FREE TYROSINE LEVELS OF RAT-BRAIN AND TISSUES WITH SYMPATHETIC INNERVATION FOLLOWING ADMINISTRATION OF L-TYROSINE IN THE PRESENCE AND ABSENCE OF LARGE NEUTRAL AMINO-ACIDS

The tyrosine concentration of fasted rats was measured in plasma, brain and tissues receiving sympathetic innervation after L-tyrosine (200 mg/kg) was administered alone or in the presence of an equimolar cocktail containing isoleucine, leucine and valine. In the samples taken at 15, 30, 45, 60, 120 or 180 min, the highest concentrations of L-tyrosine were observed in plasma, heart, adrenal gland and kidney at 15 min, but in interscapular brown adipose tissue at 15 and 30 min and in brain at 15-60 min. The decline from peak concentrations was slower in brain, kidney and interscapular brown adipose tissue than in plasma, but in all tissues examined, control levels of free tyrosine were attained by 180 min postadministration. Competing large neutral amino acids reduced the maximal uptake of tyrosine in the brain by 48% but had no effect in the other tissues examined.