Acute Effects of Beta-Adrenergic Agonists on Porcine Adipocyte Metabolism In Vitro3

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Abstract
Backfat was obtained at slaughter from market weight hogs to study the acute effects of clenbuterol (CB), ractopamine (RAC) or epinephrine (EPI), in the presence and absence of theophylline (THEO) or adenosine deaminase (ADA), on rates of lipolysis and fatty acid synthesis in vitro. Only EPI increased lipolytic rate in the absence of THEO or ADA. In the presence of THEO or ADA, RAC and CB were lipolytic, although CB had a lower maximal response. With THEO present, RAC and EPI increased lipolysis with a similar potency and responsiveness. Lipolytic responses from all agonists were prevented by propranolol. Insulin stimulated glucose incorporation into fatty acids 50 to 100%; stimulated rates were not influenced by any agonist, either alone or in the presence of ADA. When THEO was present, EPI and RAC inhibited fatty acid synthesis approximately 50%. Clenbuterol was not inhibitory under any conditions. Results indicate that, under appropriate conditions, β-adrenergic agents increase lipolysis and decrease lipogenesis in porcine adipocytes. Combined evidence suggests that lipolysis is more sensitive to β-adrenergic stimulation than is insulin-stimulated lipogenesis. Finally, RAC and CB possess only partial agonist activity relative to EPI, CB being least active.