IP(3)-Independent release of Ca(2+) 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 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, approximately 80% of the taste cells in the mudpuppy, Necturus maculosus, are bitter-responsive. In this study, we used Ca(2+) 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 Ca(2+) level in most taste cells. The DEX-induced Ca(2+) increase was inhibited by thapsigargin, an inhibitor of Ca(2+) transport into intracellular stores, but not by U73122, an inhibitor of phospholipase C, or by ryanodine, an inhibitor of ryanodine-sensitive Ca(2+) stores. Increasing intracellular cAMP levels with a cell-permeant cAMP analogue and a phosphodiesterase inhibitor enhanced the DEX-induced Ca(2+) 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-beta-S, a blocker of G-protein activation. DEX also inhibited voltage-gated Ca(2+) 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 Ca(2+) from intracellular stores by a phospholipase C-independent mechanism. We speculate that the membrane-permeant DEX may enter taste cells and interact directly with Ca(2+) stores. Comparing transduction of DEX with that of denatonium, both compounds release Ca(2+) 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.
Ogura
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Department of Anatomy and Neurobiology, Colorado State University, Fort Collins, Colorado 80523, USA.
Kinnamon
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D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000242:Cyclic AMP / Q000031:analogs & derivatives / Q000494:pharmacology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors; D003915:Dextromethorphan / Q000494:pharmacology*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004963:Estrenes / Q000494:pharmacology; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000502:physiology*; D007546:Isoquinolines / Q000494:pharmacology; D008959:Models, Neurological; D009337:Necturus; D011760:Pyrrolidinones / Q000494:pharmacology; D012433:Ryanodine / Q000494:pharmacology; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D013449:Sulfonamides; D013649:Taste / Q000502:physiology*; D013650:Taste Buds / Q000187:drug effects / Q000502:physiology*; D019284:Thapsigargin / Q000494:pharmacology; D013873:Thionucleotides / Q000494:pharmacology; D010738:Type C Phospholipases / Q000037:antagonists & inhibitors
D004791:Enzyme Inhibitors; D004963:Estrenes; D007546:Isoquinolines; D011760:Pyrrolidinones; D013449:Sulfonamides; D013873:Thionucleotides; C060229:1-(6-((3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione; D012433:Ryanodine; C015929:8-((4-chlorophenyl)thio)cyclic-3',5'-AMP; D019284:Thapsigargin; D003915:Dextromethorphan; D015544:Inositol 1,4,5-Trisphosphate; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D010738:Type C Phospholipases; C063509:N-(2-(4-bromocinnamylamino)ethyl)-5-isoquinolinesulfonamide; D002118:Calcium; D015056:1-Methyl-3-isobutylxanthine