title: Evidence for an osmoregulatory role of thyroid hormones in the freshwater mozambique tilapia Oreochromis mossambicus.
abstract: The existing equivocal reports on the osmoregulatory role of triiodothyronine (T(3)) and thyroxine (T(4)) in teleosts prompted a reinvestigation of their osmoregulatory function in the euryhaline teleost Oreochromis mossambicus. Evidence is presented for thyroidal involvement in hydromineral balance in freshwater tilapia. Dose- and tissue-related responses to various T(3) and T(4) concentrations were observed in the branchial and renal tissues. The branchial Na(+),K(+)-ATPase activity, known to reflect sodium pump dynamics, increased significantly after the administration of low doses of T(3) (20 and 40 ng. g(-1)) or T(4) (40 and 80 ng. g(-1)). Higher doses of T(3) and T(4) (>160 ng. g(-1)) did not change the enzyme activity, compared to sham-injected fish. Conversely, the specific activity of renal Na(+),K(+)-ATPase decreased significantly at all doses of T(3) or T(4). Further, immunoreactive Na(+),K(+)-ATPase in T(4)-treated fish increased in branchial chloride cells and this was coupled with a significant increase in the size of chloride cells. T(4) treatment, however, did not change branchial chloride cell density. Plasma osmolality, [Na(+)], and [Cl(-)] increased, whereas [K(+)] decreased following low doses of T(3) or T(4). As expected, plasma levels of T(3) and T(4) increased significantly in a dose-dependent manner after a single injection of either T(3) or T(4). The basal levels of T(3) and T(4) were 4.45 +/- 0.49 and 1.25 +/- 0.26 nmol. L(-1), respectively. This study shows that physiological concentrations of T(3) (<10.57 nmol. L(-1)) and T(4) (<6.64 nmol. L(-1)) enhance branchial Na(+) pump activity and chloride cell morphometric dynamics, favoring hyperosmoregulatory capacity in freshwater tilapia. These data are consistent with the hypothesis that thyroid hormones perform a role in hydromineral regulation in freshwater teleosts.
authors.affiliation: Department of Animal Physiology, University of Nijmegen, Toernooiveld 1, Nijmegen, 6525 ED, The Netherlands. subash@sci.kun.nl
authors.forename: M C
authors.initials: MC
authors.lastname: Subash Peter
authors.forename: R A
authors.initials: RA
authors.lastname: Lock
authors.forename: S E
authors.initials: SE
authors.lastname: Wendelaar Bonga
mesh_terms: D000818:Animals; D001916:Brachial Artery; D002712:Chlorides / Q000097:blood; D005260:Female; D007150:Immunohistochemistry / Q000662:veterinary; D008297:Male; D009073:Mozambique; D012964:Sodium / Q000097:blood; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D013963:Thyroid Hormones / Q000502:physiology*; D013974:Thyroxine / Q000502:physiology; D017210:Tilapia / Q000502:physiology*; D014284:Triiodothyronine / Q000502:physiology; D014882:Water-Electrolyte Balance / Q000502:physiology*
chemical_list: D002712:Chlorides; D013963:Thyroid Hormones; D014284:Triiodothyronine; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D013974:Thyroxine