Fluorescent indicators of ion concentrations.

TLDR: Ion-concentration gradients across cell membranes are central to any understanding of biological energetics and signal transduction and the characterization of ion gradients requires knowing both extracellular and intracellular ion activities and free concentrations.

Abstract: Publisher Summary Ion-concentration gradients across cell membranes are central to any understanding of biological energetics and signal transduction. The proton electrochemical gradient is generally accepted to be the key intermediary linking electron transport to adenosine triphosphate (ATP) synthesis in bacteria, chloroplasts, and mitochondria. Cytoplasmic pH is a powerful regulator of enzyme activities and is known to alkalinize in many types of cells undergoing mitogenic activation. The characterization of ion gradients requires knowing both extracellular and intracellular ion activities and free concentrations. The Na+ gradient is the energy source for secondary active transport of nutrients into animal cells and many co- or counter-transport systems for other ions. Cl- gradients are vital to the function of epithelia, the control of cell volume, and to the actions of many inhibitory neurotransmitters. Determination of extracellular ions usually poses much less of a problem than intracellular, because the extracellular medium is much more accessible. Extracellular concentrations of all the simple inorganic ions except K+ are comparable to or higher than intracellular, so detection is easier extracellularly.