GAP-JUNCTIONS EQUALIZE INTRACELLULAR NA+ CONCENTRATION IN ASTROCYTES

Gap junctions between glial cells allow intercellular exchange of ions and small molecules. We have investigated the influence of gap juncti on coupling on regulation of intracellular Na+ concentration ([Na+](i) ) in cultured rat hippocampal astrocytes, using fluorescence ratio ima ging with the Na+ indicator dye SBFI (sodium-binding benzofuran isopht halate). The [Na+](i) in neighboring astrocytes was very similar (12.0 +/- 3.3 mM), and did not fluctuate under resting conditions. During u ncoupling of gap junctions with octanol (0.5 mM), baseline [Na+](i) wa s unaltered in 24%, increased in 54%, and decreased in 22% of cells. Q ualitatively similar results were obtained with two other uncoupling a gents, heptanol and alpha-glycyrrhetinic acid (AGA). Octanol did not a lter the recovery from intracellular Na+ load induced by removal of ex tracellular K+, indicating that octanol's effects on baseline [Na+](i) were not due to inhibition of Na+, K+-ATPase activity. Under control conditions, increasing [K+](o) from 3 to 8 mM caused similar decreases in [Na+](i) in groups of astrocytes, presumably by stimulating Na+, K +-ATPase. During octanol application, [K+](o)-induced [Na+](i) decreas es were amplified in cells with increased baseline [Na+](i), and reduc ed in cells with decreased baseline [Na+](i). This suggests that basel ine [Na+](i) in astrocytes ''sets'' the responsiveness of Na+, K+-ATPa se to increases in [K+](o). Our results indicate that individual hippo campal astrocytes in culture rapidly develop different levels of basel ine [Na+](i) when they are isolated from one another by uncoupling age nts. In astrocytes, therefore, an apparent function of coupling is the intercellular exchange of Na+ ions to equalize baseline [Na+](i), whi ch serves to coordinate physiological responses that depend on the int racellular concentration of this ion. (C) 1997 Wiley-Liss, Inc.