Figure 5-14 illustrates the tools at the disposal of a prototypic cell for managing its intracellular composition. Cells in different tissues—and even different cell types within the same tissue—have different complements of channels and transporters. Epithelial cells and neurons may segregate specific channels and transporters to different parts of the cell (e.g., apical versus basolateral membrane or axon versus soma/dendrite). Thus, different cells may have somewhat different intracellular ionic compositions.
FIGURE 5-14 Ion gradients, channels, and transporters in a typical cell.
The Na-K pump keeps [Na+] inside the cell low and [K+] high
The most striking and important gradients across the cell membrane are those for Na+ and K+. Sodium is the predominant cation in ECF, where it is present at a concentration of ~145 mM (see Fig. 5-14). Na+ is relatively excluded from the intracellular space, where it is present at only a fraction of the extracellular concentration. This Na+ gradient is maintained primarily by active extrusion of Na+ from the cell by the Na-K pump (see Fig. 5-14, No. 1). In contrast, potassium is present at a concentration of only ~4.5 mM in ECF, but it is the predominant cation in the intracellular space, where it is accumulated ~25- to 30-fold above the outside concentration. Again, this gradient is the direct result of primary active uptake of K+ into the cell by the Na-K pump. When the Na-K pump is inhibited with ouabain, [Na+]i rises and [K+]i falls.
In addition to generating concentration gradients for Na+ and K+, the Na-K pump plays an important role in generating the inside-negative membrane voltage, which is ~60 mV in a typical cell. The Na-K pump accomplishes this task in two ways. First, because the Na-K pump transports three Na+ ions out of the cell for every two K+ ions, the pump itself is electrogenic. This electrogenicity causes a net outward current of positive charge across the plasma membrane and tends to generate an inside-negative Vm. However, the pump current itself usually makes only a small contribution to the negative Vm. Second, and quantitatively much more important, the active K+ accumulation by the Na-K pump creates a concentration gradient that favors the exit of K+ from the cell through K+ channels (see Fig. 5-14, No. 2). The tendency of K+ to exit through these channels, with unmatched negative charges left behind, is the main cause of the inside-negative membrane voltage. When K+ channels are blocked with an inhibitor such as Ba2+, Vmbecomes considerably less negative (i.e., the cell depolarizes). In most cells, the principal pathway for current flow across the plasma membrane (i.e., the principal ionic conductance) is through K+ channels. We discuss the generation of membrane voltage in Chapter 6.
The inside-negative Vm, together with the large concentration gradient for Na+, summates to create a large, inwardly directed Na+ electrochemical gradient that strongly favors passive Na+ entry. Given the large amount of energy that is devoted to generation of this favorable driving force for Na+ entry, one might expect that the cell would permit Na+ to move into the cell only through pathways serving important physiological purposes. The simple passive entry of Na+ through channels—without harnessing of this Na+ entry for some physiological purpose—would complete a futile cycle that culminates in active Na+extrusion. It would make little teleological sense for the cell to use up considerable energy stores to extrude Na+ only to let it passively diffuse back in with no effect. Rather, cells harness the energy of Na+ entry for three major purposes:
1. In certain epithelial cells, amiloride-sensitive Na+ channels (ENaCs) are largely restricted to the apical or luminal surface of the cell (see Fig. 5-14, No. 3), and the Na-K pumps are restricted to the basolateral surface of the cell. In this way, transepithelial Na+ transport takes place rather than a futile cycling of Na+ back and forth across a single plasma membrane.
2. In excitable cells, passive Na+ entry occurs through voltage-dependent Na+ channels (see Fig. 5-14, No. 4) and plays a critical role in generation of the action potential. In such cells, Na+ is cycled at high energy cost across the plasma membrane for the important physiological purpose of information transfer.
3. Virtually every cell in the body uses the electrochemical Na+ gradient across the plasma membrane to drive the secondary active transport of nutrients and ions (see Fig. 5-14, No. 5).
