Ractopamine and age alter oxygen use and nitrogen metabolism in tissues of beef steers

Authors: EISEMANN, JOAN - North Carolina State University; NIENABER, JOHN - Retired ARS Employee; HUNTINGTON, GERALD - North Carolina State University

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/9/2022
Publication Date: 11/1/2022
Citation: Eisemann, J.H., Nienaber, J.A., Huntington, G.B. 2022. Ractopamine and age alter oxygen use and nitrogen metabolism in tissues of beef steers. Journal of Animal Science. 100(11). Article skac304. https://doi.org/10.1093/jas/skac304.
DOI: https://doi.org/10.1093/jas/skac304

Interpretive Summary: Beta agonist have been fed to cattle to increase carcass weight. It has been shown that metabolic rate increases when they are fed, and there have been mixed reports on how they affect protein synthesis. The response of different tissues to beta agonist and the influence of age on those responses have not been well defined. Oxygen consumption by the portal-drain viscera and liver of young steers were greater than older steers, and inclusion of Ractopamine increased oxygen consumption of the hindquarter of older steers. Ractopamine increased protein deposition in the hindquarters of both age groups, but the mechanism for the increase differed. Increased protein deposition in older steers was due to increased protein synthesis and increased deposition in younger steers was due to decreased protein degradation.

Technical Abstract: The objective was to quantify the effects of age and ractopamine (RAC) on whole body oxygen consumption and Leu flux, and oxygen flux and metabolism of nitrogenous compounds by the portal-drained viscera (PDV), liver, and hindquarters (HQ) of steers. Multicatheterized steers were fed a high energy diet every 2 h in 12 equal portions. Five younger steers (body weight, (BW) = 223 ± 10.1 kg) were six mo old and 5 older steers (BW = 464 ± 16.3 kg) were 14 mo old. Treatments were control (Cont) or 80 mg RAC per kg diet in a crossover design. Nitrogen (N) balance was measured on d 9-13. Whole body oxygen consumption and net flux were measured on d 11 and d13, and net flux of N variables, Phe and Leu kinetics were measured on d 13. Whole body oxygen consumption increased (P < 0.05) in response to RAC in older but not younger steers. Retained N was greater (P = 0.009) for younger than older steers and increased (P = 0.010) with RAC in both ages of steers. Nitrogen retained as a percentage of N apparently absorbed increased (P < 0.05) in the older steers but not the younger steers in response to RAC. Oxygen uptake was greater (P < 0.05) in PDV, liver and total splanchnic tissues in the younger steers and there was no response to RAC. In contrast, oxygen uptake in HQ increased (P < 0.05) with RAC in the older but not the younger steers. Concentration and net PDV release of a-amino N (AAN) were not affected by age or RAC. Uptake of AAN by liver decreased with RAC (P = 0.001). Splanchnic release of AAN was greater in younger steers (P = 0.020) and increased (P = 0.024) in response to RAC. For HQ tissues, uptake (P = 0.005) and extraction (P = 0.005) of AAN were lesser in older than younger steers and both increased (P = 0.001) in response to RAC. Based on Phe kinetics in HQ, RAC increased (P < 0.05) protein synthesis in older steers but not in younger steers. In contrast, protein breakdown decreased (P < 0.05) in response to RAC in younger steers. In response to RAC, protein degradation was less (P < 0.05) in younger than older steers. Based on Leu kinetics, whole body protein synthesis was greater in the younger steers (P = 0.022) but not altered in response to RAC. Ractopamine enhanced lean tissue growth by increasing supply of AAN to peripheral tissues and altering protein metabolism in HQ. These metabolic responses are consistent with established responses to RAC in production situations.