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Renal Physiology

Homeostasis, the Milieu Inte´rieur, and the Wisdom of the

Melanie P. Hoenig and Mark L. Zeidel

Abstract The concept of has been inextricably linked to the function of the kidneys for more than a century when it was recognized that the kidneys had the ability to maintain the “internal milieu” and allow organisms the “physiologic freedom” to move into varying environments and take in varying diets and fluids. Early ingenious, Division of , Beth albeit rudimentary, experiments unlocked a wealth of secrets on the mechanisms involved in the formation of Israel Deaconess and renal handling of the gamut of , as well as that of , acid, and protein. Recent scientific Medical Center, advances have confirmed these prescient postulates such that the modern clinician is the beneficiary of a rich Harvard Medical understanding of the nephron and the ’s critical role in homeostasis down to the molecular level. This School, Boston, review summarizes those early achievements and provides a framework and introduction for the new CJASN Massachusetts series on renal physiology. – Correspondence: Clin J Am Soc Nephrol 9: 1272 1281, 2014. doi: 10.2215/CJN.08860813 Dr. Melanie P. Hoenig, Division of Nephrology, Department of Introduction advance our understanding. In this overview, we Medicine, Beth Israel Critical advances in our understanding of renal phys- will describe, all too briefly, the ingenious methods Deaconess Medical iology are unfolding at a rapid pace. Yet, remarkably, used by early investigators and the secrets they Center Clinic, Fa 8/ the lessons learned from early crude measurements and unlocked to help create the in-depth understanding 185 Pilgrim Road, careful study still hold true; indeed, classic articles still Boston, MA 02215. of renal physiology and that we Email: MHoenig@ serve as the basis for introductory textbooks on renal enjoy today. Additional details will follow in the bidmc.harvard.edu physiology and provide a solid working knowledge to new CJASN series of review articles on the physiology clinicians. Drawings with just a handful of transporters of the kidney. at each nephron segment, known for more than half a century, are sufficient to understand basic mecha- nisms of autoregulation, , and the effects of The Milieu Inte´rieur and the Kidney’s Essential —the tools needed to care for patients. Yet we Role clinicians also benefit from a treasure trove of subse- In the early 1800s, Darwin’s Theory of Evolution quent scientific advances, which have given us a de- combined with the recognition that the body chemis- tailed and comprehensive understanding of how the tries of many disparate species were remarkably kidney maintains stable body chemistries and volume similar led Claude Bernard to develop his theory balance. of the Milieu Intérieur: “The constancy of the inter- Thelayersofcomplexityandthemysteriesthat nal environment is the condition of a free and in- continue to unravel make it difficult to stay abreast dependent existence” (3). By this, he meant that the of current research. Still, the modern nephrologist is ability of our ancestor organisms to leave the oceans in good company. In 1959, a medical student wrote required that they develop the ability to “carry the to Homer Smith, the uncontested patriarch of modern oceanwiththem” in the form of an internal ocean, nephrology at the time, to inquire about his recti- bathing their cells constantly in fluids that resemble linear depiction of the nephron (Figure 1) and why the very seas from which they evolved. This concept, he failed to mention the counter current theory in although reminiscent of the notion of bodily humors his famous 1956 textbook, the Principles of Renal (4,5), marked an enormous advance because Bernard Physiology (1,2). Indeed, the structure of the loop described both the features of bodily fluids and the of Henle had been well known since the mid- need to maintain that internal milieu. Maintenance of 1800s, but the importance of that eponymous struc- the internal milieu was first called the “wisdom of ture, the gradient that it generated, and its role in the body” by Starling (6), who recognized that or- the final product urine was only just elucidated at ganisms must maintain the constancy of this internal thetimeofthestudent’s correspondence. Before ocean despite great fluctuations in diet, fluid intake, this, Homer Smith felt that the hairpin turn was and other environmental conditions. The term homeo- just a vestige of embryology. This student’smis- stasis was later coined by behaviorist Walter Cannon sive was a curiosity, rather than a criticism. Care- to describe the physiologic processes that, in ag- fully framed questions have always served to gregate, maintain the constancy of the internal

