Published online before print March 15, 2007, doi: 10.1161/01.RES.0000264102.00706.4e
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Submitted on November 1, 2006
Revised on March 2, 2007
Accepted on March 7, 2007
Calcium-Independent Negative Inotropy by 
-Myosin Heavy Chain Gene Transfer in Cardiac Myocytes
Todd J. Herron *;
From the Department of Molecular & Integrative Physiology (T.J.H., R.V., E.F., J.M.M.) and Department of Internal Medicine-Cardiology (T.J.H. and J.M.M.), University of Michigan, Ann Arbor. Current address for R.V.: Brock University, St. Catharines, Ontario, Canada.
* To whom correspondence should be addressed. E-mail: toddherr{at}umich.edu.
Increased relative expression of the slow molecular motor of the heart (
-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 -MyHC expression on intact cardiac myocyte contractility, however, is unclear. To determine the direct effects of increased relative -MyHC expression on cardiac contractility, we used acute genetic engineering with a recombinant adenoviral vector (AdMYH7) to genetically titrate -MyHC protein expression in isolated rodent ventricular cardiac myocytes that predominantly expressed 
-MyHC (fast molecular motor). AdMYH7-directed -MyHC protein expression and sarcomeric incorporation was observed as soon as 1 day after gene transfer. Effects of -MyHC expression on myocyte contractility were determined in electrically paced single myocytes (0.2 Hz, 37°C) by measuring sarcomere shortening and intracellular calcium cycling. Gene transfer-based replacement of -MyHC with -MyHC attenuated contractility in a dose-dependent manner, whereas calcium transients were unaffected. For example, when -MyHC expression accounted for 
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 -MyHC, leading to 38% -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 -MyHC expression (18%) have Ca2+ transient-independent physiologically relevant effects to decrease intact cardiac myocyte function. We conclude that -MyHC is a negative inotrope among the cardiac myofilament proteins.
Key words: adenovirus • contractility • gene transfer • intracellular calcium • muscle contraction • myosin • ventricular myocytes
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