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• Cl−−OH− exchanger facilitated base extrusion. • U-II induced a concentration-dependent increase in the activity of Na+-H+ exchanger and Cl−−OH− exchanger. • U-II did not change the activity of Na+−HCO 3 − co-transporter and H+-linked monocarboxylate transporters. • U-II did not change the resting pH i under HEPES- and HCO 3 −/CO 2 -buffered solution. The regulation of intracellular pH (pH i) plays a vital role in various cellular functions. We previously demonstrated that three different acid extruders, the Na+-H+ exchanger (NHE), Na+−HCO 3 − co-transporter (NBC) and H+-linked monocarboxylate transporter (MCT), functioned together in cultured human radial artery smooth muscle cells (HRASMCs). However, the functions of acid-loading transporters in HRASMCs remain poorly understood. Urotensin II (U-II), one of the most potent vasoconstrictors, is highly expressed in many cardiovascular diseases. The aim of this present study was to determine the concentration effect of U-II (3 pM∼100 nM) on the functional activity of pH i regulators in HRASMCs. Cultured HRASMCs were derived from segments of human radial arteries obtained from patients undergoing bypass grafting. Changes in pH i recovery due to intracellular acidification and alkalization induced by NH 4 Cl prepulse and Na-acetate prepulse, respectively, were detected by microspectrofluorimetry with the pH-sensitive fluorescent dye BCECF. Our present study showed that (a) U-II increased the activity of NHE in a concentration-dependent manner but did not change that of NBC or MCT or resting pH i , (b) the Cl−−OH− exchanger (CHE) facilitated base extrusion, and (c) U-II induced a concentration-dependent increase in the activity of CHE. In conclusion, for the first time, our results highlight a concentration-dependent increase in the activity of NHE and CHE, but not NBC and MCT, induced by U-II in HRASMCs. [ABSTRACT FROM AUTHOR]