Sarcomere relaxation in intact cardiac muscle.

The process of tension decay in the absence of sarcomere motion was studied in intact heart muscle isolated from rats. Sarcomere length, observed by an infrared light diffraction technique, was controlled and the effect on force dissipation measured. Distribution of the diffracted light suggested that sarcomere length was uniform immediately after contraction than prior to the next. When sarcomeres - as distinct from the total muscle length - are kept isometric, the rate of tension decay (a) reaches its maximum sooner, and (b) appears to be constant throughout relaxation. Slow stretch (about 3% sarcomere length) prolonged cardiac muscle tension during early relaxation without direct evidence of sarcomere 'yielding'. The dynamics of isometric relaxation and the effect of stretch were qualitatively the same at external calcium concentrations of 0.6 and 1.9 mM. For a specified sequence of sarcomere length changes during relaxation, the dynamics of tension decay were independent of preloaded sarcomere length when the sarcomeres are kept isometric. The data suggest that lowering contractile tension enhanced relative relaxation rate (sec-1) at a sarcomere length of 2.0 micron. The results clarify the physiological mechanism regulating the dynamics of myocardial fiber relaxation.