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

Plasma kinetics for cortisol, 3,5,3′-triiodo-L-thyronine (T 3 ), and L-thyroxine (T 4 ), 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 H 2 SO 4 ), an ambient pH of 4.7 plus Al (AlKSO 4 , 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 T 3 and T 4 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 T 4 plasma degradation rate but decreased the T 3 plasma clearance rate, T 3 plasma appearance rate, and 5′-monodeiodinase activity by increasing K m and decreasing V max ; 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 T 3 production. Depressed T 3 production through 5′-monodeiodinase inhibition may contribute to the poor growth of Al-exposed fish.