Question: 

Among others the Ca²⁺ selective cation channel TRPV4 is one of the most abundant TRP channels in the sarcolemma of mouse skeletal muscle fibres. The aim of this study was to investigate the contribution of TRPV4 to Ca²⁺ homeostasis, muscle fatigue and motor function. Therefore we investigated muscles and muscle fibres of wildtype (WT) and TRPV4 deficient mice (TRPV4^-/-) using the specific TRPV4 activator 4a-phorbol-12,13-didecanoate (4a-PDD).

Methodology: 

Single fibres were prepared from mouse interosseus muscles. Background calcium entry into fibres and decline of KCl induced Ca²⁺ transients were estimated by using the Ca²⁺ indicator Fura-2. For judging the background Ca²⁺ entry the extracellular Ca²⁺ was replaced by 0.5 mM Mn²⁺. Quench of Fura-2 fluorescence was recorded in response to excitation at 360 nm. Transient changes in [Ca²⁺]_i were induced by puff application of KCl (110 mM) and the changes in the ratio of fluorescence intensity after 340 and 380 nm excitation was measured. To study muscle fatigue, whole M. solei were stimulated with a fatigue protocol consisting of 500 ms lasting tetani (125) of 50 Hz stimulation followed by 1.5 s pauses. As a test of general motor skills we applied the wire hang test.

Results: 

In WT fibres application of 4-aPDD caused an increase in the quench rate of about 45 % (4.27 ± 0.24 %/min, N = 87 vs. 6.28 ± 0.39 %/min, N = 41, p<0.01). This effect was completely absent in TRPV^-/- fibres. The half time of decay of KCl induced Ca²⁺ transients was significantly increased in WT fibres after application of 4-aPDD (from 4.46 ± 0.2 s, N=67 to 5.92 ± 0.28 s, N=23, p < 0.01). In contrast to WT fibres in TRPV4^-/- muscle fibres half time of decay was decreased after application of 4-aPDD indicating a side effect of 4a-PDD drug which seems to be masked in WT fibres. Muscle fatigue was significantly attenuated in the presence of 4a-PDD, an effect which was not observed in soleus muscles of TRPV^-/- mice. No difference between TRPV4^-/- and WT animals was observed in wire hang test.

Conclusion: 

We conclude that TRPV4 is functionally expressed in mouse skeletal muscle, contributes to background calcium entry and can modulate depolarisation induced Ca²⁺ transients. Activity of TRPV4 can attenuate muscle fatigue but its lack does not significantly affect gross motor skills of TRPV4 deficient mice.