TRP channels are involved in mediating hypercapnic Ca2+ responses in rat glia-rich medullary cultures independent of extracellular pH

The medulla contains central chemosensitive cells important for the maintenance of blood gas and pH homeostasis. To identify the intrinsic chemosensitive cells, we measured responses of intracellular Ca2+ ([Ca2+]i) and H+ ([H+]i), and membrane potential of rat primary-cultured medullary cells to 6-s exposure to acidosis.The cells showed transient [Ca2+]i increases to extracellular pH 6.8, which was inhibited by the specific ASIC1a blocker (psalmotoxin-1), but did not respond to pH 7.1 in the HEPES-buffered solution. Isocapnic acidosis induced no changes in [Ca2+]i, whereas hypercapnic acidosis induced a remarkable Ca2+ response and an increase in membrane potential in the HCO3−-buffered solution (pH 7.1). In glia-rich cultures, intracellular acidification preceded the hypercapnic acidosis-induced Ca2+ response, and acetazolamide, a carbonic anhydrase inhibitor suppressed these responses. Transient receptor potential (TRP) channel broad-spectrum blockers Ni2+ and ruthenium red, and a TRPV1- and TRPM8-specific blocker N-(4-tertiarybutylphenyl)-4-(3-chloropyridin-2-yl)-tetrahydropyrazine-1(2H)-carbox-amide attenuated the hypercapnic acidosis-induced Ca2+ response. Subpopulations of cells that exhibited the hypercapnic acidosis-induced Ca2+ response also responded to the application of capsaicin (TRPV1 agonist) and menthol (TRPM8 agonist).These results suggest that the TRP channel family partially mediates the fast hypercapnic acidosis-induced Ca2+ response via changes in [H+]i and is a candidate of central chemosensing proteins.