Hormonal control of late pregnancy and parturition in beef cattle and pigs

Pregnant and hysterectomized Meishan gilts were used to investigate the production and secretion of relaxin, progesterone, prolactin and grov^^h hormone during different reproductive states. Six gilts were bred to Meishan boars on the first day of the second observed estrus and unmated gihs were hysterectomized on days 6-8 (estrus = day 0). Blood samples (10 ml) were collected twice daily (0800 and 2000 h) fi"om d 90 to 120 and every 20 min within a 3-h period on days 112-116. From days 99 to 109 relaxin concentrations remained consistently greater (6 ± 1.0 ng/ml) in hysterectomized compared with pregnant (2 ± 0.4 ng/ml, P < 0.05) gilts. Relaxin concentrations peaked on day 114 in both pregnant (66 ± 10.9 ng/ml) and hysterectomized (34 ± 4.2 ng/ml) gihs, by day 1 postpartum relaxin concentration decreased fi'om a peak of 66 ± 10.9 ng/ml to 1 ± 0.3 ng/ml and remained low until day 120. In hysterectomized gilts, relaxin levels declined sharply fi-om a peak of 34 ± 4.2 ng/ml to 13 ± 3.2 ng/ml within a day but remained higher (P < 0.05) than in lactating pigs until day 120. Peripheral plasma progesterone concentrations were higher (P < 0.05) in hysterectomized compared with pregnant pigs from days 99 to 109. Progesterone levels decreased by half from day 109 to day 115 and remained at 16 ng/ml until day 120 in hysterectomized pigs. The decline in progesterone levels in hysterectomized pigs coincided with the marked decline observed before parturition in pregnant pigs. Prolactin concentration in hysterectomized gilts remained low (4 ±0.8 ng/ml) throughout the period of the study, whereas in pregnant gilts PRL levels increased steadily from 16 ±0.8 ng/ml on day 99 to 39 ±10 ng/ml at parturition (day 114) and remained at high concentrations during lactation. Growth hormone levels were similar in hysterectomized and pregnant gihs from day 99 to day 114, however GH concentrations were consistently higher (P < 0.05) in lactating compared with hysterectomized gilts (2.6 ± 0.6 vs 1.0 ± 0.3 ng/ml, respectively). This study confirms the study by Felder et al. (1986) who showed that in pregnant pigs there is a timed peak relaxin release that coincides with a decline in progesterone secretion and a similar secretory pattern of these hormones is observed in hysterectomized pigs at comparable times. The study further suggests that PRL and GH secretions are different in pregnant and hysterectomized pigs probably because of the physiological changes necessitated by the onset of lactation. The adinohypophyseal secretion of PRL and GH together with the shift in relaxin and progesterone secretions near parturition implies that the ovary, central nervous system and the pituitary glands may have roles in the regulation of the hormone secreted during late pregnancy and early lactation in the pig. Introduction The production of relaxin and progesterone by corpora lutea in the pig occurs during normal estrogen cycle (Sherwood and Rutherford, 1981), pregnancy (Anderson et al., 1973, 83) and after hysterectomy (Anderson et al., 1983). Relaxin is primarily known as a hormone with important physiological roles on the reproductive tract particularly near parturition, some of the principal functions being to enhance the birth process and also to increase the survival rate of the newborn (Nara et al., 1982). There are, however, recent reports that show that relaxin is involved in other biological actions that are not directly related to reproduction (StLouis and Massicotte, 1985; Kakouris et al., 1992). In the pig, progesterone is required for the maintenance of pregnancy while relaxin is of particular importance for normal delivery of the young. Blood concentrations of progesterone increased to peak values by day 12 after mating and remain elevated until a few days before parturition, thereafter progesterone levels decline to basal concentrations (Masuda et al., 1967; Robertson and King, 1974). Peripheral plasma relaxin concentrations are detectable during early pregnancy and a sustained steady increase of this hormone is observed as pregnancy advances, this