The Renal Tubule Reabsorbs Filtered Substances

The bulk of the ultrafiltrate formed in the glomerulus must be reabsorbed by the renal tubules rather than excreted in the urine. To understand the importance of tubule reabsorption of filtered substances, consider the 10-kg beagle that forms 53.3 L of glomerular filtrate each day. The ultrafiltrate contains virtually the same concentration of salts and glucose as plasma; without tubular reabsorption, the urinary loss of sodium, chloride, potassium, bicarbonate, and glucose alone would total more than 500 g of solute. Without tubular reabsorption, the beagle would need to replace these losses constantly throughout the day by eating more than a pound of salts and drinking more than 50 L of water at the same rate as the urinary loss to maintain fluid and salt balance.

Fortunately, the renal tubule efficiently retrieves these and other constituents of the ultrafiltrate. Figure 42-1 illustrates the percentages of various filtered substances that remain in the tubule fluid at different points along the tubule. One hundred percent of the filtered glucose is reabsorbed by the proximal tubule; by the time the final urine is formed in the terminal collecting duct, approximately 99% of the filtered water and sodium has been retrieved.

Renal Tubule Function May Be Assessed by Determining Fractional Excretion Rate

The net rate of tubular reabsorption and secretion of a filtered substance is expressed as the fractional excretion rate. The fractional excretion rate of a substance X is the ratio of the urinary concentration of X (U_X) to the plasma concentration of X (P_X) divided by the urinary/plasma (U/P) ratio of a reference substance that is neither secreted nor reabsorbed. Relating U_X/P_X to the U/P ratio of a reference substance eliminates the confounding effect of water reabsorption on the urinary concentration of X. In experimental settings the urinary and plasma concentrations of inulin during a constant inulin infusion may be used for reference. However, it is more practical in clinical situations to use creatinine as the reference substance. Therefore the fractional excretion rate of X (FE_X) is determined by the following equation:

FEX=UX/PX÷Ucreatinine/Pcreatinine

where U_creatinine and P_creatinine are the urinary and plasma concentrations of creatinine. By multiplying FE_X by 100, the fractional excretion rate is expressed as the percentage of filtered X that is excreted. The fractional excretion rate, typically of sodium, can be used to assess the functional integrity of the renal tubules in clinical cases of acute renal failure.

The Proximal Tubule Reabsorbs the Bulk of Filtered Solutes

The rate of reabsorption and secretion of filtered substances varies among segments of the renal tubule. In general, the proximal tubule reabsorbs more of the ultrafiltrate than the other tubule segments combined, at least 60% of most filtered substances.

The structure of the proximal tubule and its proximity to the peritubular capillary facilitate the movement of tubule fluid components into the blood through two pathways: the transcellular pathway and the paracellular pathway. The tubule fluid flows over the apical surface of the proximal tubule epithelial cell. Substances transported through the transcellular pathway cross the apical plasma membrane, cytoplasm, and the basolateral plasma membrane into the interstitial fluid. Movement across the apical and basolateral plasma membranes occurs largely by carrier-mediated transport. The vast plasma membrane surface area of the proximal tubule contributes to transcellular transport. The apical plasma membrane has extensive microprojections, called microvilli, which collectively create the brush border (Figures 42-2 and 42-3). On the blood side of the cell, the basolateral plasma membrane has complex infoldings that enhance the surface area; the basolateral surface area equals that of the apical surface area in portions of the proximal tubule. The benefits of the enhanced plasma membrane surface area include increased capacity for the multitude of solute transporters and increased exposure to the luminal and interstitial fluids.

The second route of transport in the proximal tubule is the paracellular pathway. Substances pass through the paracellular pathway from the tubule fluid across the zonula occludens, a permeable structure that attaches the proximal tubule cells to each other at the junction of the apical and basolateral plasma membrane domains (Figure 42-4). Paracellular transport occurs by passive diffusion or by solvent drag, which is the entrainment of solute by the flow of water. Substances crossing the zonula occludens enter the lateral intercellular space, which is thought to communicate freely with the interstitial fluid; from there, reabsorbed substances can be taken up by the peritubular capillary.

