Noninvasive measurement of chloride concentration in rat olfactory receptor cells with use of a fluorescent dye

Inwardly directed Ca(2+)-dependent chloride currents are thought to prolong and boost the odorant-induced transient receptor currents in olfactory cilia. Cl(-) inward current, of course, requires a sufficiently high intracellular Cl(-) concentration (Cl(-)](i)). In previous measurements using a fluorescent Cl(-) probe, N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide (MQAE), Cl(-)](i) of newt olfactory cells was estimated to be only 40 mM. This low value led us to reexamine the Cl(-)](i) by an improved procedure. When isolated rat olfactory neurons were bathed in Tyrode's solution (150 mM Cl(-)) at room temperature, the Cl(-)] was 81.5 +/- 13.5 mM (mean +/- SE) in the tip of the dendrite (olfactory knob) and 81.8 +/- 10.2 mM (mean +/- SE) in the soma. The corresponding Cl(-) equilibrium potentials were -15.4 and -15.3 mV, respectively. Therefore, at resting potentials in the range of -90 to -50 mV, Cl(-) currents are predicted to be inward and capable of contributing to the depolarization induced by odorants. Yet, if the cell was depolarized beyond -15 mV, somal Cl(-) currents would be outward and facilitate repolarization during excitation. The measured Cl(-)] in soma and knob are of interest, because in the cilia the chloride content may be expected to equilibrate with that of the knob in the resting state. They provide a starting point for the decrease in ciliary Cl(-)] predicted to occur during transduction.