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

Inwardly directed Ca2+-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.