Are the galactopoietic effects of growth hormone-releasing factor in dairy cattle mediated solely through somatotropin

Three studies were conducted in lactating Holstein cows to determine if growth hormone-releasing factor (GRF)-induced increases in milk yield and serum concentrations of somatotropin (ST) and insulin-like growth factor-I (IGF-I) could be sustained over a 60-d period, and to compare the effects of GRF and bovine ST (bST) on milk yield and serum concentrations of ST and IGF-I.;In Experiment 1, relative to controls, 1, 3 or 12 mg GRF/d increased milk yield and serum ST and IGF-I in a dose dependent manner for 60 d. Furthermore, response of serum ST to 3 and 12 mg GRF/d was sustained over 60 d, thus there was no evidence of refractoriness to GRF. Following withdrawal of 12 mg GRF/d, milk yield remained elevated for at least 15 d.;In Experiment 2, the galactopoietic effects of GRF (12 mg/d) and bST (14 mg/d) were compared. In previous independent studies these doses of GRF and bST optimally increased milk yield. Relative to controls, bST increased milk yield and serum concentrations of ST and IGF-I. Relative to bST, GRF increased milk yield and serum concentrations of ST and IGF-I. The pattern of response of milk yield and serum ST and IGF-I support the hypothesis that GRF induced increases in milk yield are mediated by increased serum concentrations of ST and IGF-I.;In Experiment 3, the galactopoietic effects of i.v. infusion of GRF (12 mg/d) and bST (29 mg/d), which elicited similar increases in serum ST, were compared. Relative to controls, GRF and bST increased serum concentrations of ST and IGF-I. Serum concentrations of ST and IGF-I did not differ between GRF- and bST-treated cows. Relative to controls, bST and GRF increased milk yield 28 and 41%. Relative to bST, GRF increased milk yield by 10%, despite similar concentrations of serum ST and IGF-I in both groups.;In conclusion, GRF is galactopoietic and maintains increased secretion of ST in dairy cattle for at least 60 d. GRF-induced increases in milk yield were greater than increases induced by bST. The galactopoietic action of GRF is not mediated solely by elevation of total radioimmunoassayable ST and IGF-I.