Effect of the serum concentration of the growth medium on the sodium pump site density of cultured HeLa cells.

The density of sodium pump sites in the plasma membrane of cultured HeLa cells has been measured as a function of the serum concentration of the cell growth medium. Growth in media containing increased concentrations of serum (from 1 to 20% v/v) leads to an increase in sodium pump site numbers (as measured by the specific binding of [3H]ouabain) and pump activity (as measured by the ouabain-sensitive 86Rb influx). Time-course studies show that (1) new sodium pump sites first appear some 3-6 h after transfer to medium containing an elevated serum concentration and (2) the serum-mediated increase in new sodium pump sites is completely abolished by the protein synthesis inhibitors, cycloheximide and actinomycin D. These results suggest that de novo protein synthesis is required for the development of the serum response. Preliminary characterization of the serum factor responsible for initiating the synthesis of new sodium pump sites indicates that the activity is associated with a high molecular weight serum fraction (greater than 50000). The different types of interaction seen between the serum effect and other experimental manoeuvres which initiate the synthesis of new pump sites (growth in low-K+ medium, growth in Li+ medium and pre-treatment with exogenous ATP) suggest that there may be more than one pathway for the control of sodium pump site synthesis in cultured HeLa cells.