Fig. 3
Application of purified GEPII 1.0 to determine cell viability and cell growth. a Scheme demonstrating cellular K+ loss upon cell death (left arrow) or K+ uptake of proliferating cells (right arrow). b [K+] over time determined with GEPII 1.0 within the supernatant of INS-1 cells cultured either in the presence of 10 mM glucose (white circles and black line, n = 6 ± SD) or 10 mM 2-deoxyglucose (red squares and line, n = 6 ± SD). As indicated, 50 µM digitonin was applied after 14 h. *P = 0.0178, ***P < 0.0005, unpaired t-test. c Representative FRET ratio signals over time of recombinant GEPII 1.0 within the supernatant of HeLa cells (upper red curve) were recorded simultaneously with morphological alterations (cell shrinkage) of cells (lower blue curve) on an inverted fluorescence microscope. As indicated, 30 µM digitonin was added to induce cell necrosis. d [K+] of cell culture medium over time determined with purified GEPII 1.0 (red circles, n = 5 ± SD, *P = 0.05, ***P < 0.005, one-way ANOVA test with Tukey’s Multiple Comparison Test) during bacterial (E. coli) proliferation. Cell density was recorded in parallel by measuring the optical density at 600 nm (OD600, white circles, n = 5 ± SD). As indicated [K+]ex and OD600 were determined after heating of the cells (75 °C for 1 h). e Increases of bacterial cell numbers (∆Cell number) were plotted against the respective decreases of the extracellular K+ concentrations (∆[K+]ex)
