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1 Groupe de Recherche en Néphrologie, Department of Medicine, Faculty of Medicine, Laval University, Quebec, Canada G1R 2J6; and 2 Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06510
In the shark (sa), two variants of the renal Na-K-Cl cotransporter (saNKCC2A and saNKCC2F) are produced by alternative splicing of the second transmembrane domain (tm2). In mammals, these splice variants, as well as a third variant (NKCC2B), are spatially distributed along the thick ascending limb of Henle and exhibit divergent kinetic behaviors. To test whether different tm2 in saNKCC2 are also associated with different kinetic phenotypes, we examined the ion dependence of 86Rb influx for shark and rabbit splice variants expressed in Xenopus laevis oocytes. We found that, in both species, A forms have higher cation affinities than F forms. In regard to Cl affinity, however, the A-F difference was more pronounced in rabbit, and the relationship between transport activity and Cl concentration was not always sigmoidal. These results show that the tm2 of saNKCC2 is, as in rabbit, important for Cl transport, and they suggest that the ability of the distal NKCC2-expressing segment to extract Cl from the luminal fluid differs among species. We have also found that the renal NKCC2 of distant vertebrates share similar affinities for cations. This finding points to the existence of highly conserved residues that mediate the kinetic behavior of the NKCC2 splice variants.
cation-chloride cotransporter; renal isoform; splice variant; kidney; NKCC1; NKCC2; thick ascending loop of Henle; kinetics; ion transport
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