<i>In Vitro</i> Study of L-Glutamate and L-Glutamine Transport in Retinal Pericytes: Involvement of Excitatory Amino Acid Transporter 1 and Alanine–Serine–Cysteine Transporter 2

Abstract

L-Glutamate (L-Glu) is known to be a relaxant of pericytes and to induce changes in microcirculatory hemodynamics. Since the concentration of L-Glu which induces the dilation of retinal capillaries is reported to be high compared with the estimated concentration in the retinal interstitial fluid, it is hypothesized that some systems involving concentrative L-Glu release are present in retinal pericytes. The purpose of this study was to investigate the existence of L-Glu-storing systems, which contribute to autocrine L-Glu release, in retinal pericytes using conditionally immortalized rat retinal pericytes (TR-rPCT1 Key words excitatory amino acid transporter; L-glutamate; retinal pericyte; alanine-serine-cysteine transporter; L-glutamine Retinal pericytes reside within the basal lamina of retinal capillaries and communicate with the retinal capillary endothelial cells which form the blood-retinal barrier (BRB). 1,2)It is known that the contraction and relaxation of pericytes which are induced by physiological factors leads to a change in the diameter of the capillaries. [3][4][5] Because it has been reported that the diameter of capillaries in the central nervous system (CNS) affects the blood flow rate in the capillaries and, thus, the exchange of some compounds between the circulating blood and retina, 6) it is very important to identify the regulatory mechanisms governing the contraction and relaxation of retinal pericytes.L-Glutamate (L-Glu) is known as a relaxant of pericytes and Peppiatt et al. have reported that L-Glu treatment induces dilation of the retinal capillaries. 7) Regarding the mechanism for the L-Glu-induced dilation of retinal capillaries, it has been proposed that L-Glu induces the production of nitric oxide (NO), a relaxant of the cells, and inhibits Ca 2+ influx in retinal pericytes, thereby relaxing the pericytes. 7,8) An in vitro analysis has indicated that this L-Glu-induced NO production occurs in the presence of L-Glu at a concentration of more than 1 mM. 8) In addition, the dilation of retinal capillaries accompanied with pericytes was induced by treatment with 500 µM L-Glu.7) The compounds between the retinal interstitial fluid (ISF) and vitreous humor are considered to be freely exchangeable, and the concentration of L-Glu in the vitreous humor has been reported to be 444 µM in rats.9) Because the L-Glu level in the retinal ISF seems to be low compared with the concentration which induces dilation of the retinal capillaries and relaxation in the pericytes, it is possible that some mechanisms for concentrative L-Glu release are present in retinal pericytes and/or the cells surrounded by the pericytes. In neuronal cells, the packing of L-Glu into synaptic vesicles up to concentrations as high as 100 mM and exocytotic release of L-Glu in the vesicles into the synaptic cleft have been reported. [10][11][12] It has also been reported that the uptake of L-Glu from the cytoplasm of neurons into synaptic vesicles involves the vesicular L-Glu transporters (VGLUTs) which belong to the so...

Ischemia-Triggered Glutamate Excitotoxicity From the Perspective of Glial Cells

“…Subsequently, the glutamate may be extruded across the luminal membrane into the bloodstream through another type of glutamate carrier, referred to as facilitative glutamate transporters (Hawkins and Viña, 2016). Finally, at least one study, performed in the retina, described an EAAT-mediated glutamate uptake by another brain cell type-pericytes (Akanuma et al, 2015). A scheme summarizing the mechanism of glutamate uptake in the ischemic CNS is shown in Figure 3.…”

The role of glutamate and glutamine metabolism and related transporters in nerve cells

“…The transport process involves the uptake of one molecule of glutamate, three Na + , and one H + into astrocytes, while simultaneously exporting one K + from astrocytes 19,20 . Research has indicated that the Km values for glutamate transport into astrocytes, facilitated by EAAT1 and EAAT2 in rats, are 11 and 17 μM, respectively 21 …”

“…19,20 Research has indicated that the Km values for glutamate transport into astrocytes, facilitated by EAAT1 and EAAT2 in rats, are 11 and 17 μM, respectively. 21 GLT-1 is considered the major glutamate transporter among astrocytic EAATs. It accounts for approximately 1% of the total brain protein in the CNS and is responsible for more than 90% of glutamate uptake.…”

“… 19 , 20 Research has indicated that the Km values for glutamate transport into astrocytes, facilitated by EAAT1 and EAAT2 in rats, are 11 and 17 μM, respectively. 21 …”