Effects of thyroid hormone and beta-blocking agents on plasma amino acids in rats

Experimental hyperthyroidism was induced in rats by daily intraperitoneal injections of triiodothyronine (T3; 100 micrograms/100 g bw) for 3 days (short-term experiment) or by subcutaneous injections of T3 (30 micrograms/100 g bw) on alternate days during 6 weeks (long-term experiment). Control rats received corresponding volumes of solvent. Groups of rats were fed normal pellets or pellets enriched with the selective beta-1-blocker metoprolol (8.8 mmol/kg of diet) or with the non-selective beta-blocker propranolol (3.3 mmol/kg of diet). Plasma amino acids were determined 24 hours after the last T3 injection. In the short-term experiment, most individual amino acids were increased in T3-treated rats, also if beta-blocker had been given, but the increase in plasma concentration of some amino acids (valine, leucine, lysine and histidine) was prevented by propranolol. In the long-term experiment, several amino acids showed reduced plasma concentration during T3 treatment, also when the rats received beta-blocker. The plasma concentration of threonine, however, was normalized by propranolol during T3 treatment. The results indicate that metabolic alterations induced by T3 and producing changes in plasma amino acid concentrations are mainly unaffected by beta-blockade. However, as some plasma amino acid changes were prevented by propranolol but not by metoprolol, certain T3-induced metabolic derangements may be beta-2-receptor mediated.