title: Acid efflux from retinal glial cells generated by sodium bicarbonate cotransport.
abstract: Sodium bicarbonate cotransport was studied in freshly dissociated Müller cells of the salamander retina. Variations in intracellular and extracellular pH evoked extracellular potassium concentration ([K+]o were recorded. Intracellular pH was measured by standard ratio imaging of the pH-sensitive dye BCECF, whereas extracellular pH was monitored by imaging BCECF fixed to coverslips under dissociated cells. Increasing [K+]o from 2.5 to 50 mM resulted in an intracellular alkalinization. The rate of alkalinization, 0.047 pH units/min, was reduced to 42% of control when HEPES was substituted for HCO3- and was reduced to 36% of control by the addition of 0.5 mM DIDS, a Na+/HCO3- cotransport blocker. The K(+)-evoked alkalinization was Cl(-)-independent and was not substantially reduced by amiloride or bumetanide. Increasing [K+]o to 50 mM also produced a rapid extracellular acidification, 0.01 to 0.05 pH units in amplitude. HEPES substitution and addition of 0.5 mM DIDS reduced the acidification to 7-8% of control, respectively. These results confirm the presence of a Na+/HCO3- cotransport system in salamander Müller cells and provide definitive evidence that glial cells can generate an extracellular acidification when [K+]o is increased. The K(+)-evoked extracellular acidification measured beneath cell endfeet was 304% of the amplitude of the acidification beneath cell somata, confirming that cotransporter sites are preferentially localized to the endfoot. The carbonic anhydrase inhibitor benzolamide (2 x 10(-5) M), which is poorly membrane permeant, increased the K(+)-evoked extracellular acidification to 269% of control, demonstrating that salamander Müller cells possess extracellular carbonic anhydrase.
authors.affiliation: Department of Physiology, University of Minnesota, Minneapolis 55455, USA.
authors.forename: E A
authors.initials: EA
authors.lastname: Newman
mesh_terms: D000143:Acids / Q000378:metabolism*; D000468:Alkalies / Q000378:metabolism; D000557:Ambystoma; D000818:Animals; D001692:Biological Transport / Q000502:physiology; D002256:Carbonic Anhydrases / Q000378:metabolism / Q000494:pharmacology; D004594:Electrophysiology; D005452:Fluoresceins; D005456:Fluorescent Dyes; D006863:Hydrogen-Ion Concentration; D007091:Image Processing, Computer-Assisted; D009457:Neuroglia / Q000737:chemistry / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D012160:Retina / Q000166:cytology* / Q000201:enzymology; D017693:Sodium Bicarbonate / Q000378:metabolism*
chemical_list: D000143:Acids; D000468:Alkalies; D005452:Fluoresceins; D005456:Fluorescent Dyes; C043829:2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein; D017693:Sodium Bicarbonate; D002256:Carbonic Anhydrases; D011188:Potassium