MUSCLE-FIBER COMPOSITION OF RAT VASTUS-INTERMEDIUS FOLLOWING IMMOBILIZATION AT DIFFERENT MUSCLE LENGTHS

The knee joints of one hind limb of male Wistar rats (250-300 g) were immobilised in a plaster cast for up to 3 weeks with the limb in either the fully extended or flexed position Serial frozen sections of vastus intermedius muscle were stained histochemically for myosin ATPase, succinic dehydrogenase, phosphorylase and capillaries. Muscle fibres were classified as either slow twitch oxidative (SO), fast twitch oxidative glycolytic (FOG) or fast twitch glycolytic (FG). Fibre area of SO fibres (3019 μ2) decreased by 50% following 3 weeks' immobilisation in the stretched (1532 μ2) and shortened (1517 μ2) positions. Atrophy of FOG fibres (2456 μ2) was greater in muscles immobilised in the stretched (750 μ2) than shortened (1185 μ2) positions. Evidence has been obtained that muscle fibre number remained unchanged following immobilisation. Control muscles contained 70.4% SO fibres, 29.6% FOG fibres and >0.5% FG fibres. Immobilisation produced an increased proportion of high myosin ATPase staining fibres. The percentage occurrence of FOG fibres increased to 65% and 83% respectively, in muscles immobilised for 3 weeks in the stretched and shortened positions. Mechanisms for the transformation of muscle fibre types and the influence of muscle length on the properties of immobilised muscle are discussed. © 1979 Springer-Verlag.