Ontogeny of Renal Sodium Transport Michel Baum*† and Raymond Quigley*
One of the main functions of the adult kidney is to maintain a constant extracellular fluid balance. The adult kidney does this, by and large, by filtering a massive quantity of fluid and reabsorbing the solutes needed to maintain volume and electrolyte homeostasis, while leaving the waste products to be excreted in the urine. One of the most precisely regulated functions of the adult kidney is to maintain sodium balance. The challenge of the neonatal kidney is even greater. It must maintain a positive salt balance for growth while the neonate is fed a diet that is very low in sodium. This review focuses on how the neonatal kidney reabsorbs NaCl with a special emphasis on the differences between the neonatal and adult kidney. © 2004 Elsevier Inc. All rights reserved. f the 150 L the adult kidney filters a day, only ⬃1% is excreted in the urine. Thus, one of the primary roles of the renal tubules is to reabsorb most of the salt and water filtered by the glomerulus. The problem is much more complicated as the kidney has to maintain NaCl balance in the face of a variable salt intake. If an adult changes his diet from a moderate NaCl intake to a diet rich in pepperoni pizzas the kidney will excrete this salt load to maintain salt balance. If NaCl output did not match NaCl intake hypertension, edema and congestive heart failure would be the result of this dietary indiscretion. On the other hand, the kidney can adapt to a low sodium intake or states of volume depletion to excrete almost no sodium. This balance of sodium intake to excretion maintains a constant extracellular fluid balance. Unlike the adult who has a variable salt intake, the neonate only drinks milk, a fluid that is very low in sodium and chloride. While a major task of the adult kidney is to maintain salt balance, this would be ill suited for a neonate that must constantly be in positive NaCl balance in order to grow. Thus, the neonatal kidney is challenged to maintain a positive salt balance in spite of the fact that the dietary intake of salt is extremely low. The difference in the neonatal and adult kidney is exemplified by an experiment where neonatal and adult dogs were both given an isotonic NaCl load equal to 10% of their weight.1 While the adult dog excreted 50% of the sodium load within 2 hours, the newborn dog had excreted only 10% by that time.1 Sodium handling is quite different in extremely premature neonates. The challenge of reabsorbing all of the filtered sodium is not met in these neonates, let alone maintaining positive
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sodium and chloride balance for growth. The renal salt wasting is due to tubular immaturity, which results in hyponatremia unless extra sodium is provided or unless the neonate is fluid restricted. How does the neonatal kidney maintain positive salt balance in spite of the fact that mother’s milk is so low in sodium? The kidney of a term infant only filters one twenty-fifth the volume of fluid as that of the adult. The glomerular filtration rate is much lower than that of the adult even if one corrects for the body surface area. The low glomerular filtration rate makes the job of NaCl reabsorption a lot easier because less salt is delivered to the tubules. None-the-less the renal tubules of the neonate must reabsorb almost all of the filtered salt and water. In this short review, the mechanisms of renal salt reabsorption are discussed with an emphasis on the differences between the neonatal and adult kidney. As a general rule of thumb, almost all transport is secondary to the sodium pump (Na⫹-K⫹ATPase) on the basolateral membrane. The Na⫹-K⫹-ATPase transports 3 sodium ions out of the cell in exchange for 2 potassium ions into the cell. This results in a cell with sodium conFrom the Department of *Pediatrics; †Internal Medicine, The University of Texas Southwestern Medical Center, Dallas, TX. This work was supported by National Institute of Diabetes and Digestive and Kidney Disease Grant DK-41612 (to M. B.). Address reprint requests to Michel Baum, MD, Professor of Pediatrics and Internal Medicine, Sara M. and Charles E. Chair in Pediatric Research, Department of Pediatrics, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd, Dallas, Texas 752359063; e-mail:
[email protected] © 2004 Elsevier Inc. All rights reserved. 0146-0005/04/2802-0002$30.00/0 doi:10.1053/j.semperi.2003.11.008
Seminars in Perinatology, Vol 28, No 2 (April), 2004: pp 91-96
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centration approximately one tenth that of the blood and with a potential difference of -60 mV. Most solute transport, either directly or indirectly, uses this huge electrochemical gradient as the driving force for solute entry. Most solute exit across the basolateral membrane is via passive diffusion. The transporters responsible for NaCl transport along the nephron are shown in Figure 1.
