Rubidium uptake was determined as described in Figueroa et al. (91). Specifically, cells were seeded in six-well plates and allowed to attach overnight in normal culture medium. Immediately before the assay, cells were washed three times with 1 ml of rubidium assay buffer [130 mM NaCl, 5 mM KCl, 2 mM CaCl2, 1 mM MgCl2, 0.5 mM NaH2PO4, 10 mM d-glucose, 12 mM NaHCO3, and 10 mM Hepes (pH 7.4)]. Cells were then incubated for 1 or 2 hours at 37°C in rubidium assay buffer containing 270 μM RbCl. Incubation was carried out with and without 1 mM ouabain. Subsequently, cells were washed three times in rubidium buffer and scraped into 1 ml of ultrapure water. Ten microliters of 100 parts per billion cerium was added as an internal control. Afterward, 1.4 g of 70% nitric acid (Sigma-Aldrich, 225711) was added, and samples were boiled at 100°C for 30 min. Then, 250 μl of H2O2 (Sigma-Aldrich, 95321) was added, and samples were boiled again at 100°C for 30 min. As a last step, samples were diluted 1:10 with 1% nitric acid before analysis by ICP-MS. The measured 85Rb+ concentrations were normalized to sulfur (S) concentration and the internal cerium control.

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