Purification and Application of Genetically Encoded Potassium Ion Indicators for Quantification of Potassium Ion Concentrations within Biological Samples

Vital cells maintain a steep potassium ion (K + ) gradient across the plasma membrane. Intracellular potassium ion concentrations ([K + ]) and especially the [K + ] within the extracellular matrix are strictly regulated, the latter within a narrow range of ∼3.5 to 5.0 mM. Alterations of the extracellular K + homeostasis are associated with severe pathological alterations and systemic diseases including hypo‐ or hypertension, heart rate alterations, heart failure, neuronal damage or abnormal skeleton muscle function. In higher eukaryotic organisms, the maintenance of the extracellular [K + ] is mainly achieved by the kidney, responsible for K + excretion and reabsorption. Thus, renal dysfunctions are typically associated with alterations in serum‐ or plasma [K + ]. Generally, [K + ] quantifications within bodily fluids are performed using ion selective electrodes. However, tracking such alterations in experimental models such as mice features several difficulties, mainly due to the small blood volume of these animals, hampering the repetitive collection of sample volumes required for measurements using ion selective electrodes. We have recently developed highly sensitive, genetically encoded potassium ion indicators, the GEPIIs, applicable for in vitro determinations of [K + ]. In addition to the determination of [K + ] within bodily fluids, GEPIIs proved suitable for the real‐time visualization of cell viability over time and the exact determination of the number of dead cells. © 2019 The Authors.