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 junction coupling on regulation of intracellular Na⁺ concentration (Na⁺]_i) in cultured rat hippocampal astrocytes, using fluorescence ratio imaging with the Na⁺ indicator dye SBFI (sodium-binding benzofuran isophthalate). The Na⁺]_i in neighboring astrocytes was very similar (12.0 ± 3.3 mM) and did not fluctuate under resting conditions. During uncoupling of gap junctions with octanol (0.5 mM), baseline Na⁺]_i was unaltered in 24%, increased in 54%, and decreased in 22% of cells. Qualitatively similar results were obtained with two other uncoupling agents, heptanol and α-glycyrrhetinic acid (AGA). Octanol did not alter the recovery from intracellular Na⁺ load induced by removal of extracellular 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 decreases were amplified in cells with increased baseline Na⁺]_i, and reduced in cells with decreased baseline Na⁺]_i. This suggests that baseline Na⁺]_i in astrocytes “sets” the responsiveness of Na⁺, K⁺-ATPase to increases in K⁺]_o. Our results indicate that individual hippocampal astrocytes in culture rapidly develop different levels of baseline Na⁺]_i when they are isolated from one another by uncoupling agents. In astrocytes, therefore, an apparent function of coupling is the intercellular exchange of Na⁺ ions to equalize baseline Na⁺]_i, which serves to coordinate physiological responses that depend on the intracellular concentration of this ion. GLIA 20:299–307, 1997. © 1997 Wiley-Liss, Inc.