in the distal nephrons of euryhaline and seawater pufferfishes. Am J Physiol Regul Integr Comp Physiol 300: R284 -R297, 2011. First published November 17, 2010 doi:10.1152/ajpregu.00725.2009.-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. In the mammalian distal nephron, NaCl absorption is mediated by Na ϩ -K ϩ -Cl Ϫ cotransporter 2 (NKCC2, Slc12a1) in the thick ascending limb, Na ϩ -Cl Ϫ cotransporter (NCC, Slc12a3) in the distal convoluted tubule, and epithelial sodium channel (ENaC) in the collecting duct. In this study, we compared the expression profiles of these proteins in the kidneys of euryhaline and seawater pufferfishes. Mining the fugu genome identified one NKCC2 gene and one NCC gene, but no ENaC gene. RT-PCR and in situ hybridization analyses demonstrated that NKCC2 was highly expressed in the distal tubules and NCC was highly expressed in the collecting ducts of euryhaline pufferfish (mefugu, Takifugu obscurus). On the other hand, the kidney of seawater pufferfish (torafugu, Takifugu rubripes), which lacked distal tubules, expressed very low levels of NCC, and, in the collecting ducts, high levels of NKCC2. Acclimation of mefugu to seawater resulted in a 2.7ϫ decrease in NCC expression, whereas NKCC2 expression was not markedly affected. Additionally, internalization of NCC from the apical surface of the collecting ducts was observed. These results suggest that NaCl reabsorption in the distal nephron of the fish kidney is mediated by NCC and NKCC2 in freshwater and by NKCC2 in seawater. distal tubule; collecting duct; NKCC2; NCC; fish kidney; dilute urine FRESHWATER FISHES LIVE IN hypo-osmotic environments, resulting in significant influx of water into the body, mainly across the gills. To maintain body fluid homeostasis, freshwater fishes import ions from surrounding water through the gills (13,22) or from the diet, and excrete excess water in hypotonic urine (ϳ10 mM Na ϩ and Cl Ϫ ) through the kidneys (30). To increase urine volume (i.e., water excretion), freshwater fishes secrete NaCl in the proximal tubule and draw interstitial water into the tubular lumen (9); this proximally introduced NaCl should be reabsorbed in the distal nephron. Reabsorption of Na ϩ and Cl Ϫ ions from urine under water-impermeable conditions is, therefore, essential for net water excretion and the survival of fishes in freshwater. Na ϩ and Cl Ϫ reabsorption is also important for seawater fishes. Unlike freshwater fishes, however, seawater fishes experience passive water efflux. To balance water loss, seawater fishes absorb water and ions through the intestine by drinking large amounts of seawater (17). Surplus ions are excreted mainly through the gills (13, 22), although divalent ions (Mg 2ϩ and SO 4 2Ϫ ) are excreted through isotonic urine (4, 30). To produce urine with high concentrations of Mg 2ϩ (ϳ140 mM) ...
Functional analysis using the Xenopus oocyte expression system showed that Rhp2 has transport activity for methylammonium, an analog of ammonia. This transport activity was inhibited by NH 4 Cl but not trimethylamine-N-oxide and urea. These results suggested that Rhp2 is involved in ammonia reabsorption in the kidney of the elasmobranch group of cartilaginous fish comprising the sharks and rays.Members of the Rhesus (Rh) 4 glycoprotein family are membrane proteins present in a broad range of eukaryotes including insects, sea squirts, fish, birds, amphibians, and mammals. Six clusters of the Rh family (RhAG, RhBG, RhCG, RH30, Rhp1, and Rhp2 (p is for primitive)) have been defined by an extensive phylogenetic analysis (1). Among the family members, RhAG, RhBG, and RhCG are present in most vertebrates and are known to be capable of transporting ammonia 5 and methylammonium (2-9). The Rhp1 members are present in several invertebrates including aquatic crab Carcinus maenas (10), sea squirt Ciona intestinalis, and the fruit fly Drosophila melanogaster (1). Furthermore, Rhp1 was shown to be essential for embryonic development and hypodermal function in Caenorhabditis elegans (11). In contrast, Rhp2 genes were identified in genomes of non-mammalian vertebrates only by data base mining, and the function and localizations of their protein products (Rhp2) have not been characterized in any species. We previously isolated the cDNA fragments of two Rhp2s (synonym for Rhag-like1 and Rhag-like2) from puffer fish; however, no mRNA transcripts were observed in any tissues examined (12).Rhag, Rhbg, Rhcg1, and Rhcg2 are expressed in the gill of puffer fish, where they excrete ammonia to eliminate nitrogenous waste (12). Moreover, Rh glycoproteins of rainbow trout (13-16), killifish (17), and zebrafish (18 -20) have been characterized and were expressed in the tissues that are implicated in ammonia secretion. These observations strongly suggested that Rh glycoproteins are involved in ammonia excretion in teleost fish. In contrast, Rh glycoproteins of the elasmobranch fishes, which include sharks and rays, have not yet been identified. Nitrogen metabolism in the elasmobranch fishes differs greatly from the teleost fishes. Elasmobranch fish utilize ammonia to produce urea rather than directly excreting
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.