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 phenethanolamine lean enhancer being developed for commercial use in finishing pigs, on beta-adrenoceptor (beta-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. beta-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 < or = .05) linear effects of Rac in middle and outer SQ adipose tissue, and also a significant (P < or = .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 beta-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 beta-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.
Spurlock
M E
ME
Department of Animal Sciences, Purdue University, West Lafayette, IN 47907.
Cusumano
J C
JC
Ji
S Q
SQ
Anderson
D B
DB
Smith
C K
CK
Hancock
D L
DL
Mills
S E
SE
D000273:Adipose Tissue / Q000187:drug effects* / Q000378:metabolism; D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000818:Animals; D001667:Binding, Competitive; D015536:Down-Regulation; D006133:Growth Substances / Q000494:pharmacology; D008024:Ligands; D008297:Male; D009132:Muscles / Q000187:drug effects* / Q000378:metabolism; D010627:Phenethylamines / Q000494:pharmacology*; D011869:Radioligand Assay; D011897:Random Allocation; D011941:Receptors, Adrenergic / Q000032:analysis / Q000187:drug effects / Q000378:metabolism; D013552:Swine / Q000378:metabolism*
D000318:Adrenergic beta-Agonists; D006133:Growth Substances; D008024:Ligands; D010627:Phenethylamines; D011941:Receptors, Adrenergic; C060897:ractopamine