To explore the compatibility of skeletal and cardiac programs of gene expression, transgenic mice that express a skeletal muscle myogenic regulator, bmyf5, in the heart were analyzed. These mice develop a severe cardiomyopathy and exhibit a significantly shorter life span than do their nontransgenic littermates. The transgene was expressed from day 7.5 post coitum forward, resulting in activation of skeletal muscle genes not normally seen in the myocardium. Cardiac pathology was not apparent at midgestation but was evident by day 2 of postnatal life, and by 42 days, hearts exhibited multifocal interstitial inflammation, fibrosis, cellular hypertrophy, and occasional myocyte degeneration. All four chambers of the heart were enlarged to varying degrees, with the atria demonstrating the most significant hypertrophy (>100% in 42-day-old mice). The transgene and several skeletal muscle-specific genes were expressed only in patchy areas of the heart in heterozygous mice. However, molecular markers of hypertrophy (such as alpha-skeletal actin and atrial myosin light chain-1) were expressed with a wider distribution, suggesting that their induction was secondary to the expression of the transgene. In older (28-week-old) mice, lung weights were also significantly increased, consistent with congestive heart failure. The life span of bmyf5 mice was significantly shortened, with an average life span of 109 days, compared with at least a twofold longer life expectancy for nontransgenic littermates. Expression of the transgene was associated with an increase in Ca2+-stimulated myofibrillar ATPase in myofibrils obtained from the left ventricles of 42-day-old bmyf5 mice. Myocardial bmyf5 expression therefore induces a program of skeletal muscle gene expression that results in progressive cardiomyopathy that may be due to incompatibility of heart and skeletal muscle structural proteins.

Injury to cardiac myocytes often leads to the production of anti-myosin antibodies. While these antibodies are a marker of myocardial injury, their contribution to pathogenesis in diseases such as autoimmune myocarditis or rheumatic fever is much less clear. We demonstrate in this report that monoclonal anti-myosin antibodies can mediate myocarditis in a susceptible mouse strain. Additionally, we show disease susceptibility depends on the presence of myosin or a myosin-like molecule in cardiac extracellular matrix. This study demonstrates that susceptibility to autoimmune heart disease depends not only on the activation of self-reactive lymphocytes but also on genetically determined target organ sensitivity to autoantibodies.

BALB/c mice develop autoimmune myocarditis after immunization with mouse cardiac myosin, whereas C57B/6 mice do not. To define the immunogenicity and pathogenicity of cardiac myosin in BALB/c mice, we immunized mice with different forms of cardiac myosin. These studies demonstrate the discordance of immunogenicity and pathogenicity of myosin heavy chains. The cardiac alpha-myosin heavy chains of BALB/c and C57B/6 mice differ by two residues that are near the junction of the head and rod in the S2 fragment of myosin. Myosin preparations from both strains are immunogenic in susceptible BALB/c as well as in nonsusceptible C57B/6 mice; however, BALB/c myosin induces a greater incidence of disease. To further delineate epitopes of myosin heavy chain responsible for immunogenicity and disease, mice were immunized with fragments of genetically engineered rat alpha cardiac myosin. Epitopes in the region of difference between BALB/c and C57B/6 (residues 735-1032) induce disease in both susceptible and nonsusceptible mice. The data presented here demonstrate that pathogenic epitopes of both mouse and rat myosin residue in the polymorphic region of the S2 subunit. In addition, these studies suggest that polymorphisms in the autoantigen may be part of the genetic basis for autoimmune myocarditis.

The mechanisms by which the aged heart adapts to a superimposed pressure load such as hypertension have not been described. We therefore investigated biochemical and molecular genetic adaptations in the 24-month-old rat heart subjected to renovascular hypertension. Compared with 4-month-old rats, aging was associated with a 68% increase in left ventricular mass without any change in heart weight-to-body weight ratio, a 33% reduction in calcium-activated myosin ATPase activity, and a shift from a V1 to a V3 predominant myosin heavy chain (MHC) isoform distribution. A 46% reduction in alpha-MHC mRNA and a reciprocal increase in beta-MHC mRNA was seen. When hypertension was superimposed, there was a further 75% increase in ventricular mass, a 63% increase in heart weight-to-body weight ratio, and a 19% reduction in myosin ATPase. Myosin isozyme distribution was further shifted to V3, and the ratio of alpha-MHC to beta-MHC mRNA was reduced. In addition, with hypertension a significant (greater than 50%) reduction in the mRNA level of the cardiac sarcoplasmic reticular calcium-activated ATPase was seen. These data demonstrate that the aged myocardium is able to respond to a superimposed pressure load with a molecular genetic and protein synthetic pattern of hypertrophy analogous to that seen in younger animals.

We have previously shown that physical conditioning in the rat improves cardiac mechanics and biochemistry and normalizes the cardiac contractile protein abnormalities associated with renovascular hypertension. Since chronic adrenergic stimulation with dobutamine simulates some aspects of physical conditioning, this study was undertaken to investigate the effects of chronic dobutamine administration on normal and hypertensive rat hearts. Four groups of female animals were studied: controls, dobutamine-treated (2 mg/kg twice daily), renovascular hypertensives, and dobutamine-treated hypertensives. Animals were killed after 8-10 weeks and cardiac histology, myosin biochemistry, and mechanics in an isolated heart perfusion apparatus were studied. Dobutamine, unlike hypertension, was not associated with histological evidence of myocardial damage but did increase cardiac mass by 10% and calcium-activated myosin ATPase activity by 13%. Hypertension was associated with a 24% increase in mass, a 24% decrease in ATPase activity, and a shift in the myosin isoenzyme pattern from V1 to V3. The combined stimuli caused additive hypertrophy (44%) and normalized myosin biochemistry and isomyosin distribution. Dobutamine treatment was not associated with significant improvements in pump or muscle function in control or hypertensive hearts. Thus chronic dobutamine treatment, like physical conditioning, induces a physiological cardiac hypertrophy in rats that is associated with improved myosin enzymology and normalization of the contractile protein abnormalities associated with hypertension. Unlike physical conditioning, however, these biochemical alterations do not result in improved contractile function as measured in an isolated buffer-perfused heart apparatus.