Absence of direct actions of norepinephrine on cardiac myosin and cardiac actomyosin

Abstract

Norepinephrine was without effect on the Mg++-activated ATPase activity of cardiac myosin nor did it influence the ATPase activities of a Ca++-sensitive reconstituted cardiac actomyosin during the clearing phase and after superprecipitation. Neither the rate nor extent of superprecipitation of cardiac actomyosin was altered by this agent. It is concluded that the action of catecholamines on the cardiac contractile proteins is most likely as indirect one, resulting from a primary effect on the excitation-contraction coupling mechanism of the heart.