RELATIONSHIPS BETWEEN MUSCLE MEMBRANE-LIPIDS, FIBER-TYPE, AND ENZYME-ACTIVITIES IN SEDENTARY AND EXERCISED RATS

Insulin resistance in skeletal muscle is associated with 1) relative i ncreases in the proportion of glycolytic and fast-twitch muscle fibers and decreases in the proportion of more oxidative fibers and 2) a hig her proportion of the saturated fatty acids in membrane structural lip ids. Exercise is known to improve insulin action. The aims of the curr ent studies were 1) to investigate the relationship between muscle fib er type and membrane fatty acid composition and 2) to determine how vo luntary exercise might influence both variables. In sedentary Wistar r ats in experiment 1, increased amounts of unsaturated fatty acids were found in the more oxidative insulin-sensitive red quadriceps and sole us muscles, whereas reduced levels of polyunsaturated fatty acids were found in primarily glycolytic white quadriceps muscles. In experiment 2, voluntary running-wheel exercise by adult female rats over 45 days resulted in reduced proportions of type IIb fibers (P = 0.01) and inc reased proportions of type IIa/IIx fibers (P = 0.03) in extensor digit orum longus muscle. The magnitude of these changes was related to the distance run (r = -0.73, P = 0.04; r = 0.79, P = 0.02, respectively). Exercise significantly increased oxidative capacity, as assessed by th e proportion of intensely NADH-stained fibers (P = 0.0004) and citrate synthase (P = 0.003) and hexokinase (P = 0.04) activities. Citrate sy nthase activity was also increased by exercise in soleus muscle, where , as expected, no fiber type changes were detected. No significant dif ferences in the fatty acid profile of soleus and extensor digitorum lo ngus were found between groups. Voluntary exercise may play a role in improving insulin sensitivity through beneficial alterations in muscle fiber type and oxidative capacity. Membrane fatty acid composition ca n be improved by diet but appears unaffected by moderate exercise. Tak en together, these results suggest that diet and exercise may improve insulin action through separate and synergistic mechanisms.