Interrenal, Thyroidal, and Carbohydrate Responses of Rainbow Trout (<i>Salmo gairdneri</i>) to Environmental Acidification

Abstract

Exposure to acid-treated water (H 2 SO 4 , pH 6.0–4.2) for 21 d altered interrenal and thyroid function in immature rainbow trout, Salmo gairdneri. At a pH &lt; 5.2 plasma Cortisol increased, implying sustained interrenai Cortisol release. Interrenal histology showed hyperplasia and elevated nuclear diameter at pH 4.7. Eight days of acid exposure (pH 4.7) were required to raise plasma Cortisol significantly. At a pH &lt; 4.7 the ratio of plasma T 4 (L-thyroxine) to T 3 (3,5,3′-triiodo-L-thyronine) tended to increase in relation to the controls. Depending on the experiment, this was due to either a significant elevation in plasma T 4 or a decrease in plasma T 3 . No histological changes were evident in the thyroid of acid-treated trout. Eight days of acid exposure (pH 4.7) were required to depress plasma T 3 . Coincidental with higher interrenal activity, plasma glucose was elevated in acid-treated fish (pH &lt; 5.2) after 4 d of exposure. The possible relationships of the endocrinde changes to accompanying acid-induced elevations in plasma glucose and protein and hepatocyte histochemistry are considered, and their possible roles as indices of acid stress are discussed.