Cation leak in Xenopus laevis oocytes expressing wild-type or mutant glut1. (A) Intracellular sodium and potassium ion concentrations were measured by flame photometry on washed, extracted oocytes (3 replicates of 5 oocytes per condition) after 72 hours of incubation at 19°C in MBS containing ouabain (0.5mM) and bumetanide (5μM). Data, expressed in μmol/g of dry weight, are the mean values ± SEMs for 4 separate experiments; n = 12. *P < .05, **P < .01, and ***P < .001. (B) Li⁺ influx (as a surrogate for Na⁺) was measured 2 days after injection. Oocytes (7 per condition) were incubated for 2 hours at 19°C in medium in which NaCl was substituted by LiNO₃ in the presence of ouabain (0.5mM) and bumetanide (5μM). Li⁺ content in each oocyte extract was measured by atomic absorption spectrometry with a Perkin Elmer AAS 3110. The graph shows the data from 7 repeat experiments. To normalize the data, in each individual experiment NI was taken as 100%, and the Li uptake in wild-type, glut1-G286D, and glut1-ΔI435 was expressed as a percentage of NI. The 7 values were then averaged and plotted, ± SEM; n = 7. *P < .05, **P < .01, and ***P < .001. Inset: Glut1 expression levels in Xenopus laevis oocytes were assessed by immunoblotting with the use of an antibody to the C-terminal region of glut1.