Alpha-adrenergic stimulation contributes to the inotropic effect of noradrenaline

The alpha- and beta-adrenoceptor mediated inotropic effects of noradrenaline were determined in rat heart papillary muscle. The pure alpha-adrenergic inotropic effect, obtained in the presence of the beta-blocker timolol, was about half the pure beta-adrenergic inotropic effect, obtained in the presence of the alpha-blocker prazosin. The maximal inotropic effect of noradrenaline in the absence of blockers was, however, of the same magnitude as the pure beta-adrenergic effect. The alpha- and beta-adrenergic components of the inotropic response to noradrenaline were determined as those reversed by prazosin and timolol, respectively. They were smaller than the pure effects evoked separately. When beta-stimulation by noradrenaline was attenuated by carbachol, the alpha-adrenergic inotropic component was increased. Thus the alpha-effect is increased when the beta-adrenergic effect is antagonized. The alpha-adrenergic inotropic component of noradrenaline seems to play a variable role depending upon the degree of concomitant beta-receptor stimulation in the heart.