1272 Copyright © 2014 by the American Society of Nephrology www.cjasn.org Vol 9 July, 2014 Clin J Am Soc Nephrol 9: 1272–1281, July, 2014 Homeostasis and the Nephron, Hoenig and Zeidel 1273

Figure 1. | Study of the kidney requires consideration of both the role of each segment and the three-dimensional architecture. (A) Homer Smith’s rectilinear nephron mimics the straight trajectory of the fish nephron. (B) The human nephron is more intricate; the distal nephron greets its own before it returns to the environs established by the . TX, treatment. A is modified from reference 1, with permission. chemistries, as well as BP, body temperature, and energy Studies performed in frogs, rabbits, and dogs in the early balance (7). 1900s showed that the constituents of and urine The earliest insights into renal physiology came from the differed because urine contained urea, , and assiduous study of anatomy because, to a large degree, renal salts, whereas blood contains protein, , and function follows structure. Meticulous drawings and histo- very little urea. Furthermore, balance studies suggested logic study of the animal and human kidney from William that the volume and constituents of urine changed depend- Bowman (8,9), Jacob Henle (10), and others, complete with ing on changing intake or experimental infusions. In his capsule, , and convoluted tubules were available “The Secretion of Urine” monograph published in 1917, in Bernard’s time, yet the mechanisms for the formation of Cushny summarized the available literature to date and urine and the kidney’s role in homeostasis were not em- described the brisk that followed sodium braced until the next century. Until the 1920s, a debate raged infusions. He also reported findings that showed that the on the mechanism of urine formation. Some researchers kidney produced acid urine in humans and the carnivora, championed a filtration doctrine and others ascribed secre- whereas the herbivora had alkaline urine unless fed a pro- tory power to both the glomerulus and tubules (11,12). The tein diet (14). Despite this careful review and his presen- secretory theory was more popular, however, because the tation of the “modern view” that acknowledged that both sheer volume of blood that would first need to be filtered filtration and secretion could exist, Cushny struggled with and then reabsorbed by the kidney was enormous. Homer the data and felt that the theories were “diametrically op- Smith noted that the filter- strategy “seemed posed” because secretion and reabsorption would result in extravagant and physiologically complicated” (2). opposing currents along the renal epithelium. Clearly, methods were needed that could allow direct measure- ment of the filtrate and its modification along the nephron. Early Investigations Form the Framework When Wearn and Richards introduced the micropunc- An interest in comparative physiology and the advent of ture technique to the study of the kidney (first in amphibia, the marine biologic laboratories that studded the Atlantic which had large renal structures amenable to manipula- coast at the turn of the century helped frame an early tion), the debate on the formation of urine was resolved understanding of the kidney and its role in evolution. (11,15). By sampling the fluid elaborated from the glomer- Remarkably, increasingly complex fish with salty interiors ular capsule of a frog, the team demonstrated a protein- adapted to fresh water, whereas amphibians rose from the free filtrate that was otherwise similar to blood. By contrast, sea to face the challenges mandated by scarce water. In his the frog bladder urine had a different composition from famed opus, Smith summarized the observations to date the blood and was free of glucose. These findings, in light and waxed poetic (and philosophic), declaring, “Superfi- of earlier data, supported the notion that urine is formed cially, it might be said that the function of the kidneys is to by glomerular filtration, and the urine is then modified in make urine; but in a more considered view, one can say the tubules, by a combination of reabsorption and secre- that the kidneys make the stuff of philosophy itself” (13). tion. Subsequent studies by Walker and others inserted oil This impression, the sense of wonder at the intricate deal- “plugs” or “blocks” in various segments of the nephron ings of the kidney, permeates the scientific writings from and distal to the sampling pipette so that the investigators that time to this day. could avoid contamination but still study urine from 1274 Clinical Journal of the American Society of Nephrology

