| Assay Method Information | |
| | Cell-Based Assay of NHE-3 Activity (Persistent Inhibition) |
| Description: | The ability of compounds to inhibit human and rat NHE-3-mediated Na+ dependent H+ antiport after application and washout was measured using a modification of the pH sensitive dye method described above. PS120 fibroblasts stably expressing human NHE3 and NHERF2 were obtained from Mark Donowitz (Baltimore, Md.). Opossum kidney (OK) cells were obtained from the ATCC and propagated per their instructions. The rat NHE-3 gene was introduced into OK cells via electroporation, and cells were seeded into 96 well plates and grown overnight. Medium was aspirated from the wells, cells were washed once with NaCl-HEPES buffer (100 mM NaCl, 50 mM HEPES, 10 mM glucose, 5 mM KCl, 2 mM CaCl2, 1 mM MgCl2, pH 7.4), then overlayed with NaCl-HEPES buffer containing 0-30 μM test compound. After a 60 min incubation at room temperature, the test drug containing buffer was aspirated from the cells. Following aspiration, cells were washed once with NaCl-HEPES buffer without drug, then incubated for 30 min at 37° C. with NH4Cl-HEPES buffer (20 mM NH4Cl, 80 mM NaCl, 50 mM HEPES, 5 mM KCl, 2 mM CaCl2, 1 mM MgCl2, pH 7.4) containing 5 μM BCECF-AM. Cells were washed once with Ammonium free, Na+-free HEPES (100 mM choline, 50 mM HEPES, 10 mM glucose, 5 mM KCl, 2 mM CaCl2, 1 mM MgCl2, pH 7.4) and incubated in the same buffer for 10 minutes at room temperature to lower intracellular pH. NHE-3-mediated recovery of neutral intracellular pH was initiated (10 min after compound washout) by addition of Na-HEPES buffer. For the rat NHE3 assay, the Na-HEPES buffer contained 0.4 ethyl isopropyl amiloride (EIPA, a selective antagonist of NHE-1 activity that does not inhibit NHE-3). Changes in intracellular pH were monitored using a FLIPR Tetra (Molecular Devices, Sunnyvale, Calif.) by excitation at λex 439 to 505 nm, and measuring BCECF fluorescence at λem 538 nm. The initial rate of the fluorescence ratio change was used as a measure of NHE-mediated Na+/H+ activity, and reported as the change in fluorescence ratio per minute. Initial rates were plotted as the average of 2 or more replicates, and pIC50 values were estimated using GraphPad Prism. |
| Affinity data for this assay | |
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