Blockade of the rate and force responses to phenylephrine on rat atria with labetalol.

Evidence suggests that the combined alpha and beta adrenergic agonist phenylephrine influences cardiac tissue through stimulation of both alpha and beta adrenoceptors. Phenylephrine effected a positive inotropic and chronotropic response in paced (1 Hz) left and spontaneously beating right atria, respectively. In the presence of the combined alpha and beta adrenoceptor antagonist labetalol (5 x 10(-6)M) the rate and force responses to phenylephrine were completely abolished. The data suggest that phenylephrine exerts its inotropic and chronotropic effects through both alpha and beta adrenoceptors in such a way that both adrenoceptors must be blocked in order to completely abolish the responses induced by the dual agonist.