5: Renal Handling of Organic Solutes

• ▸ State the physiological utility of either excreting or reabsorbing organic solutes.

• ▸ State the general characteristics of the proximal tubular systems for active reabsorption or secretion of organic solutes.

• ▸ Describe the renal handling of glucose and state the conditions under which glycosuria is likely to occur.

• ▸ Describe the renal handling of proteins and small peptides.

• ▸ Describe the secretion of para-aminohippurate.

• ▸ Outline the handling of urate.

• ▸ Describe the secretion of organic cations.

• ▸ Describe how tubular pH affects the excretion and reabsorption of weak acids and bases.

• ▸ Describe the renal handling of urea, including the medullary recycling of urea from the collecting duct to the loop of Henle.

As pointed out in Chapter 1, a major function of the kidneys is the excretion of organic waste, foreign chemicals and their metabolites. As the kidneys excrete these substances they also filter large amounts of organic substances that they do not excrete, such as glucose and amino acids. Therefore, the kidneys must discriminate between what to keep and what to discard. While the collective concentration of the useful organic solutes that should be kept is small in comparison with inorganic ions like sodium and chloride, the large amounts filtered means that processes must exist to reabsorb them.

Some organic solutes handled by the kidneys are neutral molecules; most are anions or cations. As the useful metabolites are recovered from the filtrate, the waste and foreign substances are not only let go, but actively secreted. In dealing with organic solutes the kidneys perform a kind of triage. They (1) reabsorb metabolites that should not be lost, (2) eliminate waste products and unwanted foreign organic substances, and (3) partially reabsorb others. An analysis of the renal handling of every one of these organic substances would be prohibitive, so we will discuss a few key solutes and establish generalities about the others.

One organic substance, urea, is unique in this regard. It is a waste product that must be excreted to prevent accumulation. However, it also plays a key role in renal regulation of water balance. The renal handling of urea is briefly discussed later in this chapter and again in the following chapter in the discussion of renal handling of water.

General Properties of Organic Solute Transport

Several generalizations apply to the handling of small organic solutes by the kidney.

1. While there is a strikingly large number of organic solute species, there is a far smaller number of transport protein species, meaning that many transporters are promiscuous, accepting multiple solutes, sometimes over 100 different ones. This allows the kidneys to operate without expressing a separate transporter for each and every solute.

2. Most organic solutes are transported only in the proximal tubule. Those that are secreted or escape reabsorption in the proximal tubule end up being excreted (an exception, as covered later in this chapter, is when charged species become neutral as a result of changes in tubular pH and are reabsorbed passively in regions beyond the proximal tubule).

3. Transport involves a cascade of interrelated transport events always beginning with active extrusion of sodium across the basolateral membrane by the Na-K-ATPase. Neutral or negatively charged organic solutes then enter via symporters with sodium, while cations enter via uniporters driven by the negative membrane potential. The resulting intracellular accumulation of the solute in question establishes a favorable gradient for its efflux. The accumulated solutes then leave through a variety of pathways across the opposite membrane from which they entered or couple via an antiporter to the influx of another organic solute.

Most of the useful organic nutrients in the plasma that should not be lost in the urine are freely filtered. These include glucose, amino acids, acetate, Krebs cycle intermediates, some water-soluble vitamins, lactate, acetoacetate, β-hydroxybutyrate, and many others. The proximal tubule is the major site for reabsorption of the large quantities of these organic nutrients filtered each day by the renal corpuscles.

Glucose

Under most circumstances, it would be deleterious to lose glucose in the urine, particularly in conditions of prolonged fasting. Thus the kidneys normally reabsorb all the glucose that is filtered. A typical plasma glucose level is about 90 mg/dL (5 mmol/L). It rises transiently to well over 100 mg/dL during meals and falls somewhat during fasting. Usually all the filtered glucose is reabsorbed in the proximal tubule. This involves taking up glucose from the tubular lumen across the apical membrane via sodium-glucose symporters, followed by its exit across the basolateral membrane into the interstitium via a GLUT uniporter. Most of the glucose is reabsorbed by a high-capacity, low-affinity sodium-glucose symporter (SGLT-2) that has a stoichiometry of 1 sodium per glucose. Then the last remaining glucose is taken up in the late proximal tubule (S3 segment) by a low-capacity, high-affinity transporter (SGLT-1) that transports 2 sodium ions per glucose (Figure 5–1 top). This 2-for-1 stoichiometry provides additional energy to move glucose up its concentration gradient in the region where the luminal concentration is normally very low. Unlike the case for sodium and many other solutes, the tight junctions are not significantly permeable to glucose. Therefore, as glucose is removed from the lumen and the luminal concentration falls, there is no backleak, resulting in virtually complete reabsorption.

