Muscle contraction is principally triggered by Ca²⁺ release from the sarcoplasmic reticulum. However, the long-term maintenance of muscle function requires Ca²⁺ influx into muscle fibres from extracellular. Candidate channels to control such a background Ca²⁺ entry are members of the transient receptor potential (TRP) family of cation channels. TRPV4 is a Ca²⁺-selective cation channel that can be activated by a variety of stimuli, such as cell swelling, heat, phorbol esters and, probably, by endogenous ligands. Recently, immunofluorescence staining of tibial muscle cross sections showed the presence of TRPV4 in the sarcolemma of mouse muscle fibres (Krüger et al., Neuromuscul Disord 18:501–513, 2008). The aim of the current study was to investigate whether one of the most abundant muscular TRP channels, TRPV4, contributes to cellular Ca²⁺ homeostasis and maintenance of muscle force during prolonged tetanic stimulation. Isolated fibres of the m. interosseus muscle were used to study the entry of divalent cations and the m. soleus was prepared to investigate contractile muscle function. Western blot analyses confirmed the expression of TRPV4 in both, soleus and interosseus muscles. To estimate the background calcium entry we used the manganese-quench technique applied to isolated interosseus fibres. In the presence of standard external solution, the quench rate was determined to 3.98 0.28 per min (n= 61), while in the presence of the selective TRPV4-channel activator 4a-phorbol-12,13-didecanoate (4a-PDD, 5 mM) it was 6.33 0.42 per min (n = 38). Thus, a 60% increase of basal Ca²⁺ entry could be achieved by TRPV4 activation. Electrical stimulation of isolated soleus muscles did not reveal changes in isometric twitches upon application of 4a-PDD (1 mM). However, the amplitudes of titanic contractions (120 Hz) were slightly increased by about 10% in the presence of 4a-PDD. To study effects of 4a-PDD on muscle fatigue, we stimulated mouse soleus muscles repetitively with 500 ms-lasting trains at 50 Hz. Trains were given every two seconds for 7 min. In the course of this protocol isometric force declined up to 80% with a half-maximal titanic force at 159 8 s (n = 14). In the presence of 4a-PDD (1 mM) muscle fatigue was attenuated. On average the time of half-maximal titanic force was prolonged to 206 19 s (n = 9). We conclude that TRPV4 can be functionally activated in mouse muscle fibres leading to an increased permeability of the sarcolemma for divalent cations. TRPV4 dependent increased Ca²⁺ influx during repetitive stimulation seems to attenuate muscle fatigue of isolated muscles. These data point to the importance of a background Ca²⁺ entry into skeletal muscle fibres that seems to be partly exerted by TRPV4.

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