Concept explainers
The GHK equation is sometimes abbreviated to exclude chloride, which plays a minimal role in membrane potential for most cells. In addition, because it is difficult to determine absolute membrane permeability values for Na+ and K+, the equation is revised to use the ratio of the two ion permeabilities, expressed as α = PNa/PK:
Thus, if you know the relative membrane permeabilities of the two ions and their intracellular (ICF) and extracellular (ECF) concentrations, you can predict the membrane potential for a cell.
Using a calculator with log function or the free online Nernst/Goldman equation simulator from the University of Arizona (www.nernstgoldman.physiology.arizona.edu/), do the following calculations.
- (a) A resting cell has an alpha (a) value of 0.025 and the following ion concentrations:
Na+: ICF = 5 mM, ECF = 135 mM
K+: ICF = 150 mM, ECF = 4 mM
What is the cell’s membrane potential?
- (b) The Na+ permeability of the cell in (a) suddenly increases so that α = 20. Now what is the cell’s membrane potential?
- (c) Mrs. Nguyen has high blood pressure, and her physician puts her on a drug whose side effect decreases her plasma (ECF) K+ from 4 mM to 2.5 mM. Using the other values in (a), calculate the membrane potential with decreased plasma K+.
- (d) The physician prescribes a potassium supplement for Mrs. Nguyen, who decides that if two pills are good, four must be better. Her plasma (ECF) K+ now goes to 6 mM. What happens to her membrane potential?
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