Identification of TRPM5 Ion Channels in Type-II Taste Cells of Mice |
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Authors: | R A Romanov M F Bystrova O A Rogachevskaya S S Kolesnikov |
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Institution: | (1) Department of Biological Sciences, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA;(2) Department of Neuroscience, Mount Sinai School of Medicine, New York, NY 10029, USA;(3) Department of Biomedical Sciences, Cornell University, Ithaca, NY 14853, USA |
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Abstract: | TRPM5 are ion channels belonging to the TRP family, which demonstrate a nonselective permeability for monovalent cations and
are activated by an increase in the intracellular calcium level. TRPM5 are present in taste receptor cells of type II responsible
for reception of bitter, sweet, and umami taste sensations. Knockout of the trpm5 gene in mice results in a nearly complete loss of sensitivity to taste stimuli of the above-mentioned modalities (taste blindness).
The physiological activity of TRPM5 in taste receptive cells has practically not been studied. Using a patch-clamp technique,
we carried out a comparative analysis of the properties of recombinant TRPM5 and Ca2+-activated membrane channels in type-II taste cells in mice. Dialysis of the studied cells with a high-Ca2+ solution and application of a calcium ionophore, ionomycin, caused activation of outward-rectification ion channels permeable
for Na+, Cs+, and K+ in CHO-strain cells with exogenous TRPM5. These channels were blocked by 100 μM triphenylphosphine oxide (TPPO). Calcium-activated
channels in type-II taste cells also possessed analogous properties. Application of the calcium ionophore ionomycin or a stepwise
increase in the intracellular Ca2+ level using photolysis (uncaging) caused activation of channels nonselective with respect to Na+ and Cs+ and impermeable for N-methyl-D-glucamine (NMDG+). These channels had the current–voltage characteristics of outward rectification and a high thermosensitivity (Q10 = 6.7 ± 0.5); they could be blocked by TPPO. It should be emphasized that TRPM5 were specific with respect to type-II cells.
An increase in the intracellular calcium level induced the appearance of Cl– current in type-I cells and did not influence
the basic current in type-III cells. |
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