Action of Thyroid Inhibitor Propyl Thiouracil on Thyroid and Interrenal Axes in the Freshwater Tilapia Oreochromis mossambicus Peters

The actions of propyl thiouracil (PTU), a thyrostatic drug, on thyroid hormones (THs) and cortisol production were examined in freshwater tilapia (Oreochromis mossambicus) to examine whether the hypothyroid state affects the function of interrenal axes in this fish. The plasma levels of triiodothyronine (T₃), thyroxine (T₄), and cortisol along with branchial and renal Na⁺, K⁺-ATPase activities were measured after feeding varied doses (5-20 μg g^-1) of PTU for over 10 or 15 days. Feeding low dose (5 μg g^-1) of PTU produced significant rise in plasma T₄ level but both T₃ and T₄ levels were decreased significantly after feeding a high dose (20 μg g^-1) of PTU for 15 days. All doses of PTU failed to alter the plasma cortisol. No significant correlation was found between the plasma levels of T₃, T₄ and cortisol after varied doses of PTU. Treatment of T₃ or T₄ (40 ng g^-1) to the high dose PTU-fed tilapia produced a significant rise in plasma cortisol, suggesting a link to the thyroid and interrenal axes in this fish. The rise in T₃ or T₄ level in the PTU-treated tilapia after TH injections correlated with the branchial and renal Na⁺, K⁺-ATPase activities, which imply an effective Na handling, though a tight regulation of Na⁺ and K⁺ transport was maintained in the plasma of these fish. The data show that high dose of PTU inactivates the thyroid axis resulting in the decreased production of T₃ and T₄, though a low dose of PTU activates the thyroid axis. Our results provide evidence that exogenous T₃ activates interrenal axis to produce cortisol and both exogenous T₃ and T₄ promote branchial and renal Na handling in PTU-induced hypothyroid tilapia.