Imaging of Cytosolic Sodium and pH Using SBFI and BCECF

Abstract

Although a great deal of attention has been focused on how the cytosolic Ca2+ concentration ([Ca2+]c) is temporally and spatially regulated within cells, tight regulation of other key ions is vital to the survival and normal function of cells. In particular, a considerable amount of their energy (20–70%) is expended via the plasma membrane Na+/K+ATPase to ensure that the cytosolic Na+ concentration ([Na+]c) is relatively low compared with that in the extracellular milieu (≈10 mM and 140 mM respectively). Correspondingly cytosolic K+ ([K+]c) is kept high compared with the extracellular K+ ([K+]o; 130 mM versus 3–5 mM respectively). Both excitable and non-excitable cells use these transmembrane gradients of Na+ and K+ to great effect. The influx of Na+ down its concentration gradient is coupled to the extrusion of Ca2+ and H+ via the Na+:Ca2+ and Na+:H+ exchangers,1,2 to the uptake of ions, sugars and amino acids via co-transporters,3 and to the rapid changes in membrane potential which propagate electrochemical signals in excitable cells.4 These latter depolarisations ultimately lead to elevation of [Ca2+]c in the presynaptic terminals, neuromuscular junctions and cell bodies via the opening of voltage-gated Ca2+ channels.5