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ABSTRACT. The usefulness of Rb+ was investigated as an analog of K+ in membrane transport and in supporting the growth of HeLa cells. On replacement of extracellular K+ by Rb+, intracellular Rb+ content increased linearly at 15 min. K+ content declined in contrast with Rb+, but Na+ content was unaffected. In Rb+-preloaded cells, Rb+ and K+ were exchanged after replacement of extracellular Rb+ with K. A kinetic analysis of intracellular Rb+ content showed that most K+ was exchangeable with Rb+, but a small part of K+ was not. Both Rb+ and K+ influxes were equally inhibited by ouabain or furosemide. Rb+ efflux was repressed by furosemide, whereas K+ efflux was not influenced. When 90 % of K+ was replaced by Rb+ in serum-free chemically defined medium, cell growth and protein synthesis were inhibited. After total replacement of extracellular K+ by Rb+, cell growth ceased and cellular protein content decreased, while a small part of intracellular K+ remained unexchanged. HeLa cells seemed to accumulate Rb+ in preference to K+.

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