The cardiac muscle is capable to adjust to rapid changes in demands. Some studies suggest that cardiac economy is a function of the sarcomere length or the preload, and that the economy increases with the preload. They also related the length dependent economy to the Frank-Starling Law of the heart. Muscle economy is determined by the cross-bridge cycling dynamics. The study integrates theoretical analysis of the sarcomeric control of contraction with isolated rat trabecula studies to investigate the dependence of the cross-bridge dynamics on the sarcomere length.
The dependence of cross-bridge dynamics on the sarcomere length was investigated by testing the force responses to identical ramp shortenings that were imposed at different sarcomere lengths, and by testing the rate of force redevelopment at various sarcomere lengths. Steady activations and sarcomere lengths were imposed by tetanic contractions, utilizing cyclopiazonic acid to slow the rate of calcium sequestration by the sarcoplasmic reticulum. Sarcomere length was measured by laser diffraction technique and controlled by fast servomotor. Ramp shortenings of 0.1 micrometer were imposed at the various sarcomere lengths (from 1.85 to 2.1 micrometer).
Results show four distinct phases in the force response; two phases during the ramp shortenings and two during the muscle-isometric force redevelopments (after the ramp shortening). Until now force redevelopment rate was usually described by a single exponential function. The first fast kinetic phases of force decline during the ramp shortening and force redevelopment are attributed to fast changes in the force per cross-bridge. The second slower phases relate to the kinetics of cross-bridge cycling. Interestingly, an identical slow phase response during the ramp shortenings was obtained at all the different sarcomere lengths.
The results negate the suggested length dependent cardiac economy and suggest that cardiac muscle works at constant and high economy at all sarcomere lengths.
