Hypoosmotic volume regulation of astrocytes in elevated extracellular potassium

Abstract

Cellular volume and potassium contents were determined in rat astrocytes from primary culture following suspension in isoosmotic (269 mOsm) and hypoosmotic (136 mOsm) phosphate-buffered saline (PBS) containing various potassium concentrations. Within 1 min of suspension in hypoosmotic PBS, cells swelled to 135% of their volume in isoosmotic PBS. This initial swelling was not altered by varying the potassium concentration of the hypoosmotic PBS. After suspension in hypoosmotic PBS containing 3.2 mM potassium, a regulatory volume decrease (RVD) was observed. Higher concentrations of potassium in hypoosmotic PBS inhibited RVD following osmotic swelling. Cells swollen in hypoosmotic PBS containing 50 mM potassium continued to swell for 7 min, reaching a volume of 141% of their initial isoosmotic volume. After 7 min, these cells demonstrated a subsequent decrease in volume. The swelling observed between 1-7 min after suspension in hypoosmotic PBS containing 50 mM potassium was not affected by 10 microM gadolinium, 1 mM quinine, 1 mM DIDS (4,4'-diisothiocyanato-2,2'-stilbenedisulfonic acid), 1 mM SITS (4-acetamido-4'-isothiocyanato-2,2'-stilbenedisulfonic acid), 1 mM furosemide, or 100 microM bumetanide. Normal RVD was obtained in hypoosmotic PBS containing 50 mM potassium, if chloride was replaced with gluconate (but not nitrate) to reduce the extracellular K.Cl product to that of hypoosmotic PBS containing 3.2 mM potassium. The volume decrease seen between 7-30 min after exposure to hypoosmotic PBS containing 50 mM potassium was blocked by 1 mM DIDS, 1 mM SITS, or 1 mM furosemide. Cellular potassium content was elevated by approximately 60% after 7 min exposure to isoosmotic or hypoosmotic PBS containing 50 mM potassium. In hypoosmotic PBS, this increase in cellular potassium was reduced with replacement of chloride by gluconate, but not by nitrate. The results indicate that astrocytes swollen in PBS containing elevated potassium concentrations continue to swell, in part, by accumulation of potassium plus chloride mediated by an approach to Donnan equilibrium. Cotransport carriers or stretch-activated channels do not play a role in the enhanced swelling observed in hypoosmotic PBS containing 50 mM potassium. We suggest that a voltage-sensitive chloride channel mediates this continuation of cell swelling. This mechanism may be important in the persistent swelling of astrocytes observed in pathologic conditions such as trauma and seizures where extracellular potassium is elevated, or when other factors are present which may cause astroglial depolarization.

Real‐time passive volume responses of astrocytes to acute osmotic and ischemic stress in cortical slices and in vivo revealed by two‐photon microscopy

“…In cell culture or in dissociated preparations, astrocytes are reported to not only change their volume in response to strong osmotic stress, but to also then regulate their volume over a period ranging from seconds to minutes (Olson et al, 1995; Ordaz et al, 2004). in vivo , osmotic volume regulation develops over many hours and days (see Introduction) but there is no evidence to date that it is evoked over seconds to minutes in the intact brain.…”

Delayed K + clearance associated with aquaporin-4 mislocalization: Phenotypic defects in brains of α-syntrophin-null mice

“…In principle, spatial buffering of K ϩ should be independent of water movement, because the net ion flux is very small (24). However, K ϩ uptake occurs by several mechanisms, some of which have been shown to lead to cell swelling and water uptake in isolated cells (25)(26)(27)(28). K ϩ transport that generates an osmotic gradient will be facilitated if accompanied by water flux.…”

Turning down the volume: Astrocyte volume change in the generation and termination of epileptic seizures

“…As described above, astrocyte swelling may be a catalyst for inducing and/or augmenting seizures, so the following discussion will focus specifically on RVD subsequent to cell swelling. RVD has been repeatedly observed in cultured astrocytes swollen by hypoosmolar or high K + media (Eriksson et al 1992; Kimelberg and Frangakis 1985; Olson et al 1995; Vitarella et al 1994). Disagreement exists as to whether astrocytic RVD occurs in intact tissue, as various groups have reported astrocytic swelling in brain slices without RVD (Andrew et al 1997; Hirrlinger et al 2008; Risher et al 2009).…”