Abstract 1626: The Rate of Tension Recovery in Cardiac Muscle Correlates with the Relative Residual Tension Prevailing after Re-stretch
Numerous experiments have shown that isolated cardiac muscles generate tension more quickly at higher levels of Ca2+ activation. We investigated the molecular mechanisms underlying this effect in permeabilized rat myocardial preparations by measuring the rate of tension recovery following brief shortening/re-stretch perturbations. Separate series of experiments used Ca2+-activating solutions with different pH values (pH 6.75, 7.00 and 7.25) and different phosphate (Pi) concentrations (0, 2.5 and 5.0 mmol L−1 added Pi) to modulate the kinetics of the recovery time-course. Analysis of 1130 records from 60 preparations showed that the measured rate of tension recovery (ktr) correlated (p<0.001) with the relative residual tension, that is, the minimum tension measured immediately after re-stretch normalized to the steady-state isometric tension for the experimental condition. This new finding suggests that the rate at which cardiac muscles develop tension increases with the proportion of cross-bridges bound to the thin filament and is strong evidence of cooperative contractile activation. Altering the pH and/or the Pi concentration in the myofibrillar space appears to influence cross-bridge kinetics by changing the number of cross-bridges that remain bound between the myofilaments during imposed length changes.