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, treatment, however, did not change branchial chloride cell density. Plasma osmolality, [Na+], and [Cl-] increased, whereas [K+] decreased following low doses of T, or T,. As expected, plasma levels of T, and T, 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. (C) 2000 Academic Press.