different segments of the nephron and, therefore, charac- of the on either side of the epithelium (18). Care- terize each segment’sfunction,theionsabsorbed,andthe ful transport studies using model epithelia from nonmam- osmolarity of the fluid (16). These investigators then de- mals, including the toad bladder, the turtle bladder, and veloped the “stop flow” technique in which they the flounder bladder, which anatomically and functionally placed a pipette distal to the oil droplet, infused fluid model collecting duct principal cells, collecting duct inter- into that segment, and then sampled the fluid at the end calated cells, and distal tubule cells, respectively, gave of that segment to determine how the fluid had been important insights into transport mechanisms in these altered (Figure 2A) (17). segments. In the late 1960s, Burg and colleagues devel- This meticulous work was confirmed in mammals by oped methods for isolating and perfusing individual extension of the micropuncture technique to rat and guinea mammalian nephron segments, first from rabbits and pig kidneys (Figure 2B). However, despite the ingenious then from mice. These preparations, along with the ability use of oil blocks to prevent upstream tubular fluid from to measure minute quantities of and volumes from reaching downstream segments and then substituting ar- these tubules with -specific electrodes, including the tificial perfusates, micropuncture studies did not permit picapnotherm (which measures minute quantities of carbon control of the composition of fluids on both sides of the dioxide), permitted investigators to examine in detail the tubular epithelium. This limitation was remedied after mechanisms, driving forces, and regulation of transport World War II, when Hans Ussing developed his famed across individual nephron segments (19). With painstaking chamber methods (Figure 3). With this strategy, transport effort, investigators dissected tubules, perfused the segment across isolated epithelia could be studied quantitatively by with fluid of specific ion concentrations, and collected the systematically altering the ionic composition and voltages “waste” fluid from the other end of the segment (Figure 4).

Figure 2. | Micropuncture and “stop flow” techniques were used to help define the role of each segment of the nephron. (A) The from the kidney of the aquatic salamander is illustrated here. A micropipette removes the filtrate at a point just proximal to a “plug” of mineral oil. To de- termine the role of the tubule in handling of individual constituents (reabsorption, secretion, or ), fluid was injected into the tubule at different locations and then collected distally. This “artificial” fluid could be altered to differ from the normal filtrate by one or more constituents. (B) A sketch of a camara lucida drawing of a guinea pig nephron after microdissection (these drawings were created with the aid of a light projector because pho- tomicrographs were not readily available at the time). Oil or mercury blocks could be inserted at various points along the nephron and fluid from the lumen could be collected and studied. A is modified from reference 17, with permission; B is modified from reference 16, with permission. Clin J Am Soc Nephrol 9: 1272–1281, July, 2014 Homeostasis and the Nephron, Hoenig and Zeidel 1275

Figure 3. | The Ussing Chamber can be used to measure ion transport between the two sides of an epithelial membrane by polarized cells. Here, a monolayer of epithelial cells separates two compartments. Fluid in the two compartments is identical to eliminate the contribution of passive paracellular diffusion driven by differences in concentration, osmotic pressure, or hydrostatic pressure. Voltage electrodes placed near the epithelial membrane maintain the potential difference at zero so that the current measured by the current electrodes reflects the movement of ions by through the epithelial cells.

