title: Morphological and molecular investigations of the holocephalan elephant fish nephron: the existence of a countercurrent-like configuration and two separate diluting segments in the distal tubule.
abstract: In marine cartilaginous fish, reabsorption of filtered urea by the kidney is essential for retaining a large amount of urea in their body. However, the mechanism for urea reabsorption is poorly understood due to the complexity of the kidney. To address this problem, we focused on elephant fish (Callorhinchus milii) for which a genome database is available, and conducted molecular mapping of membrane transporters along the different segments of the nephron. Basically, the nephron architecture of elephant fish was similar to that described for elasmobranch nephrons, but some unique features were observed. The late distal tubule (LDT), which corresponded to the fourth loop of the nephron, ran straight near the renal corpuscle, while it was convoluted around the tip of the loop. The ascending and descending limbs of the straight portion were closely apposed to each other and were arranged in a countercurrent fashion. The convoluted portion of LDT was tightly packed and enveloped by the larger convolution of the second loop that originated from the same renal corpuscle. In situ hybridization analysis demonstrated that co-localization of Na(+),K(+),2Cl(-) cotransporter 2 and Na(+)/K(+)-ATPase α1 subunit was observed in the early distal tubule and the posterior part of LDT, indicating the existence of two separate diluting segments. The diluting segments most likely facilitate NaCl absorption and thereby water reabsorption to elevate urea concentration in the filtrate, and subsequently contribute to efficient urea reabsorption in the final segment of the nephron, the collecting tubule, where urea transporter-1 was intensely localized.
authors.affiliation: Laboratory of Physiology, Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwa-no-ha, Kashiwa, Chiba, 277-8564, Japan. kakumura@marine.fs.a.u-tokyo.ac.jp.|Laboratory of Aquatic Animal Physiology, Department of Aquatic Bioscience, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo, Tokyo, 113-8657, Japan. kakumura@marine.fs.a.u-tokyo.ac.jp.
authors.forename: Keigo
authors.initials: K
authors.lastname: Kakumura
authors.affiliation: Laboratory of Physiology, Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwa-no-ha, Kashiwa, Chiba, 277-8564, Japan.
authors.forename: Souichirou
authors.initials: S
authors.lastname: Takabe
authors.affiliation: Laboratory of Physiology, Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwa-no-ha, Kashiwa, Chiba, 277-8564, Japan.
authors.forename: Wataru
authors.initials: W
authors.lastname: Takagi
authors.affiliation: Laboratory of Physiology, Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwa-no-ha, Kashiwa, Chiba, 277-8564, Japan.
authors.forename: Kumi
authors.initials: K
authors.lastname: Hasegawa
authors.affiliation: Department of Biological Science, Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama, 930-8555, Japan.
authors.forename: Norifumi
authors.initials: N
authors.lastname: Konno
authors.affiliation: Institute for Marine and Antarctic Studies, The University of Tasmania, Taroona, TAS, 7053, Australia.
authors.forename: Justin D
authors.initials: JD
authors.lastname: Bell
authors.affiliation: School of Life and Environmental Sciences, Deakin University, Geelong, VIC, 3217, Australia.
authors.forename: Tes
authors.initials: T
authors.lastname: Toop
authors.affiliation: School of Life and Environmental Sciences, Deakin University, Geelong, VIC, 3217, Australia.
authors.forename: John A
authors.initials: JA
authors.lastname: Donald
authors.affiliation: Laboratory of Aquatic Animal Physiology, Department of Aquatic Bioscience, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo, Tokyo, 113-8657, Japan.
authors.forename: Toyoji
authors.initials: T
authors.lastname: Kaneko
authors.affiliation: Laboratory of Physiology, Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwa-no-ha, Kashiwa, Chiba, 277-8564, Japan.
authors.forename: Susumu
authors.initials: S
authors.lastname: Hyodo
mesh_terms: D000818:Animals; D003001:Cloning, Molecular; D004555:Electric Fish / Q000033:anatomy & histology* / Q000378:metabolism*; D029941:Fish Proteins / Q000235:genetics / Q000378:metabolism; D007150:Immunohistochemistry; D017403:In Situ Hybridization; D007685:Kidney Tubules, Collecting / Q000033:anatomy & histology* / Q000378:metabolism*; D008954:Models, Biological; D010802:Phylogeny; D021122:Protein Subunits / Q000235:genetics / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism
chemical_list: D029941:Fish Proteins; D021122:Protein Subunits; D012333:RNA, Messenger
keywords: Cartilaginous fish; Diluting segment; Kidney; Membrane transporter; Urea reabsorption