Ammonia selectively stimulates neutral amino acid transport across blood-brain barrier

The response to increased blood NH4+ of three blood-brain barrier transport systems, which are altered after portacaval anastomosis, was studied. NH4+ acetate was infused for 4 or 22 h to raise blood and brain NH4+, and brain glutamine, to levels similar to those observed after portacaval anastomosis. While brain glutamine content was much higher (16-20 mumol/g) than normal (6 mumol/g) at both times, the permeability of the blood-brain barrier to the neutral amino acid [14C]tryptophan was greater only after 22 h of infusion. After discontinuing the infusion for 5 h, tryptophan transport returned to normal, whereas brain glutamine remained elevated (13 mumol/g). Thus there seemed to be no relationship between the rate of transport and glutamine content. The permeability to [14C]sucrose was unaltered, showing that the integrity of the blood-brain barrier was maintained. Other changes that are characteristic of portacaval shunting, such as decreased basic amino acid ([14C]lysine) and monocarboxylic acid (3-[14C]hydroxybutyrate) transport, were not reproduced by 22 h of infusion. The results demonstrated that the continued presence of NH4+ could be responsible for the change in at least one of the transport systems that are affected by portacaval shunting.