This arrangement allowed investigation of individual of pharmacologic agents like chlorothiazide (24,25). In these segments of the nephron to better characterize the features detailed studies, investigators characterized vital signs, of transport, electrochemical gradients, coupling with other weight, intake, , electrolytes, clearance, plasma vol- ions, active versus , the threshold for reab- ume, and hormonal levels. These data helped solidify the sorption, and the permeability to water (1). The resulting concepts of the steady state in homeostasis (26). flurry of studies, spanning nearly 2 decades, defined the The stage was now set to identify specific renal trans- phenomenology and regulation of transport, and identified, porters, describe how they function, and characterize how at least functionally, the transporter proteins responsible for they are regulated. The remarkable reabsorptive task of the homeostasis (20). nephron tubules requires energy and active transport. It Meanwhile, in the clinical realm, the flame photometer was not long after Nobel laureate Jens Skou’s 1957 discov- became available in the late 1940s and this innovation made it ery of the Na-K-ATPase in crab nerve microsomes (27,28) possible to measure more than a dozen samples of blood for that this critical transporter was identified in the kidney. both sodium and potassium in under an hour. Before this Because of its abundance, the was identified in time, measurements were onerous and involved crude homogenates of the and medulla and both chemical extractions and precipitations (21). Studies of Na-K-ATPase activity was later measured in individual electrolytes and the metabolic derangements were now pos- segments of the nephron. The highest activity was in the sible; when this process was linked to an autoanalyzer that thick ascending limb and distal convoluted tubule (DCT) also provided chloride and total CO2,interestinacid-base and considerable activity was also observed in the proxi- disorders soared and the concept of “Gamblegrams” flour- mal tubule. Further study helped identify the polarity of ished (22). Dr. Gamble, a disciple of Henderson, studied a the renal epithelial cells with the Na-K-ATPase at the baso- range of different insults from gastrointestinal losses to ad- lateral membrane. This finding helped solidify the concept vanced CKD and their effect on electrolytes, and described that energy generated from this housekeeping enzyme, the kidney as the “remarkable organ of regulation, the kidney which maintains the normal cellular ion concentration, is sustains the chemical structure of extracellular fluid” (23). harnessed by the kidney to reabsorb the bulk of the filtered Later, availability of the automatic analyzers also spurred a sodium along with a host of other substances (29). large literature using metabolic balance studies to character- With a map in place for the role of each segment of the ize everything from bed rest or water immersion to the effects nephron and a solid understanding of factors that influence 1276 Clinical Journal of the American Society of Nephrology

Figure 4. | Study of isolated perfused tubular segments allowed study of each of the different nephron segments independently. (A) A photomicrograph of a portion of a rabbit proximal convoluted tubule during perfusion. (B) A schematic diagram of the technique. One end of the dissected tubule was connected to a micropipette, which was used to perfuse the lumen, and the other end was connected to a collection micropipette. Both the luminal fluid and the peritubular fluid could be controlled to assess tubular transport characteristics. A is reprinted with permission from Burg MB: Perfusion of isolated renal tubules. Yale J Biol Med 45: 321–326, 1972. the actions in those segments, research efforts by a multitude proteins that form the filtration slit diaphragm are charac- of investigators have set out to characterize the wide array terized and defects in several of these proteins can predict- of transporters in the nephron, their molecular structure, ably cause the nephrotic syndrome. Details of the complex their distribution in the nephron, and their role in normal meshwork of the basement membrane, a joint effort by