Distribution, elimination, and residues of [14C]clenbuterol HCl in Holstein calves

Clenbuterol HCl is a beta-adrenergic agonist that has been used illegally in Europe and the United States by some livestock producers to increase carcass leanness. The objectives of this study were to determine the metabolic disposition, distribution of radioactivity, and the concentrations of parent clenbuterol in tissues after a single oral dose of [14C] clenbuterol HCl in calves. [14C]Clenbuterol HCl (1.59 microCi/mg, 3 mg/kg BW) was administered to a 74- and a 96-kg Holstein bull calf as a single oral dose. Blood samples were taken at 1, 3, 6, 12, 24, 36, and 48 h after dosing; urine and feces were collected separately and placed into respective pools from 0 to 6, 6 to 12, 12 to 24, 24 to 36, and 36 to 48 h after dosing. At 48 h after dosing, calves were anesthetized and exsanguinated, and visceral organs, bile, eyes, brain, skeletal muscle, skin, adipose tissue, and the remainder of the carcass were collected. Tissues were processed by coarse grinding through a Hobart grinder, followed by homogenization on dry ice. Eyes were dissected and the various tissues and excreta were assayed for total radiocarbon content by combustion analysis and(or) liquid scintillation counting. Parent clenbuterol was measured in selected tissues by HPLC after solvent extraction. Urinary, fecal, and carcass radioactivity averaged 41.5 +/- 8.1, 2.4 +/- .4, and 52.3 +/- 8.7% of the dose, respectively (mean +/- SD.). Average total recovery of radiocarbon was 96.2 +/- .4%. Radioactive residues present in carcasses averaged (ppm; mean +/- SD.): blood, .6 +/- .2; heart, 1.4 +/- .0; lungs, 8.4 +/- 1.7; spleen, 2.6 +/- .3; liver, 5.0 +/- .4; kidney, 5.9 +/- .0; brain, 1.9 +/- .4; adipose tissue, 1.1 +/- .2; rumen, reticulum, omasum, abomasum, 2.3 +/- .4; small intestine, 3.2 +/- .3; large intestine, 4.0 +/- .4; skeletal muscle, 1.0 +/- .2; bile, 12.5 +/- 4.0; white skin, .7 +/- .1; black skin, 4.0 +/- .7; remainder of the carcass, 1.0 +/- .2. Ocular residues were as follows: aqueous humor, 6.3 +/- 1.2; cornea, 13.5 +/- 8.6; iris, 255.8 +/- 167.0; lens, 2.3 +/- 1.5; vitreous humor, 2.2 +/- .4; retina/choroid, 84.5 +/- 34.1; sclera, 11.1 +/- 2.1. Mean concentrations of parent clenbuterol in tissues were (ppm; mean +/- SD): lung, 6.8 +/- .9; liver, 2.2 +/- .5; kidney, 3.7 +/- .5; and heart, .9 +/- .1. Parent clenbuterol represented from 43.9% of the total residue in liver to 81.2% of the total residue in lung.