TRPV4 Channels in Mouse Skeletal Muscle: Gene Expression, Role for Ca2+ Homeostasis and Muscle Force and Fatigue

The cation channel TRPV4 is one of the most abundant TRP channels in mouse skeletal muscle. Here, we investigated the contribution of TRPV4 to Ca2+ homeostasis, muscle force and fatigue. For this we used muscles of wildtype (WT) and TRPV4-/- mice. To estimate background Ca2+ influx we applied the Mn2+ quench technique. Muscle force was tested on isolated soleus muscles. Gene expression was analyzed by RT-PCR and data normalized to that of 18s rRNA. In WT fibers application of 4a-PDD, a TRPV4 activator, caused an increase in the Mn2+ quench rate of about 45 % (4.27 ± 0.24 %/min vs. 6.28 ± 0.39 %/min, p<0.01). This effect was completely absent in TRPV4-/- fibers. However, basic quench was not reduced in TRPV4 deficient fibers. Decay time of KCl induced Ca2+-transients was increased in WT fibers after application of 4-αPDD (from 4.46 ± 0.2 s, n=67 to 5.92 ± 0.28 s, n=23, p < 0.01). This effect was likewise not observed in fibers of TRPV4-/- animals. Muscle fatigue was attenuated in the presence of 4α-PDD, an effect which was not observed in muscles of TRPV4-/- mice. No difference between TRPV4-/- and WT animals was seen in wire hang test. Gene expression analysis showed that TRPV4 and TRPV2 were the most abundant TRPV channels in muscle, their mRNA concentration being about 5-fold higher in soleus muscle that in fast twitch EDL. In soleus muscles of TRPV4-/- mice TRPV2 mRNA was significantly increased. We conclude that TRPV4 is functionally expressed in mouse skeletal muscle, contributes to background calcium entry and can modulate Ca2+ transients. TRPV4 activity can attenuate fatigue, but its lack does not affect gross motor skills. TRPV4 knockout causes a compensatory upregulation of TRPV2.