Abstract
1. Rainbow trout ( Salmo gairdneri) given a single intraperioneal injection of propanolol (a β-adrenoceptor inhibitor) had plasma triiodothyronine (T3) levels which were significantly ( P < 0.01) lower than in control fish 8 hr after the injection, but not after 24hr; thyroxine (T4) levels and T4/T3 ratios were not significantly affected. 2. Isoproterenol (a β-adrenoceptor stimulator) had no effect on T4, T3 or T4/T3 ratios. 3. A single intraperitorial injection of TSH and prolactin given together induced a significant increase ( P < 0.05) in plasma T3 levels 24 hr after the injection. 4. TSH and prolactin given to propanol-treated fish caused a further significant increase in T3 levels compared with TSH + prolactin-, TSH-, prolactin-, or solvent-injected groups ( P < 0.01). 5. The data suggest that the inhibition of peripheral monodeiodination of T4 to T3 by propranolol, well documented in mammals, does not occur in rainbow trout. On the contrary, T3 levels after propranolol administration are increased in conditions where thyroid simulation occurs.
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