Rapid ionic events and the initiation of growth in serum-stimulated neuroblastoma cells.

Rapid effects of serum stimulation on electrical and ionic membrane properties and their relationship to the initiation of DNA synthesis and cell division have been investigated in mouse N1E-115 neuroblastoma cells. Addition of 10% fetal calf serum to serum-deprived N1E-115 cells results in the initiation of DNA synthesis after a lag of approximately 10 hr. The earliest events following serum addition include: transient membrane potential and resistance changes, detectable within seconds and lasting 5--10 min; a persistent increase in the initial rate of 22Na+ influx, the major part of which is not of electrodiffusional origin, and which is potentiated by weak acid anions; and an external Na+-dependent increase in the rate of the Na+, K+ pump. In the absence of serum the stimulation of the Na+, K+ pump can be mimicked by increasing net Na+ influx with monensin or neurotoxins. Growth-depleted serum fails to induce any of the electrical and ionic events. The diuretic amiloride (0.4 mM) inhibits serum-induced Na+ influx, Na+, K+ pump stimulation and DNA synthesis, but does not affect the electrical response or the basal influx rates. The results suggest that serum growth factors act, at least in part, by stimulating an electroneutral, amiloride-sensitive Na+/H+ exchange mechanism. The enhanced Na+ influx then results in the observed stimulation of the Na+, K+ pump, while the simultaneous efflux of protons may raise the intracellular pH.