The Ca pump and the Na-Ca exchanger keep intracellular [Ca2+] four orders of magnitude lower than extracellular [Ca2+]
Whereas the concentration of Ca2+ in the extracellular space is ~1 mM (10−3 M), that in the ICF is only ~100 nM (10−7 M), a concentration gradient of 104-fold. Because of the inside-negative membrane voltage of a typical cell and the large chemical gradient for Ca2+, the inwardly directed electrochemical gradient for Ca2+ across the plasma membrane is enormous, far larger than that for any other ion. Many cells have a variety of Ca2+ channels through which Ca2+ can enter the cell (see Fig. 5-14, No. 6). In general, Ca2+ channels are gated by voltage (see pp. 189–190) or by humoral agents (see pp. 323–324) so that rapid Ca2+ entry into the cell occurs only in short bursts. However, given the existence of pathways for passive Ca2+ transport into cells, we may ask what transport mechanisms keep [Ca2+]i low and thus maintain the enormous Ca2+ electrochemical gradient across the plasma membrane.
Ca Pump (SERCA) in Organelle Membranes
Ca pumps (ATPases) are present on the membranes that surround various intracellular organelles, such as the sacroplasmic and endoplasmic reticulum (see Fig. 5-14, No. 7). These pumps actively sequester cytosolic Ca2+ in intracellular stores. These stores of Ca2+ can later be released into the cytoplasm in bursts as part of a signal-transduction process in response to membrane depolarization or humoral agents. Even though Ca2+ sequestration in intracellular stores is an important mechanism for regulating [Ca2+]i in the short term, there is a limit to how much Ca2+ a cell can store. Therefore, in the steady state, Ca2+ extrusion across the cell membrane must balance the passive influx of Ca2+.
Ca Pump (PMCA) on the Plasma Membrane
The plasma membranes of most cells contain a Ca pump that plays a major role in extruding Ca2+ from the cell (see Fig. 5-14, No. 8). It would seem that rising levels of intracellular Ca2+ would stimulate the Ca pump to extrude Ca2+ and thereby return [Ca2+]i toward normal. Actually, the pump itself is incapable of this type of feedback control; because it has such a high Km for [Ca2+]i, the pump is virtually inactive at physiological [Ca2+]i. However, as [Ca2+]i rises, the Ca2+ binds to a protein known as calmodulin (CaM; see p. 60), which has a high affinity for Ca2+. The newly formed Ca2+-CaM binds to the Ca pump, lowers the pump's Km for [Ca2+]i into the physiological range, and thus stimulates Ca2+ extrusion. As [Ca2+]i falls, Ca2+-CaM levels inside the cell also fall, so that Ca2+-CaM dissociates from the Ca pump and the pump is thereby returned to its inactive state. At resting [Ca2+]i levels of ~100 nM, the Ca pump is the major route of Ca2+ extrusion.
Na-Ca Exchanger (NCX) on the Plasma Membrane
The Na-Ca exchanger (see Fig. 5-14, No. 9) plays a key role in extruding Ca2+ only when [Ca2+]i rises substantially above normal levels. Thus, NCX is especially important in restoring low [Ca2+]i when large influxes of Ca2+ occur. This property is most notable in excitable cells such as neurons and cardiac muscle, which may be challenged with vast Ca2+ influxes through voltage-gated Ca2+ channels during action potentials.
In most cells, [Cl−] is modestly above equilibrium because Cl− uptake by the Cl-HCO3 exchanger and Na/K/Cl cotransporter balances passive Cl− efflux through channels
The [Cl−] in all cells is below the [Cl−] in the extracellular space. Virtually all cells have anion-selective channels (see Fig. 5-14, No. 10) through which Cl− can permeate passively. In a typical cell with a 60-mV inside-negative membrane voltage, [Cl−]i would be a tenth that of [Cl−]o if Cl− were passively distributed across the plasma membrane. Such is the case for skeletal muscle. However, for most cell types, [Cl−]i is approximately twice as high as that predicted for passive distribution alone, which indicates the presence of transport pathways that mediate the active uptake of Cl− into the cell. Probably the most common pathway for Cl− uptake is the Cl-HCO3 exchanger (see Fig. 5-14, No. 11). Because is several-fold higher than if it were passively distributed across the cell membrane, the outwardly directed electrochemical potential energy difference for can act as a driving force for the uphill entry of Cl− through Cl-HCO3 exchange. Another pathway that can mediate uphill Cl− transport into the cell is the Na/K/Cl cotransporter (see Fig. 5-14, No. 12), which is stimulated by low [Cl−]i.
