Effects of Indomethacin on Intracellular pH and Na⁺/H⁺ Exchanger in the Human Monocytes

The ability to maintain optimal intracellular pH (pH(i)) is an essential requirement for all cells. Na⁺-H⁺ exchanger (NHE), a ubiquitously expressed transmembrane protein, has been found widely as a major acid extruder in many different cell types, including human monocytes. We therefore investigated the mechanism of the active pH(i) recovery from intracellular acidosis (induced by NH₄Cl prepulse) using intracellular 2',7'-bis (2-carboxethyl)-5(6)-carboxyl-fluorescein (BCECF) fluorescence in cultured human monocytes. Indomethacin is a potent, nonselective inhibitor of cyclooxygenases. Due to its toxicity, the clinical use of indomethacin as an analgesic-antipyretic agent is limited. However, it has recently been found that indomethacin can effectively treat many inflammatory/immune disorders. In this study, we further investigated the effect of indomethacin on the pHi and explored the underlying mechanism. In HEPES (nominally HCO₃⁻-free) Tyrode solution, a pH(i) recovery from induced intracellular acidosis could be blocked completely by 30 μM HOE 694, a specific NHE1 inhibitor, or by removing [Na⁺]₀. Therefore, in the present study, we provided functional evidence, physiologically and pharmacologically, that the HCO₃⁻-independent acid extruder was mostly likely the NHE1 which was involved in acid extrusion in the human monocytes. Moreover, indomethacin (1 μM-1 mM) decreased pH(i) levels in a concentration-dependent manner and significantly suppressed the activity of the NHE1, suggesting that indomethacin-induced intracellular acidosis is caused both by the inhibition of NHE1 activity and the non-specified NHE1-independent acidifying mechanism. In conclusion, our present study demonstrates that NHE1 exists functionally in human monocytes, and the indomethacin-induced pHi decreasing is summation effects on NHE1-dependent and -independent mechanism.