Because the sodium-glucose symporters are saturable (T_m systems), abnormally high-filtered loads overwhelm the reabsorptive capacity (exceed the T_m; Figure 5–1 bottom). This occurs when plasma glucose approaches 200 mg/dL, a situation often found in untreated diabetes mellitus. In very severe cases, plasma glucose can exceed 1000 mg/dL, or over 55 mmol/L, leading to a significant loss of glucose.

Assume that the glucose T_m is 375 mg/min (a typical value). With a glomerular filtration rate (GFR) of 125 mL/min (1.25 dL/min) and a normal plasma glucose of 90 mg/dL, the filtered load is 1.25 dL/min × 90 mg/dL = 112.5 mg/min, well below the T_m of 375 mg/min. Thus the kidneys easily reabsorb the entire filtered load. When plasma glucose reaches 200 mg/dL, the filtered load becomes 1.25 dL/min × 200 mg/dL = 250 mg/min. At this point, some individual nephrons have reached the upper limit of what they can reabsorb, and a little glucose begins to spill into the urine. Further increases in plasma glucose saturate the remaining transporters and any amount filtered above 375 mg/min is excreted. This leads to loss of glucose and an unwanted osmotic diuresis that we discussed in Chapter 4. One can appreciate that any glucose not reabsorbed is an osmole in the tubule that has consequences for water reabsorption.

Although we sometimes say the glomerular filtrate is protein free, it is not truly free of all protein; it just has a total protein concentration much lower than plasma. First, peptides and smaller proteins (eg, angiotensin, insulin), although present at low concentrations in the blood, are filtered in considerable quantities. Second, while the movement of large plasma proteins across the glomerular filtration barrier is extremely limited, a small amount does make it through into Bowman's space. For albumin, the plasma protein of highest concentration in the blood, the concentration in the filtrate is normally about 1 mg/dL, or roughly 0.02% of the plasma albumin concentration (5 g/dL). Because of the huge volume of fluid filtered per day, the total filtered amount of protein is not negligible. Normally all of these proteins and peptides are reabsorbed completely, although not in the conventional way. They are enzymatically degraded into their constituent amino acids, which are then returned to the blood.

For the larger proteins, the initial step in recovery is endocytosis at the apical membrane. This energy-requiring process is triggered by the binding of filtered protein molecules to specific receptors on the apical membrane. The rate of endocytosis is increased in proportion to the concentration of protein in the glomerular filtrate until a maximal rate of vesicle formation, and thus, the T_m for protein uptake is reached. The pinched-off intracellular vesicles resulting from endocytosis merge with lysosomes, whose enzymes degrade the protein to low-molecular-weight fragments, mainly individual amino acids. These end products then exit the cells across the basolateral membrane into the interstitial fluid, from which they gain entry to the peritubular capillaries.

To understand the potential problem associated with a failure to take up filtered protein, remember that for a healthy young adult

If this protein was not removed from the lumen, the entire 1.8 g would be lost in the urine. In fact, most of the filtered protein is endocytosed and degraded so that the excretion of protein in the urine is normally only 100 mg/day. The endocytic mechanism by which protein is taken up is easily saturated, so a large increase in filtered protein resulting from increased glomerular permeability causes the excretion of large quantities of protein.

Discussions of the renal handling of protein logically tend to focus on albumin because it is, by far, the most abundant plasma protein. There are, of course, many other plasma proteins. Although present in lower levels than albumin, they are smaller and thus more easily filtered. For example, growth hormone (molecular weight, 22,000 Da) is approximately 60% filterable, and the smaller insulin is 100% filterable. The total mass of these filtered hormones is insignificant; however, because even tiny levels in the plasma have important signaling functions in the body, renal filtration becomes an important influence on concentrations in the blood. Relatively large fractions of these smaller plasma proteins are filtered and then degraded in tubular cells. The kidneys are major sites of catabolism of many plasma proteins including peptide hormones. Decreased rates of degradation occurring in renal disease may result in elevated plasma hormone concentrations.

