The chloride concentration in the lateral intercellular spaces of MDCK cell monolayers

We measured the Cl concentration of the lateral intercellular spaces (LIS) of MDCK cell monolayers, grown on glass coverslips, by video fluorescence microscopy. Monolayers were perfused at 37 degrees C either with HEPES-buffered solutions containing 137 mM Cl or bicarbonate/CO2-buffered solutions containing 127 mM Cl. A mixture of two fluorescent dyes conjugated to dextrans (MW 10,000) was microinjected into domes and allowed to diffuse into the nearby LIS. The Cl-sensitive dye, ABQ-dextran, was selected because of its responsiveness at high Cl concentrations; a Cl-insensitive dye, Cl-NERF-dextran, was used as a reference. Both dyes were excited at 325 nm, and ratios of the fluorescence intensity at spectrally distinct emission wavelengths were obtained from two intensified CCD cameras, one for ABQ-dextran the other for Cl-NERF-dextran. LIS Cl concentration was calibrated in situ by treating the monolayer with digitonin or ouabain and varying the perfusate Cl between 0 and 137 mM (HEPES buffer) or between 0 and 127 mM (bicarbonate/CO2 buffer). LIS Cl in HEPES-buffered solutions averaged 176 +/- 19 mM (n = 12), calibrated with digitonin, and 170 +/- 9 mM (n = 12), calibrated with ouabain. LIS Cl in bicarbonate/CO2-buffered solutions averaged 174 +/- 10 mM (n = 7) using the ouabain calibration. The Cl concentration of MDCK cell domes, measured with Cl-sensitive microelectrodes and by microspectrofluorimetry, did not differ significantly. Images of the LIS at 3 focal planes, near the tight junction, midway and basal, failed to reveal any gradients in Cl concentration along the LIS. LIS Cl changed rapidly in response to perfusate Cl with characteristic times of 0.8 +/- 0.1 min (n = 21) for Cl decrease and 0.3 +/- 0.04 min (n = 21) for Cl increase. In conclusion, (i) Cl concentration is higher in the LIS than in the bathing medium, (ii) no gradients of Cl along the depth of LIS are detectable, (iii) junctional Cl permeability is high.