Movement of water and solute from the interstitial fluid into the bloodstream is driven by Starling’s forces (see Chapter 23) and aided by the proximity of the peritubular capillary. In mammals the peritubular capillary originates at the glomerular efferent arteriole, subdivides, and wraps closely around the basal aspect of the proximal tubule (Figure 42-5). The plasma leaving the glomerulus has a high oncotic pressure because water and salts are filtered, but proteins are retained in the capillary. The peritubular capillary has low resistance, and thus the hydrostatic pressure in the capillary is low. Both these conditions—high peritubular plasma oncotic pressure and low peritubular capillary hydrostatic pressure—favor fluid and solute uptake from the interstitium into the bloodstream.

In birds the effect of the peritubular blood supply on tubular reabsorption and secretion is complicated by the presence of a renal portal circulation. Renal portal veins anastomose with the efferent glomerular arterioles and supply peritubular blood to the reptilian-type nephrons and the proximal and distal tubules of mammalian-type nephrons; thus these tubules, but not the loops of Henle of the mammalian-type nephrons, are supplied with a mixture of portal venous and arterial blood. The rate of flow to the renal portal supply varies and is controlled by a smooth muscle valve.

Reabsorption of solutes takes place by a number of mechanisms, including primary active transport, carrier-mediated secondary active transport, solvent drag, and passive diffusion. (Transport mechanisms are described in Chapter 1.) In the proximal tubule, most solute reabsorption is driven by the active transport of sodium ions (Na⁺) by the sodium-potassium–adenosinetriphosphatase (Na⁺,K⁺-ATPase) pump, which is located in the basolateral plasma membrane. The Na⁺,K⁺-ATPase extrudes 3 Na⁺ ions and takes up 2 K⁺ ions on each turnover of the pump (Figure 42-6).

Na⁺,K⁺-ATPase activity reduces the intracellular Na⁺ concentration and increases the intracellular K⁺ concentration. Outward diffusion of K⁺ down its chemical gradient through K⁺ channels makes the cell interior electrically negative relative to the exterior. These two factors create an electrochemical gradient for Na⁺ across the apical plasma membrane, favoring Na⁺ uptake from the tubule fluid into the cell. Na⁺ uptake across the apical plasma membrane is facilitated by specific transporters in the membrane that couple the movement of other solutes either in the same direction as Na⁺ (co-transport) or in the opposite direction (counter-transport). Specific Na⁺-dependent transporters for glucose (SGLT1, SGLT2), amino acids (EAAT3, SIT1, and more), phosphate (NaPi2a, NaPi2c, PiT-1), sulfate (NaS1), and citrate (NaDC1, NaDC3) mediate their uptake from the proximal tubule fluid by this mechanism of secondary active transport. The uptake of these substances increases their intracellular concentration, and they move across the basolateral plasma membrane and into the blood down their electrical or chemical gradient, facilitated by solute specific transporters and partly by passive diffusion. The list of solute transporters in the apical and basolateral plasma membranes continues to grow as more are discovered by researchers. Several of the apical Na⁺-coupled solute co-transporters and corresponding basolateral exit mechanisms are illustrated in Figure 42-6.

Bicarbonate (HCO₃⁻) reabsorption in the proximal tubule is also driven by the Na⁺ gradient, although indirectly. The chemical gradient for Na⁺ drives Na⁺ and proton (hydrogen ion, H⁺) counter-transport across the apical plasma membrane through a Na⁺/H⁺ exchanger (NHE3). Secreted H⁺ combines with filtered HCO₃⁻ in the tubule fluid to form water (H₂O) and carbon dioxide (CO₂), catalyzed by the enzyme carbonic anhydrase in the apical plasma membrane of proximal tubule cells. CO₂ enters the cell across the apical plasma membrane, in part facilitated by the integral membrane protein, aquaporin 1 (AQP1). Cytoplasmic carbonic anhydrase catalyzes the hydroxylation of CO₂ with OH⁻ donated from H₂O, forming H⁺ and HCO₃⁻ in the cell. HCO₃⁻ crosses the basolateral plasma membrane through a Na⁺,3-(HCO₃⁻) co-transporter (NBCe1) and a Na⁺-dependent HCO₃⁻/Cl⁻ exchanger. The majority of H⁺ is transported into the tubule fluid through the Na⁺/H⁺ antiporter (NHE3); the electrogenic proton pump, H⁺ATPase, also contributes to proton secretion. By this complex mechanism, illustrated in Figure 42-7, the proximal tubule reabsorbs 60% to 85% of filtered HCO₃⁻.

