Activation of a cation conductance by acetic acid in taste cells isolated from the bullfrog

The ionic mechanism of the conductance activated by acetic acid was analyzed in isolated bullfrog taste cells under whole-cell voltage-clamp. Bath-application of acetic acid (pH 3.9-4.7) induced an inward current in about 80% of the taste cells. The current occurred in external 80 mmol l-1 Ba2+ and internal 100 mmol l-1 Cs+, which completely blocked the delayed outward K+ current. The concentration-response relationship for the acid-activated current was consistent with that of the gustatory neural response. Prolonged adaptation of the surface of the tongue to HCl prior to taste cell isolation decreased the acid-induced current to about 20% of the control value without decreasing NaCl-induced neural responses and voltage-activated Na+ currents. The results suggest that the transduction mechanism of the acid response might be different from that of the response to salt. The I-V relationship of the acid-induced response was nearly linear at membrane potentials between -80 and 80 mV. The acid-induced conductance was permeable to alkali metal and alkali earth metal ions. The permeability ratios were PCa:PBa:PSr:PNa:PCs = 1.87:1.17:0.73:0.99:1.00. The present study suggests that the acid-induced receptor current in bullfrog taste cells is generated by an increase in a cation conductance in the apical taste membrane.