Calcium-Independent Negative Inotropy by {szligbeta}-Myosin Heavy Chain Gene Transfer in Cardiac Myocytes

• Published in: Circulation research Vol. 100; no. 8; pp. 1182 - 1190
• Main Authors: Herron, Todd J, Vandenboom, Rene, Fomicheva, Ekaterina, Mundada, Lakshmi, Edwards, Terri, Metzger, Joseph M
• Format: Journal Article
• Language: English
• Published: 01.04.2007
• ISSN: 0009-7330; 1524-4571

Increased relative expression of the slow molecular motor of the heart ({szligbeta}-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 {szligbeta}-MyHC expression on intact cardiac myocyte contractility, however, is unclear. To determine the direct effects of increased relative {szligbeta}-MyHC expression on cardiac contractility, we used acute genetic engineering with a recombinant adenoviral vector (AdMYH7) to genetically titrate {szligbeta}-MyHC protein expression in isolated rodent ventricular cardiac myocytes that predominantly expressed alpha -MyHC (fast molecular motor). AdMYH7-directed {szligbeta}-MyHC protein expression and sarcomeric incorporation was observed as soon as 1 day after gene transfer. Effects of {szligbeta}-MyHC expression on myocyte contractility were determined in electrically paced single myocytes (0.2 Hz, 37 degree C) by measuring sarcomere shortening and intracellular calcium cycling. Gene transfer-based replacement of alpha -MyHC with {szligbeta}-MyHC attenuated contractility in a dose-dependent manner, whereas calcium transients were unaffected. For example, when {szligbeta}-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 plus or minus 10.7 [control] versus 87.0 plus or minus 5.4 nm [AdMYH7 transduced]); and genetic titration of {szligbeta}-MyHC, leading to 38% {szligbeta}-MyHC content, attenuated shortening by 57% (138.9 plus or minus 13.0 versus 59.7 plus or minus 7.1 nm). Maximal isometric cross-bridge cycling rate was also slower in AdMYH7-transduced myocytes. Results indicate that small increases of {szligbeta}-MyHC expression (18%) have Ca super(2+) transient-independent physiologically relevant effects to decrease intact cardiac myocyte function. We conclude that {szligbeta}-MyHC is a negative inotrope among the cardiac myofilament proteins.