increase is correlated with the accumulation of electron-dense cytoplasmic granules in luteal tissue which are known to store relaxin (Belt et al., 1971; Sherwood et al., 1975). Depletion of the electron-dense cytoplasmic granules coincides with the prepartum peak release of relaxin (Sherwood et al., 1981; Felder et al., 1986). Hysterectomy of nongravid pigs early in the luteal phase (day 6-8) maintains luteal function until about day 150 (Anderson et al., 1983; Musah et al., 1984). Peripheral plasma levels of both progesterone and relaxin in hysterectomized gilts follow similar patterns to those of pregnant gilts at comparable stages (Sherwood et al., 1981; Anderson et al., 1983). PRL has biological actions in a number of organs, in mammals it appears that the fiindamental function of PRL is the regulation of reproduction (Jahn et al., 1991). Several studies have demonstrated that PRL is luteotrophic in the pig (Bramely and Menzies, 1987; Szafranska and Tilton, 1993). The GH concentration increase during lactation has been shown to alter nutrient utilization in muscle and mammary tissue in a manner that favors increased nutrient supply for milk synthesis (McDowell et al., 1987; Jahn et al., 1993). The hysterectomized pig model has been used to investigate mechanisms that regulate secretion of relaxin and progesterone by aging corpora lutea in Yorkshire gilts (Felder et al., 1986). The overall objective of this study was to examine hormone profiles in Chinese Meishan pigs during different reproductive states. Specifically, the present study investigates: 1) relaxin and progesterone secretion relationship by corpora lutea during late pregnancy and early parturition and 2) adenohypophysial secretion of PRL and GH during pregnancy and early lactation. Materials and Methods Meishan gilts exhibiting two consecutive normal estrous cycles averaging 21± 1 d (mean ± SE) were used in this experiment. Six gilts were bred to Meishan boars on the first day of the second observed estrus. Eleven unmated gilts were hysterectomized via the midventral laparatomy (Anderson et al., 1961) on day 8, estrus was designated day 0. After a 24h fasting period animals were restrained by nasal snare on the day of surgery, and anesthesia was administered by ear vein injection of thiamylal sodium (0.8-1.Og; Surital, Parke-Davis, Detroit, MI). Animals were maintained under anesthesia by a closed circuit system of halothane (3-10%, Ayerst Laboratories, Rouses Point, NY.) and oxygen (400-1000 cc/min) for the rest of surgery. At the time of hysterectomy, the number of corpora lutea and the diameter of each were recorded. Four randomly selected corpora lutea were marked on each ovary with a loop of black silk suture for later identification. Reexamination of the state of the ovarian structures was performed at the end of the experiment through mid-ventral laparatomy. The experiment described here was reviewed and approved by the Iowa State University Laboratory Animal Care Advisory Committee. Experimental groups Two days before the start of the blood sample collection period, gihs were anesthetized by the ear vein injection of thiamylal sodium and maintained in a closed circuit system of halothane and oxygen. Cannulation (id, 1.27 mm; od, 2.29 mm; Tygon microbore tubing. Fisher Scientific, Pittsburgh, PA) of cranial vena cava was performed to allow repeated blood sampling. The catheter was tunneled under the skin and exteriorized at the junction of the scapula according to the procedure described by Ford and Maurer (1978). Hysterectomized gilts were confined to individual indoor pens in a controlled environment (18-220C) during the remainder of the experimental period. Mated gihs were housed in individual farrowing units (18-220C) where they remained with their off-spring during lactation. Blood samples were collected fi-om the cranial vena cava beginning on day 90 afl;er estrus or mating. Blood samples (10ml) were collected in disposable borosilicate culture tubes (16 X 100 mm) containing 0.2 ml of a solution of heparin sodium salt (100 lU/ml); NBCO Biochemicals, Cleveland, OH). Sequential blood samples were collected once daily