Effects of the beta-adrenergic agonist clenbuterol in cows: lipid metabolism, milk production, pharmacokinetics, and residues.

The effects of the beta-adrenergic agonist clenbuterol on lipid metabolism, milk production, pharmacokinetics, and residues were studied in six lactating Brown Swiss cows. Four of these were treated with the growth-promoting dose of 5 micrograms of clenbuterol/kg of BW, mixed within the concentrate, and administered twice a day for 3 wk. The remaining two cows served as controls. All animals were in their third phase of lactation and were fed diets containing corn silage, hay, and concentrate according to individual milk production level and body weight. Milk and blood samples were collected for analysis following a rigid time schedule. Milk production and milk contents (triglycerides, protein, and lactose) were quantitatively identical in both treated and control animals, whereas significant qualitative changes occurred in the fatty acid composition of milk lipids in clenbuterol-treated animals. Compared with the controls, the relative amount (percentage of total fatty acids) of unsaturated fatty acids, particularly oleic acid (cis-delta 9-octadecaonate, 18:1), increased considerably with a simultaneous decrease of the transition chain length fatty acids (lauric acid [12:0] and myristic acid [14:0]). Plasma glucose and FFA concentrations were elevated. Concentrations of insulin-like growth factor I remained unchanged. We conclude that the physiological effect of clenbuterol is limited to the repartitioning effect in body composition, and that milk production is affected only slightly by clenbuterol. An immediate increase of clenbuterol concentrations in plasma (3.4 +/- 2.0 ng/mL) and milk (10.8 +/- 4.7 ng/mL) could be observed at the commencement of treatment. Clenbuterol concentration peaked after 10 d and remained constant until the end of the treatment.(ABSTRACT TRUNCATED AT 250 WORDS)