Beta-agonists – ractopamine and zilpaterol – have clear and consistent responses when fed as recommended to cattle and swine at the end of the finishing phase, according to Drs. Todd See of North Carolina State University and R. Rathman of Texas Tech University during a preconference symposium at the 2014 Joint Annual Meeting of the American Society of Animal Science and American Dairy Science Assn. in Kansas City, Mo.

See and Rathman discussed live animal and carcass trait effects of beta-agonists, but first Dr. Brad Johnson of Texas Tech explained some of the compounds’ mode of action on muscle tissue.

Beta-agonists bind to specific, naturally-occurring receptors in tissue in order for there to be a response, Johnson said, noting that these receptors have been found in almost all mammalian tissues. This is a different mode of action than steroidal implants that are used in cattle production, which elicit an endocrine response.

Johnson, who has extensively studied both approved beta-agonists, pointed out that beta-agonists have a direct effect on bovine muscle tissue in that the compounds increase the amount of protein synthesis in muscle fibers. Considering that protein accretion is the result of protein synthesis minus protein degradation (beta-agonists have some effect on protein degradation but is not a major focus), the increased protein synthesis results in more protein in muscle tissue, and therefore, larger muscle mass, he explained.

Beta-agonists also have effects on adipose tissue development, but those effects are not as clear, explained Dr. Steve Smith of Texas A&M University. He said research studies going back to the 1980s are not clear if beta-agonists have a direct role in adiposity or if the effects are a result of more energy being diverted to muscle tissue so that there is less available for fat development.

Smith did note that beta-agonists appear to act through a combination of lipid cell hypertrophy (larger cells) and hyperplasia (number of cells). Many studies have found fewer, larger fat cells, which means leaner carcasses, he said.

One theory is that beta-agonists act through a “dilution effect” in which the same number of marbling  cells are present in a steak, but that steak is larger, Smith said.

These inconsistent effects on lipogenesis are probably secondary effects of beta-agonists to muscle growth, Smith concluded.