title: Spatially distributed alternative splice variants of the renal Na-K-Cl cotransporter exhibit dramatically different affinities for the transported ions.
abstract: Three splice variants of the renal Na-K-Cl cotransporter (NKCC2 F, A, and B) are spatially distributed along the thick ascending limb of the mammalian kidney. To test whether NKCC2 splice variants differ in ion transport characteristics we expressed cDNAs encoding rabbit NKCC2 F, A, and B in Xenopus oocytes and determined the ion dependence of bumetanide-sensitive (86)Rb influx. The three splice variants of NKCC2 showed dramatic differences in their kinetic behavior. The medullary variant F exhibited 3-4-fold lower affinity than variants A and B for Na(+) and K(+). Chloride affinities also markedly distinguish the three variants (K(m)F = 111.3, K(m)A = 44.7, and K(m)B = 8.9 mm Cl(-)). Thus, the kinetic properties of the NKCC2 splice variants are consistent with the spatial distribution of the variants along the thick ascending limb as they are involved in reabsorbing Na(+), K(+), and Cl(-) from a progressively diluted fluid in the tubule lumen. Variant B also showed an anomalous inhibition of rubidium influx at high extracellular Na(+) concentrations, possibly important in its highly specialized role in the macula densa. The adaptation of the kinetic characteristics of the NKCC2 variants to the luminal concentrations of substrate represents an excellent example of functional specialization and diversity that can be achieved through alternative mRNA splicing.
authors.affiliation: Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06511, USA. ignacio.gimenez@yale.edu
authors.forename: Ignacio
authors.initials: I
authors.lastname: Giménez
authors.forename: Paul
authors.initials: P
authors.lastname: Isenring
authors.forename: Biff
authors.initials: B
authors.lastname: Forbush
mesh_terms: D017398:Alternative Splicing; D000595:Amino Acid Sequence; D000818:Animals; D002712:Chlorides / Q000378:metabolism*; D007668:Kidney / Q000378:metabolism*; D008969:Molecular Sequence Data; D011188:Potassium / Q000378:metabolism*; D011817:Rabbits; D012964:Sodium / Q000378:metabolism*; D028021:Sodium-Potassium-Chloride Symporters / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D064466:Solute Carrier Family 12, Member 1; D014982:Xenopus laevis
chemical_list: D002712:Chlorides; D028021:Sodium-Potassium-Chloride Symporters; D064466:Solute Carrier Family 12, Member 1; D012964:Sodium; D011188:Potassium