An isoform of the Na+/Ca2+ exchanger (SDNCX1.10) was cloned from mesangial cells of Sprague-Dawley rat. Regulation of this isoform was compared to two other clones that were derived from the Dahl/Rapp salt sensitive (SNCX) and salt resistant rat (RNCX). All isoforms differ at the alternative splice site and at amino acid 218 for SNCX. PKC activates RNCX but not SNCX while SDNCX1.10 was also activated by PKC. Regulation of exchanger activities by intracellular calcium ([Ca2+]i), pH, and kinases was assessed using Na-dependent 45Ca2+ uptake assays in OK-PTH cells expressing the vector, RNCX, SNCX, or SDNCX1.10. [Ca 2+]i was elevated from 50 to 125 nM (n = 4) with thapsigargin (40 nM) and reduced from 50 to 29 nM (n = 4) and 18 nM (n = 4) with 10 or 20 μM BAPTA, respectively. RNCX was active at all three [Ca 2+]i while SNCX and SDNCX1.10 were only active at lower [Ca2+]i. Varying extracellular pH (pHe, without nigericin) or pHe and intracellular pH (pHi, with 10 μM nigericin) from pH 7.4 to 6.2, 6.8, or 8.0 showed that SNCX activity was attenuated at both low and high pHs. SDNCX1.10 activity was attenuated only at pH 6.2 and 6.8 (with or without nigericin) while RNCX activity was attenuated at pH 6.2 (with or without nigericin) and pH 6.8 (with nigericin). Finally, only SDNCX1.10 activity was stimulated by 250 μM CPT-cAMP or 250 μM DB-cGMP treatment. Thus the differential regulation of [Ca 2+]i by these exchangers is dependent upon the pattern of cellular Na+/Ca2+ exchanger isoform expression. © 2003 Wiley-Liss, Inc.