Abstract

There is strong evidence that growth hormone (GH) does not enhance lactation through a direct effect on the udder but rather that the effect is mediated by insulin-like growth factors (IGF) -I an/or IGF-II produced by the liver. 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.

GC, Waghorn

Proceedings of the New Zealand Society of Animal Production, Volume 48, , 151-156, 1988