophan in Group 3 increasedOTconcentrations (P < 0.001) to values similar to those observed during spontaneous luteolysis (50 to 70 pg ml–1).PGFMconcentrations in this group also increased (P < 0.001), but were lower (P < 0.05) than the values in groups 1 and 2.CAPgiven prior to oestrophan decreased bothPGFMelevation (P < 0.06) and its area under the curve (P < 0.01), compared to the saline pretreated heifers. In experiment 2NA(4 mg) was infused twice for 30 minutes at five hour intervals to releaseOTon day 17 of the oestrous cycle (n = 6). However, during hormone analysis it appeared that three of six heifers had elevatedPGFMconcentrations (group 1) and three others did not (group 2).NAcaused the correlated increase of progesterone andOTsecretion (r = 0.68; P < 0.05) in both groups but it only influencedPGFsecretion in group 1 only (P < 0.05).We postulate thatOTcan amplify and modulate the course of induced luteolysis as a regulator of the amplitude of pulsatilePGFsecretion.PGFanalogue stimulates secretion of endogenousPGFfrom the uterus in cattle and this may be an important component of the luteolytic response to exogenousPGF.