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 absence 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.