Very small peptides are catabolized into amino acids or di- and tri-peptides within the proximal tubular lumen by peptidases located on the apical surface of the plasma membrane. These products are then reabsorbed by the same transporters that normally reabsorb filtered amino acids.

Finally, in certain types of renal damage, proteins released from damaged tubular cells may appear in the urine and provide important diagnostic information.

So far, we have described reabsorption of useful organic substances the body does not normally excrete. There are of course many organic cations that does excrete, both endogenously produced waste products and foreign chemicals (see Table 5–1 for a partial listing). Many of these organic cations are filterable at the renal corpuscles, with proximal secretion adding to the amount filtered. Others are extensively bound to plasma proteins and undergo glomerular filtration only to a limited extent; accordingly, proximal tubular secretion constitutes the only significant mechanism for their excretion.

The proximal tubules possess several closely related transport systems for organic cations. Because there are a number of different transporters that are relatively nonselective as to which solute species they accept, a substantial number of foreign and endogenous organic cation species are transported. Although the transporters manifest T_m limitation, in many cases over 90% of a given cation species entering the renal circulation is removed, indicating that the transport capacity is high. The process begins with the Na-K-ATPase, which establishes a potassium concentration gradient and resulting negative membrane potential. Organic cations enter across the basolateral membrane via one of several uniporters, members of the OCT family (Organic Cation Transporter) driven energetically by the negative membrane potential. This raises the cytosolic concentration of the cation well above that in the interstitium. The cations then exit into the lumen via an antiporter that exchanges a proton for the organic cation (Figure 5–2). Because this antiporter exchanges 2 univalent cations, it is electroneutral and unaffected by the membrane potential.

The active secretory pathway for many organic anions in the proximal tubule uses the recycling of α-ketoglutarate (αKG) as a tool. First, αKG, which is a divalent anion, is actively taken up from both the lumen and interstitium by a sodium-αGK symporter (stoichiometry of 3 sodium per αKG), which raises the cellular levels of αKG. Then αKG effluxes across the basolateral membrane via an antiporter that imports an organic anion that is destined to be secreted. This antiporter is a member of the OAT family (Organic Anion Transporter) of basolateral membrane proteins. The αKG keeps recycling, entering with sodium and effluxing back to the interstitium in exchange for the other organic solute. Finally, the second organic solute is secreted across the apical membrane via one of several pathways, including the multidrug resistance protein MDR-2, which is an ATPase that drives the efflux of many different organic anions (Figure 5–2).

Analogous to the transporters for cations, the basolateral membrane of proximal convoluted tubule epithelial cells contains several OAT species, each one accepting multiple solutes to be transported. The proximal tubule thus has the capacity to secrete all the organic anions listed in Table 5–2 and many more. These organic anions are not significantly permeable through tight junctions or lipid membranes, and their transport is characterized by T_m. If the plasma concentration of an organic anion is too high, it will not be efficiently removed from the blood by the kidneys.

Metabolic transformations in the liver are very important, where many foreign (and endogenous) substances are conjugated with either glucuronate or sulfate. The addition of these groups renders the parent molecule far more water-soluble. These conjugates are actively transported by the organic anion secretory pathway.

Urate

Urate, an anion that is the base form of uric acid, provides a fascinating example of the renal handling of organic anions that is particularly important for clinical medicine and is illustrative of renal pathology. An increase in the plasma concentration of urate can cause gout and is thought to be involved in some forms of heart disease and renal disease; therefore, its removal from the blood is important. However, instead of excreting all the urate it can, the kidneys actually reabsorb most of the filtered urate. Urate is freely filterable. Almost all the filtered urate is reabsorbed early in the proximal tubule, primarily via antiporters (URAT1) that exchange urate for another organic anion. Further on in the proximal tubule urate undergoes active tubular secretion. Then, in the straight portion, some of the urate is once again reabsorbed. Because the total rate of reabsorption is normally much greater than the rate of secretion, only a small fraction of the filtered load is excreted.

Although urate reabsorption is greater than secretion, the secretory process is controlled to maintain relative constancy of plasma urate. In other words, if plasma urate begins to increase because of increased urate production, the active proximal secretion of urate is stimulated, thereby increasing urate excretion.

Given these mechanisms of renal urate handling, the reader should be able to deduce the 3 ways by which altered renal function can lead to decreased urate excretion and hence increased plasma urate, as in gout: (1) decreased filtration of urate secondary to decreased GFR, (2) excessive reabsorption of urate, and (3) diminished secretion of urate.

