Myosin heavy chain (MyHC) expression was examined in regenerating fast extensor digitorum longus (EDL) and slow soleus (SOL) muscles of adult rats. Myotoxic bupivacaine was injected into SOL and EDL and the muscles were either denervated or neuromuscularly blocked by tetrodotoxin (TTX) on the sciatic nerve. Three to 10 or 30 days later, denervated SOL or EDL, or innervated but neuromuscularly blocked EDL received a slow 20 Hz stimulus pattern through electrodes implanted on the muscles or along the fibular nerve to EDL below the TTX block. In addition, denervated SOL and EDL received a fast 100 Hz stimulus pattern. Denervated EDL and SOL stimulated with the same slow stimulus pattern expressed different amounts of type 1 MyHC protein (8% versus 35% at 10 days, 13% versus 87% at 30 days). Stimulated denervated and stimulated innervated (TTX blocked) EDL expressed the same amounts of type 1, 2A, 2X and 2B MyHC proteins. Cross-sections treated for in situ hybridization and immunocytochemistry showed expression of type 1 MyHC in all SOL fibres but only in some scattered single or smaller groups of fibres in EDL. The results suggest that muscle fibres regenerate from intrinsically different satellite cells in EDL and SOL and within EDL. However, induction by different extrinsic factors arising in extracellular matrix or from muscle position and usage in the limb has not been excluded. No evidence for nerve-derived trophic influences was obtained.

ABSTRACT. Muscle regeneration is a potentially useful model for defining the mechanisms responsible for nerve-dependent myosin isogene regulation in skeletal muscle. As a first step towards this goal we have characterized the pattern of expression of the four myosin heavy chain (MHC)genes, MHC-^/slow, -2A, -2X and -2B isogenes, during early stages of muscle regeneration both in the presence and in the absence of the nerve. Muscle degeneration/regeneration was induced by intramuscular injection of the myotoxic drug, bupivacaine, in the rat slow soleus muscle. MHC transcripts were identified by in situ hybridization with specific riboprobes during the period from day 3 to day 7 after muscle injury. The four genes are not detected at day 3, when the regenerating muscle contains predominantly embryonic and neonatal MHC isoforms. MHC-2X and -2B transcripts are first detected at day 4 in both innervated and denervated muscles. These transcripts remain as major transcripts in denervated muscles whereas they are down-regulated by day 5 and disappear by day 6-7 in the presence of the nerve. Innervation induces strong up-regulation of MHC-2Aat day 4 and MHC-^/slow transcripts at day 5. MHC-2A transcripts are first homogeneously expressed in most fibers then become segregated in a minor population of fibers by day 6-7 while MHC-^/slow transcripts increase in most fibers. In the absence of the nerve MHC-^/slowtranscripts are never expressed and MHC-2A transcripts are detected in rare fibers at days 5-7.

The aim of our study was to explore the fibre type composition of the human psoas major muscle at different levels of its origin, from the first lumbar to the fourth lumbar vertebra, and to compare the muscle fibre size and distribution of different fibre types between levels with respect to its complex postural and dynamic function. Muscle samples were collected from 15 young males (younger than 35 years). Serial transverse sections (5 lm) of the samples were cut by cryomicrotome. Type I, IIA and IIX muscle fibres were typed using myosin heavy chain identification. The serial sections were analysed using a light microscope with a magnitude of 100·. The differences between measurements were evaluated using a repeated-measures ANOVA and Scheffé test for post-hoc analysis. Our study showed that the human psoas major muscle was composed of type I, IIA and IIX muscle fibres. It had a predominance of type IIA muscle fibres, whereas type I muscle fibres had the largest cross-sectional area. Type IIX muscle fibres were present as a far smaller percentage and had the smallest crosssectional area. Moreover, the fibre type composition of the psoas major muscle was different between levels of its origin starting from the first lumbar to the fourth lumbar vertebra. We conclude that the fibre type composition of the psoas major muscle indicated its dynamic and postural functions, which supports the fact that it is the main flexor of the hip joint (dynamic function) and stabilizer of the lumbar spine, sacroiliac and hip joints (postural function). The cranial part of the psoas major muscle has a primarily postural role, whereas the caudal part of the muscle has a dynamic role.

The results suggest that the Reg3G gene plays a major role in communication between injured axons and muscle and may play a significant role in skeletal muscle and peripheral nerve regeneration.

Treatment with lipoic acid during 4 weeks has shown effects on the redistribution of muscle fibers, and can prevent atrophy in slow and fast diabetic muscle.