Membrane potential and sodium flux in neuroblastoma X glioma hybrid cells: Effects of amiloride and serum

The Na + uptake into neuroblastoma x glioma hybrid cells was measured in Hepes‐buffered EMEM containing 10% calf serum and 5 mM ouabain in the presence and absence of amiloride (1.0 mM). Amiloride was found to markedly inhibit net Na + influx (by approximately 50%). Examination of the effect of amiloride on net Na + influx in the absence of calf serum revealed that a significant amiloride‐sensitive Na + influx remains even under serum‐deprived conditions, although the degree of amiloride inhibition (35%) is substantially lower than that found in the presence of serum. The amiloride‐insensitive portion of Na + influx was found to be independent of serum effects. Estimation of resting membrane potential was made by measurement of the steady state distribution of the lipophilic cation, TPP + , in the presence and absence of amiloride. A large, immediate increase in TPP + uptake, indicative of a membrane hyperpolarization, was seen upon addition of amiloride. Determination of the effect of amiloride on resting membrane potential of serum‐deprived cells showed that cells are hyperpolarized to a greater extent in the presence than in the absense of amiloride, and that serum exerts a depolarizing effect on the cells. Thus, serum‐stimulation of Na + influx results in a depolarization of resting membrane potential, while amiloride inhibition of Na + influx causes a hyperpolarization. These data strongly suggest that NG108‐15 cells possess an electrogenic Na + influx pathway that is sensitive to amiloride inhibition and enhanced by serum.