Abstract
The effects induced by dietary clenbuterol (20 micrograms kg-1 body weight day-1 for 40 days) on beta-adrenergic receptor (beta-AR) subtypes in the heart, bronchial smooth muscles and the CNS of veal calves were investigated using a binding method. Clenbuterol exposure caused a significant (P < 0.05, P < 0.01, P < 0.001) decrease in beta 1-AR and beta 2-AR in both cardiac atria and ventricles of treated animals (excluding the beta 2-AR of the right atrium). Similarly, a significant (P < 0.01, P < 0.001) down-regulation of beta-AR subtypes in bronchial smooth muscles of treated calves was observed. In the CNS (cerebral cortex, hippocampus, hypothalamus and cerebellum) the down-regulation was limited to beta 2-AR, with the exception of the hippocampus in which both beta 1-AR and beta 2-AR concentrations were significantly (P < 0.05; P < 0.01) reduced. Scatchard analysis of the binding of the beta-AR antagonist, (-) [3H]CGP 12177, revealed that the down-regulation of beta-AR was not associated with any modification in binding affinity, as Kd values were unaffected by clenbuterol treatment. Data obtained indicated that prolonged clenbuterol exposure induced a remarkable beta-AR down-regulation in the heart, bronchi and brain of veal calves.
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