Acute shifts in relaxin, progesterone, prolactin, and growth hormone secretion in Chinese Meishan gilts during late pregnancy and after hysterectomy

Pregnant and hysterectomized Chinese Meishan gilts were used to investigate mechanisms regulating the production and secretion of relaxin, progesterone, prolactin (PRL), and growth hormone (GH) during different reproductive states. Gilts were bred to Meishan boars on the 1st d of the second observed estrus, and unmated gilts were hysterectomized on d 8 (estrus = d 0). Blood samples (10 mt) were collected twice daily (0800 and 2000) from d 90 to 120 and every 20 min within a 3-h period on d 112 to 116. Relaxin plasma concentrations were consistently greater in hysterectomized than in pregnant(6 vs 2 ng/mL, P < .05) gilts on d 99 to 109. The surge relaxin release on d 114 was greater in pregnant (66 ng/mL) than in hysterectomized gilts (34 ng/mL, P < .05). Thereafter, relaxin remained consistently increased (12 ng/mL) in hysterectomized gilts, whereas it was basal (< .5 ng/mL) during lactation. Progesterone concentrations decreased by half from d 109 to 115 and remained at 16 ng/mL through d 120 in hysterectomized pigs, whereas in pregnant pigs progesterone became basal after parturition. Prolactin in hysterectomized gilts remained at 4 ng/mL throughout the study period, whereas in pregnant gilts PRL increased steadily from 16 ng/mL on d 99 to 39 ng/mL at parturition and remained increased during lactation. Growth hormone concentrations were similar in hysterectomized and pregnant gilts from d 99 to 114; however, GH concentrations were consistently greater (P < .05) in lactating than in hysterectomized gilts (2.6 vs 1.0 ng/mL, respectively). These results indicate that PRL and GH secretions differ in pregnant and hysterectomized pigs because of the physiological changes necessitated by the onset of lactation.