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 alpha-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.