The myosin heavy chain (MHC) isoforms expressed in the left ventricle are primary determinants of the work capacity of mammalian hearts. Until recently, the prevailing view was that adult human ventricles expressed virtually 100% beta MHC (a slow isoform), but contemporary studies have shown that normal human ventricles express small amounts of alpha MHC (a fast isoform) on a predominantly beta MHC background and that failing human ventricles express 100% beta MHC and virtually no alpha MHC. Questions that arise from these results are whether expression of alpha MHC contributes to the rate of rise of pressure in normal human ventricles and whether loss of alpha MHC contributes to the depressed contractile performance of human failing hearts. This project will address these possibilities based on the working hypothesis that expression of small amounts of alpha MHC significantly speeds the kinetics of force development in normal myocardium, and conversely, reduced expression of alpha MHC contributes to depressed kinetics of force (pressure) development in heart failure. The proposed experiments will assess the effects on contractile properties due to altered ratios of alpha and betaa MHC expression in (i) rodent myocardium in which MHC expression is varied by manipulating thyroid status of the animals and (ii) myocardium from normal human hearts (donor hearts not used for transplant) and failing hearts (explanted hearts from transplant recipients). Rate constants of force development and relaxation will be assessed from changes in force following photolysis of caged Ca 2+ chelators. Results will then be used to model the effects of altered myosin isoform expression on twitch kinetics and amplitude in both rat and human myocardium. Additional mechanical experiments will investigate the roles of alpha MHC and beta MHC in determining the power and stretch activation responses of myocardium. Immunohistochemistry using MHC-isoform specific antibodies and SDS-PAGE of muscle samples will quantify MHC isoform expression patterns in regions of the left ventricle to test the idea that MHC expression varies in relation to systolic wall stress. Results from these studies should provide new insights into mechanisms of depressed myocardial function in human heart failure and should also suggest new therapeutic targets for treatment of this disease.
