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Nitrogen and Protein Metabolism and Metabolites in Plasma and Urine of Beef Steers Treated with Somatotropin^1,2,3,4,

U.S. Department of Agriculture, Clay Center, NE 68933 and Beltsville, MD 20705 and Cornell University^,8, Ithaca, NY 14850

Abstract

The objectives of this study were to determine the effects of daily injection of bovine somatotropin (bST) on the metabolism of N and 1-[¹⁴C]leucine and on hormone and metabolite concentrations in growing beef steers. Injection of bST increased N retention (P < .05) primarily through decreased (P < .10) urinary N excretion. Plasma concentration of somatotropin, insulin and glucose increased (P < .01) and of urea-N (P < .01) and -amino-N (P < .10) decreased with bST compared with excipient injection. Total leucine flux was not altered by treatment; however, the partition of flux was. Leucine oxidation decreased (P < .05) and leucine used for protein synthesis (P < .10) increased, with bST compared with excipient injection. during excipient injection, 10.3 g protein were synthesized for each gram crude protein deposited, whereas during bST injections only 6.4 g were required. The average maximum contribution of myofibrillar protein degradation to whole body protein degradation, calculated from excretion of 3-methylhistidine, was 16%. Although the ratio of protein deposition/protein synthesis was low for both excipient- and bST-injected steers, the incremental efficiency of protein deposition was 50%, reflecting a dilution of protein synthe- sis required for turnover and a proportionately greater increase in protein synthesis than protein degradation with bST injection. In growing beef steers, bST stimulated whole body protein synthesis and decreased leucine oxidation. The change in partition of leucine flux, but not of total flux (irreversible loss), demonstrates a chronic redirection in metabolism consistent with homeorhetic control. These data from steers injected with bST suggest mechanisms by which bST affects metabolism during normal growth.

¹ The authors would like to express appreciation to R. F. Olsen and The Upjohn Company for donating the somatotropin used in this study.

² Supported in part by USDA Cooperative Agreement No. 58-32U4-3-453.

³ Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the USDA and does not imply its approval to the exclusion of other products that also may be suitable.

⁴ The authors acknowledge T. H. Elsasser for assistance with assay of somatotropin and assay of insulin. The authors wish to thank J. Grinsted, J. Haley, and the animal operations staff for care and feeding of the steers; C. Felber, D. Hucht, A. Kozak, B. Lee, M. Pello, J. Ward, B. Phillips and E. Williams for technical assistance; and J. Rosch for typing the manuscript.

⁵ Roman L. Hruska U.S. Meat Anim. Res. Center, ARS, USDA, Clay Center. To whom reprint requests should be addressed.

⁶ Beltsville Agric. Res. Center.

⁷ Present address: USDA, ARS, Subtropical Agric. Res. Sta., P.O. Box 46, Brooksville, FL 34605.

⁸ Dept. of Anim. Sci.

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