The control of b-agonists misuse received extra attention after outbreaks of food poisoning in 1990 in Spain caused by consumption of bovine liver. This was the first time that pharmacological residues found in slaughtered cattle were found to have caused acute intoxication in consumers. As therapeutic drugs, they have been first used as a bronchodilator for the treatment of pulmonary diseases in humans and animals. b-Agonists act specifically by binding to cell membrane b-receptors, and therefore, by activating the adenylate cyclase-cyclic AMP system. Physiological responses to stimulation of the b-receptors are increased lipolysis and reduced lipogenesis in adipose cells, increased glycogenolysis, increased protein synthesis and reduced proteolysis in striated muscle fibres, and vasodilatation, bronchodilatation and relaxation of smooth muscle fibres. Although clenbuterol is still probably the most popular b-agonist illegally used on farms, other b-agonists have been pointed out on the ''black market''. b-Agonists belong to the group of arylethanolamines, and can be catalogued in three chemical classes according to their chemical structure: the anilines (e.g., clenbuterol), resorcinols (e.g., terbutaline) and phenols (e.g., salbutamol). Zilpaterol ((7 )-trans-4,5,6,7-tetrahydro-7-hydroxy-6-(isopropylamino)imidazo [1jk,4,5]-benzazepin-2(1H)-one) is a new approved b-agonistic drugs (USA, Mexico, South Africa) characterized by a cyclic chemical structure.
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