Length-dependent electromechanical coupling in single muscle fibers

In single muscle fibers from the giant barnacle, a small decrease in muscle length decreases both the calcium activation and the peak isometric tension produced by a constant current stimulus. The effect is most pronounced if the length change immediately precedes the stimulation. In some cases, the decrease in tension with shortening can be accounted for almost entirely by a decrease in calcium release rather than changes in mechanical factors such as filament geometry. During the constant current stimulation the muscle membrane becomes more depolarized at longer muscle lengths than at the shorter muscle lengths. Under voltage clamp conditions, when the membrane potential is kept constant during stimulation, there is little length dependence of calcium release. Thus, the effect of length on calcium release is mediated through a change in membrane properties, rather than an effect on a subsequent step in excitation-contraction coupling. Stretch causes the unstimulated fiber membrane to depolarize by about l mV while release causes the fiber membrane to hyperpolarize by about the same amount. The process causing this change in potential has an equilibrium potential nearly 10 mV hyperpolarized from the resting level. This change in resting membrane potential with length may account for the length dependence of calcium release.