Proximal Tubule Transport The proximal tubule receives an ultrafiltrate of plasma and reabsorbs all the filtered glucose and amino acids, three quarters of the filtered bicarbonate, and two thirds of the filtered chloride. There are several different mechanisms for the reabsorption of NaCl by the proximal tubule. The early proximal tubule preferentially reabsorbs glucose, amino acids, and bicarbonate over chloride ions.2,3 This leaves the late proxi-
mal tubule luminal fluid without organic solutes and with a very low concentration of bicarbonate. Sodium and chloride are essentially the only solutes in the lumen of the distal half of the proximal tubule.3 The proximal tubule is the hardest working tubular segment, yet it does try to get as much NaCl transport reabsorption as possible while doing the minimum amount of work. The trick that the proximal tubule uses is to separate the reabsorption of solutes into the 2 phases described above. During the first phase where all the glucose and amino acids are reabsorbed, solute entry into the proximal tubule cell is via sodium-dependent transporters. The reabsorption of the positively charged sodium along with glucose or amino acids leaves a lumen negative transepithelial potential difference. This negative potential provides a driving force for passive chloride transport across the paracellular pathway. Thus for each glucose molecule reabsorbed
Figure 1. The nephron with the transporters responsible for sodium transport in the various nephron segments.
Ontogeny of Renal Sodium Transport
one also gets the reabsorption of a Na and Cl molecule. The vast majority of NaCl transport occurs in the late proximal tubule where in the adult segment half of NaCl transport is active and transcellular and half is passive and paracellular.4-6 The active component of NaCl transport is mediated by the parallel operation of a Na⫹/H⫹ exchanger and a Cl⫺/OH⫺ exchanger.6-10 When both of these transporters turn over, we have the net reabsorption of a sodium and a chloride ion and the secretion of a H⫹ and a OH- ion or simply a water molecule. The Na⫹/H⫹ exchanger actually plays a role not only in sodium reabsorption but it is the primary mechanism for luminal proton secretion and thus the reabsorption of bicarbonate in the early proximal tubule. As mentioned above the preferential reabsorption of organic solutes and bicarbonate in the initial phase of proximal tubule reabsorption results in a luminal fluid that is essentially composed of NaCl. This provides another way of getting something for nothing. In the adult proximal tubule, the chloride permeability of the tight junction is very high.11 Because the luminal chloride concentration is higher than that in the peritubular plasma, the chloride concentration gradient provides a driving force for passive chloride diffusion from the lumen across the tight junction into the peritubular plasma. The paracellular diffusion of the negatively charged chloride results in a lumen positive potential difference, which provides the driving force for passive paracellular NaCl transport.3 Well, how about the neonatal proximal tubule? There are several important differences in transport between the adult and the neonate. The overall theme, not only in the proximal tubule, but down the entire nephron is that the transporters responsible for solute absorption are exactly the same, but the abundance is lower in the neonatal tubule. This is true for the first part of NaCl transport as well as active NaCl transport where the apical Na⫹/H⫹ exchanger and Cl⫺/OH⫺ exchangers are in lower abundance,5,12-14 as well as the basolateral Na⫹-K⫹ATPase and the basolateral chloride transporters.15,16 This is no tragedy because the amount of fluid filtered and delivered to the tubules is much less in the neonate. Studies have shown that the fraction of the total fluid reabsorbed in the proximal tubule is the same in the neonate as the adult proximal tubule.
