title: Distribution and dynamics of branchial ionocytes in houndshark reared in full-strength and diluted seawater environments.
abstract: In teleost fishes, it is well-established that the gill serves as an important ionoregulatory organ in addition to its primary function of respiratory gas exchange. In elasmobranchs, however, the ionoregulatory function of the gills is still incompletely understood. Although two types of ionocytes, Na(+)/K(+)-ATPase (NKA)-rich (type-A) cell and vacuolar-type H(+)-ATPase (V-ATPase)-rich (type-B) cell, have been found in elasmobranch fishes, these cells were considered to function primarily in acid-base regulation. In the present study, we examined ion-transporting proteins expressed in ionocytes of Japanese-banded houndshark, Triakis scyllium, reared in full-strength seawater (SW) and transferred to diluted (30%) SW. In addition to the upregulation of NKA and Na(+)/H(+) exchanger type 3 (NHE3) mRNAs in the type-A ionocytes, we found that Na(+), Cl(-) cotransporter (NCC, Slc12a3) is expressed in a subpopulation of the type-B ionocytes, and that the expression level of NCC mRNA was enhanced in houndsharks transferred to a low-salinity environment. These results suggest that elasmobranch gill ionocytes contribute to NaCl uptake in addition to the already described function of acid-base regulation, and that NCC is most probably one of the key molecules for hyper-osmoregulatory function of elasmobranch gills. The existence of two types of ionocytes (NHE3- and NCC-expressing cells) that are responsible for NaCl absorption seems to be a common feature in both teleosts and elasmobranchs for adaptation to a low salinity environment. A possible driving mechanism for NCC in type-B ionocytes is discussed.
authors.affiliation: Laboratory of Physiology, Atmosphere and Ocean Research Institute, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan. Electronic address: takabe@aori.u-tokyo.ac.jp.
authors.forename: Souichirou
authors.initials: S
authors.lastname: Takabe
authors.affiliation: Laboratory of Physiology, Atmosphere and Ocean Research Institute, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan.
authors.forename: Mayu
authors.initials: M
authors.lastname: Inokuchi
authors.affiliation: Laboratory of Physiology, Atmosphere and Ocean Research Institute, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan; Hawai'i Institute of Marine Biology, University of Hawai'i, 46-007 Lilipuna Road, Kaneohe, HI 96744, USA.
authors.forename: Yoko
authors.initials: Y
authors.lastname: Yamaguchi
authors.affiliation: Laboratory of Physiology, Atmosphere and Ocean Research Institute, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan.
authors.forename: Susumu
authors.initials: S
authors.lastname: Hyodo
mesh_terms: D000064:Acclimatization; D000818:Animals; D003001:Cloning, Molecular; D018940:Elasmobranchii / Q000378:metabolism / Q000502:physiology*; D029941:Fish Proteins / Q000235:genetics* / Q000378:metabolism*; D005786:Gene Expression Regulation; D005880:Gills / Q000166:cytology* / Q000378:metabolism; D064587:Osmoregulation; D010802:Phylogeny; D054712:Salinity; D012623:Seawater; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers / Q000235:genetics / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000235:genetics / Q000378:metabolism; D064466:Solute Carrier Family 12, Member 1 / Q000235:genetics / Q000378:metabolism; D064486:Solute Carrier Family 12, Member 3 / Q000235:genetics / Q000378:metabolism; D025262:Vacuolar Proton-Translocating ATPases / Q000235:genetics / Q000378:metabolism
chemical_list: D029941:Fish Proteins; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers; D064466:Solute Carrier Family 12, Member 1; D064486:Solute Carrier Family 12, Member 3; D025262:Vacuolar Proton-Translocating ATPases; D000254:Sodium-Potassium-Exchanging ATPase
keywords: Elasmobranch; Gill; Houndshark; Ionocytes; Ionoregulation; Na(+), Cl(−) cotransporter