Differential effect of chronic undernutrition on the fiber type composition of fascicles in the extensor digitorum longus muscles of the rat

Several studies have shown that chronic low food consumption alters the composition and metabolism of the extensor digitorum longus muscle (EDLm) fiber types. EDLm is constituted by four independent fascicles (F2-F5) of different sizes; their constitution and metabolism, however, as well as how chronic undernourishment affects these is virtually unknown. Thus, the aim of this study is to evaluate the relative fiber type composition and metabolism of each independent fascicle in the EDLm, using control and chronically undernourished young male rats by using the alkaline ATPase and NADH-TR histochemical techniques. Our results indicate that all control fascicles showed a higher percentage of intermediate fibers (P < 0.001), except for F5, which had a higher percentage of fast fibers (P < 0.001). After chronic undernutrition, the proportion of intermediate fibers decreased in F4 (P < 0.05) and increased in F5 (P < 0.001), whereas fast fibers decreased in F3 (P < 0.05) and F5 (P < 0.001). When we investigated fiber metabolism we found that F3 and F4 had a similar composition (mainly glycolytic), whereas F2 and F5 predominantly contained oxidative fibers. All fascicles of chronic undernourished rats showed a general decrease in oxidative fibers (P > 0.05), except for F3, in which oxidative fibers increased (P < 0.05). After determining the possible predominant metabolism expressed in intermediate fibers, we propose that chronic undernutrition induces the transformation of fast-glycolytic to intermediate-oxidative/glycolytic fibers, mainly in F3 and F5. Our observations confirm that chronic undernourishment differentially affects the fiber types of each fascicle in the EDLm, which could alter their individual physiological contractile properties.