| Assay Method Information | |
| | Cell-Based Assay of NHE-3 Activity (Pre-Incubation Inhibition) |
| Description: | Rat and human NHE-3-mediated Na+-dependent H+ antiport was measured using a modification of the pH sensitive dye method originally reported by Paradiso (Proc. Natl. Acad. Sci. USA. (1984) 81(23): 7436-7440). 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 (GenBank M85300) was introduced into OK cells via electroporation, and cells were seeded into 96 well plates and grown overnight. Medium was aspirated from the wells 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 with 0-30 μM test compound. After incubation, NHE-3-mediated recovery of neutral intracellular pH was initiated by addition of Na-HEPES buffer containing 0.4 μM 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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