CONTRACTION HISTORY MODULATES ISOTONIC SHORTENING VELOCITY IN SMOOTH-MUSCLE

The effect of contraction history on the isotonic shortening velocity of canine tracheal smooth muscle was investigated. Muscles were contra cted isometrically for 20 s at initial lengths of L(o) (length of maxi mal active force), 85% L(o), or 70% L(o) using electrical field stimul ation. Muscles were then allowed to shorten isotonically under differe nt afterloads either with or without first being subjected to a step d ecrease in length to 70% L(o). Instantaneous velocities were plotted a gainst instantaneous muscle length during isotonic shortening. Regardl ess of protocol, the velocity at any muscle length during shortening w as lower when the muscle was initially activated at a longer length. T he isotonic shortening velocity decreased progressively during shorten ing at a nearly linear rate with respect to instantaneous muscle lengt h under all conditions. Results suggest that a longer muscle length at the time of activation leads to the development of higher loads on th e contractile element during subsequent shortening, resulting in a slo wer shortening velocity. This plasticity of the force-velocity relatio nship may result from cytostructural reorganization of the smooth musc le cells in response to contractile activation at different muscle len gths.