Functional differences in the growth hormone and insulin-like growth factor axis in cattle and pigs: implications for post-partum nutrition and reproduction.

Growth hormone (GH) and insulin-like growth factor-I (IGF-I) control growth and lactation in cattle and swine. Insulin participates in the endocrinology of growth and lactation because insulin and GH are antagonistic in their actions. Dairy cows experience a period of negative energy balance during the first 4-8 weeks post-partum. During this period, their somatotropic axis (comprised of GH, the GH receptor and IGF-I) becomes uncoupled and there is elevated GH and diminished IGF-I in the circulation. Blood insulin concentrations are low as well. Sows are different from dairy cows because their somatotropic axis remains coupled during lactation and both GH and IGF-I are elevated. Nonetheless, sows that become catabolic during lactation will have reduced IGF-I concentrations. Sows are inseminated after weaning so their metabolic state is different from post-partum beef and dairy cows that are inseminated when they are lactating. Dairy cows are fed ad libitum and naturally have low IGF-I during lactation. Sows have elevated IGF-I when they are well-fed. A threshold of IGF-I protein in follicular fluid may be met by local ovarian (paracrine/autocrine) and endocrine sources of IGF-I. Nutritionally induced changes in insulin and in liver IGF-I secretion that arise from perturbations of the somatotropic axis have a direct effect on the ovary through the endocrine actions of insulin and IGF-I. Sows and cows that are nutritionally compromised have low concentrations of insulin and IGF-I in their blood and this theoretically reduces ovarian responsiveness to gonadotropins. Although sows are inseminated after weaning, there appear to be carry-over effects of the previous lactation on the ovarian follicular populations that develop after the sow is weaned. Understanding the mechanisms through which metabolic hormones control ovarian function may lead to improved reproductive management of both pigs and cattle because lactation and post-partum reproduction are closely tied in both species.