IP3-Independent Release of Ca2+From Intracellular Stores: A Novel Mechanism for Transduction of Bitter Stimuli

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Abstract
A variety of substances with different chemical structures elicits a bitter taste. Several different transduction mechanisms underlie detection of bitter tastants; however, these have been described in detail for only a few compounds. In addition, most studies have focused on mammalian taste cells, of which only a small subset is responsive to any particular bitter compound. In contrast, ∼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 dextromethorphan (DEX), a bitter antitussive, with transduction of the well-studied bitter compound denatonium. Bath perfusion of DEX (2.5 mM) increased the intracellular Ca2+level in most taste cells. The DEX-induced Ca2+increase was inhibited by thapsigargin, an inhibitor of Ca2+transport into intracellular stores, but not by U73122, an inhibitor of phospholipase C, or by ryanodine, an inhibitor of ryanodine-sensitive Ca2+stores. Increasing intracellular cAMP levels with a cell-permeant cAMP analogue and a phosphodiesterase inhibitor enhanced the DEX-induced Ca2+increase, which was inhibited partially by H89, a protein kinase A inhibitor. Electrophysiological measurements showed that DEX depolarized the membrane potential and inhibited voltage-gated Na+and K+currents in the presence of GDP-β-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 channels, 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 enter taste cells and interact directly with Ca2+stores. Comparing transduction of DEX with that of denatonium, both compounds release Ca2+from intracellular stores. However, denatonium requires activation of phospholipase C, and the mechanism results in a hyperpolarization rather than a depolarization of the membrane potential. These data support the hypothesis that single taste receptor cells can use multiple mechanisms for transducing the same bitter compound.