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There are surprising differences in the fraction of NaCl transported across the paracellular pathway in the neonatal proximal tubule compared to the adult proximal tubule. Although half of chloride transport is passive and paracellular in the adult, because of the concentration gradient and the high chloride permeability in the adult segment, the neonatal proximal tubule has no passive chloride transport.11 While a gradient develops favoring passive diffusion in the neonatal tubule as it does in the adult, the neonatal proximal tubule is completely impermeable to chloride.11 The above issues raise the question as to what causes the maturational changes in transporter abundance and paracellular permeability. There are a number of hormonal changes that occur during the postnatal period. The neonate has low serum levels of glucocorticoids and thyroid hormone that increase to adult levels around the time of weaning in most animals. As an example of how the neonatal transporters change, the authors discuss the apical Na⫹/H⫹ exchanger because this is an important transporter and we know the most about how this changes during development. The newborn kidney has extremely low Na⫹/H⫹ antiporter activity compared to adults.6,10,14,17-20 If one adrenalectomizes a neonate to prevent the maturational increase in glucocorticoids, the postnatal increase in Na⫹/H⫹ exchanger activity, mRNA and brush border membrane protein abundance is markedly attenuated.21 On the other hand, if one administers glucocorticoids to the fetus and studies the neonate after delivery, the rate and abundance of the Na⫹/H⫹ exchanger is comparable to that of the adult tubule.14,19 Thus, glucocorticoids appear to be the main factor causing the maturation of not only this but most transporters along the nephron. The factors causing the maturational changes that occur in the paracellular pathway are quite different. The tight junction is made up of proteins that shake hands with the neighboring cells to cause the ion selective and restrictive permeability barrier. The changes in abundance in tight junction proteins during postnatal maturation cause the change in the permeability barrier to chloride transport seen in the proximal tubule. It appears that the maturational change in thyroid hormone causes the change in the permeability properties of the proximal tubule to chloride ions.6 Animals made hypothyroid at
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birth never increase their proximal tubule paracellular permeability to chloride ions, whereas neonates that are administered thyroid hormone have a paracellular chloride permeability comparable to that of adults. Finally, we must discuss water transport across the proximal tubule. The glomerulus produces an ultrafiltrate of plasma with an osmolality the same as that of blood. By the end of the proximal tubule most of the glomerular ultrafiltrate has been absorbed yet the osmolality of the tubular fluid has not changed. Thus, the proximal tubule must be very permeable to water. The adult tubule transports water across the apical and basolateral membranes through water channels called aquaporins. The neonatal proximal tubule is as permeable to water as the adult segment, despite the fact that the abundance of aquaporin water channels is very low. Water transport in the neonatal tubule is via a nonaquaporin mediated mechanism, likely via other transporters promiscuous for water, the paracellular pathway, or a more water permeable cell membrane. Whereas the apical and basolateral membrane are less permeable to water in the neonatal segment, the cytoplasm of the neonate allows water to pass with less restriction than the adult segment.22-28 The postnatal increase in aquaporin expression is also mediated by glucocorticoids.23
Thick Ascending Limb and Distal Convoluted Tubule Because the neonate drinks a hypotonic fluid, the neonatal kidney must have a way of producing dilute urine. The neonate can actually dilute urine to the same minimal osmolality as that of an adult, 50 mOsm/kg water. To do this there must be a nephron segments that reabsorb salt but are impermeable to water. These segments are the thick ascending limb and distal convoluted tubule that reabsorb 25% and 5% of the filtered sodium, respectively. The thick ascending limb reabsorbs sodium via an electroneutral sodium-potassium-2 chloride cotransporter (NKCC2). This is the transporter that is inhibited by furosemide and bumetanide. This is also one the transporters that has been found to be mutated in Bartter’s Syndrome along with the basolateral chloride channel and luminal potassium channel.29-33 This transporter is electroneutral, but because of the
apical membrane potassium channel, potassium diffuses back from the cell into the tubular lumen. This leaves the tubular lumen with a large lumen positive potential difference. The tight junction of the thick ascending limb is very permeable to cations and the positive potential difference provides a driving force for the paracellular absorption of calcium, magnesium, potassium, and sodium. This is clinically significant since administration of furosemide will result in an increase in not only sodium excretion, but also magnesium and calcium excretion. The hypercalciuria that results from furosemide administration is the reason that neonates develop nephrocalcinosis if treated for extended periods of time with furosemide. On the other hand, the fact that it inhibits calcium absorption makes it a useful drug to treat hypercalcemia in a wellhydrated patient. The distal convoluted tubule reabsorbs sodium chloride via an electroneutral NaCl cotransporter. This cotransporter results in the reabsorption of 5% of the filtered NaCl and is the site of action of the thiazide diuretics. Gitelman’s syndrome, an autosomal recessive salt wasting disease, is caused by a mutation of this transporter.32-36 Patients with this disorder have a hypokalemic alkalosis, as with Bartter’s syndrome, but patients with Gitelman’s syndrome are less severely affected. They are distinguished from Bartter’s Syndrome by presentation outside of the neonatal period, low calcium excretion rates, and profound hypomagnesemia. Thiazide diuretics result in an increase in calcium reabsorption in this segment and are thus clinically useful as a diuretic in neonates to prevent nephrocalcinosis. Although it is not completely clear how thiazides increase calcium absorption, one theory is that by decreasing NaCl absorption they increase chloride entry across the basolateral membrane via passive diffusion resulting in a more negative transcellular potential difference. This negative potential increases the driving force for calcium transport into the cell, the rate-limiting step in calcium absorption. All of the transporters in the thick ascending limb and distal convoluted tubule, that have been studied, are in lower abundance than that of the mature segment.15,37-39 The maturation of the Na⫹-K⫹-ATPase in the thick ascending limb is mediated by perinatal increase in glucocorticoids.38 While all of the transporters are in lower abundance, one of the primary function of this
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segment is to reabsorb salt without water and generate a dilute urine so that infants do not develop hyponatremia when drinking mothers milk; a function that these segments perform adequately with the relatively small amount of sodium delivered to them.
