Sarcomere function and crossbridge cycling.

The power of the heart is dictated by the force development and velocity of shortening (V) of the cardiac sarcomere. Both depend on the amount of Ca++ released by the sarcoplasmic reticulum during the action potential. We have investigated the inter-relationship between force (F) sarcomere length (SL) and V and the intracellular Ca++ concentration ([Ca++]i) in trabeculae isolated from the right ventricle of rat heart. Activation of the contractile filaments during a normal heartbeat requires approximately 30 microM Ca++ ions, which rapidly bind to cytosolic ligands. Consequently the [Ca++]i transient detected by intracellular probes is less than 2 microM. Length dependent binding of Ca++ to Troponin-C is responsible for the shape of the F-SL relationship. Ca++ ions are bound to Troponin-C long enough to allow the F-SL relationship, and consequently the end-systolic pressure volume relationship in the intact ventricle, to be largely--but not completely--independent of the loading conditions. V increases hyperbolically with decreasing load during contraction against a load. Stiffness studies reveal that the number of attached crossbridges increases in linear proportion to an increase of the external load. At low external loads the V was large enough to induce a substantial viscoelastic load within the sarcomere itself. The F-V relationship of a single crossbridge appeared to be linear after correction for the observed viscoelastic properties of the muscle and for load dependence of the number of crossbridges. Maximal V of sarcomere shortening without an external load (Vo), depends on the level of activation by Ca++ ions because of the internal viscous load. Our studies of the rate of ATP hydrolysis by the actin-activated S1 fragment of myosin suggest that Vo is limited by the detachment rate of the crossbridge from actin. These studies also suggest that the difference between the fast (V1) and slow (V2) myosin iso-enzyme can be explained by a difference in the amino acid domain on S1 involved in binding of the crossbridge to the actin filament.