Residual Force Enhancement in Cardiac Myofibrils

Abstract

Residual force enhancement (RFE) is a property of muscle where an activated muscle is stretched from a short to a long length resulting in greater force than is produced isometrically at the long length. This history-dependent property has been identified across skeletal muscle hierarchy including whole muscle, fascicles, fibres, and myofibrils. However, RFE has not been investigated in cardiac muscle. Therefore, the purpose of this study was to determine if RFE was present in cardiac myofibrils.Rabbit hearts were dissected and strips of left ventricle were skinned overnight with 1% Triton skinning solution and stored at −20°C. On the day of experiments, the cardiac tissue was blended and a myofibril with a good striation pattern was identified and suspended between a glass needle and a nanolever allowing for length changes and force measurement. The myofibril was set at a sarcomere length (SL) of 2.4 µm and activated to establish a reference contraction before being passively stretched to a SL of 3.2 µm. After a rest period, the myofibril was activated at a SL of 2.4 µm, actively stretched to a SL of 3.2µm, held for one minute and then relaxed. RFE was calculated as the difference between the steady-state force obtained after active stretch and the corresponding predicted isometric force at 3.2 µm based on the reference force, calculated according to the force-length relationship scaled to the filament lengths in rabbit muscles, and accounting for passive force.All myofibrils (n=5) produced more force when stretched actively compared to the calculated reference isometric force, indicating the presence of RFE in cardiac myofibrils. The average RFE was 54.8±10.8%.The presence of RFE in cardiac myofibrils supports that RFE is a universal muscle property. Investigation in cardiac muscle may shed new light into the mechanisms underlying RFE.