Ramp-rate dependent external work during ramp-load release in cardiac muscle

When the load of frog cardiac muscle was decreased linearly from the maximum force to the resting force (ramp-load release), the muscle shortened with various velocities calculated from the instantaneous lengths, which showed the instantaneous force-velocity relationship. These velocities increased with the increase of the ramp rate. To study these dependency of ramp rate, the parameters obtained from Huxley model in 1957 during quick load-release were fixed throughout the calculations, but even if the shortening of the series elastic component (SEC) was taken into account, the model did not seem to explain these velocities. If the ramp rate was reduced, velocities were more decreased as expected, especially at smaller load, which suggested that the cross-bridges should bear some extra load. Thus the internal load was added in calculation. The velocities in the modified model well explained those measured even the ramp rate varied widely, especially in higher force. At lower force, the calculated velocities increased rapidly than those measured, mainly due to the shortening of SEC which were calculated from the initial shortening of quick release. On the force-sarcomere length plane, the calculated sum-force (internal-plus external-load) was almost linearly decreased versus length with the initial slope of 2.66 (mean). Adding the original dependency (= 1), the value of 3.66 was similar to 3.5 in skeletal muscle. The external work during the ramp-load release was also calculated and decreased with the increase of ramp rate, and showed the peak or plateau, which suggested the existence of optimal work.