Somatotropin and endocrine regulation of metabolism during lactation

Administration of somatotropin to lactating ruminants elicits a number of changes in nutrient partitioning and utilization; the end result is improved supply of metabolites to the mammary gland for milk synthesis. In the short term, the galactopoietic response to somatotropin occurs in the absence of marked changes in feed intake and the physiological response of the animal appears to depend on nutritional status. Key endocrine changes induced by somatotropin include antagonism of the action of insulin or alterations of the sensitivities of body tissues to insulin and also stimulation of the release of somatomedins, in particular, insulin-like growth factor-I. The effects of somatotropin on insulin lead to altered metabolism of key nutrients. There is evidence that the somatomedins may affect mammary blood flow. Prolonged administration of somatotropin leads to increased feed intake after 2 to 3 wk, which enables the maintenance of increased milk production without adverse effects on the body as a whole.