THE GALACTOPOIETIC EFFECT OF BOVINE GROWTH-HORMONE IN GOATS IS ASSOCIATED WITH INCREASED CONCENTRATIONS OF INSULIN-LIKE GROWTH FACTOR-I IN MILK AND MAMMARY TISSUE

Lactating goats exhibiting widely divergent responses to short-term (4 days) treatment with bovine GH (bGH) were retrospectively divided into two groups based on the magnitude of this response. There was no difference between groups in terms of the pretreatment milk yield, but by day 4 of treatment milk secretion had increased by 4.99 +/- 2.5 (S.E.M.) ml/h (P > 0.05 compared with pretreatment) for group 1 and 22.9 +/- 2.4 ml/h (P < 0.001) for group 2. Plasma GH increased in both groups, but concentrations were significantly higher both before and during treatment in group 1 compared with group 2. Plasma concentrations of insulin-like growth factor-I (IGF-I) increased significantly during bGH treatment for both groups and there was no significant difference between the two until day 4 of treatment when levels of IGF-I in group 1 began to decline, whereas those from group 2 were maintained. Concentrations of IGF-I in milk from goats in group 1 were not significantly altered by GH administration, whereas those in goats in group 2 were increased by 40% (P < 0.01 compared with pretreatment). Levels of IGF-I in mammary secretory tissue from four animals from group 1 were not altered by bGH (2.8 +/- 0.2 and 2.77 +/- 0.08 nmol/kg tissue before and after treatment respectively), but were significantly (P < 0.05) increased in four animals from group 2 (2.80 +/- 0.2 and 9.9 +/- 1.1 nmol/kg tissue). Thus, it appears that the galactopoietic response in goats was associated with significantly lower levels of GH in plasma after 3 days of treatment and, more strikingly, greater amounts of IGF-I in milk and mammary tissue. This latter observation is consistent with the hypothesis that the effects of bGH on the mammary gland itself are mediated by IGF-I and that the availability of IGF-I to mammary tissue is an important component of the overall galactopoietic response to bGH.