A SUB-POPULATION OF RAT MUSCLE FIBERS MAINTAINS AN ASSESSABLE EXCITATION-CONTRACTION COUPLING MECHANISM AFTER LONG-STANDING DENERVATion, DESPITE LOST CONTRACTILITY

To define the time-course and potential effects of electrical stimulation on permanently denervated muscle, we used a rat model to evaluate the excitation-contraction coupling (ECC) status of leg muscles during progression to long-term denervation by: i) ultrastructural analysis; ii) specific binding methodologies to measure dihydropyridine receptors (DHPR), ryanodine receptors (RYR)-1, Ca2+-channels and extrusion Ca2+-pumps; iii) gene transcription and translation of Ca2+-handling proteins; iv) in vitro mechanical properties; and v) electrophysiological analyses of sarcolemmal passive properties and L-type Ca2+ current (ICa) parameters. We show that in response to long-term denervation: i) isolated muscle, unable to twitch in vitro by electrical stimulation, have very small size, but may show a slow caffeine contracture; ii) only roughly half of the muscle fibers having “voltage dependent Ca2+ channel activity” are able to contract; iii) the ECC mechanisms are still present and, in part, functional; iv) ECC related gene expression is up-regulated; and v) at any time point, there are muscle fibers that are more resistant than others to denervation atrophy and disorganization of the ECC apparatus. Altogether our results support the hypothesis that prolonged “resting” [Ca2+] may drive progression of muscle atrophy to degeneration, and that electrical stimulation-induced [Ca2+] modulation may mimic the lost nerve influence, playing a key role in modifying gene expression of denervated muscle. Hence, our work provides a potential molecular explanation of the muscle recovery that occurs in response to a rehabilitation strategy that was developed as a result of empirical clinical observations.