Journal of Neuroscience, Vol 11, 3972-3983, Copyright © 1991 by Society for Neuroscience
Sodium-bicarbonate cotransport in retinal Muller (glial) cells of the salamander
EA Newman
Department of Physiology, University of Minnesota, Minneapolis 55455.
An electrogenic Na+/HCO3- cotransport system was studied in freshly
dissociated Muller cells of the salamander retina. Cotransporter currents
were recorded from isolated cells using the whole-cell, voltage-clamp
technique following the block of K+ conductance with external Ba2+ and
internal Cs+. At constant pHo, an outward current was evoked when
extracellular HCO3- concentration was raised by pressure ejecting a
HCO3(-)-buffered solution onto the surface of cells bathed in nominally
HCO3(-)-free solution. The HCO3(-)-evoked outward current was reduced to
4.4% of control by 0.5 mM DIDS (4,4'-
diisothiocyanatostilbene-2,2'-disulfonate), to 28.8% of control by 2 mM
DNDS (4,4'-dinitrostilbene-2,2'-disulfonate), and to 28.4% of control by 2
mM harmaline. Substitution of choline for Na+ in bath and ejection
solutions reduced the response to 1.3% of control. Bicarbonate-evoked
currents of normal magnitude were recorded when methane sulfonate was
substituted for Cl- in bath, ejection, and intracellular solutions.
Similarly, an outward current was evoked when extracellular Na+
concentration was raised in the presence of HCO3-. The Na(+)-evoked
response was reduced to 16.2% of control by 2 mM DNDS and was abolished by
removal of HCO3- from bath and ejection solutions. Taken together, these
results (block by stilbenes and harmaline, HCO3- and Na+ dependence, Cl-
independence) indicate that salamander Muller cells possess an electrogenic
Na+/HCO3- cotransport system. Na+/HCO3- cotransporter sites were localized
primarily at the endfoot region of Muller cells. Ejection of HCO3- onto the
endfoot evoked outward currents 10 times larger than currents evoked by
ejections onto the opposite (distal) end of the cell. The reversal
potential of the cotransporter was determined by DNDS block of cotransport
current. In the absence of a transmembrane HCO3- gradient, the reversal
potential varied systematically as a function of the transmembrane Na+
gradient. The reversal potential was -0.1 mV for a [Na+]o:[Na+]i ratio of
1:1 and -25.2 mV for a Na+ gradient ratio of 7.4:1. Based on these values,
the estimated stoichiometry of the cotransporter was 2.80 +/- 0.13:1 (HCO3-
:Na+). Possible functions of the glial cell Na+/HCO3- cotransporter,
including the regulation of CO2 in the retina and the regulation of
cerebral blood flow, are discussed.
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