Efflux of osmolyte amino acids during isovolumic regulation in hippocampal slices

Abstract

The efflux of potassium (K(+)) and amino acids from hippocampal slices was measured after sudden exposure to 10% (270 mOsm), 25% (225 mOsm) or 50% (150 mOsm) hyposmotic solutions or after gradual decrease (-2.5 mOsm/min) in external osmolarity. In slices suddenly exposed to 50% hyposmotic solutions, swelling was followed by partial (74%) cell volume recovery, suggesting regulatory volume decrease (RVD). With gradual hyposmotic changes, no increase in cell water content was observed even when the solution at the end of the experiment was 50% hyposmotic, showing the occurrence of isovolumic regulation (IVR). The gradual decrease in osmolarity elicited the efflux of (3)H-taurine with a threshold at -5 mOsm and D-[(3)H]aspartate (as marker for glutamate) and at -20 mOsm for [(3)H]GABA. The efflux rate of [(3)H]taurine was always notably higher than those of [(3)H]GABA and D-[(3)H]aspartate, with a maximal increase over the isosmotic efflux of about 7-fold for [(3)H]taurine and 3- and 2-fold for [(3)H]GABA and D-[(3)H]aspartate, respectively. The amino acid content in slices exposed to 50% hyposmotic solutions (abrupt change) during 20 min decreased by 50. 6% and 62.6% (gradual change). Taurine and glutamate showed the largest decrease. An enhancement in (86)Rb efflux and a corresponding decrease in K(+) tissue content was seen in association with RVD but not with IVR. These results demonstrate the contribution of amino acids to IVR and indicate their involvement in this mechanism of cell volume control.

A new function for glycine GlyT2 transporters: Stimulation of γ‐aminobutyric acid release from cerebellar nerve terminals through GAT1 transporter reversal and Ca2+‐dependent anion channels

“…The glycine‐evoked GABA release studied here was largely prevented by niflumic acid and NPPB, inhibitors of anion channels. Efflux through anion channels was described in the case of anionic compounds, particularly glutamate (Franco et al, ; Raiteri et al, ; Milanese et al, ), but also in experiments with the zwitterion GABA (Lee et al, ; Peng et al, ). In recent work (Romei et al, ) it was found that GABA uptake into cerebellum nerve endings was able to stimulate release of the zwitterion glycine essentially by permeation through anion channels.…”

Mechanisms Counteracting Swelling in Brain Cells During Hyponatremia

“…Molecules involved and mechanisms in operation are not known in detail, as studies on isovolumetric regulation (IVR) are still scarce. Occurrence of IVR has been found in only two types of renal cells (47,49), in hippocampal slices (50) (fig. 3), and with lower efficiency in C6 glioma cells (51) and in cardiomyocytes (52).…”

“…In A6 cells and in myocytes, IVR stimulates K + release, but only at delayed phase of IVR, with an efflux threshold at -30% hyposmotic external osmolarity. In contrast, amino acids, particularly taurine, appear involved at earlier phases of volume regulation, showing efflux thresholds at approximately -10 to 12% hyposmolarity (50) (fig. 3).…”

Intracellular levels of glutamate in swollen astrocytes are preserved via neurotransmitter reuptake and de novo synthesis: implications for hyponatremia

“…; Franco et al . ). Observations on the preferential release of taurine from glia, and findings that in taurine‐depleted cells cell volume regulation is impaired, lead to the suggestion that this atypical amino acid serves as a major regulatory osmolyte, and that its release is regulated via a distinct mechanism (Moran et al .…”