Comparison of 3,5,3'-triiodo-L-thyronine and L-thyroxine absorption from the intestinal lumen of the fasted rainbow trout, Oncorhynchus mykiss.

The absorptions of 3,5,3'-triiodo-L-thyronine (T3) and L-thyroxine (T4) from the intestinal lumen of the rainbow trout were compared in vivo. Tracer doses of [(125)I]T4 ((+)T4) or [(125)I]T3 ((*)T3) were injected through an anal cannula into the duodenum of trout fasted for 3 days at 12°C, and radioactivity was measured in blood and tissues at 4-48 h. (*)T3 was removed more extensively than (*)T4 from the intestinal lumen and more radioactivity was absorbed into the blood and tissues of u+T3-injected trout than (*)T4-injected trout. HPLC analysis showed that a high proportion of the radioactivity in the plasma, liver, kidney and intestinal lumen of (*)T3-injected trout remained as the parent (*)T3. However, in (*)T4-injected trout most plasma radioactivity was in the form of (125)I(-), and by 24 h a high proportion of luminal radioactivity was (125)I(-). By 48 h, over 4% of the injected (*)T3 and 1% of the injected (*)T4 dose resided in the gall bladder, primarily as derivatives of (*)T3 or (*)T4. We conclude that T3 is absorbed more effectively than T4 from the intestinal lumen of fasted trout, indicating the potential for an enterohepatic T3 cycle.