Effect of calcium on force-velocity-length relations of heart muscle of the cat.

The influence of calcium concentration in the bathing solution on the force-velocity relations of the isolated, electrically excited cat papillary muscle was examined. Shortening velocity was directly measured at minimum loads by unloading or load clamping the muscle from the preload (at the length, Lmax, where maximum active tension was developed) to 0 (zero-load clamp), 2, 5, 10, and 15% of the total tension, Po. All measurements were made in the physiological length range between Lmax and 12.5% below Lmax. Increasing the calcium concentration augmented the measured maximum unloaded shortening velocity and shifted the normalized force-velocity relation when force was expressed as a fraction of Po. These observations suggest that the activating calcium has an action more complex than the binding to troponin alone.