Control of the Activity of Highly Purified Cardiac Actomyosin by Ca 2+ , Na + and K +

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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.