Canine cardiac myosin with special reference to pressure overload cardiac hypertrophy. II. Myosin ATPase.

Normal canine cardiac myosin exhibits Ca”+ATPase of 0.13 pmol of Pi/min*mg and (K+,EDTA)-ATPase of 0.47 pm01 of Pi/min*mg. Specific activities are similar in myosin preparations from normal left and right ventricles, contrary to the prior report by Wikman-Coffelt et al. (Wikman-Coffelt, J., Fenner, C., Smith, A., and Mason, D. T. (1975) J. Biol. Chem. 250,1257-1262). At 5 weeks after aortic banding, (K+,EDTA)-ATPase of myosin is normal or slightly diminished in mild hypertrophy, and depressed approximately 50% in moderate hypertrophy, whereas Ca’+ATPase appears unaltered. Similar activities are obtained for left and right ventricles from the same heart. At 13 weeks after aortic banding, activities appear normal in left and right ventricle myosin. Provided that EDTA is present, rates of denaturation during storage of cardiac myosin are too slow to account for the differences observed in freshly prepared myosin. The changes in myosin ATPase during pressure overload hypertrophy may reflect denaturation or other modification of myosin in vivo, although changes in the heavy chains cannot be excluded. There is some evidence for reversible denaturation of myosin ATPase. When cardiac myosin is dialyzed against 10 mM pyrophosphate and then returned to original solvent conditions, ATPase activities are augmented with respect to control myosin; the effect is relatively greater in stored myosin than freshly prepared myosin. In addition, pyrophosphate treatment fractionates myosin into two solubility classes, with gradual conversion from a pyrophosphate soluble fraction to a pyrophosphate gel phase upon storage of myosin in 0.5 M KCl; both fractions exhibit augmented activities.