Role of TRPC1 channel in skeletal muscle function.

Skeletal muscle contraction is reputed not to depend on extracellular Ca2+. Indeed, stricto sensu, excitation-contraction coupling does not necessitate entry of Ca2+. However, we previously observed that, during sustained activity (repeated contractions), entry of Ca2+ is needed to maintain force production. In the present study, we evaluated the possible involvement of the canonical transient receptor potential (TRPC)1 ion channel in this entry of Ca2+ and investigated its possible role in muscle function. Patch-clamp experiments reveal the presence of a small-conductance channel (13 pS) that is completely lost in adult fibers from TRPC1(-/-) mice. The influx of Ca2+ through TRPC1 channels represents a minor part of the entry of Ca(2+) into muscle fibers at rest, and the activity of the channel is not store dependent. The lack of TRPC1 does not affect intracellular Ca2+ concentration ([Ca2+](i)) transients reached during a single isometric contraction. However, the involvement of TRPC1-related Ca2+ entry is clearly emphasized in muscle fatigue. Indeed, muscles from TRPC1(-/-) mice stimulated repeatedly progressively display lower [Ca2+](i) transients than those observed in TRPC1(+/+) fibers, and they also present an accentuated progressive loss of force. Interestingly, muscles from TRPC1(-/-) mice display a smaller fiber cross-sectional area, generate less force per cross-sectional area, and contain less myofibrillar proteins than their controls. They do not present other signs of myopathy. In agreement with in vitro experiments, TRPC1(-/-) mice present an important decrease of endurance of physical activity. We conclude that TRPC1 ion channels modulate the entry of Ca(2+) during repeated contractions and help muscles to maintain their force during sustained repeated contractions.