Ionic basis of receptor potential of frog taste cells induced by acid stimuli

1. The ionic mechanism underlying the receptor potential in frog taste cells induced by acid stimuli was studied with single microelectrodes by replacing superficial and interstitial fluids of the tongue with modified saline solutions. 2. The removal of Na+, Ca2+ and Cl- from the normal interstitial fluid did not affect the receptor potential induced by acid stimuli. Interstitial 100 mM-K+ saline did not affect the acid response. 3. The receptor potential was reduced greatly when Ca2+ was removed from the superficial saline, but was increased when the Ca2+ concentration was elevated. The removal of superficial Cl- did not affect the receptor potential. The receptor potential elicited by superficial Ca2+-free saline was partly due to Na+. Li+, K+, NH4+ or choline + substituted for Na+ in producing the receptor potential. The amiloride-sensitive Na+ channel on the receptor membrane did not contribute to the receptor potential. With pure water adaptation of the tongue surface, the mean magnitude of the acid response was 35% of the control. 4. The receptor potential was unaffected by superficial tetrodotoxin (TTX) but was blocked by superficial Ca2+ antagonists such as Co2+ and Cd2+. Sr2+ substituted for Ca2+ in generating the receptor potential. 5. The receptor potentials observed under various concentrations of superficial Ca2+ became smaller when Na+ was present in the superficial fluid, indicating a competition between Ca2+ and Na+. 6. It is concluded that a large portion of the receptor potential induced by acid stimuli is produced by cations passing through a tastant-gated Ca2+ channel on the taste receptor membrane. Both divalent (Ca2+, Sr2+) and monovalent (Na+, Li+, K+, NH4+, choline+) cations can pass through the Ca2+ channel. The other mechanism responsible for the remaining part of the receptor potential is discussed.