Thus far we have examined how cells transport solutes and H2O across their membranes and thereby control their intracellular composition. We now turn our attention to how the body controls the milieu intérieur, namely, the ECF that bathes the cells. Just as the cell membrane is the barrier between the ICF and ECF, epithelia are the barriers that separate the ECF from the outside world. In this subchapter, we examine the fundamental principles of how epithelial cells transport solutes and H2O across epithelial barriers.
An epithelium is an uninterrupted sheet of cells that are joined together by junctional complexes (see p. 43). These junctions serve as a selectively permeable barrier between the solutions on either side of the epithelium and demarcate the boundary between the apical and basolateral regions of the cell membrane. The apical and basolateral membranes are remarkably different in their transport function. This polarization allows the epithelial cell to transport H2O and selected solutes from one compartment to another. In other words, the epithelium is capable of vectorial transport. In many cases, transport of solutes across an epithelium is an active process.
Membranes may be called by different names in different epithelia. The apical membrane can be known as the brush border, the mucosal membrane, or the luminal membrane. The basolateral membrane is also known as the serosal or peritubular membrane.
The epithelial cell generally has different electrochemical gradients across its apical and basolateral membranes
Imagine an artificial situation in which an epithelium separates two identical solutions. Furthermore, imagine that there is no difference in voltage across the epithelium and no difference in hydrostatic pressure. Under these circumstances, the driving forces for the passive movement of solutes or H2O across the epithelium would be nil. Because the apical and basolateral membranes of the cell share the same cytosol, the electrochemical gradients across the apical and basolateral membranes would be identical.
However, this example is virtually never realistic for two reasons. First, because the composition of the “outside world” is not the same as that of ECF, transepithelial concentration differences occur. Second, transepithelial voltage is usually not zero. Thus, the electrochemical gradients across the apical and basolateral membranes of an epithelial cell are generally very different.
Electrophysiological methods provide two major types of information about ion transport by epithelial cells. First, electrophysiological techniques can define the electrical driving forces that act on ions either across the entire epithelium or across the individual apical and basolateral cell membranes. Second, these electrical measurements can define the overall electrical resistance of the epithelium or the electrical resistance of the individual apical and basolateral cell membranes.
The voltage difference between the solutions on either side of the epithelium is the transepithelial voltage (Vte). We can measure Vte by placing one microelectrode in the lumen of the organ or duct of which the epithelium is the wall and a second reference electrode in the blood or interstitial space (Fig. 5-20A). If we instead insert the first microelectrode directly into an epithelial cell (see Fig. 5-20A), the voltage difference between this cell and the reference electrode in blood or the interstitial space measures the basolateral cell membrane voltage (Vbl). Finally, if we compare the intracellular electrode with a reference electrode in the lumen (see Fig. 5-20A), the voltage difference is the apical cell membrane voltage (Va). Obviously, the sum of Va and Vbl is equal to the transepithelial voltage (see Fig. 5-20B). It is also possible to insert ion-sensitive microelectrodes into the lumen or the epithelial cells and thereby determine the local activity of ions such as Na+, K+, H+, Ca2+, and Cl−.
FIGURE 5-20 Measurement of voltages in an epithelium. A, The transepithelial voltage difference between electrodes placed in the lumen and interstitial space (or blood) is Vte. The basolateral voltage difference between electrodes placed in the cell and interstitial space is Vbl. The apical voltage difference between electrodes placed in the lumen and cell is Va. B, Relative to the reference voltage of zero in the interstitial space, the voltage inside the cell in this example is −70 mV, and the voltage in the lumen is −3 mV. These values are typical of a cell in the renal proximal tubule or a small intestine.
By using the same voltage electrodes that we introduced in the preceding paragraph, we can pass electrical current across either the whole epithelium or the individual apical and basolateral membranes. From Ohm's law, it is thus possible to calculate the electrical resistance of the entire wall of the epithelium, or transepithelial resistance (Rte); that of the apical membrane, or apical resistance (Ra); or that of the basolateral membrane, or basolateral resistance (Rbl).
