Intracellular Na+ activity in cultured mouse oligodendrocytes

Na(+)-selective double-barrelled microelectrodes were used to measure the intracellular Na+ activity (aiNa) and membrane potential (Em) in oligodendrocytes from cultures of embryonic mouse spinal cord. In Na(+)-free solutions aiNa rapidly fell from its baseline of about 15 mM to values below 1 mM. Elevation of the K+ concentration in the bath ([K+]o) from 5.4 to 15 or 50 mM elicited an aiNa decrease of 4.7 or 9.0 mM, respectively. Ouabain blocked the aiNa decrease in response to 50 mM K+ by 37%. Bath application of 1 mM glutamate resulted in a membrane depolarization of 4.5 mV and a concomitant rise of aiNa by 8.6 mM. aiNa increased by approximately 11 mM after washout of a solution containing 20 mM NH4+. This aiNa increase was not blocked by amiloride, excluding a major contribution of a Na+/H+ antiporter. We conclude that, in cultured oligodendrocytes, transmembraneous Na+ movements are involved in pH regulation, glutamate causes an influx of Na+, and that the Na+/K+ pump and passive KCl uptake contribute to K+ accumulation.