The Chloride Pump

Because the resting membrane potential of a cell is not at either the sodium or potassium equilibrium potentials, there is a continuous net flux of sodium across the membrane. As we have just seen, metabolic energy must be expended in order to maintain the ion gradients for sodium and potassium. What about chloride? The equilibrium potential for chloride given the internal and external concentrations shown in Table 2-1 would be about -80 mV, but the resting membrane potential is about -71 mV. Thus, we would expect that there would be a steady influx of chloride into the cell because of this imbalance between the electrical and concentration gradients for chloride. Eventually, this influx would raise the internal chloride concentration to the point where the new chloride equilibrium potential would be -71 mV, the same as the resting membrane potential. At that point the concentration gradient for chloride would be reduced sufficiently to come into balance with the resting membrane potential. We can calculate from the Nernst equation that chloride would have to rise to about 7.5 mM from its usual 5 mM in order for this new equilibrium state to be established.

In some cells, this does indeed appear to happen: chloride reaches a new equilibrium governed by the resting membrane potential of the cell. (The cell would also gain the same small amount of potassium; because there is so much potassium inside, a change of a few millimolar in potassium concentration makes very little change in the potassium equilibrium potential, however.) In other cells, however, the chloride equilibrium potential remains different from the resting membrane potential, just as the sodium and potassium equilibrium potentials remain different from Em. The only way this nonequilibrium condition can be maintained is by expending energy to keep the internal chloride constant that is, there must also be a chloride pump similar in function to the sodium-potassium pump. In most cells, the chloride pump moves chloride ions out of the cell, so that the chloride equilibrium potential remains more negative than the resting membrane potential. In a few cases, however, an inwardly directed chloride pump has been discovered. Less is known about the molecular machinery of the chloride pump than that of the sodium-potassium pump. It is thought to involve an ATPase in some instances, so that the energy released by hydrolysis of ATP is the immediate driving energy for the pumping. In other cases, the pump may use energy stored in gradients of other ions to drive the movement of chloride.

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