Tight and leaky epithelia differ in the permeabilities of their tight junctions
One measure of how tightly an epithelium separates one compartment from another is its resistance to the flow of electrical current. The range of transepithelial electrical resistance is quite large. For example, 1 cm2 of a rat proximal tubule has a resistance of only 6 Ω, whereas 1 cm2 of a rabbit urinary bladder has a resistance of 70,000 Ω. Why is the range of Rte values so great? The cells of these epithelia do not differ greatly in either their apical or basolateral membrane resistances. Instead, the epithelia with low electrical resistances have a low-resistance pathway located in their tight junctions. Epithelia are thus classified as either “tight” (high electrical resistance) or “leaky,” depending on the relative resistance of their tight junctions. In other words, the tight junctions of leaky epithelia are relatively more permeant to the diffusion of ions than the tight junctions of tight epithelia.
Now that we have introduced the concept that solutes and H2O can move between epithelial cells through tight junctions, we can define two distinct pathways by which substances can cross epithelia. First, a substance can cross through the cell by sequentially passing across the apical and then the basolateral membranes, or vice versa. This route is called the transcellular pathway. Second, a substance can bypass the cell entirely and cross the epithelium through the tight junctions and lateral intercellular spaces. This route is called the paracellular pathway.
As might be expected, leaky epithelia are not so good at maintaining large transepithelial ion or osmotic gradients. In general, leaky epithelia perform bulk transepithelial transport of solutes and H2O in a nearly isosmotic fashion (i.e., the transported fluid has nearly the same osmolality as the fluid from which it came). Examples include the small intestine and the proximal tubule of the kidney. As a general rule, tight epithelia generate or maintain large transepithelial ion concentration or osmotic gradients. Examples include the distal nephron of the kidney, the large intestine, and the tightest of all epithelia, the urinary bladder (whose function is to be an absolutely impermeable storage vessel).
In addition to tight junctions, epithelia share a number of basic properties. First, the Na-K pump is located exclusively on the basolateral membrane (Fig. 5-21). The only known exception is the choroid plexus, where the Na-K pump is located on the apical membrane.
FIGURE 5-21 Models of epithelial solute transport.
Second, most of the K+ that is taken up by the Na-K pump generally recycles back out across the basolateral membrane through K+ channels (see Fig. 5-21). A consequence of the abundance of these K+ channels is that the K+ gradient predominantly determines Vbl, which is usually 50 to 60 mV, inside negative.
Third, as in other cells, [Na+]i, typically 10 to 30 mM, is much lower in an epithelial cell than in the ECF. This low [Na+]i is a consequence of the active extrusion of Na+ by the Na-K pump. The large, inwardly directed Na+ electrochemical gradient serves as a driving force for Na+ entry through apical Na+ channels and for the secondary active transport of other solutes across the apical membrane (e.g., by Na/glucose cotransport, Na-H exchange, Na/K/Cl cotransport) or the basolateral membrane (e.g., by Na-Ca exchange).
Epithelial cells can absorb or secrete different solutes by inserting specific channels or transporters at either the apical or basolateral membrane
By placing different transporters at the apical and basolateral membranes, epithelia can accomplish net transepithelial transport of different solutes in either the absorptive or secretory direction. For example, the renal proximal tubule moves glucose from the tubule lumen to the blood by using an Na/glucose cotransporter (SGLT) to move glucose into the cell across the apical membrane, but it uses facilitated diffusion of glucose (GLUT) to move glucose out of the cell across the basolateral membrane. Clearly, the proximal tubule cell could not use the same Na/glucose cotransporter at both the apical and basolateral membranes because the electrochemical Na+ gradient is similar across both membranes.
We will now look at four examples to illustrate how epithelia can absorb or secrete various solutes by using the transporters discussed above in this chapter.
Consider the model in Figure 5-21A, which is similar to that first proposed by Hans Ussing and coworkers to explain NaCl absorption across the frog skin. The basolateral Na-K pump pumps Na+ out of the cell, thereby lowering [Na+]i and generating an inward Na+ electrochemical gradient across the apical membrane. This apical Na+ gradient, in turn, provides the driving force for Na+ to enter the cell passively across the apical membrane through ENaC Na+ channels. The Na+ that enters the cell in this way is pumped out across the basolateral membrane in exchange for K+, which recycles back out across the basolateral membrane. Note that the Na-K pump generates a current of positive charge across the cell from lumen to interstitium. This current, in turn, creates a lumen-negative transepithelial voltage that can then provide a driving force for passive Cl− absorption across the tight junctions—through the so-called paracellular pathway. The net result is NaCl absorption. This process contributes to NaCl reabsorption in the collecting tubule of the kidney.