Chloride ion (Cl⁻) reabsorption in the proximal tubule is also indirectly powered by the Na⁺,K⁺-ATPase pump and occurs through both paracellular and transcellular routes. As Na⁺, HCO₃⁻, glucose, amino acids, and other solutes are selectively reabsorbed and water is taken up along with these solutes, the concentration of Cl⁻ in the tubule fluid rises, establishing a chemical gradient for Cl⁻ movement toward the blood side of the epithelium. In addition, in the early proximal tubule, the selective uptake of Na⁺ exceeds that of anions, resulting in a net positive charge on the blood side. This creates a small electrical gradient favoring anion reabsorption. Thus, in the early proximal tubule, the chemical and electrical gradients favor Cl⁻ reabsorption. The zonula occludens is highly permeable to Cl⁻, thus passive, paracellular transfer of Cl⁻ from the tubule lumen to the interstitial fluid occurs. Transcellular Cl⁻ absorption also occurs in the proximal tubule. Cl⁻-coupled transporters in both the apical and basolateral plasma membranes and Cl⁻ channels in the basolateral plasma membrane facilitate transmembrane Cl⁻ transport, which is also driven by electrical and chemical gradients established by Na⁺,K⁺-ATPase activity.

In distal portions of the proximal tubule, the tubule fluid becomes depleted of many of the substances necessary for Na⁺ reabsorption by co-transport. There, Na⁺,K⁺-ATPase continues to move Na⁺ from the cell into the interstitial fluid; Na⁺ uptake from the lumen occurs predominantly by electrically neutral sodium chloride (NaCl) uptake facilitated by coordinated Na⁺– and Cl⁻-coupled transporters and by passive reabsorption of Na⁺ through the paracellular pathway. Paracellular transport of Na⁺ is made possible here by the chemical gradient for Cl⁻ established by the selective reabsorption of other solutes in the early proximal tubule. As Cl⁻ moves down its chemical gradient from the tubule lumen to the blood side, it carries Na⁺ along with it by electrostatic attraction. The passage of Cl⁻ down its chemical gradient also abolishes the small, lumen-negative charge and in fact establishes a small, lumen-positive charge in the late proximal tubule, which further favors the passive transfer of Na⁺ to the blood side.

Other filtered solutes, such as potassium (K⁺) and calcium (Ca²⁺) ions, are present in the tubule fluid in low concentrations and are reabsorbed by the proximal tubule. Approximately 65% of filtered Ca²⁺ is reabsorbed in the proximal tubule. About 90% of the Ca²⁺ uptake in the proximal tubule is paracellular because of a favorable electrochemical gradient in the late proximal tubule and solvent drag. The majority of K⁺ reabsorption in the proximal tubule also occurs by passive mechanisms, primarily through the paracellular route.

The proximal tubule also reabsorbs filtered peptides and low–molecular-weight proteins. A large proportion of filtered peptides are degraded to amino acids by peptidases in the proximal tubule brush border and are reabsorbed by co-transport with Na⁺ across the apical plasma membrane. Short-chain peptides are themselves transported through co-transport with H⁺ on specific transporters (PEPT1 and PEPT2) in the proximal tubule brush border, driven by the proton gradient between the tubule fluid and cytoplasm. Most of these dipeptides and tripeptides are degraded by intracellular peptidases, although some may exit intact to the blood side through another peptide transporter.

Low–molecular-weight proteins are avidly reabsorbed by the proximal tubule, but by a different mechanism. Filtered proteins such as insulin, glucagon, parathyroid hormone, and many more are taken up at the apical plasma membrane by receptor-mediated endocytosis (see Figure 42-4). The proteins bind receptors (megalin and cubilin) in the plasma membrane, are endocytosed, and delivered by the endocytic vesicles to intracellular organelles called lysosomes while the receptors are recycled to the apical plasma membrane (Figure 42-8). Proteolytic enzymes in the lysosomes degrade the reabsorbed proteins; the amino acids that are the end products are transported into the interstitial fluid and returned to the blood. Diseased glomeruli often leak protein into the filtrate; in these instances, the proximal tubule endocytic machinery is upregulated and the lysosomal compartment is expanded, often to the extent that the increased number and size of lysosomes in proximal tubules are appreciable in histologic sections.