Effects of NaCl removal on osmolyte fluxes and regulatory volume decrease in cultured astrocytes.

Cultured cerebellar astrocytes exhibit regulatory volume decrease (RVD) after anisosmotic swelling by extrusion of potassium (K), chloride (Cl), and organic osmolytes. The volume decay curve after swelling in 50% hyposmotic solutions has a first-order rate constant k min(-1) of 0.18 and 70% cell volume recovery is attained after 15 min. Replacing NaCl in the hyposmotic medium by sugars and polyalcohols markedly accelerated RVD increasing k to 0.37-0.39 min(-1) and full recovery in 3-5 min. NaCl removal increased (80%) K efflux (measured as 86Rb), delayed the inactivation of Cl efflux (measured as 125I) and enhanced taurine and D-aspartate release by 25%. These effects appear due to NaCl removal rather than to specific actions of the organic molecules. Replacing Na by other cations or Cl by gluconate increased k to 0.22 min(-1) and 0.26 min(-1), respectively, and 86Rb efflux by 4-23% and 39, respectively. It is concluded that the electrochemical gradient after NaCl removal increasing the K driving force, delaying the equilibrium of Cl and increasing amino acid efflux, accounts for the observed RVD acceleration. Other consequences of NaCl removal possibly modulating the K efflux pathway include changes in the surface charge screening, decreased ion strength, a rise in [Ca]i and a decrease in intracellular Na.