With slight alterations, the same basic cell model can explain K+ secretion as well as Na+ absorption (see Fig. 5-21B). Adding K+ channels to the apical membrane allows some of the K+ that is taken up by the Na-K pump across the basolateral membrane to be secreted across the apical membrane. This mechanism is the basis of K+ secretion in the collecting tubule of the kidney. Such a model accurately predicts that drugs such as amiloride, which blocks apical ENaC Na+ channels in these cells, will inhibit K+ secretion as well as Na+ reabsorption.
The small intestine and proximal tubule absorb nutrients that are present in the luminal compartment by secondary active cotransport of Na+ with organic solutes. An example is Na+ cotransport with glucose by SGLT (see Fig. 5-21C). The inwardly directed electrochemical Na+ gradient across the apical membrane, generated by the Na-K pump, now drives the entry of both Na+ and glucose. Glucose, which has accumulated in the cell against its concentration gradient, exits passively across the basolateral membrane by a carrier-mediated transporter (GLUT) that is not coupled to Na+. Again, the flow of positive current across the cell generates a lumen-negative transepithelial voltage that can drive passive Cl− absorption across the tight junctions. The net effect is absorption of both NaCl and glucose.
If the Na+-coupled Cl− entry mechanism is located on the basolateral membrane, the same basic cell model can account for NaCl secretion into the lumen (see Fig. 5-21D). The inwardly directed Na+electrochemical gradient now drives secondary active Cl− uptake across the basolateral membrane by the Na/K/Cl cotransporter NKCC1. Cl− accumulated in the cell in this way can then exit across the apical membrane passively through Cl− channels such as CFTR. Notice that negative charges now move across the cell from interstitium to lumen and generate a lumen-negative voltage that can drive passive Na+secretion across the tight junctions (paracellular pathway). The net process is NaCl secretion, even though the primary active transporter, the Na-K pump, is pumping Na+ from the cell to the interstitium. Secretory cells in the intestine and pulmonary airway epithelium use this mechanism for secreting NaCl.
Water transport across epithelia passively follows solute transport
In general, H2O moves passively across an epithelium in response to osmotic gradients. An epithelium that secretes salt will secrete fluid, and one that absorbs salt will absorb fluid. The finite permeability of the bare lipid bilayer to H2O and the presence of AQPs in most cell membranes ensure that osmotic equilibration for most cells is rapid. In addition, particularly in leaky epithelia, tight junctions provide a pathway for H2O movement between the epithelial cells. However, epithelial H2O permeability (hydraulic conductivity) varies widely because of differences in membrane lipid composition and in abundance of AQPs. The presence of AQPs in the plasma membrane may be either constitutive or highly regulated.
Absorption of a Hyperosmotic Fluid
If the epithelium absorbs more salt than its isotonic equivalent volume of H2O, the absorbate is hyperosmotic. An example is the thick ascending limb of the loop of Henle in the kidney, which reabsorbs a large amount of salt but relatively little H2O. As a result, dilute fluid is left behind in the lumen, and the renal interstitium is rendered hyperosmotic.
Absorption of an Isosmotic Fluid
In certain epithelia, such as the renal proximal tubule and small intestine, net H2O movement occurs with no detectable osmotic gradients across the epithelium (Fig. 5-22). Moreover, the reabsorbed fluid appears to be nearly isosmotic with respect to luminal fluid. Of course, fluid absorption could not really occur without the requisite solute driving force across the epithelium. Two explanations, which are not exclusive, have been offered.
FIGURE 5-22 Model of isotonic water transport in a leaky epithelium. Na-K pumps present on the lateral and basal membrane pump Na+ into two restricted spaces: the lateral intercellular space and restricted spaces formed by infoldings of the basal membrane. The locally high osmolality in the lateral intercellular space pulls water from the lumen and the cell. Similarly, the locally high osmolality in the restricted basal spaces pulls water from the cell. The solution that emerges from these two restricted spaces—and enters the interstitial space—is only slightly hypertonic (virtually isotonic) compared with the luminal solution.
First, the H2O permeability of epithelia performing isosmotic H2O reabsorption might be extremely high because of the high constitutive expression of AQPs in the apical and basolateral membranes. Thus, modest transepithelial osmotic gradients (perhaps only 1 to 2 mOsm), which are the product of solute absorption, are sufficient to drive H2O transport at the rates observed. Measurements cannot distinguish such small osmotic gradients from no gradient at all.
