Regulatory volume decrease in cultured astrocytes. II. Permeability pathway to amino acids and polyols

Abstract

The permeability of the hyposmolarity-activated pathway to amino acids and polyols in cultured astrocytes was examined following the change in rate and direction of regulatory volume decrease (RVD) when the extracellular concentration of the osmolytes was increased to reverse their intracellular-extracellular concentration gradient. Activation of the pathway by swelling would allow those permeable osmolytes to enter the cell and inhibit RVD. The pathway was found to be permeable to neutral amino acids, with beta-amino acids (beta-alanine = taurine > gamma-aminobutyric acid) more permeable than alpha-amino acids. Glycine, alanine, threonine, phenylalanine, and asparagine, but not glutamine, were permeable through this pathway. Aspartate was more permeable than glutamate, and K+ and not Na+ must be the accompanying cation. Basic amino acids were excluded. The dimension of the amino acid pore activated by hyposmolarity seems to be at the limit of glutamate-glutamine size. Influx rather than efflux of amino acids was observed when extracellular concentration was greater than intracellular concentration, with differences in the amount accumulated by cells correlating with their efficiency as RVD blockers. Influx of taurine (as representative of permeable amino acids) was inhibited by the Cl- channel blockers/exchangers 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (40%) and dipyridamole (85%) , and it is suggested that amino acids permeate through an anion channel. Sorbitol and mannitol, but not inositol, exhibited a small inhibitory effect on the later phase of RVD, whereas inositol slightly accelerated RVD.

Intracellular delivery of 2-deoxy-D-glucose into tumor cells by long-term cultivation and through swelling-activated pathways: implications for radiation treatment

“…The observed inhibition of RVD implies a substantial uptake of 2DG by tumor cells. Earlier studies justify the use of RVD inhibition as an indicator of swelling-activated membrane permeability for small extracellular solutes (19,32). Moreover, the secondary swelling observed in the tested cell lines (Fig.…”

Target cell-specific modulation of neuronal activity by astrocytes

“…Although several studies have suggested that GABA or GABA-like substances can be secreted from glial cells in culture (28,30,34,35), it is generally assumed that glial cells do not contain the enzymes necessary for the synthesis of GABA and that, if present, GABA is taken up from the extracellular medium by GABA transporters. Our results suggest that GABA is most likely the transmitter involved, even though other transmitters might substitute (36). Taurine would be a possible candidate because it is abundant in the olfactory bulb (37,38) and induces a slow, ''nondesensitizing'' inhibition of mitral cells during a prolonged bath application (39).…”

Hydrogen Peroxide Potentiates Volume-sensitive Excitatory Amino Acid Release via a Mechanism Involving Ca2+/Calmodulin-dependent Protein Kinase II

“…VRACs, which are typically activated in response to cell swelling, are ubiquitously expressed and permeable toward small organic molecules in astrocytes and other cell types (10,36,64,65). Two recent publications have shown that H 2 O 2 induces a gradual increase in Cl Ϫ conductance in non-swollen cells, which exhibits the typical current/voltage relationship and pharmacological properties of VRACs (15,16).…”