The use of an oxytocin antagonist to study the function of ovarian oxytocin during luteolysis in cattle

Abstract

The importance of ovarian oxytocin (OT) in cattle during luteolysis and the mid-luteal phase using a highly specific OT antagonist (CAP-527) was studied. To establish the effective dose of CAP, heifers (n = 4) were infused with saline for 30 min, followed by 50 IU OT into the abdominal aorta on Days 17 and 18 of the estrous cycle. After 5.5 hours later, either 4, 6, 8 or 10 mg of CAP was infused for 30 min, followed by 50 IU OT. Plasma concentrations of 15-keto-13, 14-dihydro-prostagiandin F 2α (PGFM) increased after 4 and 6 mg CAP. Therefore, in Experiment 2, 8 mg of CAP was infused and 50 IU OT was given after 3, 4, 6 and 9 h to define how long CAP saturates OT receptors. Concentrations of PGFM increased after 6 and 9 h of OT treatment only. We concluded that 8 mg CAP effectively blocked uterine OT receptors for 4 h in our model; hence in further experiments this dosage of CAP was used. In Experiment 3, CAP was given to 4 heifers every 4 h on Days 15 to 22 of the cycle, and 4 additional heifers received saline and served as the control. The CAP treatment changed neither the duration of the cycle, progesterone, PGFM nor OT plasma concentrations compared with that of the controls. Experiment 4 was designed to study the involvement of OT in noradrenaline (NA)-stimulated progesterone secretion. It was found earlier that ovarian OT stimulates progesterone secretion, and that NA was able to evoke concomitant release of both progesterone and OT. Therefore, in Experiment 4, NA was infused on Days 11 and 12 of the cycle in heifers (n = 4) but it was preceded with 8 mg CAP or with a saline (control) infusion. Concentrations of plasma progesterone concentrations increased after NA treatment in both the experimental and control groups. Thus, we conclude that if ovarian OT and uterine/ovarian OT receptors are involved in luteolysis and steroidogenesis in cattle, they play a more facilitating than mandatory role.