Second, the lateral intercellular spaces between the epithelial cells (lateral interspaces; see Fig. 5-22, option 1) as well as the spaces between the infoldings of the basal membrane (basal labyrinth; see Fig. 5-22, option 2) might be modestly hyperosmotic as a consequence of the accumulation of absorbed solutes in a localized region. The resulting localized osmotic gradient would pull H2O into the lateral interspaces from the cell (across the lateral portion of the basolateral membrane) or from the lumen (across the tight junction). Similarly, a localized osmotic gradient would pull H2O into the basal labyrinth from the cell (across the basal portion of the basolateral membrane). By the time that the fluid emerges from these spaces and reaches the interstitium, it would have become nearly isosmotic.
Absorption of a Hypo-osmotic Fluid
If both sides of an epithelium are bathed by isosmotic solutions, it is not possible to concentrate the fluid in the lumen. You might imagine that the epithelium could accomplish the task by absorbing a hypo-osmotic fluid. However, this would require absorbing more H2O than solute, which would require H2O transport to “lead” rather than to follow solute transport. Indeed, active transport of H2O does not occur, N5-21 and H2O cannot move against an osmotic gradient. Hypo-osmotic fluid absorption does indeed occur in the body but requires that the osmolality of the basolateral compartment exceed that of the apical compartment. As discussed on p. 816, the medullary collecting duct uses this approach to concentrate the urine. The collecting duct absorbs a hypo-osmotic fluid because (1) the interstitial fluid in the renal medulla is hyperosmotic and (2) the H2O permeability of the renal collecting duct is high due to the insertion of AQP2—under hormonal control—into the apical membrane.
Epithelia can regulate transport by controlling transport proteins, tight junctions, and the supply of the transported substances
A large range of physiological stimuli regulate the rates at which specific epithelia transport specific solutes. Virtually all known intracellular signaling cascades (see Chapter 3) have been implicated in mediating these regulatory effects. Ultimately, these cascades must affect the rates at which specific solutes move through transporters or channels.
Increased Synthesis (or Degradation) of Transport Proteins
One approach for modifying transport activity is to change the number of transport molecules in the cell. For example, the hormone aldosterone directly or indirectly increases the transcription rate of genes that encode Na-K pump subunits, thereby increasing Na-K pump synthesis in the distal nephron of the kidney.
Recruitment of Transport Proteins to the Cell Membrane
Cells can also change the functional activity of transporters by storing some of them in an intracellular organelle “pool” and then inserting them into the cell membrane. For example, histamine causes cytoplasmic “tubulovesicles” containing H-K pumps (the pool) to fuse with the apical membrane of gastric parietal cells, which initiates gastric acid secretion.
Post-translational Modification of Pre-existing Transport Proteins
Another approach for modulating the transporter rate is to change the activity of pre-existing transport proteins. For example, increases in the level of intracellular cAMP enhance the phosphorylation of apical membrane Cl− channels that are involved in NaCl secretion by intestinal and airway epithelia. The cystic fibrosis gene product (CFTR) is a Cl− channel whose function is regulated by phosphorylation. A defect in the regulation of apical membrane Cl− channels is the primary physiological abnormality in cystic fibrosis.
Changes in the Paracellular Pathway
The passive movement of solutes across the tight junction can contribute to either “forward” transepithelial movement of the solute or backleak of the solute, depending on the solute gradients. Thus, the epithelium can modulate net transport by changing the permeability of the paracellular pathway. For example, the Na+ permeability of the tight junctions of the proximal tubule increases when ECF volume increases. This increase in the permeability of the paracellular pathway may lower net Na+ reabsorption because of increased backleak of absorbed Na+ from the lateral interspace, across the tight junction, and into the lumen.
Luminal Supply of Transported Species and Flow Rate
Changes in the concentration of transported solutes can have profound effects on rates of net solute transport. As fluid moves along the renal proximal tubule, for example, the very process of glucose absorption depletes glucose from the lumen and thereby slows further glucose absorption. Increasing the rate at which fresh high-glucose fluid enters the proximal-tubule lumen raises the glucose concentration at the site of glucose uptake and thus increases the rate of glucose absorption. In addition, flow itself—most likely sensed by bending of the central cilium (see p. 43) present at the apical membrane of most types of epithelial cells—can trigger signaling cascades resulting in alteration of transporter and channel function.