Regional Blood‐Brain Barrier Permeability to Amino Acids After Portacaval Anastomosis

Abstract

The influx of phenylalanine, tryptophan, leucine and lysine across the blood-brain barrier of individual brain structures was studied in rats 7-8 wk after a portacaval shunt or sham operation. The method involved a brief infusion of labeled amino acid in tracer quantity and quantitative autoradiography. Clearance rates of phenylalanine, tryptophan and leucine were increased in proportion to each other in every region examined, but not by the same factor. Tryptophan clearance increased the most (.apprx. 200%) and leucine the least (.apprx. 30%), compared with phenylalanine (.apprx. 80%). This was unexpected, as all 3 amino acids are believed to be transported by the same mechanism. The changes were most marked in several limbic structures and the reticular formation; the hypothalamus was least affected. Plasma clearance of lysine was decreased in all areas by .apprx. 70%. Since the circulating lysine concentration was decreased by 13%, the actual rate of lysine influx was even more reduced. Results demonstrate specific alterations in 2 different amino acid transport systems. The resulting excess brain neutral amino acids, some of which are neurotransmitter precursors, and reduced basic amino acid availability, may be of etiological significance in hepatic encephalopathy.