Distal nephrons of euryhaline and seawater pufferfishes

Abstract

The nephrons of freshwater and euryhaline fishes are composed of a glomerulus, a proximal tubule, a distal tubule, and a collecting duct, whereas nephrons in seawater fishes lack the distal tubule. The process of NaCl reabsorption in the distal nephron allows freshwater fishes to excrete hypotonic urine and seawater fishes to excrete urine containing high concentrations of divalent ions; the relevant transporters, however, have not yet been identified. To identify transporters involved in the NaCl reabsorption, a seawater pufferfish torafugu (Takifugu rubripes), whose genome sequence is available, and a closely related euryhaline species mefugu (Takifugu obscurus) were analyzed and compared. Mining the fugu genome identified one NKCC2 (Na+-K+-Cl− cotransporter 2, Slc12a1) gene and one NCC (Na+-Cl− cotransporter, Slc12a3) gene. RT-PCR and in situ hybridization analyses demonstrated that (a) the kidney of euryhaline pufferfish mefugu highly expressed NKCC2 in the distal tubules and NCC in the collecting ducts; and (b) the kidney of seawater pufferfish torafugu highly expressed NKCC2 in the collecting ducts. Renal NCC expression was decreased in seawater-acclimated mefugu, and internalization of NCC from the apical surface of the collecting ducts was observed. These results suggest that NaCl reabsorption in the fish distal nephron is mediated by NCC and NKCC2 in freshwater and by NKCC2 in seawater.