the physiology and disease. This effort has been aided by endothelial and epithelial cells, can result in thin basement sophisticated and rapidly evolving techniques with PCR, membrane disease or hereditary syndromes, such as Alport cloning and amplification of cDNAs, and expression in syndrome. The elixir, vascular endothelial growth factor, ap- Xenopus oocytes, knockout mice, genome-wide association pears to support the endothelium; when it is compromised, studies, and other models. endothelial cells cannot sustain the regular challenges re- quired to support normal structure and function of this unique bed and, thus, thrombosis and endothelial The Ultrastructure of the Glomerulus, the Concept of injury can occur. In addition, direct micropuncture of glo- Clearance, and Autoregulation meruli allowed a detailed description of normal glomerular Once the debate on the formation of urine was settled hemodynamics and identified the derangements in glomer- and it was clear that urine was formed by filtration, ular function that occur with disease. Application of glo- reabsorption, and secretion, mysteries of the elegant filtra- merular micropuncture in animals with glomerular tion design were explored and the concept of clearance was damage caused by hypertension, , or loss of renal developed. Use of scanning electron microscopy allowed mass led directly to current therapies, including dietary researchers to appreciate the three-dimensional structure of protein restriction and use of -converting the cells and the ingenious design of the glomerulus. enzyme inhibition. Podocytes were visualized extending their primary, sec- The role of the glomerulus in “clearance” has provoked ondary, and tertiary projections to interdigitate with neigh- significant inquiry as well. Early clinical investigators ex- boring “feet” and form the filtration slit diaphragm. plored the clearance of urea and after ingestion Meanwhile, within the capillary bed, the delicate fenes- or determined the clearance with the infusion equilibrium trated endothelium drapes the basement membrane. A methods, first with and later with a host of radio- series of investigations into the dynamics of glomerular active markers, such as 125I-iothalamate. When it became ultrafiltration, first in a unique strain of Wistar rats with possible to measure the low concentrations of creatinine in surface glomeruli and later in primates, helped define serum, use of endogenous creatinine to calculate and later factors that create the net driving force for filtration estimate clearance became possible. The pitfalls of this and provided the mathematical framework for our current strategy (e.g., contribution from secretion, variability based understanding of these forces (30). In addition, studies on on muscle mass, and the changes in serum levels related to the permselectivity of the glomerular capillary wall using diet and volume status, all of which make it less precise ferritin molecules of various sizes that were neutral, anionic, than the measured GFR) are more than balanced by the or cationic revealed that particles were restricted based on convenience (33). both charge and size; this explained the limited clearance of It was soon evident that massive daily glomerular albumin at 39 A°, which is smaller than the 42 A° pores filtration could translate into life-threatening losses if there observed in the glomerular endothelial cell (31). were no mechanisms in place to limit them in the event of More recently, new techniques have helped further low perfusion. Some researchers heralded these strategies characterize these observations. For example, models of as “acute renal success” (34); however, further study of the nephron development, such as the zebrafish, transparent integrated response that maintains the GFR over a wide and rapidly growing fish with a single pair of , range of perfusion pressures provided an understanding have been indispensable to determining the effects of single of the roles of the juxtaglomerular apparatus in autoregu- defects on kidney function and development (32). Myriad lation and tubuloglomerular feedback (35). Clin J Am Soc Nephrol 9: 1272–1281, July, 2014 Homeostasis and the Nephron, Hoenig and Zeidel 1277

