Brain capillary endothelial cells form the blood-brain barrier (BBB). They are connected by extensive tight junctions, and are polarized into luminal (blood-facing) and abluminal (brain-facing) plasma membrane domains. The polar distribution of transport proteins mediates amino acid (AA) homeostasis in the brain. The existence of two facilitative transporters for neutral amino acids (NAAs) on both membranes provides the brain access to essential AAs. Four Na(+)-dependent transporters of NAA exist in the abluminal membranes of the BBB. Together these systems have the capability to actively transfer every naturally occurring NAA from the extracellular fluid (ECF) to endothelial cells and from there into circulation. The presence of Na(+)-dependent carriers on the abluminal membrane provides a mechanism by which NAA concentrations in the ECF of brain are maintained at approximately 10% those of the plasma. Also present on the abluminal membrane are at least three Na(+)-dependent systems transporting acidic AAs (EAAT) and a Na(+)-dependent system transporting glutamine (N). Facilitative carriers for glutamine and glutamate are found only in the luminal membrane of the BBB. This organization promotes the net removal of acidic- and nitrogen-rich AAs from the brain and accounts for the low level of glutamate penetration into the central nervous system. The presence of a gamma-glutamyl cycle at the luminal membrane and Na(+)-dependent AA transporters at the abluminal membrane may serve to modulate movement of AAs from blood to the brain. The gamma-glutamyl cycle is expected to generate pyroglutamate (synonymous with oxyproline) within the endothelial cells. Pyroglutamate stimulates secondary active AA transporters at the abluminal membrane, thereby reducing the net influx of AAs to the brain. It is now clear that BBB participates in the active regulation of the AA content of the brain.

Na؉ -dependent transporters for glutamate exist on astrocytes (EAAT1 and EAAT2) and neurons (EAAT3). These transporters presumably assist in keeping the glutamate concentration low in the extracellular fluid of brain. Recently, Na ؉ -dependent glutamate transport was described on the abluminal membrane of the bloodbrain barrier. To determine whether the above-mentioned transporters participate in glutamate transport of the blood-brain barrier, total RNA was extracted from bovine cerebral capillaries. cDNA for EAAT1, EAAT2, and EAAT3 was observed, indicating that mRNA was present. Western blot analysis demonstrated all three transporters were expressed on abluminal membranes, but none was detectable on luminal membranes of the blood-brain barrier. Measurement of transport kinetics demonstrated voltage dependence, K ؉ -dependence, and an apparent K m of 14 M (aggregate of the three transporters) at a transmembrane potential of ؊61 mV. Inhibition of glutamate transport was observed using inhibitors specific for EAAT2 (kainic acid and dihydrokainic acid) and EAAT3 (cysteine). The relative activity of the three transporters was found to be approximately 1:3:6 for EAAT1, EAAT2, and EAAT3, respectively. These transporters may assist in maintaining low glutamate concentrations in the extracellular fluid.

The efficacy of clay modeling compared with cat dissection for human muscle identification was examined over two semesters at LaGuardia Community College in Queens, NY. The 181 students in 10 sections in this study were randomly distributed into control (cat dissection) and experimental (clay modeling) groups, and the results of the muscle practical examination were analyzed. The clay-modeling group was significantly better at identifying human muscles on human models than the cat-dissection group, and was as good at identifying muscles on their self-made clay mannequins as the cat-dissection group was at identifying cat muscle on their specimens. This study demonstrated that clay modeling is more effective than cat dissection for learning human muscles at the community college level.

Cationic amino acid (CAA) transport is brought about by two families of proteins that are found in various tissues: Cat (CAA transporter), referred to as system y+, and Bat [broad-scope amino acid (AA) transporter], which comprises systems b0,+, B0,+, and y+L. CAA traverse the blood-brain barrier (BBB), but experiments done in vivo have only been able to examine the BBB from the luminal (blood-facing) side. In the present study, plasma membranes isolated from bovine brain microvessels were used to identify and characterize the CAA transporter(s) on both sides of the BBB. From these studies, it was concluded that system y+ was the only transporter present, with a prevalence of activity on the abluminal membrane. System y+ was voltage dependent and had a Km of 470 +/- 106 microM (SE) for lysine, a Ki of 34 microM for arginine, and a Ki of 290 microM for ornithine. In the presence of Na+, system y+ was inhibited by several essential neutral AAs. The Ki values were 3-10 times the plasma concentrations, suggesting that system y+ was not as important a point of access for these AAs as system L1. Several small nonessential AAs (serine, glutamine, alanine,and glycine) inhibited system y+ with Ki values similar to their plasma concentrations, suggesting that system y+ may account for the permeability of the BBB to these AAs. System y+ may be important in the provision of arginine for NO synthesis. Real-time PCR and Western blotting techniques established the presence of the three known nitric oxide synthases in cerebral endothelial cells: NOS-1 (neuronal), NOS-2 (inducible), and NOS-3 (endothelial). These results confirm that system y+ is the only CAA transporter in the BBB and suggest that NO can be produced in brain endothelial cells.

