Fluorescence-optical measurements of chloride movements in cells using the membrane-permeable dye diH-MEQ

Fluorescence-optical measurements of the intracellular chloride concentration facilitate identification of chloride movements across the cell membrane of living cells. The two main dyes used for this purpose are 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ) and 6-methoxy-quinolyl acetoethyl ester (MQAE). The use of both substances is impaired by their poor membrane permeability and therefore limited loading of the cells to be studied. Here we report the use of 6-methoxy-N-ethylquinolinium iodide (MEQ), a chloride-sensitive dye for which a membrane-permeable form is easily prepared. This makes the loading procedure as easy as with the acetoxymethyl (AM) forms of other dyes for sensing intracellular ions. In addition, the original method, which described absolute concentration measurements of chloride in the cytosol, was modified in so far as only relative measurements were made. This avoids the known limitations of single wavelength excitation and emission dyes with respect to exact concentration measurements. Moreover, to enhance the signal-to-noise ratio the driving force for chloride was considerably increased by changing the original direction of the anion flux in the cells under investigation. We verified the method by using fibroblasts and activating ICln, a putative chloride channel cloned from epithelial cells and of paramount importance in the regulatory volume decrease in these cells. In the presence of SCN- the MEQ quench measured in NIH 3T3 fibroblasts is dramatically enhanced in hypotonically challenged cells compared with cells under isotonic conditions. Antisense oligodeoxynucleotides sensing ICln considerably impeded the swelling-induced chloride current (ICl) in NIH 3T3 fibroblasts. Accordingly, the chloride movement measured by the SCN- quench of the MEQ signal was significantly reduced. Similar results can be obtained in the presence of 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) or 4, 4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), two known blockers of chloride transport in the plasma membrane of a variety of cells. In conclusion, fluroscence-optical measurements using MEQ as the chloride-sensitive dye provide a reliable and easy-to-use method for measuring changes of the chloride flux across the cell membrane of living cells.