Stimulation of K⁺ transport systems by Ha-ras

The expression of Ha-ras in quiescent NIH3T3 cells carrying a glucocorticoid-inducible human Ha-ras gene (Val-Gly mutation at codon 12) stimulates total ⁸⁶Rb⁺ influx. This effect is predominantly due to an elevated ⁸⁶Rb⁺ uptake through an ouabain-resistant, furosemide-sensitive system. The ouabain-sensitive Na⁺/K⁺-ATPase is less affected. The transport which is resistant to both inhibitors is not altered by Ha-ras. Overexpression of the Ha-ras proto-oncogene causes only a marginal increase in total ⁸⁶Rb⁺ uptake. The stimulation of the furosemide-sensitive influx by Ha-ras is paralleled by an increase in mean cell volume which can be inhibited by furosemide. A rapid stimulation of the furosemide-sensitive Rb⁺ influx is also observed after addition of bombesin to growth-arrested cells. Furosemide inhibits the mitogenic response after expression of Ha-ras or addition of bombesin. Both the Ha-ras and the bombesin-induced stimulation of the furosemide-sensitive Rb⁺ transport can be blocked by protein kinase C depletion or the protein kinase C inhibitor staurosporine. In contrast to bombesin-induced phosphatidylinositol-4,5-bisphosphate hydrolysis which is down-modulated by Ha-ras, the stimulation of the furosemide-sensitive Rb⁺ influx by bombesin is elevated in Ha-ras-expressing cells. This is in accordance with the increased mitogenic activity of bombesin in Ha-ras-expressing cells.