Renal Transport of Sodium During Development

Introduction

The kidney is responsible for maintaining a constant composition and volume of the extracellular fluid. This steady state is achieved by the remarkable capacity of the kidney to maintain sodium balance between what is absorbed in the intestine and what is excreted in the urine. To achieve this, the adult kidney filters approximately 150 L of an ultrafiltrate of plasma from which the tubules reabsorb the vast majority of the filtered solutes and water. This leaves the urine with not only waste products but also the amount of sodium and water virtually equal to that consumed.

The glomerular filtration rate of the term neonatal kidney is approximately 2 mL/min. When factored for an adult’s surface area, the glomerular filtration rate is still only 25% of the adult’s rate of 100 to 120 mL/min/1.73 m ² . The adult glomerular filtration rate, corrected for body surface area of 1.73 m ² , is reached at approximately 2 year of age. Thus, during the course of postnatal renal development, there is a substantial increase in the glomerular filtration rate that must be matched by an increase in the capacity to reabsorb sodium, which is called glomerular-tubular balance . In other words, the developmental increase in sodium absorption matches the developmental increase in glomerular filtration rate. Postnatal renal development is not only characterized by an increase in the abundance of sodium transporters along the nephron but also by isoform changes in some key transporters, as well as maturational changes in the hormones that regulate sodium transport to match sodium intake and to protect the composition and volume of the extracellular fluid during times of stress such as volume depletion.

The very premature neonate has glomerular-tubular imbalance. Although the glomerular filtration rate in a premature infant may be only a fraction of that of the term infant, it nonetheless filters an ultrafiltrate of plasma containing sodium at a rate greater than the immature tubule’s capacity to reabsorb sodium. ^, Shown in Fig. 97.1 is the fractional excretion of sodium, the percentage of filtered sodium excreted in the urine, in premature compared with term infants. As can be seen, very premature infants excrete 5% to 10% of the filtered sodium, whereas the term infant has urine that has approximately 0.1% to 0.2% of the filtered sodium in the urine. Thus there is substantive salt wasting by the premature neonate. Glomerular-tubular imbalance is also found for glucose, in which neonates less than 30 weeks gestation often have significant glucosuria. ^, Neonates normally consume breast milk, which has a very low concentration of sodium. Thus very premature infants will develop hyponatremia and volume depletion if their milk is not supplemented with sodium. Fig. 97.2 depicts that premature neonates will be in negative sodium balance and will thus require sodium supplementation. Another important point shown in Fig. 97.2 is that term neonates are in slightly positive salt balance, which is essential for growth.

The term neonate is able to maintain a positive sodium balance over a wide range of sodium intake. ^, In addition, the neonate is less able to excrete a sodium load than an adult. This was exemplified by studies comparing the ability of neonatal and adult dogs to excrete a volume of saline equal to 10% of their weight. Neonatal dogs had a blunted natriuretic response to saline loading compared with adults excreting only 10% of the sodium infused in 2 hours compared with 50% of the sodium infused in adult dogs. The ability to excrete a sodium load was not explained by the difference in glomerular filtration rate (GFR) with saline expansion in the two groups. The authors concluded that the difference was due to enhanced distal sodium absorption in the neonate.

Principles of Membrane Sodium Transport

The huge volume of isotonic fluid filtered each day by the glomerulus is delivered to the proximal tubule. The glomerular filtrate contains the same concentration of solutes as that in the blood, but it by and large is lacking the large molecular weight proteins in plasma. The proximal tubule reabsorbs approximately two thirds of the glomerular filtrate in an isotonic fashion. In other words, the glomerular ultrafiltrate has a sodium concentration of 140 mEq/L and osmolality of 290 mOsm/kg water. By the end of the proximal tubule the luminal fluid sodium concentration and osmolality are unchanged. However, the composition of the fluid changes substantively.

The proximal tubule predominantly reabsorbs organic solutes, sodium bicarbonate and sodium chloride. Reabsorption of solutes without water would result in a hypotonic luminal fluid if it were not for the fact that the proximal tubule has abundant expression of aquaporin 1, a water channel found on the apical and basolateral membranes of most cells. ^, Thus the high water permeability of the proximal tubular cell results in osmotic equilibration of the luminal fluid to that of the blood in the peritubular capillaries. The osmotic permeability of the neonatal proximal tubule is higher than that of the adult tubule. A cartoon of the nephron is shown in Fig. 97.3 .

Following the proximal tubule is the thin descending limb that also expresses aquaporin 1 and is thus highly permeable to water. The expression of aquaporins on the apical and basolateral membrane allows water to flow into the hypertonic medulla and concentrate the luminal fluid. At the bend of the thin limb the characteristics of the tubule changes significantly. The thin ascending limb is water impermeable, and it has a high sodium permeability. The high sodium permeability of the thin ascending limb results in the accumulation of NaCl into the interstitium of the medulla. Neither the thin descending limb nor the thin ascending limb actively transports solutes. This hairpin tubular structure is part of the countercurrent multiplication system that contributes to a hypertonic medullary interstitium.

As the tubular fluid ascends the loop of Henle, it flows into the thick ascending limb that is also water impermeable. The thick ascending limb reabsorbs salt without water. Approximately 25% of the filtered sodium is reabsorbed in the thick ascending limb via the electroneutral sodium-potassium-2 chloride co-transporter. The transepithelial potential difference is lumen positive due potassium recycling (secretion) into the lumen via the apical membrane potassium channel. The lumen positive potential difference provides a driving force for calcium and magnesium transport across the paracellular pathway. The fluid leaving the thick ascending limb has an osmolality of 50 mOsm/kg water regardless of the final urinary osmolality. This tubular fluid then flows into the distal convoluted tubule, where an additional 5% to 10% of the filtered sodium is reabsorbed via a sodium chloride co-transporter that is electroneutral. Finally, 1% to 3% of the filtered sodium is reabsorbed by the collecting tubule across the epithelial sodium channel, which is under the control of aldosterone. Whether the urine will be hypertonic or hypotonic will be dependent on whether vasopressin is present to cause the insertion of aquaporin 2, another water channel, into the apical membrane of the collecting duct.

