Physiology of the somatotropic axis with particular reference to the ruminant

The physiology of growth hormone and the insulin-like growth factors are reviewed with particular reference to the dairy industry. Growth hormone secretion in the ruminant is pulsatile in nature and nutritional factors have a major impact on its secretion. Isolation of growth hormone-releasing factor has allowed further progress in understanding the mechanisms underlying growth hormone release. The receptors appear to be under active endocrine and metabolic control, and nutritional influences on the somatotropic axis are in large part mediated through changes in somatotropic receptors. The mode of action of growth hormone to induce acute metabolic affects and lipolysis remains to be resolved, but there is increasing evidence that its anabolic actions are mediated by the insulin-like growth factors. Recent studies of measurement of insulin-like growth factor-1 and -2 in the ruminant and the role of growth hormone, nutrition, insulin, and sex steroids in their regulation are reviewed. The relative role of the two factors and the multiple forms of their receptors remain to be resolved. It is well-documented that growth hormone is galactopoietic. The evidence that this effect is largely due to enhanced nutrient supply to the mammary gland is not convincing. Effects of growth hormone are indirect and may be mediated by the insulin-like growth factors. The potential is considerable for manipulating the growth hormone insulin-like growth factor axis to enhance lactation.