Nonlinear force response of active smooth muscle subjected to small stretches

Isometrically mounted and electrically stimulated strips of rabbit mesotubarium superius muscles were subjected to sinusoidal length perturbations (delta L) of variable amplitude (0-1% of Lmax) at constant frequency (80 Hz). In both the plateau phase of long-maintained (steady-state) contractions and in brief contraction-relaxation cycles the force perturbation amplitude (delta F) response was directly proportional to delta L (and to the developed force, F) for delta L values less than approximately 0.30% of Lmax. At larger delta L values the magnitude of the delta F response deviated downward from a linear relationship. Statistical analysis of the relationship characterized a "deviation point," the value of delta F and delta L beyond which the delta F response departed from linearity. The value of delta F at the deviation point depended strongly on the developed force, whether it was varied by changing muscle rest length, stimulus strength, or time during a phasic contraction-relaxation cycle. The value of delta L at the deviation point was independent of developed force. Restricting the length of muscle subjected to delta L (while not changing cellular dimensions) showed that the slope of delta F/delta L relationship (the active stiffness) depended inversely on the length of muscle in the system. These findings are consistent with a model in which the active elasticity arises from cross bridges that are borne on myofilaments and that both generate and transmit the muscle force.