Isoprenaline increases brain concentrations of administered L-dopa and L-tryptophan in the rat

A small dose of isoprenaline or saline was administered intraperitoneally to rats 20 min before the administration of one of the amino acids L-dopa or L-tryptophan. Isoprenaline caused a marked increase in the brain concentration of the administered amino acid. Isoprenaline has previously been shown to cause a decrease in at least some of those plasma amino acids which compete with L-dopa and tryptophan for carrier-mediated transport into the brain. The effect of isoprenaline on the concentrations of dopa and tryptophan in the brain is suggested to be at least partly caused by a change in the relationship between endogenous and administered amino acids. It is also possible that a direct effect of isoprenaline on the blood-brain barrier transport system contributes to the effect. The reported finding might be of clinical interest in view of the therapeutic importance of aromatic amino acids with a central site of action.