[The mechanics of sarcomeres studied using laser diffraction. A study of relaxation]

Myocardial tissues act as an optical filter to light. This is due to their regular striation--periodically alternating clear and dark anisotropic bands. When a narrow band of rat right ventricle is illuminated with a Helium-Neon laser (lambda = 633 nm) diffraction bands are observed, the spacing of which is inversely proportional to the sarcomere length. After muscular stimulation, the displacement of the diffraction bands allows measurement in real time of the contraction and relaxation of the sarcomeres. Sarcomere relaxation comprises two successive experimental phases, one rapid and the other one slow. The time constant of the rapid phase (tau 1) increases linearly with the total load; that of the slow phase (tau 2) decreases. The end of the rapid phase and tau 1 depend on the conditions of load. Our results suggest that the recaptation of calcium by the sarcoplasma reticulum and the affinity of troponine C for calcium depend on the level and changes of load. The affinity of TnC for calcium increases with high load and/or low amplitude of sarcomere shortening: it decreases with low loads and/or high amplitudes of sarcomere shortening.