Velocity of sarcomere shortening in rat cardiac muscle: relationship to force, sarcomere length, calcium and time.

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Abstract
The relation between force and velocity was determined in 16 trabeculae of rat right ventricle as a function of time during a twitch, of sarcomere length and of external Ca2+ concentration, [Ca2+]o. The trabeculae were studied in modified Krebs-Henseleit solution at 25.degree. C. Force was measured with a semiconductor strain gauge. Sarcomere length was measured with a laser diffraction system. A servomotor system was used in which control could be switched between sarcomere length, muscle length and force. Force-velocity relations were derived from load clamps and from contractions in which sarcomere length was initially held constant followed by a quick release and slower release of the sarcomeres at controlled velocity. Results are compatible with the hypothesis that V0 [velocity] is more sensitive than P0 [force] to the amount of Ca2+ bound to the contractile proteins, and that V0 reaches a maximal value with an amount of Ca2+ bound to the contractile proteins at which P0 has obtained only about 50% of its maximal value.