Control of the Activity of Highly Purified Cardiac Actomyosin by Ca²⁺, Na⁺ and K⁺

Abstract

The responses of cardiac actomyosin to changes in the concentrations of Ca²⁺, Na⁺ and K⁺ were examined to evaluate the role that direct actions of these cations could play in the regulation of myocardial contractility. The sensitivity of the Mg²⁺-activated adenosine triphosphatase (ATPase) activity of reconstituted cardiac actomyosin to changing Ca²⁺ concentration was similar to that for skeletal actomyosin although both cardiac myosin and actomyosin were less active than the corresponding skeletal proteins at all Ca²⁺ concentrations. Activation of both myosins by either the cardiac or skeletal actintropomyosin complex was virtually absent when free Ca²⁺ concentration was below 10^-7 M. Full activation was seen at Ca²⁺ concentrations greater than 10^-5 M. These findings support the view that variations in intracellular Ca²⁺ concentration play a role in regulating myocardial contractility.

Replacement of KCl by NaCl stimulated both the ATPase activity and superprecipitation of cardiac actomyosin. The positive inotropic effects associated with replacement of intracellular K⁺ by Na⁺, e.g. the positive staircase accompanying increased heart rate, may therefore be due, in part at least, to a direct action of Na⁺ upon the contractile proteins.