IP3-independent release of Ca2+ from intracellular stores: A novel mechanism for transduction of bitter stimuli

A variety of substances with different chemical structures elicits a bitter taste. Several different transduction mechanisms underlie detection of bit ter tastants; however, these have been described in detail for only a few c ompounds. In addition, most studies have focused on mammalian taste cells, of which only a small subset is responsive to any particular bitter compoun d. In contrast, similar to 80% of the taste cells in the mudpuppy, Necturus maculosus, are bitter-responsive. In this study, we used Ca2+ imaging and giga-seal whole cell recording to compare the transduction of dextromethorp han (DEX), a bitter antitussive, with transduction of the well-studied bitt er compound denatonium. Bath perfusion of DEX (2.5 mM) increased the intrac ellular Ca2+ level in most taste cells. The DEX-induced Ca2+ increase was i nhibited by thapsigargin, an inhibitor of Ca2+ transport into intracellular stores, but not by U73122, an inhibitor of phospholipase C, or by ryanodin e, an inhibitor of ryanodine-sensitive Ca2+ stores. Increasing intracellula r cAMP levels with a cell-permeant cAMP analogue and a phosphodiesterase in hibitor enhanced the DEX-induced Ca2+ increase, which was inhibited partial ly by H89, a protein kinase A inhibitor. Electrophysiological measurements showed that DEX depolarized the membrane potential and inhibited voltage-ga ted Na+ and K+ currents in the presence of GDP-P-S, a blocker of G-protein activation. DEX also inhibited voltage-gated Ca2+ channels. We suggest that DEX, like quinine, depolarizes taste cells by block of voltage-gated K cha nnels, which are localized to the apical membrane in mudpuppy. In addition, DEX causes release of Ca2+ from intracellular stores by a phospholipase C- independent mechanism. We speculate that the membrane-permeant DEX may ente r taste cells and interact directly with Ca2+ stores. Comparing transductio n of DEX with that of denatonium, both compounds release Ca2+ from intracel lular stores. However, denatonium requires activation of phospholipase C, a nd the mechanism results in a hyperpolarization rather than a depolarizatio n of the membrane potential. These data support the hypothesis that single taste receptor cells can use multiple mechanisms for transducing the same b itter compound.