Evidence for an osmoregulatory role of thyroid hormones in the freshwater mozambique tilapia Oreochromis mossambicus.

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. Copyright 2000 Academic Press.