Elevation of plasma tryptophan by insulin in rat.

The concentration of tryptophan in rat doses of glucagon (1 mg/kg) reduce plasma increases by 30-40/0o 2 hr after plasma tryptophan concentrations. fasting animals receive insulin (2 U/kg) Lower doses are without a significant or consume a carbohydrate meal. effect. The sources of plasma trypto-These treatments decrease the con-phan responsible for its increase after centrations of all other amino acids insulin administration have not been examined: Insulin administration also identified. lowers plasma glucose levels. Large JNSULIN ADMINISTRATION 12 or the secretion of endogenous insulin 3 ' 4 depresses the plasma concentrations of most amino acids in mammals. This effect is rapid and is associated with increased amino acid uptake into muscle and accelerated incorporation into protein.5 Relatively little information is available on the effects of insulin on plasma tryptophan. A few reports have appeared showing that plasma trypotopan concentrations decrease in human subjects responding to a glucose load 6 and in dogs receiving insulin injections, 2 and increase in alligators after insulin injection. 7 The factors influencing trvptophan distribution and metabolism are of special significance considering this amino acid's scarcity in most foods 8 and its unique role in several important physiologic functions. Tryptophan is a precursor of the cofactor NAD, 9 the plasma concentration of tryptophan appears to influence the level of serotonin, a putative neurotransmitter, in the brain, l0 and the delivery of dietary tryptophan to the liver via the hepatic portal vein determines the extent to which hepatic polysomes are aggregated l l and the rate at which certain hepatic proteins are synthesized.' 2 We have utilized a sensitive fluorimetric assay for typtophan 13 to examine the effects of insulin on the concentration of this amino acid in rat plasma.l4 Tryptophan differs from all other amino acids studied to date in that its concentration in rat blood rises after injection of insulin or ingestion of a carbohydrate load.