Although system A is present at the blood-brain barrier (BBB), the physiological roles of system A have not been clarified. The efflux transport of the substrates of system A, such as L-proline (L-Pro), glycine (Gly), and α-methylaminoisobutyric acid (MeAIB), across the BBB was investigated using the in vivo Brain Efflux Index method. Over a period of 40 min, L-[3H]Pro and [3H]Gly underwent efflux from the brain, whereas [3H]MeAIB did not. The efflux of L-[3H]Pro was inhibited by the presence of unlabeled L-Pro and MeAIB, suggesting that carrier-mediated efflux transport of L-Pro across the BBB is involved in system A. L-[3H]Pro uptake by TR-BBB cells, used as an in vitro BBB model, was Na+-dependent with high-affinity (Km1 = 425 μM) and low-affinity (Km2 = 10.8 mM) saturable processes. The manner of inhibition of L-[3H]Pro uptake for amino acids was consistent with system A. Although GlnT, ATA2, and ATA3 mRNA were all expressed in TR-BBB cells, ATA2 mRNA was predominant. Under hypertonic conditions, ATA2 mRNA in TR-BBB cells was induced by up to 373%, and it activated [3H]MeAIB uptake. In light of these observations, our results indicate that L-Pro and Gly are transported from the brain across the BBB, whereas MeAIB is retained in the brain. System A is involved in efflux transport for L-Pro at the BBB. The predominantly expressed ATA2 mRNA at the BBB may play a role in maintaining the concentration of small neutral amino acids and cerebral osmotic pressure in the brain under pathological conditions.
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