Measurement of membrane potential and [Ca²⁺](i) in cell ensembles: Application to the study of glutamate taste in mice

We have studied the spectral properties of the voltage-sensitive dye, 1- (3-sulfonatopropyl)-4-[β[2-(di-n-octylamino)-6-naphtyl]vinyl] pyridinium betaine (di-8-ANEPPS), and the Ca²⁺-sensitive dye, fura-2, in azolectin liposomes and in isolated taste buds from mouse. We find that the fluorescence excitation spectra of di-8-ANEPPS and fura-2 are largely nonoverlapping, allowing alternate ratio measurements of membrane potential and intracellular calcium ([Ca²⁺](i)). There is a small spillover of di-8- ANEPPS fluorescence at the excitation wavelengths used for fura-2 (340 and 360 nm). However, voltage-induced changes in the fluorescence of di-8- ANEPPS, excited at the fura-2 wavelengths, are small. In addition, di-8- ANEPPS fluorescence is localized to the membrane, whereas fura-2 fluorescence is distributed throughout the cytoplasm. Because of this, the effect of spillover of di-8-ANEPPS fluorescence in the [Ca²⁺](i) estimate is <1%, under the appropriate conditions. We have applied this method to study of the responses of multiple taste cells within isolated taste buds. We show that membrane potential and [Ca²⁺](i) can be measured alternately in isolated taste buds from mouse. Stimulation with glutamate and glutamate analogs indicates that taste cells express beth metabotropic and ionotropic receptors. The data suggest that the receptors responding to 2-amino-4- phosphonobutyrate (L-AP4), presumably metabotropic L-glutamate receptors, do not mediate excitatory glutamate taste responses.