Acid efflux from retinal glial cells generated by sodium bicarbonate cotransport

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.