Measurement of Ion Fluxes and pH Gradients Across Cell Membranes

Movement of ions across cell membranes is associated with processes such as cell signaling, regulation of cell volume, maintenance of cell composition and pH, and energy transduction. Even in the erythrocyte, a very specialized cell with limited biological function, 11 different transport systems for Na⁺ and K⁺ have been identified (1 ). The different fluxes are characterized by their sensitivity to specific inhibitors, and can be monitored either by measuring the uptake and/or release of isotopes, such as ⁴² K or ⁸⁶ Rb, which usually acts as K-congener in cation transport systems, or by measuring the net changes in ionic composition (intra- or extracellularly) caused by the continued action of the transport system of interest. In practice, the isotope and bulk ion determination techniques often require similar experimental protocols, and they will be considered together here. Two approaches will be described: intermittent determination of isotope/cation composition of intra- and extracellular fluid (Section 2.1. . and 13 -3 ), and continuous recording of extracellular ionic composition (Section 2.2. and 3.2. ).