A stimulus-activated conductance in isolated taste epithelial membranes.

Membrane vesicles isolated from the cutaneous taste epithelium of the catfish were incorporated into phospholipid bilayers on the tips of patch pipettes. Voltage-dependent conductances were observed in approximately 50% of the bilayers and single-channel currents having conductances from 8 to greater than 250 pS were recorded. In 40% of the bilayers displaying no voltage-dependent conductances, micromolar concentrations of L-arginine, a potent stimulus for one class of catfish amino acid taste receptors, activated a nonselective cation conductance. The L-arginine-gated conductance was concentration-dependent, showing half-maximal activation in response to approximately 15 microM L-arginine. L-Arginine-activated channels had unitary conductances of 40-50 pS and reversed between -6 and +18 mV with pseudointracellular solution in the pipette and Ringer in the bath. L-Alanine, a potent stimulus for the other major class of catfish amino acid taste receptors, did not alter bilayer conductance. D-Arginine, which is a relatively ineffective taste stimulus for catfish but a good cross-adapter of the L-arginine-induced neural response, had no effect on bilayer conductance at concentrations below 200 microM. However, increasing concentrations of D-arginine from 1 to 100 microM progressively suppressed the L-arginine-activated conductance, suggesting that D-arginine competed for the L-arginine receptor, but did not activate the associated cation channel. This interpretation is consonant with recent biochemical binding studies in this system. These results suggest that L-arginine taste receptor proteins in the catfish are part of or closely coupled to cation-selective channels which are opened by L-arginine binding.