Effects of lengthened immobilization on functional and histochemical properties of rabbit tibialis anterior muscle

Abstract

SUMMARYThe imposition of long-term lengthened immobilization on rabbit tibialis anterior (TA) muscles resulted in rapid increases in slow oxidative (SO) fibre number. After 2 weeks, SO number had increased 2-fold and was 5 times greater after 6 weeks immobilization. There were also fibre-type-specific effects on SO, fast oxidative glycolytic (FOG) and fast glycolytic (FG) fibre areas. Twitch strength was unchanged throughout immobilization whilst production of tetanic tension was impaired during the initial period but had returned to control levels by 6 weeks. Twitch contraction times and isometrically determined rates of rise and relaxation were largely unaffected by immobilization despite the marked increase in expression of slow myosin. The change in phenotypic expression of the lengthened TA was not attributable to chronically increased levels of activation since integrated EMG activity was unchanged from control values throughout the immobilization period. Thus it is suggested that a chronic increase in tension consequent on the lengthening procedure is a potent stimulus for fast-to-slow myosin transformation.

Increased efficacy and decreased systemic‐effects of botulinum toxin A injection after active or passive muscle manipulation

“…The fact that fiber size was so dramatically altered by BTX-A injection highlights the extreme sensitivity of muscle fibers to neural activity. The relatively large 80% decrease in fiber size and muscle maximum tetanic tension indicates that the muscle is even more sensitive to 'disuse' by BTX-A injection compared with 'disuse' by other models, such as limb immobilization, 24,25 spaceflight, 26 and hindlimb suspension 27 where 1 month of disuse is usually accompanied by only about a 30% loss in TA function. It is tempting to speculate that loss of neuromuscular junction patency after BTX-A injection may initiate active cellular processes that promote muscle degradation via the ubiquitin-proteasome pathways.…”

Myosin gene expression in the respiratory muscles

“…The signal could act just mechanically or through the release of a messenger. However, recent studies suggest that the muscular activity itself is not sufficient for triggering the process [47]. Nevertheless, the link between the mechanical signal and changes in MyHC gene expression remains undiscovered.…”

“…It is believed that changes in the tension generated by the muscle during the effort would be the initial phenomenon involved in the transformation process [47]. A fascinating question is how changes in tension result in changes in MyHC gene expression.…”

“…At least six genes exist in this cluster [38,39], and it is believed that there is a specific The transcriptional and translational mechanisms that regulate the expression of the genes encoding MyHC isoforms are not well-known, although they are being actively investigated. It is believed that the expression is regulated fundamentally by mechanical signals, particularly by changes in the tension generated by the muscle during the effort [47]. The stretch (active tension) as well as electrical stimulation (passive tension) provoke the repression of the fast-type genes, with activation of the slow-type genes [2,48].…”

Effects of Clenbuterol and Cyclosporin A on the Myosin Heavy Chain mRNA Level and the Muscle Mass in Rat Masseter

“…The masseter muscle mass was significantly (p < 0.001) increased by either the CB or the CB + CsA treatment, but decreased with the CsA treatment (p < 0.01). These results suggest that in rat masseter muscle, CB has an anabolic action accompanying MHC mRNA I → IIa → IId/x sequence transition independently of CaN-signaling pathways, and CaN is involved in the type I fiber gene expression and the muscle mass maintenance of type IIb fiber.Key words: myosin heavy chain mRNA, muscle mass, clenbuterol, cyclosporin A, real time RT-PCR.Masticatory muscles have been known to be capable of adapting their mass and muscle phenotypes to meet a wide range of functional demands [1][2][3][4][5][6][7][8][9], as in limb muscles [10][11][12][13][14][15][16]. However, most of the studies on masticatory muscles to date have focused on describing the results rather than analyzing the molecular mechanism of muscle mass alteration and fibertype conversion.…”