RESUMEN
The mechanism through which the repartitioning agent clenbuterol increases heart rate was investigated. First, the relative importance of the beta 1- and beta 2-adrenoceptors was established in rat and bovine right atria in vitro. The positive chronotropic and inotropic effects of (+/-)isoproterenol in rat and bovine right atria, respectively, were markedly antagonized (P < .001) by the beta 1-adrenoceptor antagonist CGP 20712A but were antagonized less by the beta 2-adrenoceptor antagonist ICI 118 551 in rat (P < .01), but not in bovine atria, indicating a major role of the beta 1-adrenoceptors. Clenbuterol was only a partial agonist in rat right atria, increasing heart rate at high concentrations through stimulation of beta 1-adrenoceptors. In studies in vivo, clenbuterol decreased the plasma potassium concentration (P < .05) and increased the plasma glucose concentration (P < .05). Clenbuterol also reduced diastolic blood pressure (P < .01) and increased heart rate (P < .001). The increase in heart rate was not due to direct stimulation of cardiac beta 1-adrenoceptors by clenbuterol but was consistent with a reflex response to beta 2-adrenoceptor-mediated hypotension. This would have caused the activation of baroreceptors, which in turn would have resulted in both the release of norepinephrine to stimulate cardiac beta 1-adrenoceptors and the inhibition of cholinergic input to the heart. Thus, the effects of clenbuterol could be eliminated completely by ICI 118 551 or reduced by approximately 50% using CGP 20712A. The combination of treatment of clenbuterol and CGP 20712A could be useful. It may allow the full repartitioning effects seen with the beta 2-agonist alone, but with a markedly attenuated effect on the heart. Such a treatment regimen may also help reduce the increased energy expenditure and loss of appetite seen following the initial administration of clenbuterol.