EFFECTS OF DEXAMETHASONE ON FIBRE SUBTYPES IN RAT MUSCLE

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Abstract
The extent to which dexamethasone treatment produced atrophy of fast-twitch (EDL) and slow-twitch (SOL) muscles in the rat was investigated. The mean weight of steroid-treated EDL muscles was decreased as compared to normal; SOL muscles from normal and dexamethasone-treated animals showed no significant difference. Muscle fiber diameters also showed comparatively minor changes in SOL, which consists of Type 1 (slow oxidative) and Type 2A (fast oxidative/glycolytic) fibers. Rat EDL contains, in addition to Type 1 and Type 2A fibers, 2 sub-populations of fast glycolytic fibers (Types 2B and 2B''). These fiber types showed the most severe degree of atrophy both after dexamethasone treatment and after denervation. The mean ratio of the weights of denervated to innervated EDL muscles was lower in steroid-treated rats than in normal animals suggesting that the atrophy produced by steroid treatment in conjunction with denervation was more than simply additive. Analysis of the proportions of histochemical fiber types in SOL and EDL showed that dexamethasone treatment produced no major alterations in the fiber type constitution of these muscles. Further histochemical studies showed that there was relatively severe impairment of myophosphorylase activity in Type 2B'' (fast glycolytic) fibers as compared to other fiber types; conversely Type 1 fibers frequently contained increased myophosphorylase. Levels of .beta.-hydroxybutyrate dehydrogenase were low in both normal and steroid-treated EDL but high in SOL which also showed higher general oxidative activity. The particular susceptibility of fast glycolytic fibers to atrophy as a result of steroid treatment may be linked to the relatively severe reduction of myophosphorylase activity in these fibers; and their comparative inability to utilize alternative energy sources, especially substrates derived from free fatty acids.