Stimulation of hepatic thyroxine 5'-deiodinase activity in rainbow trout (Oncorhynchus mykiss) by Pacific salmon growth hormone.

1. Growth hormone extracted from Pacific salmon pituitaries (sGH) was injected intra-peritoneally into rainbow trout to determine sGH effects on plasma levels of thyroxine (T4), 3,5,3'-triiodothyronine (T3) and properties of the hepatic 5'-deiodinase enzyme (5'-D) responsible for T4 to T3 conversion. 2. After 24 hr, sGH (0.1 or 0.5 microgram/g) did not alter the plasma T4 level or 5'-D, Km, but elevated plasma T3 and especially 5'-D Vmax. 3. Thus sGH, like the previously tested human GH, acutely enhances the potential for extra-thyroidal T3 production in trout by increasing the functional level of hepatic 5'-D, but without changing the plasma T4 level.