Effect of osmotic stress on potassium accumulation around glial cells and extracellular space volume in rat spinal cord slices

Abstract

In rat brain and spinal cord slices, the local extracellular accumulation of K(+), as indicated by K(+) tail currents (I(tail)) after a depolarization step, is greater in the vicinity of oligodendrocytes than that of astrocytes. It has been suggested that this may reflect a smaller extracellular space (ECS) around oligodendrocytes compared to astrocytes [Chvátal et al. [1997] J. Neurosci. Res. 49:98-106; [1999] J. Neurosci. Res. 56:493-505). We therefore compared the effect of osmotic stress in spinal cord slices from 5-11-day-old rats on the changes in reversal potentials (V(rev)) of I(tail) measured by the whole-cell patch-clamp technique and the changes in ECS volume measured by the real-time iontophoretic method. Cell swelling induced by a 20 min perfusion of hypoosmotic solution (200 mmol/kg) decreased the ECS volume fraction from 0.21 +/- 0.01 to 0.15 +/- 0.02, i.e., by 29%. As calculated from V(rev) of I(tail) using the Nernst equation, a depolarizing prepulse increased [K(+)](e) around astrocytes from 11.0 to 44.7 mM, i.e., by 306%, and around oligodendrocytes from 26.1 to 54.9 mM, i.e., by 110%. The ECS volume fraction decrease had the same time course as the changes in V(rev) of I(tail). Cell shrinkage in hyperosmotic solution (400 mmol/kg) increased ECS volume fraction from 0.24 +/- 0.02 to 0.32 +/- 0.02, i.e., by 33%. It had no effect on [K(+)](e) evoked by a depolarizing prepulse in astrocytes, whereas in oligodendrocytes [K(+)](e) rapidly decreased from 52 to 26 mM, i.e., by 50%. The increase in ECS volume was slower than the changes in [K(+)](e). These data demonstrate that hypoosmotic solution has a larger effect on the ECS volume around astrocytes than around oligodendrocytes and that hyperosmotic solution affects the ECS volume around oligodendrocytes only. This indicates that increased K(+) accumulation in the vicinity of oligodendrocytes could be due to a restricted ECS. Oligodendrocytes in the CNS are therefore most likely surrounded by clusters of "compacted" ECS, which may selectively affect the diffusion of neuroactive substances in specific areas and directions and facilitate spatial K(+) buffering.

Diffusion in Brain Extracellular Space

“…These detailed results have been supported by several studies that used a more limited range of osmotic variation (11, 147, 299, 369, 401) (Table 5). A study (166) on the CA3 region of the rat hippocampus at age P5 noted that α changed from 0.41 to 0.17 while λ increased from 1.38 to 1.49 after exposure to 215 mosmol kg −1 solution (Table 5).…”

“…To study the effects of cell swelling on the [K + ] e changes evoked by a depolarizing pulse, 50 mM K + or hypotonic solution was applied. These maneuvers led to an increase in evoked [K + ] e that was much greater in the vicinity of astrocytes than near oligodendrocytes (66, 401). These results indicate that swelling is more pronounced in astrocytes than in oligodendrocytes, and suggests that astrocytes are responsible for the majority of the cell volume changes seen in nervous tissue (66).…”

Chloride/anion channels in glial cell membranes

“…This is a requirement because one of the best-documented functions of astrocytes, potassium homeostasis seems to involve an increase of cell volume (Walz and Juurlink, 2002). There is not nearly as much known about volume regulation in oligodendrocytes as in astrocytes, despite the fact that the extracellular space around oligodendrocytes is more compact than the one around astrocytes (Vargova et al, 2001). Oligodendrocytes should therefore exhibit larger changes in extracellular osmolarity than astrocytes during normal and pathological functioning.…”

Analysis of K+ accumulation reveals privileged extracellular region in the vicinity of glial cells in situ

“…To study the changes in K + flux and accumulation in the vicinity of the glial membrane during cell swelling, spinal cord slices were exposed to hypotonic stress (see also Vargová et al, 2001). In astrocyte‐like cells during the application of 200 mmol/kg for 20 min, the reversal potential evoked by a depolarization pulse rose by 17.2 ± 1.8 mV (n = 5; Fig.…”

“…The application of Ba 2+ almost completely blocked the decaying component of the inward and outward currents, as well as tail currents after depolarizing and hyperpolarizing voltage steps, indicating that these currents are carried predominantly by K + (Berger et al, 1991; Chvátal et al, 1995). Further analysis of oligodendrocyte currents in brain slice preparations (Chvátal et al, 1997, 1999; Vargová et al, 2001) revealed that I tail observed after the offset of a depolarizing pulse are caused by the reversed shift of K + across the cell membrane from the ECS into the cell. Because the glial membrane is highly permeable for K + , the membrane potential (V m ) is close to the reversal potential (V rev ) determined by the gradient of K + outside and inside the cell according to the Nernst equation.…”