EFFECT OF AN OXYTOCIN ANTAGONIST ON PROSTAGLANDIN F 2aa SECRETION AND THE COURSE OF LUTEOLYSIS IN SOWS
The role of oxytocin (OT) in the regulation of prostaglandin F2a (PGF2a ) secre- tion 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 de - sign were used, to determine
the dose and time of application of OT and CAP. In Group I (n = 6) gilts were treated intravenously with saline or with 10, 20 and 30 IU of OT. Concentrations of the main PGF2a metabolite i.e. 13,14-dihydro-15-keto-prosta- glandin F2a (PGFM) were measured in blood samples as uterine response
to the treat- ment. Twenty IU of OT was the most effective to stimulate PGFM release and this dose was used after CAP treatment in gilts of Groups II, III and IV. Gilts of Group II (n = 3) were injected into the uterine horns (UH) with 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/gilt) followed by 20 IU of OT. All doses of CAP effectively inhibited
OT- stimulated PGF2a release, therefore 9 mg was selected for the further studies. Gilts of Group IV (n = 4) received OT 4, 6 and 8 h after CAP to define how long CAP blocks the OT receptors. Concentrations of PGFM increased after any of this period of time. Thus, we concluded that 9 mg of
CAP infused every 4 h will effectively block OT re- ceptors. In Experiment 2, gilts (n = 4) received CAP as a 30-min infusion every 4 h on Days 12Ś20 of the oestrous cycle. Control gilts (n = 3) were infused with saline. CAP infusions diminished the height of PGFM peaks (P < 0.05).
Frequency of the PGFM (P < 0.057) and OT (P < 0.082) peaks only tended to be lower in the CAP-treated gilts. Peripheral plasma concentrations of progesterone (P4 ) and oestradiol-17b (E2 ) and the time of luteolysis initiation as measured by the decrease of P4 concentration were the
same in CAP- and saline-treated gilts. The macroscopic studies of the ovaries in gilts revealed lack of differences between groups. We conclude that OT is involved in the secretion of luteolytic PGF2a peaks but its role is limited to controlling their height and frequency. Blocking of OT receptors
did not prevent luteolysis in sows.