Determination of 3,5,3′-triiodo-L-thyronine (T3) levels in tissues of rainbow trout (Salmo gairdneri) and the effect of low ambient pH and aluminum

Tissue T₃ (3,5,3′-triiodo-L-thyronine) concentrations were measured in rainbow trout, Salmo gairdneri, after digestion by Pronase or collagenase and extraction with ethanolic ammonia (99:1, v/v) followed by 2N NH₄OH and chloroform. Recoveries of ¹²⁵I]T₃ administered in vivo or in vitro were high and consistent and there was close parallelism between sample dilutions and the radioimmunoassay curve, but recoveries of unlabeled T₃ administered in vitro were low and variable. Alternatively, trout were brought to isotopic equilibrium by ¹²⁵I]T₃ infusion for 96 h, the extracted ¹²⁵I]T₃ determined by gel filtration and the tissue T₃ content calculated from the specific activity of plasma ¹²⁵I]T₃. By the latter method, tissue T₃ concentrations were: intestine (4.2 ng/g), kidney (2.5), liver (2.8), stomach (1.5), heart (1.0), muscle (0.7), gill (0.6) and skin (0.3). Muscle (67% of body weight) comprised the largest tissue T₃ pool (82% of all tissues examined). Seven days exposure of trout to water acidified with H₂SO₄ (pH 4.8) or acidified water containing aluminum (21.6 mM), decreased tissue T₃ content generally and particularly in muscle (14% of controls). In conclusion, skeletal muscle is the largest T₃ tissue pool and seems highly responsive to altered physiologic state.