Prolactin and luteinizing hormone secretion in the pregnant pig

Four pregnant, primiparous, crossbred gilts and six gilts from the same population that had been ovariectomized (OVX) for approximately 3 wk were placed in individual pens in an enclosed building. Blood samples were collected every 30 min for 12 h from all gilts via an indwelling jugular vein cannula when the pregnant gilts were at d 30, 50, 70, 90, and 110 of gestation. Serum was quantified for LH and prolactin (PRL) by RIA. The OVX gilts served as controls to ensure that any variations in serum LH and PRL concentrations observed in the pregnant animals were not due to environmental factors unrelated to pregnancy. Within the pregnant gilts, mean serum LH concentrations, mean basal serum LH concentration, and mean serum LH peak height were similar on all days; however, number of LH peaks on d 30, 50, and 70 were greater (P < .05) than on d 90 and 110, and number of LH peaks on d 50 was greater (P < .05) than that on d 70. Within the pregnant gilts, mean serum PRL concentration, mean basal serum PRL concentration, and mean PRL peak height were greater (P < .001) on d 110 than on all other days; however, number of PRL peaks were similar among days. Parameters of LH and PRL secretion in the OVX and pregnant gilts varied independently. Results of this study indicated that 1) LH secretion does not vary appreciably throughout pregnancy and 2) PRL secretion does not vary significantly during the first 90 d of pregnancy, after which it increases markedly on or before 110 d.