EFFECT OF OXYTOCIN AND PGF2-ALPHA ON PROLACTIN-RELEASE IN SOWS

Experimental sows were divided into three groups treated as follow: Group I (n = 3): on Day 12 of the oestrous cycle i.m. injection of oxytocin (OT) at a dose of 0.25 I.U./kg b.w.; Group II (n = 4): on Day 12 and 13 of the oestrous cycle i.v. injections of OT (0.03 I.U./kg b.w.), and Group III (n = 3): on Day 13 of the oestrous cycle i.m. injection of cloprostenol (prostaglandin F2-alpha analogue; PG) at a dose of 500-mu-g. Blood samples were collected from the jugular vein for 2 h before and 2 h or 4 h after OT and PG treatment, respectively. Concentration of prolactin (PRL) was determined in all studied groups, whereas 13, 14-dihydro-15-keto-PGF2-alpha (PGFM) was estimated in Group I and II. OT injections (i.v. or i.m.) on Day 12 and 13 of the oestrous cycle rised its concentrations in the peripheral blood lasting for 50 and 120 min, respectively. But short lasting rise of PRL was caused by OT in 3 sows, whereas of PGFM in 2 sows only. In all other studied sows values of PRL and PGFM were during an experiment on the pre-treatment level. However, if PGF2-alpha analogue was injected on Day 13 of the cycle, it caused significant rise of PRL concentration (P < 0.01) 20 min after treatment. PRL concentration measured after PG administration (7.0 +/- 0.7 ng/ml) was statistically higher (P < 0.01) compared to the period preceding PG injection (2.6 +/- 0.3 ng/ml). We conclude that injection of OT in the late-luteal phase of the oestrous cycle did not stimulate increase of PRL and PGFM release. On the contrary, an analogue of PGF2-alpha administered to the animals on 13th day of the oestrous cycle, enhanced PRL secretion.