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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.

To determine the temporal effect of ractopamine (Rac), a phenethanolamine, on adipose lipogenic enzyme activity and gene expression, 20 crossbred barrows were fed Rac (20 mg/kg of diet) for 0, 1, 8, or 24 d before slaughter (105 +/- 1 kg). Ractopamine had no effect (P > .05) on the activity of acetyl-coenzyme A carboxylase or malic enzyme in either the middle or outer layers of subcutaneous adipose tissue. Similarly, mRNA abundance for acetyl-coenzyme A carboxylase and the glucose transport proteins Glut 1 and Glut 4 were not affected by Rac in either adipose depot. Despite the inability of Rac to affect adipose tissue metabolism, Rac increased nitrogen retention, longissimus muscle area, and alpha-actin gene expression in skeletal muscle. Results indicate that Rac was not a functional beta-adrenergic agonist toward adipose tissue in this study. We suggest that the response to Rac in adipose tissue is masked by a combination of factors including tissue insensitivity, Rac-dose limitation, inherent partial agonism of Rac, and beta-adrenoceptor down-regulation.

Substances in evacuated blood-collection devices produce gas-chromatographic peaks with retention times similar to those of drugs. All tubes tested except the serum separator tubes contained tris(2-butoxyethyl) phosphate, as determined by mass spectrometry. In addition, the partition coefficient during the solvent-extraction step increased for some drugs by as much as 40%, while for other drugs it decreased by as much as 30%, depending on the drug and the collection tube. The Becton Dickinson serum separator tube also contains several other compounds identified by mass spectrometry to be dibasic esters used in the preparation of the polyester gel. The Corning serum separator tube and the Venoject serum tube contain still other impurities. The Becton Dickinson royal-blue-stoppered tube contained the least amount of impurities of the tubes tested. CVs of 25 commonly measured drugs in plasma pools increased from 5% for samples collected in glass tubes to greater than 20% for samples collected in evacuated blood-collection tubes. Clearly, accuracy and precision of measurements of therapeutic drugs can be seriously compromised if an inappropriate blood-collection device is used.