The effect of ractopamine on β-adrenoceptor density and affinity in porcine adipose and skeletal muscle tissue

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Abstract
We have evaluated the effect of feeding ractopamine (Rac), a phenethanolamine lean enhancer being developed for commercial use in finishing pigs, on β-adrenoceptor (β-AR) number and ligand-receptor binding affinity in adipose and muscle tissues. Pigs weighing 78 ± 1 kg were fed Rac (20 mg/ kg of diet) for 0 (control), 1, 8, or 24 d before being killed at 105 ± 1 kg BW. β-adrenoceptor density (per milligram of protein) was decreased by Rac up to approximately 50% in both the middle and the outer layers of subcutaneous (SQ) adipose tissue. Orthogonal contrasts indicated significant (P ≤ .05) linear effects of Rac in middle and outer SQ adipose tissue, and also a significant (P ≤ .05) quadratic effect of Rac in the middle layer. Ractopamine did not affect the maximal binding (Bmax) of longissimus muscle. The relative affinity with which the β-AR population of the tissues examined bound the radioligand ([3H]dihydroalprenolol) was not influenced by Rac. Likewise, feeding Rac had no effect on the affinity of the β-AR for Rac. The data indicate that a Rac-induced reduction in the Bmax of adipose tissue may account for the diminished in vitro lipolytic potency of exogenous Rac after prolonged periods of Rac feeding, and that Rac-induced desensitization differs between adipose and skeletal muscle tissues.

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