Several members of the transient potential receptor (TRP) channel superfamily are expressed in skeletal muscle and are thought to contribute to intracellular Ca²⁺-regulation of muscle fibres. In cell models and expressions systems TRPC3, one of the predominant TRP channels in skeletal muscle, is know to be modulated by various mechanisms. TRPC3 can be activated by i) G protein coupled receptor activation and subsequent release of diacylglycerol, ii) depletion of intracellular Ca²⁺ stores and iii) exocytotic insertion into the plasma membrane. To study conditions that may cause a translocation of TRPC3 to the plasma membrane we monitored the TRPC3 distribution in HEK 293 cells stably expressing yellow fluorescent protein-labelled TRPC3 (TRPC3-YFP). The investigations were extended to isolated muscle fibres by using indirect immunofluorescence staining of TRPC3. In HEK 293 cells we observed translocation of TRPC3 to the plasma membrane in response to application of epithelial growth factor (EGF), the unspecific Ca²⁺-channel blocker Gd3+ (50 mM), nifedipine (50 mM) and 2-aminoethyldiphenylborinate (2-APB, 50 mM). The increases in plasma membrane fluorescence occurred within 10–20 min and were accompanied by a decrease in intracellular YFP-dependent fluorescence. Plasma membrane translocation of TRPC3-YFP could also be stimulated in the absence of extracellular Ca2+. In isolated interosseus muscle fibres of the mouse TRPC3 translocation was observed under similar conditions as shown for HEK 293 cells. In particular, incubation with Gd3+ and nifedepine caused a marked increase in sarcolemmal localization of TRPC3. These data suggest that translocation of TRPC3 from cytoplasmic pools to the plasma membrane is an important mechanism of TRPC3 activation in muscle fibres. Exocytotic insertion of TRPC3 into the sarcolemma is stimulated by inhibition of Ca²⁺-influx into muscle fibres. Thus, TRPC3 could be coupled to a feedback mechanism that connects sub-sarcolemmal Ca²⁺ deficiency to increased activation of Ca²⁺-influx channels.

Supported by the German Ministry of Science, BMBF (MD-NET) and Foundation Benni & Co.