Cation dependencies and turnover rates of the human K+-dependent Na+-Ca2+ exchangers NCKX1, NCKX2, NCKX3 and NCKX4

Abstract The Solute Carrier Family 24 (SLC24) belongs to the CaCA super family of Ca 2+ /cation antiporters and codes for five different K + - dependent Na + - Ca 2+ exchangers (NCKX1-5). NCKX proteins play a critical role in Ca 2+ homeostasis in a wide variety of biological processes such as vision, olfaction, enamel formation, Melanocortin-4-receptor-dependent satiety and skin pigmentation. NCKX transcripts are widely found throughout the brain. In this study we examine the differences between NCKX1-4 in terms of cation dependencies. We measured changes to Ca 2+ influx via the reverse exchange mode while manipulating external Ca 2+ or K + or internal Na + concentrations (External Ca 2+ Dependence, External K + Dependence and Internal Na + Dependence respectively); we also looked at the effect of external Na + /Ca 2+ competition and 3′ 4′- Dichlorobenzamil on the transport of ions in HEK 293 cell lines. A fluorescence based assay was used to determine differences in transport kinetics of the four membrane spanning exchangers using the Michaelis-Menten constant ( K m ). Our results show that there are no significant differences between the NCKX isoforms to explain the variation in the specific expression pattern of these exchangers.