Muscle loading modulates aquaporin-4 expression in skeletal muscle

The purpose of this investigation was to determine the effect of hindlimb suspension (HS) on the expression of aquaporin-4 (AQP4) in rat soleus as well as to test the hypothesis that AQP4 can be regulated in skeletal muscle and that the expression is related to the fiber metabolism.SDSPAGE experiments showed that HS determined a twofold increase in the fast MHC isoforms, concomitant with a similar decrease in the relative amount of the slow MHC isoforms after only 8 days of HS. A marked induction in AQP4 mRNA was found by semiquantitative RT-PCR. In agreement with the increased level of AQP4 mRNA, Western blot experiments show a 90% increase in the AQP4 content. Moreover, double immunofluorescence experiments demonstrated that after 8 days of suspension, the expression of AQP4 increased twofold in parallel with a similar increase in the number of fast IIA fibers and remained at the same level after 14 and 21 days of suspension. In parallel, the number of type I fibers decreased after suspension. Finally, our analysis of the transitional fibers from type I to type IIA that were found after 4 days of HS revealed that the expression of AQP4 temporally parallels that of type IIA MHC protein. Our results demonstrate that AQP4 expression can be regulated in skeletal muscle. Furthermore, AQP4 up-regulation is associated with slow- to fast-twitch fiber conversion, and thus to the glycolitic metabolism of the fiber. Our results also suggest that AQP4 is a component of the ensemble of muscle protein involved in muscle plasticity, depending on functional demand.