Fig. 5.

Elevation of intracellular Ca²⁺ in response to apical delivery of protons in PKD2L1-YFP cells. (A) Simultaneous measurement of the change in intracellular Ca²⁺ (Upper Left) and the magnitude of current (Lower Left) in response to apical uncaging of NPE-caged proton in a PKD2L1-YFP cell. Fluorescent images taken at different time points are shown at Right. (B) Scatter plot of the change in intracellular Ca²⁺ as a function of the integrated current in response to UV uncaging from experiments as in A. Red, cells that fired action potentials; blue, cells that did not fire action potentials; black, control data measured 10 s before the UV flash. (C) Proposed model for sour taste transduction in PKD2L1-expressing cells. Proton entry through a proton-selective conductance specifically expressed on apical surface of PKD2L1-expressing cells leads to depolarization and generation of action potentials that propagate to the cell body and activate voltage-gated Ca²⁺ channels. In addition to accessing this pathway, weak acids may produce intracellular acidification and inhibit resting K⁺ conductances.