Endocrine relationships of Meishan and White composite females after weaning and during the luteal phase of the estrous cycle

Endocrine differences between European crossbred and Chinese Meishan female s have been somewhat modest. Indwelling jugular cannulas were placed in Mei shan (n = 7) and White composite (n = 6) multiparous sows before weaning, a nd blood was sampled from 4 h before to 240 h after removal of litters. Tem poral changes in FSH, triiodothyronine (T-3), and tetraiodothyronine (T-4) after weaning differed between Meishan and White composite sows. Plasma cor tisol concentrations were higher in Meishan sows than in White composite so ws (P < 0.01), but there were no temporal differences between breeds after weaning. Other hormones monitored (prolactin, GH, ICE-I, and inhibin) were not different between breeds. In the second experiment, Meishan gilts (n = 7) and sows (n = 7) and White composite sows (n = 9) were cannulated during the luteal phase of the estrous cycle and sampled after treatments consist ing of GnRH (15 and 150 ng/kg BW), ovariectomy, estradiol cypionate challen ge after ovariectomy (10 <mu>g/kg BW), and GnRH antagonist. In response to GnRH challenge, White composite sows had elevated (P < 0.05) concentrations of gonadotropins compared with Meishan. Cortisol concentrations were eleva ted in Meishan as compared with White composite females (P < 0.01) but unaf fected by GnRH treatment. After ovariectomy, LH concentrations increased 3 h sooner in White composite than in Meishan females. After GnRH antagonist, declines in gonadotropins were comparable in both breeds, but LH increased in Meishan females by 20 h hut not until after 54 h in White composite fem ales. White composite females demonstrated only a short decline in FSH in r esponse to the GnRH antagonist, but Meishan females had a prolonged decline in FSH concentrations. Consistently elevated cortisol concentrations in Me ishan females may positively impact ovarian function; thus, the assumption that high plasma cortisol concentration as an index of stress that impairs reproductive function should be reevaluated in swine. There were few endocr ine differences that would relate to or explain increased ovulation rates f ound in the Meishan breed; thus, other mechanisms must exist to explain the increase in ovarian function in the Meishan breed.