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.