Self-Inhibition in Amiloride-sensitive Sodium Channels in Taste Receptor Cells

Abstract

Electrophysiological recording techniques were used to study the Na+ dependence of currents through amiloride-sensitive sodium channels (ASSCs) in rat taste cells from the fungiform and vallate papillae. Perforated patch voltage clamp recordings were made from isolated fungiform and vallate taste receptor cells (TRCs) and Na+ transport was measured across lingual epithelia containing fungiform or vallate taste buds in a modified Ussing chamber. In isolated fungiform TRCs that contain Na+ currents sensitive to the diuretic amiloride, Na+ ions inhibit their own influx through ASSCs, a process known as sodium self-inhibition. Due to the interaction between self-inhibition and the driving force for Na+ entry, self-inhibition is most evident in whole-cell recordings at Na+ concentrations from 50 to 75 mM. In amiloride-sensitive cells, the Na permeability is significantly higher in extracellular solutions containing 35 mM Na+ than in 70 or 140 mM Na+. Compared with the block by amiloride, the development of self-inhibition is slow, taking up to 15 s to become maximally inhibited. Approximately one third of fungiform TRCs and all vallate TRCs lack functional ASSCs. These amiloride-insensitive TRCs show no signs of self-inhibition, tying this phenomenon to the presence of ASSCs. The sulfhydryl reagent, p-hydroxymercuribenzoate (p-HMB; 200 μM), reversibly removed self-inhibition from amiloride-sensitive Na+ currents, apparently by modifying cysteine residues in the ASSC. Na+ currents in amiloride-insensitive TRCs were unaffected by p-HMB. In sodium transport studies in fungiform taste bud–containing lingual epithelia, ∼40% of the change in short-circuit current (Isc) after addition of 500 mM NaCl to the mucosal chamber is amiloride sensitive (0.5 mM). p-HMB significantly enhanced mucosal NaCl-induced changes in these epithelia at mucosal Na+ concentrations of 50 mM and above. In contrast, the vallate-containing epithelia, which are insensitive to amiloride, showed no enhancement of Isc during p-HMB treatment. These findings suggest that sodium self-inhibition is present in ASSCs in taste receptor cells where it may play a crucial role in performance of salt-sensitive pathways in taste tissue during sodium stimulation. This phenomenon may be important in the process of TRC adaptation, in the conservation of cellular resources during chronic sodium exposure, or in the gustatory response to water.