DISTRIBUTION, ELIMINATION, AND RESIDUES OF [C-14] CLENBUTEROL HCL IN HOLSTEIN CALVES

Clenbuterol HCl is a beta-adrenergic agonist that has been used illega lly in Europe and the United States by some livestock producers to inc rease carcass leanness. The objectives of this study were to determine the metabolic disposition, distribution of radioactivity, and the con centrations of parent clenbuterol in tissues after a single oral dose of [C-14] clenbuterol HCl in calves. [C-14]Clenbuterol HCl (1.59 mu Ci /mg, 3 mg/kg BW) was administered to 74- and a 96-kg Holstein bull cal f 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 a nd 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 ane sthetized and exsanguinated, and visceral organs, bile, eyes, brain, s keletal muscle, skin, adipose tissue, and the remainder of the carcass were collected. Tissues were processed by coarse grinding through a H obart grinder, followed by homogenization on dry ice. Eyes were dissec ted and the various tissues and excreta were assayed for total radioca rbon content by combustion analysis and(or) liquid scintillation count ing. Parent clenbuterol was measured in selected tissues by HPLC after solvent extraction. Urinary, fecal, and carcass radioactivity average d 41.5 +/- 8.1, 2.4 +/- .4, and 52.3 +/- 8.7% of the dose, respectivel y (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; skele tal muscle, 1.0 +/- .2; bile, 12.5 +/- 4.0; white skin, .7 +/- .1; bla ck skin, 4.0 +/- .7; remainder of the carcass, 1.0 +/- .2. Ocular resi dues 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 concentrat ions 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 li ver to 81.2% of the total residue in lung.