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We have measured the EDTA and Ca2+ activated ATPase activity of myosin from hearts of normal dogs, dogs in gross heart failure and dogs with cardiac hypertrophy without failure. Heart failure was caused by systolic and/or diastolic hemodynamic loads. Under one or more assay conditions, depressed ATPase activity was consistently found for myosin from failing hearts. In contrast to myosin from hearts with diastolic loads, myosin from failing hearts with pulmonic stenosis in the presence of Ca2+ and 0.1 m KCl showed normal mean ATPase activity with several preparations having elevated activity. Elevated ATPase activities were obtained for myosin from the non-failing, but hypertrophied hearts of dogs with aortic stenosis. The data suggest that the interaction of ionic strength and pH with the ATPase active site of suggest that the interaction of ionic strength and pH with the ATPase active site of myosin from failing hearts is altered, thus modifying the ATPase activity in the presence of Ca2+ or EDTA. The altered interaction may be modified by the presence of hypertrophy, resulting in normal or increased activity in the presence of Ca2+ and decreased activity in the presence of EDTA.

We used light scattering and ultracentrifugation to study the binding ratio of myosin and actin from normal and failing dog hearts and to determine the effect of temperature and adenosine triphosphate (ATP) on the binding. For comparison, similar studies were done on myosin and actin from rabbit skeletal muscle. We found a higher combining ratio, by weight, for cardiac myosin and actin (4.8 : 1) than for skeletal myosin and actin (4.4 : 1). The dissociation constants were similar at 24°C for cardiac and skeletal proteins. But at 10°C, the dissociation constant was greater for cardiac proteins, and the difference appeared more pronounced when phosphate was used to buffer the solutions. The dissociation constant at 10°C was greater for myosin and actin from failing dog hearts. ATP caused dissociation of cardiac and skeletal F-acto-heavy meromyosin and F-actomyosin. There was evidence that the binding of cardiac proteins may not be as tight as that of skeletal muscle proteins.

This work was done to evaluate the response of cardiac myosin ATPase in the compensated heart subjected to the increased volume work of a high cardiac-output state. Dogs with bilateral femoral arteriovenous fistulas were studied either 14 days (acute arteriovenous fistulas) or a mean of 230 days (chronic arteriovenous fistulas) after construction of the fistulas. Dogs with anemia secondary to repeated bleeding (anemic) were studied 14 days after a high output state was first documented. Calcium-activated myosin adenosine-triphosphatase (ATPase) activity was significantly elevated for the anemic and acute arteriovenous fistula dogs, but the elevations were not significant for myosin from the chronic arteriovenous fistula dogs. In the presence of K + and EDTA, the cardiac myosin ATPase activity was generally elevated for the anemic, less so for the acute arteriovenous fistula and was normal for the chronic arteriovenous fistula. The Ca 2+ uptake (in the presence of oxalate) by the cardiac sarcoplasmic reticulum was significantly less than normal for all three groups.

1. 1. Myosin was extracted from dog hearts and isolated by three methods which utilize ammonium sulfate fractionation and by two modifications of SZENT-GYORGYI'S dilution-precipitation method. 2. 2. The molecular weight of the myosin was determined by the YPHANTIS meniscus-depletion equilibrium method. 3. 3. Velocity sedimentation of the myosin using schlieren optics never revealed aggregation. Calculation of the molecular weight by the YPHANTIS method, however, revealed that several myosin samples were disperse. There is some evidence that dispersity may be avoided by using a method which includes a final high speed centrifugation. 4. 4. A statistically significant difference (P < 0.025) was demonstrated between the mean molecular weight, 4.76·105, of myosin prepared by MUELLER'S modification of SZENT-GYORGYI'S method and the mean molecular weight, 5.37·105, of the myosin prepared by all the methods which utilized ammonium sulfate fractionation. 5. 5. The best estimate of the molecular weight of myosin is 4.76·105. 6. 6. The calcium-activated ATPase activity was unaffected by the method of preparation.

Large doses of desiccated thyroid were administered to dogs for from 55 to 176 days (mean 118 days) to evaluate the effect of long-term administration upon the myocardium. An increased pulmonary vascular bed and cardiac enlargement were found in most animals. Calcium-activated myosin adenosine-triphosphatase (ATPase) activity was significantly elevated at pH values of 7.0 and below. The ATPase activity of sulfhydryl-modified myosin was also altered. However, in the presence of K+ and EDTA, the myosin ATPase activity was similar to that of myosin from normal dog hearts. The calcium uptake by the cardiac sarcoplasmic reticulum (CSR) (in the presence of oxalate) was significantly less than normal for the thyroid-treated dogs. The calcium content of the CSR was also less than normal for the treated dogs. The norepinephrine content of the myocardium from the thyroid-treated dogs was less than normal, but when corrected for hypertrophy was within normal limits.