Regulatory Effects of Ca2+ and H+ on the Rat Chorda Tympani Response to NaCl and KCl

Modulatory effects of pH(i) and [Ca2+](i) on taste receptor cell (TRC) epithelial sodium channel (ENaC) were investigated by monitoring chorda tympani (CT) responses to NaCl and KCl at various lingual voltages, before and after lingual application of ionomycin and with 0-10mM CaCl2 in the stimulus and rinse solutions adjusted to pH(o) 2.0-9.7. 0.1 and 0.5M KCl responses varied continuously with voltage and were fitted to an apical ion channel kinetic model using the same parameters. ENaC-dependent NaCl CT response was fitted to the same channel model but with parameters characteristic of ENaC. A graded increase in TRC [Ca2+](i) decreased the ENaC-dependent NaCl CT response, and inhibited and ultimately eliminated its pH sensitivity. CT responses to KCl were pH(i)- and [Ca2+](i)-independent. Between +/- 60 mV applied lingual potential, the data were well described by a linear approximation to the nonlinear channel equation and yielded 2 parameters, the open-circuit response and the negative of the slope of the line in the CT response versus voltage plot, designated the response conductance. The ENaC-dependent NaCl CT response conductance was a linear function of the open-circuit response for all pH(i)-[Ca2+](i) combinations examined. Analysis of these data shows that pH(i) and [Ca2+](i) regulate TRC ENaC exclusively through modulation of the maximum CT response.