In all of the nephron segments that actively transport sodium, the cells are poised for the vectorial transport of sodium from the lumen across the epithelium to the peritubular capillaries. The driving force for all active transport of sodium is the Na ⁺ /K ⁺ -ATPase on the basolateral membrane. The Na ⁺ /K ⁺ -ATPase uses ATP to transport three sodium ions out of the cell and two potassium ions into the cell. The intracellular sodium concentration is approximately 10 mEq/L, and the intracellular potassium concentration is approximately 140 mEq/L. The low intracellular sodium concentration provides a driving force for the uptake of sodium across the apical membrane of cells along the nephron. In addition, the pump is electrogenic due to the stoichiometry of Na and K transport. With three sodium ions exiting the cell for two potassium ions entering the cell, the cellular potential difference is negative (−60 to −90 mV). This provides another driving force for sodium entry across the apical membrane.

The apical membrane of cells is a lipid bilayer that is impermeable to sodium. Thus, for sodium to enter the cell down its electrochemical gradient, there has to be a protein transporter in the membrane. There are three modes of sodium entry that are used along the nephron. The cartoon shown in Fig. 97.3 shows all three of these transporters. First, sodium can pass across an apical channel. This occurs in the collecting tubule principal cell where the epithelial sodium channel, designated ENaC, facilitates sodium entry down its electrochemical gradient. The abundance of ENaC on the apical membrane is regulated by aldosterone. Entry of the positive sodium ion leaves the lumen with a negative potential difference that serves as a driving force for potassium secretion, proton secretion, or chloride absorption. The second type of transport process is a sodium chloride co-transporter or sodium-other solute symporter. In this type of transporter, sodium enters the cell down its electrochemical gradient in the same direction as another solute. In the distal convoluted tubule, that transporter is the sodium chloride co-transporter, designated NCC. This is the transporter that is inhibited by thiazide diuretics and is electroneutral. Sodium can also enter cells along with another solute such as glucose, as occurs in the proximal tubule. In this case, there is a net positive charge entering the cell because glucose is electroneutral. This leaves the tubular lumen with a negative transepithelial potential difference. The lumen negative transepithelial potential can serve as a driving force for the passive transport of negative ions such as chloride that may be transported across the paracellular pathway. Finally, sodium may be transported in exchange for another ion. In the proximal tubule, there is a sodium hydrogen exchanger designated NHE. The NHE results in the reabsorption of sodium and secretion of a proton into the cell. NHE is electroneutral, and the secretion of a proton will titrate filtered bicarbonate and result in net bicarbonate reabsorption.

Cells are linked to the adjoining cells by a tight junction, which is the most apical structure of the paracellular pathway. The proteins that make up the tight junction are a rather ubiquitously expressed molecule designated occludin and a family of tight junction proteins called claudins . There are more than 20 claudins, and they all have four transmembrane domains, a short intracellular loop, and two extracellular loops that bind to the claudin expressed on the adjoining cell. Claudins form the pore and barrier function of the paracellular pathway. Depending on the characteristics of the first loop, claudins will affect the ability of solutes to pass across the paracellular pathway.

Some claudins deserve discussion, such as claudin 16 and 19 in the thick ascending limb. Sodium enters the thick ascending limb via a sodium, potassium, two-chloride co-transporter designated NKCC. This is the transporter that is inhibited by furosemide. This transporter is electroneutral and is described in Fig. 97.3 . Potassium recycles into the tubular lumen via a potassium channel designated ROMK, leaving the tubular lumen with a positive transepithelial potential difference. This positive potential provides a driving force for the passive paracellular absorption of magnesium and calcium. Mutations in either claudin 16 or 19 prevent the passive absorption of calcium and magnesium across the paracellular pathway, resulting in an autosomal recessive disorder called familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) . In this disorder, large amounts of magnesium and calcium are excreted in the urine, causing hypomagnesemia, nephrocalcinosis, and renal stones.

The reabsorption of glucose and other neutral organic solutes with sodium leaves the lumen of the early proximal tubule with a negative potential difference. This provides a driving force for passive paracellular chloride reabsorption in the proximal tubule. In addition, the composition of the luminal fluid changes along the proximal tubule. The preferential reabsorption of bicarbonate over chloride ions in the early proximal tubule results in a higher chloride and lower bicarbonate concentration than that of the peritubular fluid. Thus chloride diffuses down its concentration gradient across the paracellular pathway and is reabsorbed.

There are developmental changes in the passive permeability properties of the proximal tubule. These developmental changes in permeability are due to changes in the abundance of some claudins, such as claudin 2, which is more highly expressed in the neonate than the adult. There are also claudins that are expressed in the neonate, such as claudin 6, 9, and 13, which are not expressed in the adult. In the adult, approximately one third of chloride transport is passive and paracellular and is mediated by the electrochemical driving forces discussed earlier. ^, The chloride permeability of the adult proximal tubule is far greater than the neonate, which limits passive paracellular chloride transport by the neonatal segment to almost zero. ^{, ,} The lower chloride permeability in the neonate compared with the adult is likely due to the expression of claudin 6 and 9 by the neonatal proximal tubule and not by the corresponding adult segment. Expression of either claudin 6 or 9 into Madin-Darby canine kidney (MDCK) cells in vitro results in a decrease in chloride permeability. Thus not only are there maturational changes in active NaCl transport but also of passive paracellular transport.