Sodium and Water Homeostasis polyuria, and polydipsia. Bartter initially attributed his Sodium, the major extracellular cation, plays a pivotal eponymous syndrome to a state of excess role in the maintenance of extracellular fluid volume and with angiotensin resistance but when three-fourths adre- perfusion of vital organs and capillary beds. The kidney nalectomy did not resolve the defect, he focused on the has an elaborate array of sodium transporters throughout loop of Henle. Subsequent contributions from physiolo- the nephron (36). In the proximal tubule, it is linked to an gists helped distinguish this disorder from Gitelman’s syn- elegant mechanism to reabsorb the filtered drome and helped define the interplay of transporters in 1 load by excreting H ions with the electroneutral antipor- the loop of Henle and the DCT. However, it was the stun- 1 1 ters or Na /H exchangers. Reabsorption of the ample ning characterizations by geneticists that helped identify filtered sodium also plays an important role in the reab- mutations in several genes; these discoveries explained the sorption of glucose, sulfate, , and several amino subtle differences in the phenotype of these disorders and acids. The remaining fraction of filtered sodium is reabsorb- will be carefully considered within this series (38,39) ed with unique transporters in each of the subsequent An endocrinopathy was also the initial theory that Dr. nephron segments in which apical reabsorption of sodium Liddle invoked to describe a family with early onset severe is rate limiting. These transporters include the furosemide- hypertension and hypokalemia that was notable for sup- sensitive channel in the loop of Henle, the thiazide-sensitive pressed and aldosterone. As soon as the epithelial cotransporter that is primarily in the DCT, was characterized, investigators demon- and the epithelial sodium channel transporter that is located strated complete linkage in affected individuals with a primarily in the collecting tubules (Figure 5). defect in this transporter that resulted in a constitutively Our understanding of the mechanisms of sodium trans- active sodium channel (40). Insights into the molecular bi- port along the nephron comes from disparate sources. The ology, structure, function, and regulation of each of these advent of sulfonamides, investigated initially as much- sodium transporters has clearly enriched our grasp of re- needed antibiotics after World War II, were promptly nal physiology and complemented earlier predictions. recognized for their saluretic effects and soon revealed a Hormonal and sympathetic nervous input can greatly wealth of secrets regarding the transport of sodium augment sodium reabsorption, particularly by angiotensin II throughout the nephron (37). Genetic disorders also pro- in the proximal tubule and aldosterone in the distal nephron, vided important clues. Endocrinologist Frederick Bartter whereas the effect of atrial natriuretic peptide in the medullary and others described a set of youths afflicted with growth collecting duct was found to be the opposite (41). Knowledge and mental retardation, muscle cramps, salt craving, of these transporters is critical to the understanding of the

