Nitric oxide lacks direct effect on TRPC5 channels but suppresses endogenous TRPC5-containing channels in endothelial cells

TRPC5 is a member of the canonical transient receptor potential (TRPC) family of proteins that forms cationic channels either through homomultimeric assembly or heteromultimeric coordination with other TRPC proteins. It is expressed in a variety of cells including central neurones and endothelial cells and has susceptibility to stimulation by multiple factors. Here we investigated if TRPC5 is sensitive to nitric oxide. Mouse TRPC5 or human TRPC5 was over-expressed in HEK293 cells, and TRPC5 activity was determined by measuring the cytosolic Ca²⁺ concentration with an indicator dye or by recording membrane current under voltage clamp. TRPC5 activity could be evoked by carbachol acting at muscarinic receptors, lanthanum, or a reducing agent. However, S-nitroso-N-acetylpenicillamine (SNAP) and diethylamine NONOate (DEA-NONOate) failed to stimulate or inhibit TRPC5 at concentrations that generated nitric oxide, caused vasorelaxation, or suppressed activity of TRPC6 via protein kinase G. At high concentrations, SNAP (but not DEA-NONOate) occasionally stimulated TRPC5 but the effect was confounded by background TRPC5-independent Ca²⁺ signals. Endogenous Ca²⁺-entry in bovine aortic endothelial cells (BAECs) was suppressed by SNAP; TRPC5 blocking antibody or dominant-negative mutant TRPC5 suppressed this Ca²⁺ entry and occluded the effect of SNAP. The data suggest that nitric oxide is not a direct modulator of homomeric TRPC5 channels but may inhibit endogenous BAEC channels that contain TRPC5.