The differential effect of intragastric and intravenous tryptophan on plasma glucose, insulin, glucagon, GLI and GIP in the fasted rat.

L-tryptophan was given to fasted rats intragastrically or intravenously at a dose of 500 of 166 mg/kg b.w., respectively. Mean (+/- SEM) plasma insulin levels rose after both stimuli and at 10, 30 and 45 min were 63 +/- 26, 86 +/- 25, 48 +/- 7 mU/l after oral, and 28 +/- 4, 25 +/- 6, 19 +/- 6 mU/l after intravenous administration, respectively; plasma tryptophan levels at the above intervals during the oral study were 27%, 60% and 128%, respectively of those during the intravenous study. Plasma GIP levels rose only after intragastric tryptophan administration, and plasma GLI levels did not change in response to either intragastric or intravenous tryptophan. Intragastric tryptophan consistently raised plasma pancreatic glucagon levels which were significantly higher than those observed in control rats given saline, 5, 10, 30 and 45 min after administration. The rise in plasma glucagon was attributed to the glucagonotropic effect of GIP.