Many of the organic solutes handled by the kidney are weak acids or bases and exist in both, neutral and ionized forms. The state of ionization affects both the aqueous solubility and membrane permeability of the substance. Neutral solutes are more permeable than ionized solutes. As water is reabsorbed from the tubule, any substance remaining in the tubule becomes progressively more concentrated. And as described in Chapter 9, the luminal pH may change substantially during flow through the tubules. Therefore, both the progressive concentration of organic solutes and change in pH strongly influence the degree to which they are reabsorbed by passive diffusion through regions of tubule beyond the proximal tubule.

At low pH weak acids are predominantly neutral (acid form), while at high pH they dissociate into an anion and a proton. Imagine the case in which the tubular fluid becomes acidified relative to the plasma, which it does on a typical Western diet. For a weak acid in the tubular fluid, acidification converts much of the acid to the neutral form and therefore, increases its permeability. This favors diffusion out of the lumen (reabsorption). Highly acidic urine (low pH) tends to increase passive reabsorption of weak acids (and promote less excretion). For many weak bases, the pH dependence is just the opposite. At low pH they are protonated cations (trapped in the lumen). As the urine becomes acidified, more is converted to the impermeable charged form and is trapped in the lumen. Less is reabsorbed passively, and more is excreted. These events are depicted in Figure 5–3. In the top part of the figure, tubular acidification converts weak acids to the neutral form, allowing passive reabsorption, and converts weak bases to cations, trapping them in the lumen. In the bottom part of the figure, tubular alkalization keeps weak acids ionized, and thus trapped in the lumen, and keeps weak bases neutral, allowing their passive reabsorption.

Having said this, what difference does it make? Because so many medically useful drugs are weak organic acids and bases, all these factors have important clinical implications. For example, if one wishes to enhance the excretion of a drug that is a weak acid, one attempts to alkalinize the urine (because this traps the ionic form in the lumen). In contrast, acidification of the urine is desirable if one wishes to prevent excretion of the drug. Of course, exactly the opposite applies to weak organic bases. At any luminal fluid pH, increasing the urine flow increases the excretion of both weak acids and bases.

Finally, some organic solutes, although more membrane permeable in the neutral form, are less soluble in aqueous solution and tend to precipitate. This specifically applies to urate. The combination of excess plasma urate and low urinary pH, which converts urate to the neutral uric acid, often leads to the formation of uric acid kidney stones.

Urea is a very special substance for the kidney. It is an end product of protein metabolism, waste to be excreted, and also an important component for the regulation of water excretion. Urea differs from all the other organic solutes discussed in this chapter in several significant ways. (1) There are no membrane transport mechanisms in the proximal tubule; instead, it easily permeates the tight junctions of the proximal tubule where it is reabsorbed paracellularly. (2) Tubular elements beyond the proximal tubule express urea transporters and handle urea in a complex, regulated manner.

Urea is derived from proteins, which form much of the functional and structural substance of body tissues. Proteins are also a source of metabolic fuel. Dietary protein is first digested into its constituent amino acids. These are then used as building blocks for tissue protein (eg, muscle), converted to fat or oxidized immediately. During fasting, the body breaks down proteins into amino acids that are used as fuel, in essence consuming itself. The metabolism of amino acids yields a nitrogen moiety (ammonium) and a carbohydrate moiety. The carbohydrate goes on to further metabolic processing, but the ammonium cannot be further oxidized and is a waste product. Ammonium per se is rather toxic to most tissues (except the medullary interstitium, see Chapter 9) and the liver immediately converts most ammonium to urea and a smaller, but crucial amount to glutamine. While normal levels of urea are not toxic, the large amounts produced on a daily basis, particularly on a high protein diet, represent a large osmotic load that must be excreted. Whether a person is well fed or fasting, urea production proceeds continuously and constitutes about half of the usual solute content of urine.

The normal level of urea in the blood is quite variable (3–9 mmol/L),¹ reflecting variations in both protein intake and renal handling of urea. Over days to weeks, renal urea excretion must match hepatic production; otherwise plasma levels would increase into the pathological range producing a condition called uremia. On a shorter-term basis (hours to days), urea excretion rate may not exactly match production rate because urea excretion is also regulated for purposes other than keeping a stable plasma level.

