Passive and active membrane properties of mudpuppy taste receptor cells

1. Intracellular recordings were obtained from taste receptor cells and surface epithelial cells of isolated mudpuppy lingual epithelium. 2. Surface epithelial cells had a mean resting potential of -40.2 +/- 8.9 mV, a mean input resistance of 40.3 +/- 11.3 M omega, and a linear current-voltage (I-V) relationship. Taste receptor cells had a mean resting potential of -61.7 +/- 15 mV, a mean input resistance of 380.3 +/- 177.2 M omega, and the I-V relationship showed pronounced outward rectification; the outward rectification persisted in high-K+ saline, but was abolished by tetraethylammonium bromide (TEA). 3. Surface epithelial cells responded to depolarizing current injection with only passive membrane potential changes. Taste receptor cells responded to brief pulses of depolarizing current injection with regenerative action potentials characterized by an abrupt rising phase, an inflexion on the falling phase, and a prolonged after-potential. 4. The abrupt rising phase of the action potential was blocked by tetrodotoxin (TTX), suggesting that voltage-gated Na+ currents are responsible for the rising phase. 5. Long-duration action potentials were elicited from cells treated with TEA to block outward K+ currents and with TTX to block Na+ currents, and from cells bathed in isotonic CaCl2. These results suggest that the active membrane response contains a significant Ca2+ component. 6. The after-potential was blocked or greatly reduced by the addition of Ca2+ channel blockers to the bathing medium. In contrast, addition of TEA to the bathing medium greatly enhanced the after-potential. These data suggest that a significant portion of the after-potential is Ca2+ mediated. 7. The mean reversal potential for the after-potential (-76.8 +/- 6.0 mV) was significantly different from the mean reversal potential for the undershoot of the action potential (-86 +/- 5.6 mV). Superfusion with TEA reduced the reversal potential of the after-potential to -42.3 +/- 8.2 mV and abolished the undershoot. These results suggest that the after-potential results from at least two conductances, one which is blocked by TEA and the other which is Ca2+ dependent and involves ions other than, or in addition to K+. 8. Our data suggest that taste receptor cells, unlike surface epithelial cells, possess voltage-gated Na+, Ca2+, and K+ channels, as well as Ca2+-mediated channels. The role of the Ca2+ channels may be in part to regulate release of transmitter onto nerve terminals. The role of the other conductances in taste transduction is unknown.