Glutamine and glutamate transport activities were measured in isolated luminal and abluminal plasma membrane vesicles derived from bovine brain endothelial cells. Facilitative systems for glutamine and glutamate were almost exclusively located in luminal-enriched membranes. The facilitative glutamine carrier was neither sensitive to 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid inhibition nor did it participate in accelerated amino acid exchange; it therefore appeared to be distinct from the neutral amino acid transport system L1. Two Na-dependent glutamine transporters were found in abluminal-enriched membranes: systems A and N. System N accounted for ∼80% of Na-dependent glutamine transport at 100 μM. Abluminal-enriched membranes showed Na-dependent glutamate transport activity. The presence of 1) Na-dependent carriers capable of pumping glutamine and glutamate from brain into endothelial cells, 2) glutaminase within endothelial cells to hydrolyze glutamine to glutamate and ammonia, and 3) facilitative carriers for glutamine and glutamate at the luminal membrane may provide a mechanism for removing nitrogen and nitrogen-rich amino acids from brain.

No abstract

Regulation of neutral amino acid transport was studied using isolated plasma membrane vesicles derived from the bovine blood-brain barrier. Neutral amino acids cross the blood-brain barrier by facilitative transport system L1, which may allow both desirable and undesirable amino acids to enter the brain. The sodiumdependent amino acid systems A and B o,؉ are located exclusively on abluminal membranes, in a position to pump unwanted amino acids out. ␥-Glutamyl transpeptidase, the first enzyme of the ␥-glutamyl cycle, is an integral protein of the luminal membrane of the bloodbrain barrier. We demonstrate that oxoproline, an intracellular product of the ␥-glutamyl cycle, stimulates the sodium-dependent systems A and B o,؉ by 70 and 20%, respectively. Study of system A showed that 2 mM oxoproline increased the affinity for its specific substrate N-methylaminoisobutyrate by 50%. This relationship between the activity of the ␥-glutamyl cycle and system A transport may provide a short term regulatory mechanism by which the entry of potentially deleterious amino acids (i.e. neurotransmitters or their precursors) may be retarded and their removal from brain accelerated.

A transpharyngeal endoscopic approach to remove loose anterior cervical hardware is feasible and warrants further evaluation.

There remains no standard postoperative imaging algorithm for patients after complete vestibular schwannoma resection.