Pharmacologic enhancement of functional recovery in free muscle transfers

Correlation of contractile function recovery with acetylcholine receptor changes in a rat muscle flap model

The aim of this study was to investigate the correlation between contractile function recovery and changes of acetylcholine receptors (AChR) in a transferred muscle flap following reinnervation. Orthotopic transfer of the gracilis muscle flap with repair of its nerve was performed bilaterally in 48 rats. The rats were randomly divided into six experimental groups based on the time intervals for assessments (1, 4, 5, 10, 20, and 30 weeks). Sixteen gracilis muscle samples from eight rats without surgery were used as the controls. In each group, muscle contractile force and weight were measured (n = 16). The AChR numbers (n = 8) and subunits (epsilon and gamma) mRNA (n = 8) were examined using [(125)I]-alpha-bungarotoxin and fluorescent quantitative-PCR. The results showed the AChR numbers in the muscle flap increased from 4 to 20 weeks after reinnervation and correlated with recovery of the tetanic contraction force. However, correlation between the increase of AChR number with the specific tension (peak contractile force normalized to wet muscle weight) was only found from 4 to 10 weeks postoperatively. The expression of gamma-subunit mRNA increased at the early period after flap transfer and then decreased rapidly, whereas the epsilon-subunit mRNA recovered gradually since fourth week postoperatively. A small amount of gamma-subunit mRNA could still be detected at 30 weeks after surgery. In conclusion, following reinnervation of the transferred muscle flap, the contractile functional recovery is partially correlated to increase of the AChRepsilon. Our findings may provide evidence for further study of improving muscle function in functional reconstruction by targeting the AChR.

Chronic electrostimulation after nerve repair by self-anastomosis: effects on the size, the mechanical, histochemical and biochemical muscle properties

This study tests the effects of chronic electrostimulation on denervated/reinnervated skeletal muscle in producing an optimal restoration of size and mechanical and histochemical properties. We compared tibialis anterior muscles in four groups of rats: in unoperated control (C) and 10 weeks following nerve lesion with suture (LS) in the absence of electrostimulation and in the presence of muscle stimulation with either a monophasic rectangular current (LSEm) or a biphasic modulated current (LSEb). The main results were (1) muscle atrophy was reduced in LSEm (-26%) while it was absent in LSEb groups (-8%); (2) the peak twitch amplitude decreased in LS and LSEm but not in LSEb groups, whereas the contraction time was shorter; (3) muscle reinnervation was associated with the emergence of type IIC fibers and proportions of types I, IIA and IIB fibers recovered in the superficial portion of LSEb muscles; (4) the ratio of oxidative to glycolytic activities decreased in the three groups with nerve injury and repair; however, this decrease was more accentuated in LSEm groups. We conclude that muscle electrostimulation following denervation and reinnervation tends to restore size and functional and histochemical properties during reinnervation better than is seen in unstimulated muscle.

Enhanced reinnervation after neurotization with Schwann cell transplantation

We investigated the feasibility of using Schwann cell transplantation to enhance reinnervation after direct nerve-to-muscle neurotization (NMN). The denervated anterior tibial muscle was neurotized by tibial nerve implantation, and Schwann cell suspension (transplantation group) or an equivalent volume of culture medium (control group) was injected at the implantation site. In the control group, few axons invaded the muscle, demonstrating that skeletal muscle was poorly permissive to the advancement of axons. In the transplantation group, a large number of regenerating axons grew for a longer distance throughout the muscle, and reinnervated motor endplates were significantly more abundant. Enhanced reinnervation and functional recovery of the muscle in the transplantation group was confirmed by a significant increase in the compound muscle action potential and in muscle weight. These results suggest that intramuscular Schwann cell transplantation has potential as a cell therapy to improve functional recovery after NMN.

Microscopic changes at the neuromuscular junction in free muscle transfer

Free muscle transfers do not generate the same force after transfer as that at the original sites. Light and electron microscopy were used to study serially during 30 weeks the changes at the neuromuscular junction after free muscle transfer of the gracilis muscle in the adult Wistar rat. Under light microscopy, after staining with acetylthiocholine the neuromuscular junction showed changes of degeneration with withdrawal of the innervating axon terminal followed by regeneration and reconstitution of the neuromuscular junction. The newly formed neuromuscular junction still lacked the structural detail seen in the control neuromuscular junction, even after 30 weeks. With the electron microscope, mitochondrial swelling and clumping of the synaptic vesicles were followed by withdrawal of the axon terminal from the muscle membrane on denervation. The infolding of the muscle membrane at the neuromuscular junction became less prominent. With reinnervation the ultrastructure of the junction was only partially reestablished with poorly reconstituted primary and secondary folds of the muscle membrane 30 weeks after the transfer. Failure of complete reformation of the ultrastructure of the neuromuscular junction may provide another explanation for failure of full recovery of skeletal muscle function after free muscle transfer.

Qualitative and quantitative changes in acetylcholine receptor distribution at the neuromuscular junction following free muscle transfer

The qualitative and quantitative changes in acetylcholine receptor distribution were studied in the gracilis muscle of the Wistar rat following free neurovascular transfer. Even at 30 weeks after transfer, the morphology of the neuromuscular junction failed to return to the presurgical state. The number of acetylcholine receptors at the reinnervated neuromuscular junction also remained lower than the control. The persistent weakness following free neurovascular muscle transfer may be attributed to these qualitative and quantitative changes at the neuromuscular junction.

Fiber type morphometry and capillary geometry in free, vascularized muscle transfers

In order to establish the relationship among intraoperative ischemia time, the recovery of contractile function, muscle fiber morphometry, and capillary geometry following free muscle transfer, 15 male dogs underwent unilateral, orthotopic free transfer of the left gracilis muscle with microneurovascular anastomosis. Intraoperative ischemia time varied from 0 to 4 hours. After a recovery period averaging 14 months, isometric contractile function was measured in the transfers and the contralateral, control muscles. Muscle fiber type composition, fiber size, and capillary geometry were quantified from histochemical sections. Transferred and control muscles had similar type I fiber percentages, type I fiber sizes, and capillary densities. Type II fiber sizes were reduced in transfers relative to controls. There was no significant effect of intraoperative ischemia on any morphometric variable. Although contractile function was generally reduced after free, vascularized transfer, there was only a weak correlation between the recovery of tetanic tension and type I fiber percentage (r = 0.53, P less than 0.05). It is concluded that free, vascularized muscle transfer results in minimal disturbance of muscle fiber type composition, fiber size, or capillary supply and that the diminished contractile function observed in this setting is still largely unexplained.