EXOGENOUS PROLACTIN ELEVATES PLASMA PROGESTERONE CONCENTRATIONS AND REDUCES ANDROGEN CONCENTRATIONS DURING THE EARLY LUTEAL PHASE OF THE PORCINE ESTROUS-CYCLE

Sixteen crossbred sows were used after second farrowing in a study performed on a commercial unit. Three days after weaning, polyvinyl catheters were placed into both jugular veins of control (n = 9) and prolactin (PRL) treated (n = 7) sows for blood sampling and PRL administration. Blood was sampled at 2-h intervals, beginning on the first day of estrus and continuing for 6 subsequent days. Injections of 0.3 mg PRL (in 1 ml of saline) were given every 2 h for 3 days starting from the second day of blood sampling, always 3-4 min before blood collection. Controls were injected similarly with an equal volume of saline. Blood samples were assayed for luteinizing hormone (LH), PRL, progesterone (P4), androstenedione (A4) and testosterone (T) by radioimmunoassay (RIA) procedures. All data were standardized to maximum LH concentrations (Day 0). Non-linear regression analysis with known covariance matrix was used in statistical calculations of the presented data. The chi2 test was applied to verify the significance of all regression coefficients and to test the differences between the control and PRL-treated groups. In control sows, all studied hormones excluding P4 Were at their highest concentrations in the peri-estrous period. When PRL administration was started 24-48 h after the LH surge, plasma P4 concentrations in particular PRL-treated sows were elevated in comparison with those of control sows (P < 0.01). This elevation was observed during, as well as after the PRL injection period. Prolactin also significantly inhibited plasma concentrations of A4 and T in most of the treated sows. Results suggest a luteotrophic effect of exogenous PRL during the early luteal phase in sows.