Metabolic and contractile changes in fast and slow muscles of the cat after glucocorticoid-induced atrophy

The susceptibility of fast- and slow-twitch hind limb muscles to glucocorticoid-induced atrophy was investigated in adult male cats treated for 10 to 14 days with triamcinolone (4 mg/kg/day), using several histochemical, biochemical, and functional indices. After treatment, muscle weight loss in the fast-twitch muscles (medial gastrocnemius and vastus medialis) occurred to a greater extent than in the slow-twitch muscles (soleus and vastus intermedius), with the latter muscles decreasing in weight proportional to the body weight. Fast-twitch glycolytic (FG) fibers responded with similar degrees of atrophy in the muscles examined; however, slow-twitch oxidative (SO) and fast-twitch oxidative glycolytic (FOG) fibers atrophied more in the fast-twitch compared to the slow-twitch muscles. Phosphofructokinase and NADP⁺-linked isocitrate dehydrogenase specific activities decreased similarly in the fast-twitch muscles, while no change occurred in the slow-twitch muscles. Functionally, the soleus and medial gastrocnemius remained unchanged in abilities to generate tension tetanically, when this was expressed per unit muscle mass or per unit contractile protein. As a result of the treatment, however, the medial gastrocnemius fatigued faster in response to repetitive stimulation in the glucocorticoid-treated animals. The results suggest that the response of muscle to glucocorticoid-induced atrophy is not regulated by the primary metabolic pathways used for energy production. The differences in response of the SO and FOG fiber types in fast- versus slow-twitch muscles suggest basic differences in metabolic and activity profiles of the same fiber types in different muscles, which may influence susceptibility to atrophy.