Variation of thin filament length in heart muscle

Abstract

ALTHOUGH the length dependence of force generation by heart muscle has been attributed to the degree of overlap of thin and thick filaments within the sarcomere, the required accurate values of the lengths of cardiac thin filaments have not been available. The existence in frog atrial trabeculae of thick filaments of uniform 1.55 µm length, of a small number of very long thin filaments, and of rosettes containing fewer or more than six thin filaments surrounding each thick filament has been noted³ (Fig. 1), but no study of the dimensions of a statistically significant number of thin filaments has been made. Using serial transverse electron micrographs of cardiac cells we have found that the lengths of thin filaments vary by as much as 0.6 µm within a single sarcomere in a single myofibril. All other factors being constant, this broad range should increase the length of the plateau of the sarcomere length–tension relationship and allow the myocardial cells contracting in situ to operate entirely on the plateau. The sliding of myofilaments, therefore, should have little direct effect on force generation in the intact heart.