| Abstract: | We have studied the induction of an amiloride-sensitive sodium influx into C6 glioma, NIE, and NB2A neuroblastoma cell lines. In late log phase, cells grown continuously in the presence of 10% fetal calf serum showed Na+ influxes of approximately 25–30 nmol/mg protein min; < 5% of this flux was inhibited by amiloride. Removal of serum for 24 h caused a decrease in the total Na+ influx to 15–20 nmol/mg protein/min. Upon readdition of serum to the incubation medium, there was an increase in total Na+ influx, depending on the cell type, of 20–400% within 2 min. This increment in Na+ influx represented an increase in amiloride-sensitive Na+ transport with an apparent K′, of 0.4 mM. By adding serum back at various times after serum deprivation, it was determined that 4 h was required to observe a detectable increase in the amiloride-sensitive Na+ flux. Thus, serum removal results in the induction of the amiloride transport system which, however, remains latent until the reintroduction of serum to the medium. Addition of 5 μg/ml of cycloheximide blocked the increase in Na+ transport, indicating that de novo protein synthesis mediated this serum deprivation–induced increase in Na+ transport. Moreover, inhibition of de novo lipid synthesis by 0.1 mM fenfluramine also blocked the induction of this transport activity, suggesting that a coordinated synthesis of lipid and protein is required for the expression of this sodium transport site. We have also found that this serum stimulated Na+ influx did not saturate with Na+ concentration, up to 140 mM. Also, among commonly used inhibitors of passive Na+ entry into epithelial tissues, only amiloride was capable of inhibiting this transport system in these neural cell lines. |
