Renal transport of endogenous amino acids. II. Influence of treatment with triiodothyronine or dexamethasone in immature and adult rats.

Repeated administration of triiodothyronine (T3, 20 micrograms/100 g b.w., once daily for 3 days) or dexamethasone (60 micrograms/100 g b.w., once daily for 3 days) caused significant changes of amino acid plasma concentrations in young (10 days old) and adult rats (2 months old). After treatment with T3, in young animals concentrations of beta-alanine, aspartic acid, glutamic acid, ornithine, asparagine, leucine, taurine, serine, alanine, and glutamine were enhanced, whereas those of tyrosine, arginine, and threonine were significantly diminished. In adult rats only slight changes occurred after both T3 and dexamethasone. In the kidney, the reabsorbed fraction of amino acids was enhanced after both T3 and dexamethasone treatment in young rats, whereas in adult rats the two hormones were without influence on tubular reabsorption of amino acids. Taken together, both different plasma concentrations of amino acids and changes in amounts of amino acids reabsorbed in the tubuli, in adult rats the fractional excretion of amino acids was nearly unchanged following hormone treatment (exception: significant increase in taurine clearance). But in young animals the fractional excretion was reduced in 15 of 22 amino acids after dexamethasone and in 12 of 23 amino acids after T3 treatment, indicating stimulatory effects of both hormones on tubular amino acid carrier systems in immature animals.