Collecting Duct The collecting duct reabsorbs only 1% to 3% of the filtered sodium but nonetheless plays a vital role in sodium homeostasis because the final modulation of sodium reabsorption is performed here. As is shown in Figure 1, the reabsorption of sodium is via a sodium channel in principal cells in the collecting tubule. The reabsorption of sodium in this segment results in a large lumen negative potential difference that results in either the secretion of potassium, reabsorption of chloride through the paracellular pathway, or the secretion of a proton. The driving force for sodium absorption is the basolateral Na⫹-K⫹-ATPase. All of the aforementioned transporters in this segment are regulated by aldosterone. This is why aldosterone deficiency or damage to the collecting tubule from obstructive uropathy results in hyperkalemic metabolic acidosis (type IV RTA), while hyperaldosteronism results in hypertension associated with hypokalemic metabolic alkalosis. The sodium channel in this segment has been designated ENaC. Its surface expression is increased in Liddle’s syndrome,40-42 which results in hypertension associated with hypokalemic alkalosis, but low serum aldosterone levels. A defect in either the aldosterone receptor or the sodium channel itself leads to pseudohypoaldosteronism. Neonates with the channel defect are more severely affected than those with the aldosterone receptor mutation and have respiratory distress as neonates, since the reabsorption of perinatal pulmonary fluid is partially dependent on the pulmonary ENaC function.43-47 The rate of sodium reabsorption in the immature collecting tubule is much less than that of the adult segment.48-50 This is not caused by a lower plasma concentration of aldosterone or a paucity of aldosterone receptors in the neonatal segment. There is, however, a paucity of sodium channels in the neonatal collecting tubule and the ones that are there have a lower probability of being open to allow sodium passage into the cell than that in the adult segment.49 In addition
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the activity of the basolateral Na⫹-K⫹-ATPase is also lower in the neonatal cortical collecting tubule than in the adult segment.15,51
References 1. Goldsmith DI, Drukker A, Blaufox MD, et al: Hemodynamic and excretory response of the neonatal canine kidney to acute volume expansion. Am J Physiol 237: F392-F397, 1979 2. Liu FY, Cogan MG: Axial heterogeneity of bicarbonate, chloride, and water transport in the rat proximal convoluted tubule. Effects of change in luminal flow rate and of alkalemia. J Clin Invest 78:1547-1557, 1986 3. Rector FC Jr: Sodium, bicarbonate, and chloride absorption by the proximal tubule. Am J Physiol 244:F461-F471, 1983 4. Baum M, Berry CA: Evidence for neutral transcellular NaCl transport and neutral basolateral chloride exit in the rabbit convoluted tubule. J Clin Invest 74:205-211, 1984 5. Shah M, Quigley R, Baum M: Maturation of rabbit proximal straight tubule chloride/base exchange. Am J Physiol 274:F883-F888, 1998 6. Shah M, Quigley R, Baum M: Maturation of proximal straight tubule NaCl transport: Role of thyroid hormone. Am J Physiol Renal Physiol 278:F596-F602, 2000 7. Aronson PS: Ion exchangers mediating NaCl transport in the renal proximal tubule. Cell Biochem Biophys 36:147-153, 2002 8. Aronson PS, Giebisch G: Mechanisms of chloride transport in the proximal tubule. Am J Physiol 273:F179-F192, 1997 