Effect of intravenous glucose and insulin on plasma tryptophan and tyrosine concentrations in normal subjects and patients with carcinoid tumors.

We evaluated the effect of intravenous (i.v.) glucose on the plasma tryptophan (TRP) and tyrosine (TYR) concentration of 12 normal subjects, six patients with carcinoid tumors and the carcinoid syndrome (carcinoid syndrome), and five patients with carcinoid tumors without the carcinoid syndrome (tumor.) Following i.v. glucose administration, the plasma Trp concentration of the normal subjects and the tumor patients incresed, while the plasma Trp concentration of the carcinoid syndrome patients decreased. Following i.v. glucose administration, the plasma Tyr concentration of the normal subjects and the tumor patients decreased, while the plasma Tyr concentration of the carcinoid syndrome patients did not change. The response to i.v. insulin differed in some respects from the response to i.v. glucose: the plasma Trp of normal subjects did not change while the plasma Trp of carcinoid syndrome patients decreased; the plasma Tyr of the normal subjects increased while the plasma Tyr concentration of the carcinoid syndrome patients did not change. The carcinoid syndrome patients had high serum serotonin concentrations and impaired glucose tolerance and insulin secretion as compared to both normal subjects and tumor patients. We conclude that under appropriate experimental conditions, glucose administration can increase the plasma Trp concentration of normal human subjects.