Stimulation and inhibition can be achieved by several methods. If there's a formula to this, care to share?
The sodium pump is by itself electrogenic, three Na+ out for every two K+ that it imports. So if you block all sodium pump activity in a cell, you would see an immediate change in the membrane potential because you remove a hyperpolarizing current, in other words, the membrane potential becomes less negative. BUT the direct electrogenic contribution of the sodium pump to the membrane potential is tiny, so you will only see an immediate depolarization of a few mV.
Over a long period of time, things get a little more complicated. The sodium pump maintain the gradient of high extracellular Na vs low intracellular Na, and high intracellular K vs low extracellular K. The larger part of the membrane potential is due to a relatively high membrane permeability to K+ (through K+ channels). K+ flows out of the cell because of the concentration gradient, but as it flows out, the inside of the cell become increasingly negative (positive ions leave) to an extent where the negative electrical potential draws K+ back into the cell at the same rate as it leaves due to the concentration gradient. This is the basis of the membrane potential, and it only exist because the sodium pump maintains the concentration gradients of Na and K. So over a longer period of time, inhibition of the sodium pump remove the concentration gradient and hence the membrane potential.
Michael Jakob Voldsgaard Clausen
The change in membrane potential due to blocking the Na+/K+ pump is not immediate, but takes minutes (see Miura D.S. and Rosen M.R. 1978 for recordings from cardiac Purkinje fibers showing the temporal dynamics). By contrast, changing the K+ gradient (by altering the extracellular K+ concentration) changes the membrane potential immediately. That's because the principle mechanism for the resting membrane potential is the combination of selective ion permeability (principally to K+) and a corresponding ion concentration difference. The Na+/K+ pump sets up the latter condition, but it not directly responsible for the resting membrane potential (although its electrogenic stoichiometry contributes a minor part). Indeed, if a cell does not have a resting Na+ conductance (this is true for some cells), the resting membrane potential would be maintained indefinitely even if the Na+/K+ pump were eliminated, because there would be no Na+ leak, no run down of Na+ concentration difference, and no consequent run down of K+ concentration difference.
A mobile phone provides a good analogy. The K+ concentration difference (combined with selective K+ permeability) is the battery. The Na+/K+ pump is the charger. The charger is necessary to charge the battery, but once the battery is charged, you can unplug the charger and the phone continues to work for quite some time. Take out the battery, though, and the phone stops working immediately.
It should be noted that that the cell membrane potential is wentangled with the Na K ATPase fuction; that is the fubtional SYSTEM.
Prof. B. Momcilovic, MD
The point is that the Na+/K+ pump is necessary for establishing a resting membrane potential, but not sufficient. Selective K+ permeability and a K+ concentration gradient are both necessary and sufficient.
The Na+/K+ pump also has a number of other functions, such as cell volume regulation and intracellular signaling, and is coupled to the activity of other ion pumps.
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