Effects of unweighting and clenbuterol on myosin light and heavy chains in fast and slow muscles of rat

Abstract

To investigate the plasticity of slow and fast muscles undergoing slow-to-fast transition, rat soleus (SOL), gastrocnemius (GAS), and extensor digitorum longus (EDL) muscles were exposed for 14 days to 1) unweighting by hindlimb suspension (HU), or 2) treatment with the beta(2)-adrenergic agonist clenbuterol (CB), or 3) a combination of both (HU-CB). In general, HU elicited atrophy, CB induced hypertrophy, and HU-CB partially counteracted the HU-induced atrophy. Analyses of myosin heavy (MHC) and light chain (MLC) isoforms revealed HU- and CB-induced slow-to-fast transitions in SOL (increases of MHCIIa with small amounts of MHCIId and MHCIIb) and the upregulation of the slow MHCIa isoform. The HU- and CB-induced changes in GAS consisted of increases in MHCIId and MHCIIb ("fast-to-faster transitions"). Changes in the MLC composition of SOL and GAS consisted of slow-to-fast transitions and mainly encompassed an exchange of MLC1s with MLC1f. In addition, MLC3f was elevated whenever MHCIId and MHCIIb isoforms were increased. Because the EDL is predominantly composed of type IID and IIB fibers, HU, CB, and HU-CB had no significant effect on the MHC and MLC patterns.

Physiological role of myosin light and heavy chain isoforms in fast- and slow-twitch muscles: effect of exercise

“…Our study shows that the above-mentioned relation exists only between FT and ST muscles, but not between two FT muscles, EDL and Pla. It has been shown that higher apparent affinities exist between the MyLC 3 fast isoform and MyHC IIb and IId isoforms [21,26]. Our study shows that there are correlations between the relative content of MyHC IIb, IId and MyLC 3 fast isoforms in ST and FT muscles.…”

“…Even the distribution of MyLC isoforms between ST and FT muscles, as well as between FT muscles, still gives rise to serious discrepancies. It has been proposed that the functional role of MyLC isoforms is realized in combination with certain MyHC isoforms [21,26], but the distribution of MyHC and MyLC isoforms in different skeletal muscles has still not been determined with any certainty. It has been shown that exercise, depending on its character, induces differential expression of myosin protein isoforms in skeletal muscle [2].…”

Hypergravity from conception to adult stage: effects on contractile properties and skeletal muscle phenotype

“…Thus, transitions at the total MLC level were less marked than for MHC (see below) and troponin isoforms. Such lesser effects of environmental conditions on MLC changes have already been described elsewhere (Ingalls et al, 1996;Stevens et al, 2000).…”

Immunohistochemical Analysis of Laryngeal Muscles in Normal Horses and Horses With Subclinical Recurrent Laryngeal Neuropathy

“…There is no evidence in RLN for a selective loss of small myelinated fibers of the recurrent laryngeal nerve (Duncan et al 1991b) that are likely to innervate slow muscle fibers. Slow-to-fast muscle fiber transformation is well known to occur in limb muscles under conditions of altered impulse activity to slow muscles, such as spinal cord transection (Hoh et al 1980;Lieber et al 1986), zero gravity (Baldwin et al 1994;Talmadge et al 1996), and hindlimb suspension (Talmadge et al 1996;Stevens et al 2000). A similar fiber-type transformation can occur in RLN if mild axonopathy of nerve fibers innervating slow muscle fibers impairs their ability to sustain the transmission of low-frequency impulses for the prolonged periods needed to support slow fibers, while not affecting the short bursts of activity needed to support 2a fibers.…”