9. Kurtz I, Nagami G, Yanagawa N, et al: Mechanism of apical and basolateral Na(⫹)-independent Cl-/base exchange in the rabbit superficial proximal straight tubule. J Clin Invest 94:173-183, 1994 10. Shah M, Quigley R, Baum M: Neonatal rabbit proximal tubule basolateral membrane Na⫹/H⫹ antiporter and Cl-/base exchange. Am J Physiol 276:R1792-R1797, 1999 11. Quigley R, Baum M: Developmental changes in rabbit proximal straight tubule paracellular permeability. Am J Physiol Renal Physiol 283:F525-F531, 2002 12. Baum M, Quigley R: Maturation of proximal tubular acidification. Pediatr Nephrol 7:785-791, 1993 13. Baum M, Quigley R: Ontogeny of proximal tubule acidification [editorial]. Kidney Int 48:1697-1704, 1995 14. Beck JC, Lipkowitz MS, Abramson RG: Ontogeny of Na/H antiporter activity in rabbit renal brush border membrane vesicles. J Clin Invest 87:2067-2076, 1991 15. Schmidt U, Horster M: Na-K-activated ATPase: Activity maturation in rabbit nephron segments dissected in vitro. Am J Physiol 233:F55-F60, 1977 16. Schwartz GH, Evan AP: Development of solute transport in rabbit proximal tubule. III. Na-K-ATPase activity. Am J Physiol 246:F845-F852, 1984 17. Baum M: Neonatal rabbit juxtamedullary proximal convoluted tubule acidification. J Clin Invest 85:499-506, 1990 18. Baum M, Biemesderfer D, Gentry D: Ontogeny of rabbit renal cortical NHE3 and NHE1: effect of glucocorticoids. Am J Physiol 268:F815-F820, 1995
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19. Baum M, Quigley R: Prenatal glucocorticoids stimulate neonatal juxtamedullary proximal convoluted tubule acidification. Am J Physiol 261:F746-F752, 1991 20. Shah M, Gupta N, Dwarakanath V, et al: Ontogeny of Na⫹/H⫹ antiporter activity in rat proximal convoluted tubules. Pediatr Res 48:206-210, 2000 21. Gupta N, Tarif SR, Seikaly M, et al: Role of glucocorticoids in the maturation of the rat renal Na⫹/H⫹ antiporter (NHE3). Kidney Int 60:173-181, 2001 22. Mulder J, Baum M, Quigley R: Diffusional water permeability (PDW) of adult and neonatal rabbit renal brush border membrane vesicles. J Membr Biol 187:167-174, 2002 23. Mulder J, Haddad MN, Baum M, et al: Glucocorticoids increase osmotic water permeability in neonatal proximal tubule brush border membrane. J Am Soc Nephrol 12:20A(abstr), 2001 24. Quigley R, Baum M: Developmental changes in rabbit juxtamedullary proximal convoluted tubule water permeability. Am J Physiol 271:F871-F876, 1996 25. Quigley R, Baum M: Water transport in neonatal and adult rabbit proximal tubules. Am J Physiol Renal Physiol 283:F280-F285, 2002 26. Quigley R, Gupta N, Lisec A, et al: Maturational changes in rabbit renal basolateral membrane vesicle osmotic water permeability. J Membr Biol 174:53-58, 2000 27. Quigley R, Harkins EW, Baum M: Maturational changes in rabbit brush border membrane vesicle (BBMV) osmotic water permeability (Pf). Pediatr Res 41:282A, 1997 28. Quigley R, Harkins EW, Thomas PJ, et al: Maturational changes in rabbit renal brush border membrane vesicle osmotic water permeability. J Membr Biol 164:177-185, 1998 29. Simon DB, Bindra RS, Mansfield TA, et al: Mutations in the chloride channel gene, CLCNKB, cause Bartter’s syndrome type III. Nat Genet 17:171-178, 1997 30. Simon DB, Karet FE, Hamdan JM, et al: Bartter’s syndrome, hypokalaemic alkalosis with hypercalciuria, is caused by mutations in the Na-K-2Cl cotransporter NKCC2. Nat Genet 13:183-188, 1996 31. Simon DB, Karet FE, Rodriguez-Soriano J, et