Several Na+-dependent carriers of amino acids exist on the abluminal membrane of the blood-brain barrier (BBB). These Na+-dependent carriers are in a position to transfer amino acids from the extracellular fluid of brain to the endothelial cells and thence to the circulation. To date, carriers have been found that may remove nonessential, nitrogen-rich, or acidic (excitatory) amino acids, all of which may be detrimental to brain function. We describe here Na+-dependent transport of large neutral amino acids across the abluminal membrane of the BBB that cannot be ascribed to currently known systems. Fresh brains, from cows killed for food, were used. Microvessels were isolated, and contaminating fragments of basement membranes, astrocyte fragments, and pericytes were removed. Abluminal-enriched membrane fractions from these microvessels were prepared. Transport was Na+ dependent, voltage sensitive, and inhibited by 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid, a particular inhibitor of the facilitative large neutral amino acid transporter 1 (LAT1) system. The carrier has a high affinity for leucine ( Km 21 ± 7 μM) and is inhibited by other neutral amino acids, including glutamine, histidine, methionine, phenylalanine, serine, threonine, tryptophan, and tyrosine. Other established neutral amino acids may enter the brain by way of LAT1-type facilitative transport. The presence of a Na+-dependent carrier on the abluminal membrane capable of removing large neutral amino acids, most of which are essential, from brain indicates a more complex situation that has implications for the control of essential amino acid content of brain.

-dependent neutral amino acid transporters A, ASC, and N of the blood-brain barrier: mechanisms for neutral amino acid removal. Am J Physiol Endocrinol Metab 287: E622-E629, 2004. First published May 27, 2004 10.1152/ajpendo.00187.2004.-Four Na ϩ -dependent transporters of neutral amino acids (NAA) are known to exist in the abluminal membranes (brain side) of the blood-brain barrier (BBB). This article describes the kinetic characteristics of systems A, ASC, and N that, together with the recently described Na ϩ -dependent system for large NAA (Na ϩ -LNAA), provide a basis for understanding the functional organization of the BBB. The data demonstrate that system A is voltage dependent (3 positive charges accompany each molecule of substrate). Systems ASC and N are not voltage dependent. Each NAA is a putative substrate for at least one system, and several NAA are transported by as many as three. System A transports Pro, Ala, His, Asn, Ser, and Gln; system ASC transports Ser, Gly, Met, Val, Leu, Ile, Cys, and Thr; system N transports Gln, His, Ser, and Asn; Na ϩ -LNAA transports Leu, Ile, Val, Trp, Tyr, Phe, Met, Ala, His, Thr, and Gly. Together, these four systems have the capability to actively transfer every naturally occurring NAA from the extracellular fluid (ECF) to endothelial cells and thence to the circulation. The existence of facilitative transport for NAA (L1) on both membranes provides the brain access to essential NAA. The presence of Na ϩ -dependent carriers on the abluminal membrane provides a mechanism by which NAA concentrations in the ECF of brain are maintained at ϳ10% of those of the plasma. active transport; brain extracellular fluid; capillaries; endothelial cells; essential amino acids AVAILABILITY of essential neutral amino acids (NAA) is necessary for normal brain function. Because NAA are hydrophilic, transport systems are necessary to transfer them across the blood-brain barrier (BBB). Large essential NAA enter the brain on system L1 1 , a high-affinity facilitative system that is present in the luminal and abluminal membranes of the endothelial cell (23,24,26,32,36,37). Nonessential NAA, most of which are smaller, have not been found to pass from blood to brain as readily; nevertheless, the uptake of small nonessential amino acids by the brain was greater than zero, suggesting some transport or leakage at a slow rate (1,23,24).The concentrations of all naturally occurring amino acids in the cerebrospinal fluid [presumably similar to the extracellular fluid (ECF) of brain], with the exception of glutamine, are ϳ10% of those in plasma (17,19,20,35). This situation cannot be explained by the consumption of NAA by brain, because the arteriovenous differences across the brain of most NAA are imperceptible (7,8,29), as are the arteriovenous differences of ammonia, a by-product of amino acid catabolism (6). These observations indicate that NAA leave the brain against a concentration gradient at the same rate they enter.The maintenance of an NAA gradient between plasma and ECF requires the ex...

Brain capillary endothelial cells form the blood-brain barrier (BBB). They are connected by extensive tight junctions, and are polarized into luminal (blood-facing) and abluminal (brain-facing) plasma membrane domains. The polar distribution of transport proteins mediates amino acid (AA) homeostasis in the brain. The existence of two facilitative transporters for neutral amino acids (NAA) on both membranes provides the brain access to essential AA. Four Na+-dependent transporters of NAA exist in the abluminal membranes of the BBB. Together these systems have the capability to actively transfer every naturally occurring NAA from the extracellular fluid (ECF) to endothelial cells and thence to the circulation. The presence of Na+-dependent carriers on the abluminal membrane provides a mechanism by which NAA concentrations in the ECF of brain are maintained at about 10% of those of the plasma. Also present on the abluminal membrane are at least three Na+-dependent systems transporting acidic AA (EAAT) and a Na+-dependent system transporting glutamine (N). Facilitative carriers for glutamine and glutamate are found only in the luminal membrane of the BBB. This organization promotes the net removal of acidic and nitrogen-rich AA from brain, and accounts for the low level of glutamate penetration into the central nervous system (CNS). The presence of a γ-glutamyl cycle at the luminal membrane and Na+-dependent AA transporters at the abluminal membrane may serve to modulate movement of AA from blood to brain. The γ-glutamyl cycle is expected to generate pyroglutamate within the endothelial cells. Pyroglutamate stimulates secondary active AA transporters at the abluminal membrane, thereby reducing net influx of AA to the brain. It is now clear the BBB participates in the active regulation of the AA content of the brain.