Abstract
Aspects of 3,5,3′-triiodo- l-thyronine (T 3) metabolism were studied in fed rainbow trout ( Oncorhynchus mykiss) held at 11.5–14° and intraperitoneally implanted with hydrogenated corn oil (controls) or oil containing cortisol. Cortisol implants caused dose-related plasma cortisol elevations within the physiological range for 2–3 weeks, loss in body weight, and depression in plasma T 3 and free T 3 index with no consistent change in plasma thyroxine (T 4) or free T 4 index. Plasma T 3 clearance rate and plasma T 3 appearance rate were both increased by cortisol, with no change in hepatic microsomal T 4 5′-monodeiodinase activity ( K m or V max), but with a significant decrease in muscle T 3 concentration. It is concluded that chronic physiologic cortisol treatment enhances plasma T 3 clearance without change in hepatic T 4 to T 3 conversion, resulting in a decline in T 3 concentration in both plasma and tissue (muscle) compartments.
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