ABSENCE OF EFFECTS OF CARDIAC GLYCOSIDES ON CARDIAC MYOSIN AND A Ca⁺⁺-SENSITIVE RECONSTITUTED CARDIAC ACTOMYOSIN

¹ Department of Physiology, College of Physicians and Surgeons, Columbia University, New York, New York

Ouabain in concentrations from 10^-4 to 10^-10 M had no effect upon either the Mg⁺⁺- or Ca⁺⁺-activated adenosine triphosphatase activity of cardiac myosin. Examination of a reconstituted cardiac actomyosin which responds to physiologic variations of free Ca⁺⁺ concentration revealed no stimulatory effects of ouabain on either adenosine triphosphatase activity or superprecipitation. No evidence was obtained that ouabain potentiates the stimulatory action of free Ca⁺⁺ or exerts a direct Ca⁺⁺-like action on actomyosin. Similarly, digitoxin was without effect on these in vitro systems. The positive inotropic action of cardiac glycosides on the intact heart thus does not appear to reflect a direct interaction with the contractile proteins.