THIN FILAMENT COOPERATIVITY AS A MAJOR DETERMINANT OF SHORTENING VELOCITY IN SKELETAL-MUSCLE FIBERS

The mechanism underlying the calcium sensitivity of the velocity of sh ortening of skeletal muscle fibers was investigated using a multiple s hortening protocol: within a single contraction, skinned rabbit psoas fibers were made to shorten repetitively under a light load by briefly stretching back to their initial length at regular intervals. At satu rating [Ca2+], the initial fast shortening pattern was repeated reprod ucibly. At submaximal [Ca2+], the first shortening consisted of fast a nd slow phases, but only the slow phase was observed in later shorteni ngs. When the fibers were held isometric after the first shortening, t he velocity of the second shortening recovered with time. The recovery paralleled tension redevelopment, implying a close relationship betwe en the velocity and the number of the preexisting force-producing cros s-bridges. However, this parallelism was lost as [Ca2+] was increased. Thus, the velocity was modified in a manner consistent with the coope rative thin filament activation by strong binding cross-bridges and it s modulation by calcium. The present results therefore provide evidenc e that the thin filament cooperativity is primarily responsible for th e calcium sensitivity of velocity. The effect of inorganic phosphate t o accelerate the slow phase of shortening is also explained in terms o f the cooperative activation.