Effect of an oxytocin antagonist on prostaglandin F-2 alpha secretion and the course of luteolysis in sows

The role of oxytocin (OT) in the regulation of prostaglandin F-2 alpha (PGF (2 alpha)) secretion during luteolysis in gilts was studied using a highly specific OT antagonist (CAP-581). In Experiment 1 gilts on Days 14 to 19 of the oestrous cycle in Latin square design were used, to determine the dose and time of application of OT and CAP. In Group I (n = 6) gilts were treat ed intravenously with saline or with 10, 20 and 30 IU of OT. Concentrations of the main PGF(2 alpha) metabolite i.e. 13,14-dihydro-15-keto-prostagland in F-2 alpha (PGFM) were measured in blood samples as uterine response to t he treatment. Twenty IU of OT was the most effective to stimulate PGFM rele ase and this dose was used after CAP treatment in gilts of Groups TT, III a nd IV. Gilts of Group II (n = 3) were injected into the uterine hems (UH) w ith saline (5 ml/horn) or CAP (2 mg, 3 mg and 4 mg; half dose/horn) and OT was injected (i.v.) 30 min thereafter. Any of the CAP doses given into the UH affected PGFM plasma concentrations stimulated by OT. in Group III (n = 4) gilts were Infused (i.v.) for 30 min with CAP (9 mg, 14 mg and 18 mg/gil t) followed by 20 IU of OT. All doses of CAP effectively inhibited OT-stimu lated PGF(2 alpha) release, therefore 9 mg was selected for the further stu dies. Gilts of Group TV (n = 4) received OT 4, 6 and 8 h after CAP to defin e how long CAP blocks the OT receptors. Concentrations of PGFM increased af ter any of this period of time. Thus, we concluded that 9 mg of CAP infused every 4 h will effectively block OT receptors. In Experiment 2, gilts (n = 4) received CAP as a 30-min infusion every 4 h on Days 12-20 of the oestro us cycle. Control gilts (n = 3) were infused with saline. CAP infusions dim inished the height of PGFM peaks (P < 0.05). Frequency of the PGFM (P < 0.0 57) and OT (P < 0.082) peaks only tended to be lower in the CAP-treated gil ts. Peripheral plasma concentrations of progesterone (P-4) and oestradiol-1 7 beta (E-2) and the time of luteolysis initiation as measured by the decre ase of P-4 concentration were the same in CAP- and saline-treated gilts. Th e macroscopic studies of the ovaries in gilts revealed lack of differences between groups. We conclude that OT is involved in the secretion of luteoly tic PGF(2 alpha) peaks bur its role is limited to controlling their height and frequency. Blocking of OT receptors did not prevent luteolysis in sows.