I n this study we analyzed the expression of aquaporin-4 (AQP4) in mammalian skeletal muscle. Immunohistochemical experiments revealed that affinity-purified AQP4 antibodies stained selectively the sarcolemma of fast-twitch fibers. By immunogold electron microscopy, little or no intracellular labeling was detected. Western blot analysis showed the presence of two immunopositive bands with apparent molecular masses of 30 and 32 kD specifically present in membrane fraction of a fast-twitch rat skeletal muscle (extensor digitorum longus, EDL) and not revealed in a slow-twitch muscle (soleus). PCR Southern blot experiments resulted in a selective amplification in EDL of a 960-bp cDNA fragment encoding for the full-length rat form of AQP4. Functional experiments carried out on isolated skeletal muscle bundle fibers demonstrated that the osmotic response is faster in EDL than in soleus fibers isolated from the same rat. These results provide for the first time evidence for the expression of an aquaporin in skeletal muscle correlated to a specific fiber-type metabolism. Furthermore, we have analyzed AQP4 expression in skeletal muscle of mdx mice in which a decreased density of orthogonal arrays of particles, a typical morphological feature of AQP4, has been reported. Immunofluorescence experiments showed a marked reduction of AQP4 expression suggesting a critical role in the membrane alteration of Duchenne muscular dystrophy.