Force Kinetics and Individual Sarcomere Dynamics in Cardiac Myofibrils after Rapid Ca 2+ Changes

Abstract

Kinetics of force development and relaxation after rapid application and removal of Ca 2+ were measured by atomic force cantilevers on subcellular bundles of myofibrils prepared from guinea pig left ventricles. Changes in the structure of individual sarcomeres were simultaneously recorded by video microscopy. Upon Ca 2+ application, force developed with an exponential rate constant k ACT almost identical to k TR, the rate constant of force redevelopment measured during steady-state Ca 2+ activation; this indicates that k ACT reflects isometric cross-bridge turnover kinetics. The kinetics of force relaxation after sudden Ca 2+ removal were markedly biphasic. An initial slow linear decline (rate constant k LIN) lasting for a time t LIN was abruptly followed by an ∼20 times faster exponential decay (rate constant k REL). k LIN is similar to k TR measured at low activating [Ca 2+], indicating that k LIN reflects isometric cross-bridge turnover kinetics under relaxed-like conditions (see also Tesi et al., 2002. Biophys. J. 83:2142–2151). Video microscopy revealed the following: invariably at t LIN a single sarcomere suddenly lengthened and returned to a relaxed-type structure. Originating from this sarcomere, structural relaxation propagated from one sarcomere to the next. Propagated sarcomeric relaxation, along with effects of stretch and P i on relaxation kinetics, supports an intersarcomeric chemomechanical coupling mechanism for rapid striated muscle relaxation in which cross-bridges conserve chemical energy by strain-induced rebinding of P i.