Effects of low ambient pH and aluminum on plasma kinetics of cortisol, T3, and T4 in rainbow trout (Oncorhynchus mykiss)

Abstract

Plasma kinetics for cortisol, 3,5,3′-triiodo-L-thyronine (T3), and L-thyroxine (T4), and other blood parameters were examined in catheterized rainbow trout (Oncorhynchus mykiss) exposed for 7 days to a sublethal ambient pH of 4.7 (adjusted with H2SO4), an ambient pH of 4.7 plus Al (AlKSO4, 20 μmol/L), or an ambient pH of 7.7 (control). Sublethal pH alone increased the cortisol level and the plasma degradation rate but did not change the plasma clearance rate; it did not modify plasma T3 and T4 levels and their kinetics or hepatic microsomal 5′-monodeiodinase activity; it increased plasma glucose and decreased plasma Cl− and osmolality with no change in packed cell volume. Low pH in combination with Al increased the cortisol plasma level, plasma clearance rate and plasma degradation rate; it increased the T4 plasma degradation rate but decreased the T3 plasma clearance rate, T3 plasma appearance rate, and 5′-monodeiodinase activity by increasing Km and decreasing Vmax; it also increased plasma glucose and depressed plasma Cl− and osmolality but increased packed cell volume. We concluded that Al exacerbates the effects of low ambient pH to increase cortisol prodcntion and decrease T3 production. Depressed T3 production through 5′-monodeiodinase inhibition may contribute to the poor growth of Al-exposed fish.