Physiological Responses of African Catfish (clarias gariepinus) to Water-Borne Ferric Iron: Effects on Thyroidal, Metabolic and Hydromineral Regulations

With a view to understand the mechanism associated with the tolerance of excess water-borne iron, thyroidal, hydromineral and metabolic responses were studied in the freshwater African catfish Clarias gariepinus after exposing them to nominated concentrations (6.2 and 62 μM) of ferric iron [Fe(III)] for 48 hr. Plasma triiodothyronine (T₃) and thyroxine (T₄) concentrations and the indices of metabolic and hydromineral regulations were analyzed in the iron-treated fish. Plasma T₄ (P<0.001) and T₃ (P<0.05) decreased in catfish after Fe(III) treatment. On the contrary, an elevated (P<0.001) plasma T₄ occurred in the fish kept for recovery in clean freshwater after iron-treatment. A significant (P<0.01) hyperglycemia was observed in 62 μM Fe(III)-treated fish whereas plasma urea concentration remained unchanged. Ouabain-dependent Na⁺, K⁺-ATPase activity increased (P<0.05) in the branchial tissue of 6.2 μM iron-treated fish but decreased in the renal (P<0.05), intestinal (P<0.01) and hepatic (P<0.01) tissues. Plasma Na⁺ and PO₄^2- decreased (P<0.05), whereas plasma K⁺ and Ca^2- increased (P<0.05) after Fe(III) treatment. Our results indicate that despite the modification in the metabolic and hydromineral regulation, water-borne Fe(III) suppresses the thyroid activity, and withdrawal of Fe(III) activates the thyroid function in African catfish, thus supporting the hypothesis that thyroid hormones are involved in iron handling in fish.