Mechano-sensitivity of normal and long term denervated soleus muscle of the rat

Introduction and objectives: Evidence showed that physical forces, as passive stretching or active contraction, may counteract various kinds of skeletal muscle atrophy due, for instance, to muscle immobilization, pathophysiology or denervation. Accordingly, active muscle contraction induced by functional electric stimulation is helpful to reduce the muscle atrophic state in denervated man. Moreover, there is evidence that also passive mechanical stimulation of the sarcolemnic membrane may reduce the atrophic muscle state. As to the mechanisms by which mechanical stimulation modulates muscle physiology and pathophysiology, there is a growing list of facts that signaling pathway to the nucleus involves stretch activated channels (SACs) of the sarcolemma and the cytoskeleton. SACS activation allowed a Ca2+ inflow that activates Ca2+ dependent molecular signals. Cytoskeleton may be activated by Ca2+-dependent and independent paths and its contraction and elongation represent not only a mechanical signal to the nucleus but also a stimulus for many molecular signals. The aim of this work was to evaluate in soleus muscle of the rat, the mechano-sensitivity of SACS before and after medium and long term denervation. Methods: Electrophysiologic experiments were made in normal and denervated Soleus muscle of Wistar rats. Currents were recorded in voltage clamp by intracellular microelectrodes inserted in a single fiber. Results: Our findings demonstrated that SACs were expressed in normal soleus muscle and that SAC currents were potentiated by muscle stretching. Another important result was that the sensitivity to stretching increased after denervation and was particularly evident in long term denervated muscles. Discussion: The reported effects are in agreement with the effects of exercise on inducing muscle hypertrophy or with the positive effects on repairing the atrophic state of skeletal muscles by mechanical stimulation or, in denervated humans, by the functional electrical stimulation (FES).