Myofibrils - a model to study dynamics of cardiac relaxation on the sarcomere level

摘要

To study the sarcomeric determinants of relaxation, force kinetics and sarcomere dynamics were measured in isolated subcellular cardiac myofibrils. Following rapid (within 10 ms) Ca~(2+)-removal, relaxation initially occurs with a slow, linear force decline (rate constant k_(LIN)) during which sarcomeres maintain their length. Relaxation then turns at a time t_(LIN) after Ca~(2+)-removal into a rapid exponential force decay with a rate constant k_(REL). This rapid, major phase of the force decay results from the mechanical relaxation, i.e., rapid lengthening of a single sarcomere. Evidence will be shown that k_(LIN) reflects the rate constant of cross-bridge detachment from actin which is slow during cross-bridge turnover under isometric conditions. In the rapid phase of relaxation the fixed-ends myofibrils evade from a continued slow relaxation by an intersarcomeric coupling mechanism in which individual sarcomeres relenghten one after another along the myofibril. This sequential relaxation of sarcomeres is likely the prerequisite for the rapid isovolumetric pressure decay of the heart.