Figure 5. | The unique transporters and cell structure of each segment of the nephron work in concert to maintain homeostasis.ENaC, epithelial sodium channel; NKCC2, Na+-K+-2Cl cotransporter; ROMK, renal outer medullary potassium. 1278 Clinical Journal of the American Society of Nephrology

clinical use of diuretics and the care of patients with a wide dictates the charged state of proteins that affects confor- variety of issues, from the patient with essential hypertension mational shape, enzymatic activity, binding, and cellular to the complex patient with cirrhosis. transport and, thus, allows proteins to perform essential In the 1960s, Guyton proposed that all hypertension, metabolic functions. Although some acid-base enthusiasts ultimately, is a result of the failure of the kidney to excrete enjoy consideration of the “strong ion difference” to rec- the excess of total body sodium with a normal pressure oncile data, the normal kidney’s remarkable response to natriuresis (42). Although this theory has been disputed subtle differences in pH or, more likely, intracellular CO2 over the years, it is notable that monogenic defects that does not take these differences into account. Although the lead to hypertension or hypotension are found exclusively exact mechanisms used to sense pH are still not yet un- in genes that encode either renal transporter proteins or derstood, it is well known that the kidney plays a domi- proteins that regulate the function of renal transporter pro- nant role in the regulation of the acid-base balance (47). teins and ultimately renal sodium handling. Indeed, with acidosis, a complex intracellular cascade en- Despite mounting evidence on the importance of sodium sues, including activation of the electroneutral sodium- in the kidney’s role in homeostasis, investigators in the coupled transporter for , an increase 1950s soon recognized that the measured serum sodium in glutamine metabolism and ammoniagenesis, as well as correlated poorly with the total body sodium by compar- an increase in the expression of the sodium anti- ing these values in heterogeneous patients with a variety porter (48,49). Additional orchestrated responses in the of chronic conditions. Instead, the serum sodium corre- distal nephron with the help of the machinery in the in- lated well with the serum osmolality (particularly when tercalated cells and neighboring principal cells contribute corrections were made for the osmotic contributions of to the kidneys’ response to the challenges of acidemia (50). glucose and nonprotein nitrogen) (43). (Of note, the same By contrast, metabolic alkalosis can be easily and rapidly investigators also recognized that the Na21-K1/total body handled by the kidney by excretion of excess filtered bi- water ratio correlated closely with “corrected” serum so- carbonate or may be perpetuated by the kidney because of a dium and explained the importance for accounting of po- response to volume depletion with secondary activation of tassium repletion during the treatment of hyponatremia.) the renin-angiotensin and aldosterone axis or from primary Maintenance of the was noted to be hyperaldosteronism. These processes are intimately linked to tightly regulated by both the release of and the regulation of potassium as well. Finally, the kidney the kidneys’ response (44). This interplay between the two responds to the challenges of primary respiratory disorders is essential for water homeostasis, a critical factor in the to offset the effects of these disturbances in a predictable maintenance of cell volume. Although cells have devel- fashion (51,52). In chronic respiratory alkalosis, the kidney oped strategies to deal with excess or insufficient water, can decrease acid excretion by decreasing ammonia produc- these volume regulatory changes require extrusion or in- tion and bicarbonate retention; in respiratory acidosis, activ- clusion of electrolytes, which alters the cellular interior ity of the Na-H antiporter is augmented so that more milieu and wreaks havoc on normal cellular function. bicarbonate is reabsorbed (53). Understanding of these com- Later adaptations allow cells to return toward normalcy plex processes helps in the care of actual patients who de- but only within a small range. Water reabsorption requires velop acid-base disorders from a wide variety of insults. the ability to both establish an osmotic gradient in the kidney and to reabsorb water from the urinary filtrate. The kidney has an elegant strategy to concentrate or dilute Potassium Homeostasis the urine by its response to vasopressin and the ability to Excretion of potassium in excess of the amount filtered is deploy to the luminal membrane (45). At least another triumph of the kidney in electrolyte homeostasis and seven isoforms are expressed in the kidney and its mechanism, another important milestone in the under- play important roles at different sites. In the proximal tu- standing of renal pathophysiology (54–56). Potassium, the bule and thin descending limb, aquaporin 1 appears to major cation in the body, must be maintained in high con- serve as the dominant gateway for water reabsorption, centrations in the intracellular space and a low concentration fi whereas traf cking of aquaporin 2 along cytoskeletal ele- in the extracellular fluid to allow both normal cellular func- ments in the collecting duct cells allows reabsorption of tion and the considerable gradient required for excitation of water and urine concentration in the principal cells of nerves and contractions of muscles. The kidney plays its role the collecting ducts (46). Detailed study of the molecular by reabsorption of nearly all of the filtered load proximally structure and cell physiology of these transporters has al- and variable secretion in the distal nephron. Along the way, lowed insight into the rare genetic diseases that affect potassium is secreted into the lumen with the help of the aquaporins, such as congenital nephrogenic diabetes insipidus renal outer medullary potassium transporter, which provides and the common acquired defects related to lithium, cal- sufficient substrate to the NaKC2 transporter, and by the cium, and even urinary obstruction. Similarly, study of the “big potassium,”“maxi,” or high conductance transporter vasopressin receptor has resulted in new strategies and (57). Both appear to play an important role in the secretion pharmacologic agents for the treatment of states of excess of potassium in the distal nephron dictated by the influence antidiuretic and polycystic kidney disease. of aldosterone and magnitude of distal flow (58).

