The ionic basis of the receptor potential of frog taste cells induced by sugar stimuli.

The ionic mechanism underlying the receptor potential in frog taste cells induced by sugar stimuli was studied with conventional microelectrodes by replacing the superficial and interstitial fluids of the tongue with modified solutions. The taste cell generated a depolarizing receptor potential accompanying a remarkable reduction of input resistance in response to stimulation with galactose and sucrose. The magnitude of the receptor potential in response to galactose solution increased linearly with decreasing pH in the pH range 6-8, but remained constant above pH8. The reversal potential was increased by only 29 mV by a 10-fold increase in the H+ concentration of the stimulus, suggesting that there are pH-dependent and pH-independent components in the mechanism generating the receptor potential. The use of Na(+)-free, Ca(2+)-free and K(+)-free interstitial fluids did not affect the receptor potential, but the elimination of Cl- from the interstitial fluid largely abolished it. Interstitial 0.1 mmol l-1 N,N'-dicyclohexyl-carbodiimide (DCCD) completely inhibited the receptor potential and interstitial 0.1 mmol l-1 N-ethylmaleimide (NEM) decreased the potential to 40% of the control value. Lowering the pH of interstitial fluid from 7.2 to 6.3 decreased the receptor potential to 30% of the control value. It is concluded that part of the receptor potential in frog taste cells induced by sugar stimuli may be produced by an inflow of H+ through the taste-receptive membrane. The intracellular pH of the taste cell may be regulated by a Cl(-)-dependent H+ pump in the basolateral membrane.