A Subpopulation of Rat Muscle Fibers Maintains an Assessable Excitation-Contraction Coupling Mechanism After Long-Standing Denervation Despite Lost Contractility

Abstract

To define the time course and potential effects of electrical stimulation on permanently denervated muscle, we evaluated excitation-contraction coupling (ECC) of rat leg muscles during progression to long-term denervation by ultrastructural analysis, specific binding to dihydropyridine receptors, ryanodine receptor 1 (RYR-1), Ca channels and extrusion Ca pumps, gene transcription and translation of Ca-handling proteins, and in vitro mechanical properties and electrophysiological analyses of sarcolemmal passive properties and L-type Ca current (ICa) parameters. We found that in response to long-term denervation: 1) isolated muscle that is unable to twitch in vitro by electrical stimulation has very small myofibers but may show a slow caffeine contracture; 2) only roughly half of the muscle fibers with "voltage-dependent Ca channel activity" are able to contract; 3) the ECC mechanisms are still present and, in part, functional; 4)ECC-related gene expression is upregulated; and 5) at any time point, there are muscle fibers that are more resistant than others to denervation atrophy and disorganization of the ECC apparatus. These results support the hypothesis that prolonged "resting" [Ca] may drive progression of muscle atrophy to degeneration and that electrical stimulation-induced [Ca] modulation may mimic the lost nerve influence, playing a key role in modifying the gene expression of denervated muscle. Hence, these data provide a potential molecular explanation for the muscle recovery that occurs in response to rehabilitation strategies developed based on empirical clinical observations.

Permanent LMN denervation of human skeletal muscle and recovery by h-b FES: management and monitoring

“…This had also been demonstrated in rodents even after a year-long permanent denervation in both hemi-diaphragm and leg muscles [26,28,80], and then confirmed in human up to 20-year after LMN denervation both without and with Functional Electrical Stimulation, which effectively reverts long-term atrophy and conceivably maintains the mass of regenerated myofibers [27,54]. Strong corroborating evidence was collected analyzing expression of myogenesis-related genes in denervated rat muscles [4,70,98]. Beside in LMN denervation due to SCI, regeneration of muscle fibers in human LMN denervated muscle had been reported in cases of free microvascular muscle transfers up to 4 years after surgery, when activation of satellite cells was evident [52,53].…”

“…The results of the EU RISE Project, and of the related animal research, provide different perspectives. Twenty out of 25 patients completed a 2 years h-b FES program [57,58], which resulted in: 1. significant increase of thigh muscle size and of the muscle fibers, with striking improvements of the ultra-structural organization of contractile material; 2. significant increase in muscle force output during electrical h-b FES of denervated muscle: management and monitoring European Journal Translational Myology -Myology Reviews 1 (3): [91][92][93][94][95][96][97][98][99][100][101][102][103][104] 2010 -95 -stimulation (knee extension torque); 3. the recovery of quadriceps m. force was sufficient to allow compliant subjects to perform FES-assisted stand-up and stepping-in-place exercises; 4. ultra structural analyses demonstrated that the shorter was the time elapsed from SCI to the beginning of h-b FES, the larger were the number and the size of recovered fibers. The study demonstrates that h-b FES of permanent LMN denervated muscle is an effective home therapy that results in rescue of muscle mass and tetanic contractility [18,55,57,58].…”

Excitation-contraction coupling and mechano-sensitivity in denervated skeletal muscles

“…An early phase with a rapid atrophy. The following medium phase is a period of slow progressive atrophy and the last phase (long term denervated muscle) the fiber sizes and ECC function of the severely atrophic muscle fibers remain stable [2,3,62]. Meanwhile, myofibers decrease in number and fibrosis increases, the residual surviving fibers conserve some functionality as excitability and ECC function with an increased mechano-sensitivity.…”

“…This decrease was very marked in the first 3 weeks, progressed slower up to 26 weeks whereas later the decrease progressed quite slowly indicating that the late loss of muscle mass is negligible [3,62]. The muscle weight-to-body weight ratio changes are paralleled by alterations involving a progressive reduction of i) fiber diameter, iii) T-tubule surface area, iv) membrane resistance and resting membrane potential, v) expression and functionality of voltagedependent Na + channels, index of a less muscle excitability, vi) expression and voltage dependence of L-CaC, index of an affected ECC apparatus, vi) loss of the sarcomeric structure and reduced contractility.…”

“…In mammals changes of the passive electrical properties of the sarcolemma [15], in transverse tubules organization [68], sarcolemmal Ca 2+ current, and the ability to sustain tension during prolonged contractions [19] were just observed in the first weeks after muscle denervation. Muscle atrophy advances faster in the early weeks after denervation but then progresses slower and it may reach a steady-state that is maintained for long time [2,62]. Within months after complete injury of the conus medullaris and cauda equina, the muscles are no longer excitable by commercially available electrical stimulators.…”