Qualitative Differences between β‐Adrenergic and α‐Adrenergic Inotropic Effects in Rat Heart Muscle

Abstract If β‐ and α‐adrenergic inotropic effects are cyclic AMP dependent and cyclic AMP independent, respectively, they may be qualitatively different. The inotropic effects of β‐receptor stimulation (isoprenaline) and α‐receptor stimulation (phenylephrine combined with propranolol) were characterized in isolated perfused rat hearts, rat atria and rat papillary muscles. The β‐effect reached its maximum before the α‐effect. The α‐effect followed a three‐phasic time‐course indicating both stimulatory and inhibitory components. The aortic pressure wave (perfused heart) indicated a shorter contraction phase after β‐stimulation than after α‐stimulation. The time to peak tension (atrium, papillary muscle) was relatively shorter after isoprenaline than after α‐stimulation, which tended to prolong it. The contraction‐relaxation cycles (atrium, papillary muscle) were examined by recording the isometric tension (T), its first (T′) and second (T″) derivatives, α‐ and β‐stimulation both increased T max , T′ max (maximal rate of tension rise), T′ min , (maximal rate of tension decline) and T″ min (maximal rate of transition from rise to decline of tension). Isoprenaline increased T min , (papillary muscle) and T″ min (atrium, papillary muscle) relatively more than did α‐stimulation, i.e. the relaxing processes were activated relatively more by β‐stimulation. The results indicate different mechanisms for the two adrenergic inotropic effects. The relatively larger activation of relaxation by β‐stimulation is assumed to be caused by cyclic AMP.