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Journal of Dairy Science Vol. 78 No. 10 2140-2149
© 1995 by American Dairy Science Association ®
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Comparison of Growth Hormone-Releasing Factor and Somatotropin: The Somatotropic Axis In Lactating Primiparous Cows

W. K. Vanderkooi 1, M. J. Vandehaar 1, B. K. Sharma 1, M. Binelli 1, H. A. Tucker 1, R. M. Akers 1, and W. M. Moseley 1

1 Department of Animal Science, Michigan State University, East Lansing 48824 and Department of Dairy Science, Virginia Polytechnical Institute and State University, Blacksburg 24061, and The Upjohn Company, 7923-25-4 Performance Enhancement Research, Kalamazoo, MI 49001

Our objective was to compare the effects of recombinant bovine growth hormone-releasing factor and recombinant bST on the somatotropic cascade in lactating dairy cows. Primiparous cows were killed after 63 d of continuous daily infusion with 12 mg of releasing factor or 29 mg of bST or no infusion (controls). Both hormone infusions similarly increased mean concentration of somatotropin in serum, but pulsatility of somatotropin in serum was greater for cows given releasing factor. Both hormone infusions increased the amounts of IGF-I in serum, IGF-I mRNA in liver, and IGF-binding protein-3 in serum and decreased IGF binding protein-2 in serum, but these effects were less for cows given releasing factor than for those given bST. Both infusions decreased the number of free binding sites for IGF-I in mammary tissue. In liver, treatment did not alter the abundance of mRNA for the somatotropin receptor or the number of free binding sites for somatotropin. Results suggest that endogenous somatotropin is less effective as an IGF-I secretagogue than is exogenous bST infused continuously, yet the releasing factor and bST increased milk yield similarly. We conclude that growth hormone-releasing factor stimulates milk synthesis mostly through the same mechanisms as bST, but that serum IGF-I alone is not a good indicator of the galactopoietic potency of the two hormones.

Key Words: somatotropin • growth hormone-releasing factor • insulin-like growth factor-I

Submitted on August 8, 1994
Accepted on May 3, 1995




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