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.
Rose
C R
CR
Department of Neurology, Yale University School of Medicine, New Haven, Connecticut 06520, USA. crose@biomed.med.yale.edu
Ransom
B R
BR
D000818:Animals; D000831:Animals, Newborn; D001253:Astrocytes / Q000201:enzymology / Q000378:metabolism*; D001572:Benzofurans; D002478:Cells, Cultured; D004988:Ethers, Cyclic; D005456:Fluorescent Dyes; D017629:Gap Junctions / Q000201:enzymology / Q000378:metabolism*; D006034:Glycyrrhetinic Acid / Q000494:pharmacology; D006624:Hippocampus / Q000166:cytology / Q000201:enzymology; D006706:Homeostasis / Q000502:physiology; D008856:Microscopy, Fluorescence; D000442:Octanols / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism
D001572:Benzofurans; D004988:Ethers, Cyclic; D005456:Fluorescent Dyes; D000442:Octanols; C061665:sodium-binding benzofuran isophthalate; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D006034:Glycyrrhetinic Acid