Potassium-induced stimulation of glutamate uptake in mouse cerebral astrocytes: the role of intracellular pH.

The Na(+)-glutamate cotransporters are believed to countertransport OH- and K+. Previous evidence that the velocity of glutamate uptake can exceed the acid extrusion capacity of astrocytes raised the question of whether intracellular pH can become rate limiting for glutamate uptake. Cytoplasmic buffering capacity and acid extrusion in astrocytes are partially HCO3- dependent. Also, it was reported recently that raising extracellular [K+] alkalinizes astrocyte cytoplasm by an HCO3- dependent mechanism. Here, we have compared glutamate uptake in HCO3(-)-buffered and HCO3(-)-depleted solutions at varying [K+]. We observed a pronounced stimulation of glutamate uptake by extracellular K+ (3-24 mM) that was substantially HCO3- dependent and affected preferentially the uptake of high concentrations (> 25 microM) of glutamate. Stimulation of uptake by low extracellular [K+] (1.5-3 mM) was less dependent on HCO3-. Potassium-induced stimulation of uptake was weaker in rat astrocyte cultures than in mouse. The effects of Ba2+ and amiloride on glutamate uptake, as well as the HCO3(-)-dependent stimulatory effects of K+ and the species difference, all related consistently to effects on intracellular pH. The effects on uptake, however, were much larger than predicted by the associated changes in electrochemical gradient of OH-. A "bimodal" scheme for glutamate transport can account qualitatively for the observed correlation between intracellular pH and velocity of glutamate uptake.