Given the presence of these transport pathways mediating Cl− uptake, why is [Cl−]i only ~2-fold above that predicted for passive distribution? The answer is that the passive Cl− efflux through anion-selective channels in the plasma membrane opposes Cl− uptake mechanisms. Another factor that tends to keep Cl− low in some cells is the K/Cl cotransporter. This KCC (see Fig. 5-14, No. 13), driven by the outward K+gradient, tends to move K+ and Cl− out of cells. Thus, the K+ gradient promotes Cl− efflux both by generating the inside-negative Vm that drives Cl− out of the cell through channels and by driving K/Cl cotransport.
The Na-H exchanger and Na+-driven transporters keep the intracellular pH and  above their equilibrium values
H+, , and CO2 within a particular compartment are generally in equilibrium with one another. Extracellular pH is normally ~7.4, is 24 mM, and is ~40 mm Hg. In a typical cell, intracellular pH is ~7.2. Because [CO2] is usually the same on both sides of the cell membrane, can be calculated to be ~15 mM. Even though the ICF is slightly more acidic than the ECF, pHi is actually much more alkaline than it would be if H+ and were passively distributed across the cell membrane. H+ can enter the cell passively and can exit the cell passively, although both processes occur at a rather low rate. H+ can permeate certain cation channels and perhaps H+-selective channels (see Fig. 5-14, No. 14), and moves fairly easily through most Cl− channels (see Fig. 5-14, No. 15). Because a membrane voltage of −60 mV is equivalent as a driving force to a 10-fold concentration gradient of a monovalent ion, one would expect [H+] to be 10-fold higher within the cell than in the ECF, which corresponds to a pHithat is 1 pH unit more acidic than pHo. Similarly, one would expect to be only one tenth of . The observation that pHi and are maintained higher than predicted for passive distribution across the plasma membrane indicates that cells must actively extrude H+ or take up .
The transport of acid out of the cell or base into the cell is collectively termed acid extrusion. In most cells, the acid extruders are secondary active transporters that are energized by the electrochemical Na+gradient across the cell membrane. The most important acid extruders are the Na-H (see Fig. 5-14, No. 16) and the Na+-driven Cl-HCO3 exchangers (see Fig. 5-14, No. 17), as well as the Na/HCO3 cotransporters with stoichiometries of 1 : 2 and 1 : 1 (see Fig. 5-11E, F). These transport systems are generally sensitive to changes in pHi; they are stimulated when the cell is acidified and inhibited when the cell is alkalinized. Thus, these transporters maintain pHi in a range that is optimal for cell functioning. Less commonly, certain epithelial cells that are specialized for acid secretion use V-type H pumps (see Fig. 5-14, No. 18) or H-K pumps on their apical membranes to extrude acid. These epithelia include the renal collecting duct and the stomach. As noted above, virtually all cells have V-type H pumps on the membranes surrounding such intracellular organelles as lysosomes, endosomes, and Golgi.
Because most cells have powerful acid-extrusion systems, one might ask why the pHi is not far more alkaline than ~7.2. Part of the answer is that transport processes that act as acid loaders balance acid extrusion. Passive leakage of H+ and through channels, as noted above, tends to acidify the cell. Cells also have transporters that generally move out of cells. The most common is the Cl-HCO3exchanger (see Fig. 5-14, No. 11). Another is the electrogenic Na/HCO3 cotransporter with an stoichiometry of 1 : 3 (see Fig. 5-14, No. 19), which moves out of the cell across the basolateral membrane of renal proximal tubules.