Effects of membrane potential on mechanical activation in skeletal muscle

The effect of subthreshold depolarization on mechanical threshold was investigated in tetrodotoxin-poisoned mammalian and amphibian skeletal muscle fibers using a two-microelectrode voltage-clamp technique. Mechanical threshold was determined with a 2-ms test pulse. The immediate effect of depolarization was inhibition of the mechanical system. The consequent increase in the test pulse threshold was linearly related to the size of the depolarization and there was, on the average, a 10% increase in threshold for a 10-mV depolarization in mammalian fibers. The duration of the inhibitory period was also related to the size of the depolarization. Inhibition was interrupted by the onset of activation (seen as a reduction in the test pulse threshold), and in rat soleus fibers this occurred within 100 ms with a 20-mV depolarization, inhibition decayed within 10 ms. The decay of activation after brief conditioning pulses was initially rapid (on the average, the test pulse threshold recovered to 80% of its control value within 1 ms) and then slow (full recovery took 100-500 ms). After long conditioning pulses, activation often decayed into a period of inhibition. When depolarization (of 20 mV or more) was maintained for several seconds, the fibers became inactivated. Rat extensor digitorum longus and sternomastoid fibers were strongly inactivated by depolarization to -40 mV and the test pulse to +40 mV did not cause contraction.