Restoring forces and relaxation of rat cardiac muscle.

Abstract

Sarcomeres in cardiac muscle can actively shorten to a length which is shorter than their length at slack length of the resting muscle. The aim of this study on rat cardiac trabeculae was to analyse the origin and magnitude of forces, which restore sarcomere length to slack length during relaxation. Three possible sources of restoring forces were considered: (1) passive elastic elements: (2) repulsive forces between thin filaments in double overlap, (3) opposing forces, which may result from deformation of thick filaments. Sarcomere length was measured and controlled with the use of light diffraction techniques. The results show that: (a) Hypotonic solutions, which led to increase in cross-sectional area of the trabeculae of 15%, i.e. equivalent to sarcomere shortening of about 0.25 μm, did not produce changes of sarcomere length at slack length of resting muscle. (b) Active force, which was developed in conventional Krebs-Hcnseleil solution at 25°C, depended on sarcomere length (SL) and has been found to be zero at SL = 1.58 μm (minimal active SL) and maximal at SL = 2.35 μm. Passive force was developed if sarcomeres were stretched beyond 2.15 μm. (c) Minimal active sarcomere length at the plateau of tension during tetani which were evoked in the presence of caffeine (10 mmol I−1) equalled 1.35 μm (d) The effect of rapid length perturbations during contractions at controlled sarcomere length consisted of diminution of force development. This deactivating effect of sarcomere motion was manifest if motion occurred after peak tension and increased sigmoidally during the twitch. (e) Minimal sarcomere velocity of lengthening (3–5μm s−1) was found to exist during unloaded contractions following a rapid length perturbation, which led to deactivation. Rcapplication of a load of less than 1% of maximal tension during the unloaded relaxation phase led to a 10-fold increase of lengthening velocity. The results suggest that total restoring force in isolated cardiac muscle which is mainly due to repulsion of thin filaments is smaller than a few percent of maximal actively developed tension. The contribution of passive elastic elements to restoring force is probably negligible. Opposing forces are not sufficiently large to prevent sarcomeres from shortening to well below the length of thick filaments. The deactivating effect of sarcomere motion and the effect of external load seem to be more important to relaxation than the presence of restoring forces at short sarcomere lengths.