Effects of Albumin, Amino Acids and Clofibrate on the Uptake of Tryptophan by the Rat Brain

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Abstract
The influences of total tryptophan concentration, albumin binding and amino acid competition on the rate of tryptophan influx into rat brain were compared using a single-pass injection technique with [3H] water as a freely diffusible reference. Omission of 3% bovine albumin from a bolus containing tryptophan in Krebs-Ringer bicarbonate buffer injected into the carotid artery increased non-albumin bound (free) tryptophan concentration 3-fold but tryptophan uptake by only 35 and 30% into forebrain and hypothalamus, respectively. Tryptophan uptake from injected rat plasma was more markedly elevated when free tryptophan concentration was raised. When free tryptophan was doubled, but total tryptophan unchanged, by in vitro addition of clofibrate to a plasma bolus, uptake was increased by 53 and 28% into forebrain and hypothalamus, respectively. When clofibrate was injected in vivo so that plasma total tryptophan concentration was decreased by 45% but neither free tryptophan nor competing amino acid concentrations were altered, then uptake from a bolus of the rat''s own plasma was unchanged. Addition of competing amino acids at physiological concentrations to tryptophan in Krebs-Ringer buffer significantly reduced tryptophan influx into both brain regions, but did not increase the effect of albumin binding. Tryptophan uptake into rat forebrain may be substantially influenced by albumin binding and competition from other amino acids, but hypothalamic uptake is less influenced by these factors.