Force kinetics and individual sarcomere dynamics in cardiac myofibrils after rapid Ca2+ changes

Kinetics of force development and relaxation after rapid application and removal of Ca2+ 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 Ca2+ 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 Ca2+ activation; this indicates that k(ACT) reflects isometric cross-bridge turnover kinetics. The kinetics of force relaxation after sudden Ca2+ removal were markedly biphasic. An initial slow linear decline (rate constant k(LIN)) lasting for a time t(LIN) was abruptly followed by an similar to20 times faster exponential decay (rate constant k(REL)). k(LIN) is similar to k(TR) measured at low activating [Ca2+], 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.