Mechanics of myocardial relaxation: Application of a model to isometric and isotonic relaxation of rat myocardium

Abstract

Using a simple model for cardiac muscle relaxation which takes into account muscle length, activation, elasticity and a rate constant for the decay of activation, we are able to use easily measured mechanical parameters to assess the state of the cardiac relaxing system. In isolated trabeculae carneae from the left ventricle of the rat, performing physiologically sequenced contractions, observations have been made (1) at varying preloads and afterloads, (2) with changes in temperature from 23° to 33°C, (3) with changes in bath Ca 2+ concentration and (4) with the addition of isoproterenol. During isometric relaxation, the slope ( S IM) of the curve relating maximum rate of decline of force ( − dF dt max ) to end-systolic muscle length is load-independent and sensitive to interventions which directly affect the cardiac relaxing system (e.g., temperature, isoproterenol); it is only slightly sensitive to bath calcium concentration. During isotonic relaxation, the maximum velocity of lengthening ( + dL dt max ) is in negative linear proportion to muscle shortening at a given preload, the slope ( S IT) of the curve relating + dL dt max to end-systolic length is sensitive to the interventions which directly affect the cardiac relaxing system but insensitive to calcium-mediated inotropic interventions. The model provides a theoretical basis for the use of S IM and S IT as measures of the relaxation process.