THE EFFECT OF RACTOPAMINE ON BETA-ADRENOCEPTOR DENSITY AND AFFINITY IN PORCINE ADIPOSE AND SKELETAL-MUSCLE TISSUE

We have evaluated the effect of feeding ractopamine (Rac), a phenethan olamine lean enhancer being developed for commercial use in finishing pigs, on beta-adrenoceptor (beta-AR) number and ligand-receptor bindin g affinity in adipose and muscle tissues. Pigs weighing 78 +/- 1 kg we re fed Rac (20 mg/kg of diet) for 0 (control), 1, 8, or 24 d before be ing killed at 105 +/- 1 kg BW. beta-adrenoceptor density (per milligra m of protein) was decreased by Rac up to approximately 50% in both the middle and the outer layers of subcutaneous (SQ) adipose tissue. Orth ogonal contrasts indicated significant (P less than or equal to .05) l inear effects of Rac in middle and outer SQ adipose tissue, and also a significant (P less than or equal to .05) quadratic effect of Rac in the middle layer. Ractopamine did not affect the maximal binding (B-ma x) of longissimus muscle. The relative affinity with which the beta-AR population of the tissues examined bound the radioligand ([H-3]dihydr oalprenolol) was not influenced by Rac. Likewise, feeding Rac. The dat a indicate that a Rac-induced reduction in the B-max of adipose tissue may account for the diminished in vitro lipolytic potency of exogenou s Rac after prolonged periods of Rac feeding, and that Rac-induced des ensitization differs between adipose and skeletal muscle tissues.