Glucocorticoid hormone renders rat glioma cells dependent on high concentrations of external Ca 2+ for growth

C6 rat glioma cells, like other tumor cells, grow optimally in Ca 2+ ‐ or Mg 2+ ‐ depleted 5% serum medium. However, the glucocorticoid hormone hydrocortisone renders these cells dependent on high concentrations of external Ca 2+ for growth. Upon Ca 2+ deprivation (30–80 μM Ca 2+ medium) hydrocortisone‐treated C6 cells undergo reversible cell‐cycle arrest at G 0 /G 1 phase. This effect is specific for glucocorticoid hormones and occurs at physiological concentrations. Growth restimulation of Ca 2+ ‐deprived, hydrocortisone‐treated C6 cells by bovine pituitary growth factors or serum growth factors only takes place if the external Ca 2+ concentration is increased. On the other hand, C6 cell growth requirement for external Mg 2+ was not increased by the glucocorticoid hormone treatment. A minimum of 80 μM of external Mg 2+ is required to keep cells adhered and spread in Ca 2+ ‐depleted (30 μM) 5% serum medium; in high Ca 2+ concentration (1.8 mM), Mg 2+ is not required for adhesion or spreading. Thus, the hormone hydrocortisone renders the cell cycle of C6 glioma cells controllable by the levels of external Ca 2+ , a minimal external Mg 2+ being necessary to warrant normal adhesion.