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