Increased secretion of insulin-like growth factor I into milk of cows treated with recombinantly derived bovine growth hormone

Six lactating, non-pregnant Jersey cows were given subcutaneous injections of recombinantly derived bovine growth hormone for 7 d. Milk yield was increased by 4.5 kg/d on d 7, compared with the average yield of 10.7 +/- 0.4 kg/d (mean +/- s.e.m.) for the 7 d preceding treatment. Concentrations of insulin-like growth factor I (IGF-I) in the milk increased from 0.44 +/- 0.04 nmol/l (mean +/- s.e.m.) during the 7 d preceding treatment to 1.6 +/- 0.2 nmol/l on d 7 of treatment. Taking the increase in milk yield into account the total increase in the secretion of IGF-I into milk of one udder half was 6-fold. Plasma concentrations of total IGF-I rose from 15.5 +/- 1.3 nmol/l (mean +/- s.e.m.) on the day preceding treatment to 56.9 +/- 3.6 nmol/l (mean +/- s.e.m.) on d 7 of treatment. Mammary plasma flow increased from 1.6 +/- 0.09 to 2.2 +/- 0.06 l/min.udder half over the same time. Estimates of the amount of IGF-I that reached the mammary gland gave values of 24 and 116 nmol/min.udder half before and during treatment respectively. IGF-I in milk of treated cows was associated predominantly with proteins ranging from 40,000 to 150,000 mol.wt, but a significant proportion (19%) of the total IGF-I was present in the free unbound form. IGF-I crosslinking studies revealed the presence in milk of one specifically labelled band at 31,000 mol.wt.