EFFECT OF EXOGENOUS PORCINE PROLACTIN (PPRL) ON LH PLASMA-CONCENTRATION AND ON THE RELEASE OF GNRH IN-VITRO FROM THE STALK MEDIAN-EMINENCE IN PREPUBERTAL GILTS

The aim of this study was to determine the effect of exogenous porcine prolactin (pPRL) induced hyperprolactinemia on the level of luteinizi ng hormone (LH) in blood, on the concentration of LH/human chorionic g onadotrophin (hCG) receptors in ovaries and on the release of gonadotr ophin-releasing hormone (GnRH) in vitro from the stalk median eminence (SME) in prepubertal gilts. The experiment was performed on 22 prepub ertal gilts (Large White), at 155 days of age (mean +/- SD body weight 60.1 +/- 3.4 kg). The animals were randomly assigned to two groups: o ne received 5 mg of PRL (BIOLACTIN, Biochefa, Sosnowiec, Poland, n = 1 1) and the other received saline (n = 11) i.m. once a day for 21 days. Blood samples were collected from the jugular vein three times a day during the first 2 weeks, four times a day in the third week of the st udy, additionally every 20 min for 4 h (08:00-12:00 h) and every 2 h f or 8 h on days 1, 7, 15 and 21 of the experiment. Concentrations of PR L and LH in plasma and GnRH in medium were analyzed by radioimmunoassa y (RIA) procedures. All gilts were slaughtered on the 21st day of trea tment. Ovaries and hypothalamus were dissected immediately following s laughter. Mean concentrations of LH in the consecutive weeks of the st udy were similar in the PRL-treated gilts whereas in the control anima ls they rose significantly (from 0.29 +/- 0.02 ng ml(-1) the first wee k to 0.40 +/- 0.02 ng ml(-1) in the third week; P < 0.01). Exogenous P RL increased mean LH concentrations during the whole period of the exp eriment (P < 0.01) but did not affect the frequency, amplitude or dura tion of the LH peaks. In the ovaries of hyperprolactinemic animals the concentration of LH/hCG receptors increased compared with control gil ts (from 1.95 +/- 0.43 fmol mg(-1) to 2.39 +/- 0.55 fmol mg(-1) of pro tein). Dissociation constants for hyperprolactinemic and control group s were similar. In vitro, basal and potassium-stimulated release of Gn RH from the SME did not differ between the treated and control groups. The results indicate that PRL treatment stimulated a release of LH fr om the pituitary gland in prepubertal gilts and slightly elevated the number of LH/hCG receptors in the ovaries. These findings suggest that PRL may influence processes connected with the onset of puberty in gi lts.