Intracellular Na + activity in cultured mouse oligodendrocytes

Na + ‐selective double‐barrelled microelectrodes were used to measure the intracellular Na + activity (a i Na ) and membrane potential (E m ) in oligodendrocytes from cultures of embryonic mouse spinal cord. In Na + ‐free solutions a i Na 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 a i Na decrease of 4.7 or 9.0 mM, respectively. Ouabain blocked the a i Na 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 a i Na by 8.6 mM. a i Na increased by approximately 11 mM after washout of a solution containing 20 mM NH 4 + . This a i Na 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 KCI uptake contribute to K + accumulation.