Acid-Base Homeostasis Divalent Cations and Phosphate Homeostasis Maintenance of pH is a critical activity of the kidney and The kidney plays a critical role in maintaining both normal is essential for normal cellular function because the pH extracellular ion levels and the vast repositories of Clin J Am Soc Nephrol 9: 1272–1281, July, 2014 Homeostasis and the Nephron, Hoenig and Zeidel 1279

calcium needed for normal intracellular function and main- Renal Physiology Has Significant Clinical Relevance tenance of the skeleton. Integrated control by parathyroid One of the considerable gifts to the field of nephrology is hormone and 1,25-dihydroxyvitamin D helps achieve this that there is a deep and growing understanding of the end (59). Calcium is reabsorbed through a paracellular route intricacies of kidney function and the ingenious methods that in the proximal tubule and the thick ascending limb, the kidney uses to govern homeostasis. These discoveries whereas there are unique, well characterized transient recep- complement observations made with careful consideration tor potential ion channels in the DCT. Each of these regions and primitive measurements in the past. Predictions on the is controlled by local effects prescribed by the calcium sens- movement of ions have been translated by detailed charac- ing receptor and modulated by the pH (60). There is also an terization, on the molecular level, of ion transporters and interesting pas de deux between sodium and calcium han- provide insight into the integrated responses of the kidney to dling. Volume depletion or decreased sodium delivery de- the maintenance of the internal milieu. Those of us who are creases urinary calcium either by increasing proximal privileged enough to care for patients with disorders of the reabsorption or by promoting reabsorption in the DCT kidney can utilize knowledge gleaned in the laboratory to with changes in the activity of the basolateral Na1/Ca1 understand real clinical concerns. exchanger or hyperpolarization of the luminal plasma Because it is difficult for any practicing nephrologist to membrane (61,62). The fate of homeostasis is stay abreast of the rapidly unfolding revelations, this new intimately linked to that of other cations. In parallel to renal physiology series will serve as an update to the calcium, the majority of filtered magnesium is reabsorbed current understanding of the nephron. CJASN will provide in the proximal tubule and thick ascending limb by para- careful reviews of the nephron, sequentially, from the glo- cellular movement mediated, in part, by the claudin pro- merulus to the collecting duct, followed by a review on the teins, which govern ion movement through the otherwise control of urinary drainage and bladder function. Next, tight junctions in those regions (63). In addition, the action there will be a series of cohesive reviews that will address of the renal outer medullary potassium transporter to sup- how the kidney factors in the integrated response to so- ply more potassium in the lumen for the NaKC2 trans- dium and water homeostasis, potassium handling, acid- porter is thought to create a lumen-positive transepithelial base homeostasis, excretion of organic cations and anions, potential difference that can favor cation reabsorption in and divalent cations and phosphate homeostasis. Protein this segment (64). By contrast, in the DCT, movement of metabolism and control of renal nitrogen excretion as well Mg21 is an active transcellular process. At this segment, as sensory functions of the kidney will follow. Finally, the cation channels from the melastin transient receptor po- role of the interstitium and hormonal function of the kid- tential subfamily play a major role in this endeavor, evi- ney will be considered. denced by Mg21 wasting seen in the rare genetic disorders In jingoistic banter that many nephrologists can echo with defects in this channel (65). with sincerity, Homer Smith asserted, “The responsibility Emerging details on phosphate metabolism have for maintaining the composition of [the internal milieu] ... identified a family of sodium phosphate transporters that devolves to the kidneys. It is no exaggeration to say that help reabsorb the bulk of the filtered phosphate in the the composition of the body fluids is determined not by proximal tubule (66). These transporters appear to be affected what the mouth takes in but by what the kidneys keep; by a series of factors, including the recently characterized they are the master chemists of our internal environment” fibroblast growth factor 23 and its obligate coreceptor Klotho, (13).With this series in hand, the CJASN reader will be which together, via the fibroblast growth factor receptor, privy to the cutting-edge science of nephrology; knowl- inhibit the reabsorption of sodium-dependent phosphate edge of the dramatic advances in our understanding will reabsorption and lower levels by downregulat- likely turn any nephrologist into a philosopher. ing the gene for 1a-hydroxylase (67). Phosphate that es- capes proximal reabsorption and is delivered distally is available to bind H1 as an important source of “titratable Authors’ Note acid” (68). The landmark works described in this article are freely accessible on the Internet for those who would like to indulge in the primary sources. These texts and manu- Protein Metabolism scripts have been made available as part of Google Scholar Even Richard Bright (69) in the 1820s recognized that in and the Internet Archive, nonprofit digital libraries with “dropsy” (or edema) of renal origin, urea was increased in the mission to allow universal access to all knowledge. the blood and decreased in the urine such that urea could These archival texts include Homer Smith’s The Principals serve as a marker of kidney failure. One hundred years later, of Renal Physiology and From Fish to Philosopher,Bowman’s Thomas Addis tried to assess renal function using urea treatise On the Structure and Use of the Malpighian Bodies of clearance and “rest” the kidneys from the “work” of clearing the Kidney, and texts by Starling, Cushny, Cannon, and proteins by prescribing a low-protein diet (70). Landmark Bernard. studies by Brenner’s group suggested that this strategy was correct because high-protein diets fed to laboratory animals Disclosures can be shown to increase renal blood flow and glomerular None. filtration and subsequently contribute to the progression of fi CKD (71). Nevertheless, the speci c mediators that lead to References fi hyper ltration and contribute to the changes seen with a 1. Smith HW: Principles of Renal Physiology, New York, Oxford high-protein diet are yet to be determined. University Press, 1956 1280 Clinical Journal of the American Society of Nephrology

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