The mechanism of action of growth hormone on milk production of ruminants

S.R. Davis, S.C. Hodgkinson, P.D. Gluckman, L.G. Moore and B.H. Breier

Ruakura Agriculture Centre, Ministry of Agriculture and Fisheries, Hamilton; Paediatrics Department, University of Auckland, Auckland and Wallaceville Animal Research, Ministry of Agriculture and Fisheries, Upper Hutt

Proceedings of the New Zealand Society of Animal Production. 1988, 48: 191-194

While plasma IGF-I concentrations rose following GH injection of dairy cows and goats, intravenous infusion of goats with IGF-I did not increase milk yield. However, GH treatment increased the relative binding of IGF-I to a high molecular weight binding protein in plasma. Plasma binding proteins may crucially affect the transport of IGF's from plasma to tissues.

Plasma IGF-II concentrations doubled during GH treatment of lactating goats. In view of the increased presence of IGF-II receptors in mammary tissue during lactation it is possible that plasma IGF-II is involved in mediation of the GH effect on lactation.

The increased partition of nutrients for milk synthesis achieved during GH treatment is facilitated by increased blood flow to the udder. There is little change in the plasma concentration of substrates for milk synthesis. It is likely that the increased blood flow occurs as a metabolic consequence of increased milk synthesis.

Keywords: NZSAPAB; Growth hormone; insulin-like growth factor; lactation