Changes in the volume of marginal cells induced by isotonic 'Cl- depletion/restoration': involvement of the Cl- channel and Na+-K+-Cl- cotransporter

Marginal cells constitute the endolymph-facing epithelium responsible for the secretion of endolymph by the stria vascularis in the inner ear. We have studied the possible involvement of Cl- conductance and Na+-K+-Cl- cotransport in the mechanism of changes in cell volume upon isotonic Cl- depletion/restoration. Changes in cell volume were estimated from video-microscopic images with the aid of an image processor. Marginal cells shrank to similar to 80% of their original volume in 30 s and to 65-70% in 90 s upon total replacement of [Cl](0) (similar to 150 mM) by gluconate(-), and the original volume of the shrunken cells was restored within 2 min after restoration of Cl-. The order of potency of anions to induce isotonic shrinkage was gluconate(-) > I- > F- > Br-. The cell shrinkage caused by Cl- depletion was partially inhibited by 5-Nitro-2-(3-phenyl-propylamino)-benzoic acid (NPPB, 0.2 mM), but not by either 4-acetamido-4'-isothiocyanato-stilbene-2,2'disulfonic acid (SITS, 0.5 mM), bumetanide (10 mu M) or ouabain (1 mM). The cell shrinkage caused by a reduction of [Cl](0) from similar to 150 mM to 7.5 mM was not affected by [K](0) in the range of 3.6 mM to 72 mM. These results suggest that the main efflux pathway(s) responsible for the 'Cl removal'-induced shrinkage depends on volume-correlated Cl- conductance (Takeuchi and Irimajiri, J. Membrane Biol. 150, 47-62, 1996) and that this pathway(s) is essentially independent of the Na+-K+-Cl- cotransporter, the Na+,K+-ATPase, and the K+-Cl- cotransporter. With regard to volume recovery after isotonic shrinkage, its critical dependence on the simultaneous presence of Na+, K+ and Cl- in the bath and its substantial inhibition by bumetanide (10 mu M) both indicate a major role for Na+-K+-Cl- cotransport. The strong influence on cell volume of solute fluxes working through the Cl- channel and the Na+-K+-Cl- cotransporter implies an essential role for these pathways in the ion transport mechanism(s) of the marginal cell.