The Load Dependence Of The Size And The Speed Of The Working Stroke Of Cardiac Myosin In Situ

Cardiac performance depends on power developed by the myocardium and mutations of cardiac myosin, which affect power output, have been proposed to be responsible for various forms of cardiomyopathy. Precise measurements of force and filament sliding generated by cardiac myosin are lacking. Here sarcomere-level mechanics are applied for the first time to intact trabeculae from the right ventricle of the rat to determine the size and the speed of the working stroke of cardiac myosin in situ and their dependency on the load. The isotonic shortening transients elicited by stepwise drops in force to 0.2-0.8 of peak force (Tp) are recorded at sarcomere length of 1.9 and 2.2 μm and external Ca2+-concentrations ([Ca2+]o) 1 and 2.5 mM (temperature 27 oC). The size and the speed of the early rapid phase of shortening (the mechanical manifestation of the myosin working stroke) increase with the reduction of the load from approximately 3 nm per half-sarcomere and 1000 s−1 at high load to 6 nm per half-sarcomere and 7000 s−1 at low load, but are not affected by the increase of sarcomere length or [Ca2+]o. Thus the two-fold increase of force and power in the range of sarcomere length and [Ca2+]o explored are solely due to the increase in number of myosin motors. These results demonstrate the possibility for in situ studies of the molecular basis of cardiomyopathies. Supported by MIUR-PRIN and Telethon (Italy).