Differential effects of nandrolone decanoate in fast and slow rat skeletal muscles

Purpose: We studied the effects of high doses of an anabolic-androgenic steroid, exercise training, and a combination of steroid and training on mammalian fast- and slow-twitch skeletal muscles at the cellular level. Methods: Thirty-two male rats were divided into sedentary and treadmill-trained groups (increased spaed and time: 18 m.min(-1), 0.5 h.d(-1), 5 d.wk(-1)). Eight animals of each group were treated with nandrolone decanoate (ND) (15 mg.kg-1.wk(-1)), and others received the same doses of solvent. The animals were killed after 5 wk, and the contractile parameters for isolated small bundles of soleus and extensor digitorum longus (edl) fibers were estimated. Results: Muscle mass, twitches, and K+ contractures were increased in soleus and edl muscles after the drug treatment and after the exercise training. Caffeine contractures were increased only after the exercise training. The combination of exercise with ND treatment produced greater effects. particularly a significant increase in sensitivity to caffeine and the amplitude of K+ contractures as well as a shortening of the time required to restore contracture. These modifications were more marked in slow than fast muscle. Conclusion: These results show that 5 wk of exercise training produced changes in the contractile responses developed by isolated skeletal muscle cells. The combination of exercise training with ND treatment potentiated these effects, suggesting that there was some modification in the excitation-contraction coupling mechanism. ND treatment also produced a more potent effect in soleus than edl sedentary muscle.