Force response to width and length perturbations in compressed skinned skeletal muscle fibers

Previous studies have shown that radial compression of calcium-activated skinned skeletal muscle fibers, with attendant reduction of filament lattice spacing, reduces isometric force generation. In relaxed skinned fibers, radial compression produces a marked increase in axial elastic modulus, and the response to a small amplitude length perturbation resembles that of a muscle in rigor. We interpret these results as indicating that radial compression of the myofilament lattice produces "hindered" cross-bridges which are load bearing but not force generating. The experiments reported here were designed to study the effect(s) of "hindered" cross-bridges on both the time course of isometric force responses following Ca2+ activation and fiber width and length perturbations. The experiments were carried out at room temperature on radially compressed skinned single rabbit soleus fibers. Force development following step-wise Ca2+ activation and step-wise changes of fiber width was "slow" (60-90 sec) compared to that in normal width fibers (approximately 1 sec), and could be approximated by a single exponential curve. Force redevelopment following a length release in compressed fibers was both more rapid and more complicated than force development following activation and width steps, and required a double exponential curve for an adequate description. The results are consistent with the notion that hindered cross-bridges form as a result of lattice compression, and that the hindered bridges affect the force responses following width and length perturbations.