We studied the effect of regeneration, altered innervation and thyroid hormone (TH) levels on fiber type transitions in slow soleus (SOL) muscles grafted (GRAFT) into host extensor digitorum longus (EDLh) muscles of euthyroid (EU), hyperthyroid (HT) and hypothyroid (HY) Lewis strain rats. SOL muscles were excised from 3-week to 4-week-old inbred Lewis rats and intramuscularly transplanted into EDLh muscles of 2-month-old female rats of the same strain. The proportions of type 1, 2A, 2X and 2B fibers of GRAFT were determined by immunohistochemistry and compared with those of EDLh muscle and EDL and SOL muscles of the unoperated contralateral hind limb. After an average regeneration period of 6-7 months and after being reinnervated by the "fast" peroneal nerve of EDLh muscle, GRAFT was transformed into a fast muscle. However, the extent of GRAFT transformation varied with different TH states. In the EU rats, GRAFT contained about 95 % of fast fibers, among which type 2X and 2B fibers predominated (about 75 %). The transition toward fast muscle phenotype was more pronounced in HT status, where the fastest type 2B fibers predominated. On the contrary, in HY status, the slow to fast transformation was less pronounced, as GRAFT contained less type 2B and 2X but more type 2A and 1 fibers. We conclude that the type of innervation is the crucial factor for the slow to fast fiber type transitions in GRAFT, but the extent of muscle transformation is further modulated by altered TH status.

Skeletal muscle fibre types, whose characteristics are determined by myosin heavy chain (MyHC) isoforms, can adapt to changed physiological demands with changed MyHC isoform expression resulting in the fibre type transitions. The endurance training is known to induce fast-to-slow transitions and has beneficial effect in carcinogenesis, whereas the effect of an excessive fat intake and its interaction with the effect of swimming are less conclusive. Therefore, we studied the effect of high-fat mixed lipid (HFML) diet and long-term (21-week) swimming on fibre type transitions and their average diameters by immunohistochemical demonstration of MyHC isoforms in slow soleus (SOL), fast extensor digitorum longus (EDL), and mixed gastrocnemius medialis and lateralis (GM, GL) muscles, divided to deep and superficial portions (GMd, GMs, GLd, GLs), of sedentary and swimming Wistar rats with experimentally (dimethylhydrazine) induced colon tumours and fed either with HFML or low-fat corn oil (LFCO) diet. HFML diet induced only a trend for fast-to-slow transitions in SOL and in the opposite direction in GMd. Swimming triggered significant transitions in unexpected slow-to-fast direction in SOL, whereas in GMs the transitions had tendency to proceed in the expected fast-to-slow direction. The average diameters of fibre types were mostly unaffected. Hence, it can be concluded that if present, the effects of HFML diet and swimming on fibre type transitions were counteractive and muscle-specific implying that each muscle possesses its own adaptive range of response to changed physiological conditions.

To build on the existing data on the pattern of myosin heavy chain (MyHC) isoforms expression in the human muscle spindles, we aimed to verify whether the 'novel' MyHC-15, -2x and -2b isoforms are co-expressed with the other known isoforms in the human intrafusal fibres. Using a set of antibodies, we attempted to demonstrate nine isoforms (15, slow-tonic, 1, α, 2a, 2x, 2b, embryonic, neonatal) in different regions of intrafusal fibres in the biceps brachii and flexor digitorum profundus muscles. The reactivity of some antibodies with the extrafusal fibres was also tested in the masseter and laryngeal cricothyreoid muscles. In both upper limb muscles, the expression of slow-tonic isoform was a reliable marker for differentiating positive bag fibres from negative chain fibres. Generally, bag1 and bag2 fibres were distinguished in isoform 1 expression; the latter consistently expressed this isoform over their entire length. Although isoform 15 was not abundantly expressed in intrafusal fibres, its expression was pronounced in the extracapsular region of bag fibres. Using a 2x isoform-specific antibody, this isoform was demonstrated in the intracapsular regions of some intrafusal fibres, particularly chain fibres. To the best of our knowledge, this study is the first to demonstrate 15 and 2x isoforms in human intrafusal fibres. However, whether the labelling with an antibody specific for rat 2b isoform reflects the expression of this isoform in bag fibres and some extrafusal ones in the specialised cranial muscles requires further evaluation. The revealed pattern of isoform co-expression only partially agrees with the results of previous, more extensive studies. Nevertheless, it may be inferred that MyHC isoform expression in intrafusal fibres varies along their length, across different muscle spindles and muscles. Furthermore, the estimation of expression may also depend on the antibodies utilised, which may also react differently with intrafusal and extrafusal fibres.

The aim of this paper was to present our experience with seven monoclonal antibodies, six of them were applied in immunohistochemistry and immunoblotting of MyHC isoforms in rats and humans, one of them, 6H1 (Lucas et al., 2000), was tested in human muscle sections only. The four antibodies specific to rat MyHC isoforms, BA-D5,SC-71, BF-35,BF-F3 (Schiaffino et al.,1989) reacted as declared both on muscle sections and immunoblots of rat except SC-71 antibody, which stained MyHC-2a and -2x bands in blots. One of the two commercially available antibodies, A4.74 antibody,reliably marked type 2a fibres of rat,but in blots it weakly stained MyHC-2a and -2x isoforms when used undiluted. The other one, F113.15F4, stained type 2a and 2x fibres and corresponding MyHC bands in blots. Therefore, using this antibody rat MyHC-2x can be additionally confirmed, which can be otherwise demonstrated only on the principle of exclusion with BF-35.Using the same set of antibodies human fast MyHC isoforms can be revealed less clearly. Namely, SC-71 and A4.74 antibodies intensively stained histochemical type 2a,predominantly expressing 2a MyHC transcripts and moderately type 2x fibres, expressing mostly 2x MyHC transcripts, in blots the antibodies recognized both fast isoforms. The 6H1antibody was the only one that selectively labelled type 2x fibres, whereas BF-35 left unstained only a variable proportion of histochemical type 2x fibres and MyHC-2x in blots. F113.15F4 did not distinguish between human fast fibre types and corresponding MyHC isoforms in blots. The negativeresults obtained with BF-F3 in muscle sections and in blots are in agreement with the absence of MyHC-2b in human skeletal muscles. Our results imply that the reactivity of antibodies specific to distinct MyHC isoforms should be carefully evaluated not only among various species but with the two different techniques used as well.

To compare the organization of human and rat ocular medial recti muscles (MR).