Comparison of the effects of administration of recombinant bovine growth hormone or N-Met insulin-like growth factor-I to lactating goats.

Five goats were injected with GH (15 mg/day), three goats received systemic infusions of insulin-like growth factor (IGF)-I (43 nmol/h) and four goats received systemic infusions of physiological saline (20 ml/h) on days 4-6 of a 10-day experimental period during mid-lactation. Milk yield increased by an average of 24% in GH-treated goats by the time of the third injection. In contrast, milk yield of IGF-I-infused goats did not differ from saline-infused animals although two of three goats did show a small increase (12%) after 36 h of IGF-I infusion. With GH and IGF-I treatments plasma IGF-I concentrations increased similarly, reaching maxima of 100-130 nmol/l within 24 h. Plasma IGF-I concentration was relatively constant in saline-infused goats at about 50 nmol/l throughout the experiment. Total IGF-I bound to 50 kDa and 150 kDa binding proteins in plasma was increased by GH and IGF-I treatments but, in contrast to IGF-I, GH increased the proportion of IGF-I bound to 150 kDa binding protein. In a second experiment, four goats received systemic infusion of IGF-I (43 nmol/h) and four goats received systemic infusion of physiological saline (20 ml/h). There was no evidence that milk yield was changed during IGF-I infusion. However, when those goats which had previously received IGF-I infusions were injected with GH, milk yield increased by 30%.(ABSTRACT TRUNCATED AT 250 WORDS)