al: Genetic heterogeneity of Bartter’s syndrome revealed by mutations in the K⫹ channel, ROMK. Nat Genet 14:152-156, 1996 32. Simon DB, Lifton RP: Ion transporter mutations in Gitelman’s and Bartter’s syndromes. Curr Opin Nephrol Hypertens 7:43-47, 1998 33. Simon DB, Lifton RP: Mutations in Na(K)Cl transporters in Gitelman’s and Bartter’s syndromes. Curr Opin Cell Biol 10:450-454, 1998 34. Cruz DN, Shaer AJ, Bia MJ, et al: Gitelman’s syndrome revisited: An evaluation of symptoms and health- related quality of life. Kidney Int 59:710-717, 2001 35. Simon DB, Lifton RP: Mutations in renal ion transporters cause Gitelman’s and Bartter’s syndromes of inherited hypokalemic alkalosis. Adv Nephrol Necker Hosp 27:343-359, 1997
36. Simon DB, Nelson-Williams C, Bia MJ, et al: Gitelman’s variant of Bartter’s syndrome, inherited hypokalaemic alkalosis, is caused by mutations in the thiazide-sensitive Na-Cl cotransporter. Nat Genet 12:24-30, 1996 37. Bachmann S, Bostanjoglo M, Schmitt R, et al: Sodium transport-related proteins in the mammalian distal nephron-Distribution, ontogeny and functional aspects. Anat Embryol (Berl) 200:447-468, 1999 38. Rane S, Aperia A: Ontogeny of Na-K-ATPase activity in thick ascending limb and of concentrating capacity. Am J Physiol 249:F723-F728, 1985 39. Schmitt R, Ellison DH, Farman N, et al: Developmental expression of sodium entry pathways in rat nephron. Am J Physiol 276:F367-F381, 1999 40. Bubien JK, Ismailov II, Berdiev BK, et al: Liddle’s disease: abnormal regulation of amiloride-sensitive Na⫹ channels by beta-subunit mutation. Am J Physiol 270: C208-C213, 1996 41. Findling JW, Raff H, Hansson JH, et al: Liddle’s syndrome: prospective genetic screening and suppressed aldosterone secretion in an extended kindred. J Clin Endocrinol Metab 82:1071-1074, 1997 42. Schild L, Lu Y, Gautschi I, et al: Identification of a PY motif in the epithelial Na channel subunits as a target sequence for mutations causing channel activation found in Liddle syndrome. EMBO J 15:2381-2387, 1996 43. Chang SS, Grunder S, Hanukoglu A, et al: Mutations in subunits of the epithelial sodium channel cause salt wasting with hyperkalaemic acidosis, pseudohypoaldosteronism type 1. Nat Genet 12:248-253, 1996 44. Fuller PJ, Rogerson FM: Pseudohypoaldosteronism: kidney, lungs and colon. Clin Endocrinol (Oxf) 56:571-572, 2002 45. Geller DS, Rodriguez-Soriano J, Vallo BA, et al: Mutations in the mineralocorticoid receptor gene cause autosomal dominant pseudohypoaldosteronism type I. Nat Genet 19:279-281, 1998 46. Prince LS, Launspach JL, Geller DS, et al: Absence of amiloride-sensitive sodium absorption in the airway of an infant with pseudohypoaldosteronism. J Pediatr 135: 786-789, 1999 47. Rossier BC, Pradervand S, Schild L, et al: Epithelial sodium channel and the control of sodium balance: Interaction between genetic and environmental factors. Annu Rev Physiol 64:877-897, 2002 48. Satlin LM: Postnatal maturation of potassium transport in rabbit cortical collecting duct. Am J Physiol 266:F57F65, 1994 49. Satlin LM, Palmer LG: Apical Na⫹ conductance in maturing rabbit principal cell. Am J Physiol 270:F391-F397, 1996 50. Vehaskari VM: Ontogeny of cortical collecting duct sodium transport. Am J Physiol 267:F49-F54, 1994 51. Constantinescu AR, Lane JC, Mak J, et al: Na(⫹)-K(⫹)ATPase-mediated basolateral rubidium uptake in the maturing rabbit cortical collecting duct. Am J Physiol Renal Physiol 279:F1161-F1168, 2000
