Cardiac myosin and congestive heart failure in the dog

Hypertrophy of the human heart at the level of fine structure. An analysis and two postulates

Muscle cells in the left ventricular walls of four markedly hypertrophied human hearts (above 600 gm) were compared with muscle cells in four non-hypertrophied hearts (up to 310 gm). Blocks of tissue obtained postmortem within 6 hours were processed for light and electron microscopy under conditions suitable for good preservation of myofibrils. A lattice parameter, q_h, was defined as the number of myosin filaments per square micron in either H zones or A bands. By the use of methods of electron microscopy, q_h was determined for perpendicular cross-sections of A bands in a large number of well preserved myofibrils of muscle cells in both groups of hearts. Statistical evaluation of the distributions of values of q_h revealed no significant difference between the two groups. Thus, the myofilament lattices in hypertrophied cells were geometrically within normal limits. Planimetric measurements of cross-sectional areas of muscle fibers were made, using photomicrographs obtained from one representative hypertrophied heart and from one control. The size-frequency distribution of the measurements showed a marked difference between the two hearts, and confirmed the presence of hypertrophy of muscle cells. Counts of the number of myofibrils per muscle cell were determined for samples from the same two hearts, evaluated statistically, and found to be significantly higher for the hypertrophied heart. It is proposed (a) that myofibrils in hypertrophied heart muscle cells have filament lattices with geometrical arrangement and macromolecular parameters that are the same as those found in myofibrils of normal heart muscle cells; and (b) that in hypertrophy the number of myofilaments increases through formation of new myofibrils, and possibly also by addition of filaments to preexisting myofibrils.

THE STUDY OF LEFT VENTRICULAR FUNCTION IN MAN BY INCREASING RESISTANCE TO VENTRICULAR EJECTION WITH ANGIOTENSIN

The performance of the left ventricle was studied in 18 patients by determining the effects of increasing the resistance to left ventricular ejection with graded infusions of angiotensin. By this means, left ventricular function curves, i.e., the relationship between ventricular end-diastolic pressure and stroke work, were constructed. In patients without left ventricular disease the ventricular function curves exhibited steep ascending limbs, large increases in stroke work occurring with small elevations of left ventricular end-diastolic pressure; in contrast, in patients with cardiac disease, the ventricular function curves were either flat, i.e., small changes in stroke work occurred with large elevations in left ventricular end-diastolic pressure, or the function curves exhibited descending limbs. Prior to the infusion of angiotensin many patients with and without depression of their left ventricular function curves had comparable values of left ventricular end-diastolic pressure and stroke work. Thus, the angiotensin test may prove useful for detecting impairment of myocardial function and for expressing the severity of the impairment in a quantitative form.

The amino acid composition of actin and myosin and Ca2+-activated myosin adenosine triphosphatase in chronic canine congestive heart failure

The Ca2+-activated myosin ATPase and the amino acid compositions of actin and myosin were determined for preparations from chronically failing dog hearts. Hypertrophy and congestive heart failure were produced by combined tricuspid valve insufficiency and pulmonary artery stenosis. Control, shamoperated, and noncardiac circulatory failure (inferior vena cava constriction) dogs also were studied. All hearts were divided into right ventricle, septum and left ventricle and each sample was individually analyzed. Calcium-activated ATPase decreased in the failing hearts and showed a distinct gradient of depression from right to left ventricles. There were no changes in ATPase activity among the other groups. The amino acid composition of actin was the same regardless of origin. The amino acid composition of myosin was unaltered except that cystine/2 residues were markedly decreased in failing heart myosin. The same gradient of depression was present as was found for Ca2+-activated myosin ATPase. This study suggests that protein metabolism is abnormal and that altered proteins are produced in hypertrophy and congestive heart failure. It appears that these changes do not affect all proteins, since actin was normal by the parameters studied. It is clear that the stressed ventricle is the most severely involved, but the entire heart is altered to some degree. Thus, we conclude that altered protein metabolism may be an important primary factor in the genesis of heart failure.

[The left ventricle in chronic cor plumonale (author's transl)]

The volume of both ventricles was estimated in 25 hearts with chronic cor pulmonale. Also, the weight of the left ventricle, including the whole septum and the free part of the right ventricle, was determined to be 100 g. Distinct hypertrophy of the right ventricular part of the septum could be demonstrated in all cases. A weight increase of the left ventricle, including the whole septum, is frequently caused by the hypertrophic ventricular part of the septum. Th relative weight of the right ventricular portion )calculated according to Muller's method) was subtracted from the weight of the left ventricle plus the whole septum. After this weight correction, a hypertropyh of the left ventricle could only be demonstrated in cases with hypertension or renal arterio= and arteriolosclerosis. In one case of left ventricular hypertrophy, hypertension could not be ruled out. In most of the cases a distinct dilation of the left ventricle could be found. The highest degrees of dilation were found in patients with brohchiectases of the lungs. Our results indicate that the structural dialation of the left ventricle may be due to an increased shunt-volume by bronchopulmonary anastomoses.

Subunits and their interactions

There is fairly general agreement that myosin isolated from rabbit skeletal muscle has a molecular weight of about 500,000. The higher values that have been reported apparently reflect protein aggregation related to the method of preparation. On the basis of present evidence, the myosin molecule has an elongate helical core of two f subunits (average weight about 215,000) that extend into a globular head region containing three g subunits (average weight about 20,000). Myosin may be dissociated into subunits by a number of methods. In 5 M guanidine, the myosin molecule is dissociated into f and g subunits, while at pH above 10, the g subunits are dissociated from the intact fibrous core of myosin. The dissociation of g subunits at pH 10 is accompanied by the loss of both ATPase activity and actin-binding capacity; however, the exact biological significance of the g subunits is presently uncertain. In preliminary studies, the f subunits appear to contain the sulfhydryl residues currently implicated in myosin ATPase, and there is some indication of allosteric regulation of enzymic activity.

Human cardiac myosin: electron microscopic observations

Solutions of human cardiac myosin contain macromolecules which were visualized in the electron microscope by the shadow-casting technic. The dominant particle was observed to be rod-shaped, to have a globular expansion at one end, and to have a mean length of about 1500 Å. The molecule of human cardiac myosin was found to be morphologically similar to myosin molecules from canine cardiac muscle and rabbit skeletal muscle.

Molecular Characteristics of Canine Cardiac Myosin

Cardiac myosin was obtained from the hearts of normal dogs by conventional extraction and precipitation techniques, followed by ammonium sulfate fractionation in the presence of 2.0 M lithium chloride. The molecular characteristics of this protein preparation are presented. Cardiac myosin prepared by this technique shows less tendency than conventionally prepared solutions to denature when stored at 0 to 4°C, and has a significantly higher ATP ase activity. Comparison of canine cardiac myosin and skeletal muscle myosin, obtained under identical conditions, revealed no differences between these two proteins in their intrinsic viscosities, S 20. w , molecular weight, and UV absorbances, although ATP ase activity was lower for cardiac myosin.