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 luteinizing hormone (LH) in blood, on the concentration of LH/human chorionic gonadotrophin (hCG) receptors in ovaries and on the release of gonadotrophin-releasing hormone (GnRH) in vitro from the stalk median eminence (SME) in prepubertal gilts. The experiment was performed on 22 prepubertal 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: one received 5 mg of PRL (BIOLACTIN, Biochefa, Sosnowiec, Poland, n = 11) 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 study, additionally every 20 min for 4 h (08:00–12:00 h) and every 2 h for 8 h on days 1, 7, 15 and 21 of the experiment. Concentrations of PRL and LH in plasma and GnRH in medium were analyzed by radioimmunoassay (RIA) procedures. All gilts were slaughtered on the 21st day of treatment. Ovaries and hypothalamus were dissected immediately following slaughter. oncentrations of LH in the consecutive weeks of the study were similar in the PRL-treated gilts whereas in the control animals they rose significantly (from 0.29 ± 0.02 ng ml−1 the first week to 0.40 ± 0.02 ng ml−1 in the third week; P < 0.01). Exogenous PRL increased mean LH concentrations during the whole period of the experiment (P < 0.01) but did not affect the frequency, amplitude or duration of the LH peaks. In the ovaries of hyperprolactinemic animals the concentration of LH/hCG receptors increased compared with control gilts (from 1.95 ± 0.43 fmol mg−1 to 2.39 ± 0.55 fmol mg−1 of protein). Dissociation constants for hyperprolactinemic and control groups were similar. In vitro, basal and potassium-stimulated release of GnRH from the SME did not differ between the treated and control groups. sults indicate that PRL treatment stimulated a release of LH from 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 gilts.