The gist of the renal handling of urea is the following: it is freely filtered. About half is reabsorbed passively in the proximal tubule. Then an amount equal to that reabsorbed is secreted back into the loop of Henle. Finally, about half is reabsorbed a second time in the medullary collecting duct. The net result is that about half the filtered load is excreted (Figure 5–4).

As a molecule, urea is small (molecular weight, 60 Da), water-soluble, and freely filtered. Because of its highly polar nature, it does not permeate lipid bilayers, but a set of uniporters (UT family) transport urea in various places beyond the proximal tubule and in other sites within the body (particularly red blood cells). Because urea is freely filtered, the filtrate contains urea at a concentration identical to that in plasma. Let us assume a normal plasma level (5 mmol/L). As water is reabsorbed, the urea concentration rises well above 5 mmol/L, driving diffusion through the leaky tight junctions. Roughly, half the filtered load is reabsorbed in the proximal tubule by the paracellular route. As the tubular fluid enters the loop of Henle, about half the filtered urea remains, but the urea concentration has increased somewhat above its level in the filtrate because proportionally, more water than urea was reabsorbed. At this point, the process becomes fairly complicated. First, conditions in the medulla depend highly on the individual's state of hydration. Second, there is a difference between superficial nephrons with short loops of Henle that only penetrate the outer medulla and juxtamedullary nephrons, with long loops of Henle that reach all the way down to the papilla. For simplicity we consider all nephrons together.

The interstitium of the medulla has a considerably higher urea concentration than does plasma (for reasons explained later). The concentration increases from the outer to the inner medulla. Since the medullary interstitial urea concentration is greater than that in the tubular fluid entering the loop of Henle, there is a concentration gradient favoring secretion into the lumen. The tight junctions in the loop of Henle are no longer permeable (as they were in the cortex), but the epithelial membranes of the thin regions of the Henle's loops express urea uniporters, members of the UT family. This permits secretion of urea into the tubule. In fact, the urea secreted from the medullary interstitium into the thin regions of the loop of Henle replaces the urea previously reabsorbed in the proximal tubule. Thus, when tubular fluid enters the thick ascending limb, the amount of urea in the lumen is at least as large as the filtered load (the loop of Henle has reversed what was accomplished in the proximal tubule). However, because about 80% of the filtered water has now been reabsorbed, the luminal urea concentration is now several times greater than in the plasma. Beginning with the thick ascending limb and continuing all the way to the inner medullary collecting ducts (through the distal tubule and cortical collecting ducts), the apical membrane urea permeability (and the tight junction permeability) is essentially zero. Therefore, an amount of urea roughly equal to the filtered load remains within the tubular lumen and flows from the cortical into the medullary collecting ducts.

During the transit through the cortical collecting ducts variable amounts of water are reabsorbed, significantly concentrating the urea. Just how much depends on factors discussed in the next chapter, but the luminal urea concentration is well above that of plasma. We indicated earlier that the urea concentration in the medullary interstitium is much greater than in plasma, but the luminal concentration in the medullary collecting ducts is even higher, so in the inner medulla the gradient now favors reabsorption and urea is reabsorbed a second time via another isoform of UT urea uniporter. It is this urea reabsorbed in the inner medulla that leads to the high medullary interstitial concentration, driving urea secretion into the thin regions of the loop of Henle. This means that some of the urea recycles, that is, it is reabsorbed from inner medullary collecting ducts and secreted into thin limbs of the loop of Henle, from where it travels within the tubule to the collecting ducts again to repeat the process. The overall result of these events is that half the original amount of filtered urea passes into the final urine, an amount that, over the long term, must match hepatic production of urea if the body is to remain in balance for urea. The concentration of urea in the final urine can be more than 50× that in plasma, depending on how much water is reabsorbed. These processes are summarized in Figure 5–4.

• Important organic metabolites are reabsorbed almost completely (saved), whereas waste products are for the most part excreted.

• Most organic solutes are transported only in the proximal tubule, usually by a combination of multiporters.

• Normal filtered loads of glucose are completely reabsorbed in the proximal tubule, but in conditions of pathological hyperglycemia the transport saturates, leading to the appearance of glucose in the urine.

• Peptides are reabsorbed either by endocytosis or as individual amino acids following enzymatic degradation on the brush border of the proximal epithelium.

• Some organic solutes, when converted to neutral forms by changes in tubular pH, can be reabsorbed passively in the distal nephron.

• Urea is reabsorbed proximally and recycled between the collecting ducts and loops of Henle in the medulla, resulting in a net excretion of about half the filtered load.