Functional alpha 1-adrenoceptors in the rat heart during beta-receptor blockade.

In absence of beta-receptor blocking agents, alpha-adrenergic inotropic effects could be demonstrated in the rat myocardium for the synthetic alpha-agonists phenylephrine and methoxamine, but not for the naturally occurring catecholamines, adrenaline and noradrenaline. Other synthetic alpha-agonists were without effects. In the presence of the beta-receptor blocking agent, propranolol or timolol, marked alpha-effects were demonstrated for adrenaline and noradrenaline in both the right and left atria and the right ventricle. The results indicate that alpha-receptors may be functionally important in the beta-blocked myocardium.