Energy metabolism of fast- and slow-twitch skeletal muscle in the rat: Thyroid hormone induced changes |
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Authors: | Colin J M Nicol and Ian A Johnston |
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Institution: | (1) Department of Physiology and Pharmacology, University of St. Andrews, Bute Medical Buildings, KY16 9TS St. Andrews, Fife, Scotland |
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Abstract: | Summary Male Wistar rats were made hypothyroid or hyperthyroid over a period of six weeks, by administration of carbimazole or triiodothyronine (T3). Serial frozen sections of soleus and extensor digitorum longus (EDL) muscle were stained histochemically for myosin ATPase, succinic dehydrogenase and phosphorylase. Muscle fibres were classified as either slow twitch oxidative (SO), fast twitch oxidative glycolytic (FOG) or fast twitch glycolytic (FG). In addition the activities of phosphorylase, phosphofructokinase (PFK), fructose-1,6-diphosphatase (FDP), lactate dehydrogenase (LDH), hexokinase, citrate synthetase, cytochrome oxidase, 3-hydroxyacyl-CoA dehydrogenase (HAD) and 5-AMP aminohydrolase were measured in both muscles.Increasing plasma levels of T3 are associated with marked alterations in the fibre type populations in both muscles. In the soleus there is conversion of SO to FOG fibres while in the EDL, FG fibres are converted to FOG fibres. The quantitative changes in metabolic enzyme activity however, are in the main restricted to the soleus. Increased T3 levels result in an increased capacity for the aerobic metabolism of both fat and carbohydrate and an increase in anaerobic glycolytic activity in the soleus muscle which parallels the change in fibre types. However, the extent of these increases cannot be explained solely on this basis and there is also an overall increase in aerobic activity in all fibres including slow oxidative ones. It is concluded that the effects of thyroid hormone on muscle phenotype and respiratory capacity involve both primary and secondary sites of action and the possible mechanisms are discussed.Abbreviations
EDL
extensor digitorum longus
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FDP
fructose-1,6-diphosphatase
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FG
fast twitch glycolytic
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FOG
fast twitch oxidative glycolytic
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HAD
3-hydroxyacyl-CoA-dehydrogenase
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LDH
lactate dehydrogenase
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PFK
phosphofructokinase
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SO
slow twitch oxidative
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T
3
triiodothyronine
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T
4
thyroxine |
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