title: Calcium-independent negative inotropy by beta-myosin heavy chain gene transfer in cardiac myocytes.
abstract: Increased relative expression of the slow molecular motor of the heart (beta-myosin heavy chain [MyHC]) is well known to occur in many rodent models of cardiovascular disease and in human heart failure. The direct effect of increased relative beta-MyHC expression on intact cardiac myocyte contractility, however, is unclear. To determine the direct effects of increased relative beta-MyHC expression on cardiac contractility, we used acute genetic engineering with a recombinant adenoviral vector (AdMYH7) to genetically titrate beta-MyHC protein expression in isolated rodent ventricular cardiac myocytes that predominantly expressed alpha-MyHC (fast molecular motor). AdMYH7-directed beta-MyHC protein expression and sarcomeric incorporation was observed as soon as 1 day after gene transfer. Effects of beta-MyHC expression on myocyte contractility were determined in electrically paced single myocytes (0.2 Hz, 37 degrees C) by measuring sarcomere shortening and intracellular calcium cycling. Gene transfer-based replacement of alpha-MyHC with beta-MyHC attenuated contractility in a dose-dependent manner, whereas calcium transients were unaffected. For example, when beta-MyHC expression accounted for approximately 18% of the total sarcomeric myosin, the amplitude of sarcomere-length shortening (nanometers, nm) was depressed by 42% (151.0+/-10.7 [control] versus 87.0+/-5.4 nm [AdMYH7 transduced]); and genetic titration of beta-MyHC, leading to 38% beta-MyHC content, attenuated shortening by 57% (138.9+/-13.0 versus 59.7+/-7.1 nm). Maximal isometric cross-bridge cycling rate was also slower in AdMYH7-transduced myocytes. Results indicate that small increases of beta-MyHC expression (18%) have Ca2+ transient-independent physiologically relevant effects to decrease intact cardiac myocyte function. We conclude that beta-MyHC is a negative inotrope among the cardiac myofilament proteins.
authors.affiliation: Department of Molecular & Integrative Physiology, University of Michigan, Ann Arbor, MI 48109-0622, USA. toddherr@umich.edu
authors.forename: Todd J
authors.initials: TJ
authors.lastname: Herron
authors.forename: Rene
authors.initials: R
authors.lastname: Vandenboom
authors.forename: Ekaterina
authors.initials: E
authors.lastname: Fomicheva
authors.forename: Lakshmi
authors.initials: L
authors.lastname: Mundada
authors.forename: Terri
authors.initials: T
authors.lastname: Edwards
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Metzger
mesh_terms: D000818:Animals; D002118:Calcium / Q000502:physiology*; D002478:Cells, Cultured; D005786:Gene Expression Regulation / Q000502:physiology; D018014:Gene Transfer Techniques; D009200:Myocardial Contraction / Q000235:genetics* / Q000502:physiology; D032383:Myocytes, Cardiac / Q000378:metabolism / Q000502:physiology*; D018995:Myosin Heavy Chains / Q000235:genetics* / Q000378:metabolism* / Q000502:physiology; D051381:Rats; D024723:Ventricular Myosins / Q000235:genetics* / Q000378:metabolism* / Q000502:physiology
chemical_list: C000594638:MYH7 protein, rat; D024723:Ventricular Myosins; D018995:Myosin Heavy Chains; D002118:Calcium