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Resting Membrane Potential -
Gibbs-Donnan Equilibrium
In addition to its role in maintaining the cytoplasmic Na+ and K+ concentrations (see Maintenance of the Membrane Potential), the Na+/K+ ATPase is also responsible for the regulation of cell volume. Thus, another consequence of inhibiting the Na+/K+ ATPase is cell swelling. In the absence of a properly functioning pump, Na+ constantly enters the cell down its electrochemical gradient. Because water follows Na+ by osmosis, this leads to cell swelling. In fact, when the Na+/K+ ATPase is inhibited, cells gain Na+, Ca2+, Cl, and water, and they lose K+. When the Na+/K+ ATPase is inhibited, a new equilibrium is established for various ions. This new equilibrium is referred to as the Gibbs-Donnan equilibrium. The Gibbs-Donnan equilibrium will not be discussed here in detail. It will only be mentioned that the Gibbs-Donnan equilibrium occurs in a cell that has no ATP supply to power the Na+/K+ ATPase (for example, under hypoxic or anoxic conditions, or by addition of metabolic poisons such as dinitrophenol or cyanide), or a cell whose Na+/K+ ATPase is inhibited (e.g., by oubain). Under these conditions, a new equilibrium is passively reached. The equilibrium is strongly influenced by the fact that the membrane of most cells is permeable to Na+, K+, Cl, and Ca2+ (the relative permeability is not important here), but it is impermeable to large cytoplasmic proteins. At Gibbs-Donnan equilibrium, this unequal permeability leads to a greater total solute (ions + proteins) concentration in the cell than in the extracellular environment. The greater osmolality in the cell, thus, draws water into the cell by osmosis, leading to cell swelling
The Gibbs-Donnan equilibrium is also important in establishing the distribution of ions and solutes across the capillary endothelium in tissue capillaries, and results from the fact that the capillary endothelium is permeable to ions and small solutes, but not to large proteins.






Posted: Saturday, February 15, 2014
Last updated: Saturday, November 25, 2017