Transient length-related mechanical states in smooth muscle

The isotonic shortening of electrically stimulated ovarian ligament smooth muscle strips from rabbits was studied by briefly applying sudden increases in afterload (force steps, 0.6 to 3.0 s long) just sufficient to halt the shortening. Upon removal of the extra afterload, the isotonic shortening velocity significantly increased compared with prestep velocities measured at the same muscle lengths. The degree of potentiation depended upon the duration of the force step. Muscles yielded initially when the step was applied, and their stiffness decreased. During the zero-velocity portion of the force step there was a steady increase in stiffness back to levels appropriate to that force. Complete force-velocity curves were made following short (0.6 s) and long (3.0 s) force steps. The values for Vmax (and all intermediate velocities) were significantly greater following a long force step. In a final experimental series, muscles were held isometric immediately after removal of the force step. Force rose monotonically, with a more rapid redevelopment following a long force step. A mathematical model is presented, according to which the effect of the duration of the force step on the poststep mechanical properties may be due to either the alteration of an internal resistance to shortening or a change in the kinetic properties of the cross-bridge array. A hypothesis is proposed relating the steady decline in isotonic shortening velocity to a partial local depletion of energy-yielding substrates.