A Physiological Analysis of Hyponatremia: Implications for Patients on Peritoneal Dialysis

The basis for hyponatremia is a negative balance for sodium (Na+) plus potassium (K+) and/or a positive balance for water. In patients with normal renal function, vasopressin is needed to prevent the excretion of electrolyte-free water. Vasopressin is not important when there is little residual renal function. If hyponatremia is accompanied by a quantitatively appropriate gain in weight, this implies that a gain of electrolyte-free water was the basis for hyponatremia. In the absence of this weight gain, a loss of salts is to be suspected. If the extracellular fluid (ECF) volume is obviously low, hyponatremia is due to a deficit of NaCl, unless there is a deficit of K+. With a KCl deficit and a contracted ECF volume, there should also be a large shift of Na+ into cells, so metabolic alkalosis would not be an expected finding. In contrast, those patients with no change in weight who have a normal or expanded ECF volume are subdivided into those with a gain of solutes restricted to the ECF compartment (glucose, mannitol), or those with a deficit of solutes of intracellular fluid origin, which implies that a catabolic state (malnutrition) may be present.

Regulation of the Renal NaCl Cotransporter and Its Role in Potassium Homeostasis

Daily dietary potassium (K+) intake may be as large as the extracellular K+ pool. To avoid acute hyperkalemia, rapid removal of K+ from the extracellular space is essential. This is achieved by translocating K+ into cells and increasing urinary K+ excretion. Emerging data now indicate that the renal thiazide-sensitive NaCl cotransporter (NCC) is critically involved in this homeostatic kaliuretic response. This suggests that the early distal convoluted tubule (DCT) is a K+ sensor that can modify sodium (Na+) delivery to downstream segments to promote or limit K+ secretion. K+ sensing is mediated by the basolateral K+ channels Kir4.1/5.1, a capacity that the DCT likely shares with other nephron segments. Thus, next to K+-induced aldosterone secretion, K+ sensing by renal epithelial cells represents a second feedback mechanism to control K+ balance. NCC’s role in K+ homeostasis has both physiological and pathophysiological implications. During hypovolemia, NCC activation by the renin-angiotensin system stimulates Na+ reabsorption while preventing K+ secretion. Conversely, NCC inactivation by high dietary K+ intake maximizes kaliuresis and limits Na+ retention, despite high aldosterone levels. NCC activation by a low-K+ diet contributes to salt-sensitive hypertension. K+-induced natriuresis through NCC offers a novel explanation for the antihypertensive effects of a high-K+ diet. A possible role for K+ in chronic kidney disease is also emerging, as epidemiological data reveal associations between higher urinary K+ excretion and improved renal outcomes. This comprehensive review will embed these novel insights on NCC regulation into existing concepts of K+ homeostasis in health and disease.

K+-induced natriuresis is preserved during Na+ depletion and accompanied by inhibition of the Na+-Cl- cotransporter

During hypovolemia and hyperkalemia, the kidneys defend homeostasis by Na(+) retention and K(+) secretion, respectively. Aldosterone mediates both effects, but it is unclear how the same hormone can evoke such different responses. To address this, we mimicked hypovolemia and hyperkalemia in four groups of rats with a control diet, low-Na(+) diet, high-K(+) diet, or combined diet. The low-Na(+) and combined diets increased plasma and kidney ANG II. The low-Na(+) and high-K(+) diets increased plasma aldosterone to a similar degree (3-fold), whereas the combined diet increased aldosterone to a greater extent (10-fold). Despite similar Na(+) intake and higher aldosterone, the high-K(+) and combined diets caused a greater natriuresis than the control and low-Na(+) diets, respectively (P < 0.001 for both). This K(+)-induced natriuresis was accompanied by a decreased abundance but not phosphorylation of the Na(+)-Cl(-) cotransporter (NCC). In contrast, the epithelial Na(+) channel (ENaC) increased in parallel with aldosterone, showing the highest expression with the combined diet. The high-K(+) and combined diets also increased WNK4 but decreased Nedd4-2 in the kidney. Total and phosphorylated Ste-20-related kinase were also increased but were retained in the cytoplasm of distal convoluted tubule cells. In summary, high dietary K(+) overrides the effects of ANG II and aldosterone on NCC to deliver sufficient Na(+) to ENaC for K(+) secretion. K(+) may inhibit NCC through WNK4 and help activate ENaC through Nedd4-2.

Effects of angiotensin II on kinase-mediated sodium and potassium transport in the distal nephron

PURPOSE OF REVIEW

The aim is to review the recently reported effects of angiotensin II (Ang II) on sodium and potassium transport in the aldosterone-sensitive distal nephron, including the signaling pathways between receptor and transporter, and the (patho)physiological implications of these findings.

RECENT FINDINGS

Ang II can activate the sodium chloride cotransporter (NCC) through phosphorylation by Ste20-related, proline-alanine rich kinase (SPAK), an effect that is independent of aldosterone but dependent on with no lysine kinase 4 (WNK4). A low-sodium diet (high Ang II) activates NCC, whereas a high-potassium diet (low Ang II) inhibits NCC. NCC activation also contributes to Ang-II-mediated hypertension. Ang II also activates the epithelial sodium channel (ENaC) additively to aldosterone, and this effect appears to be mediated through protein kinase C and superoxide generation by nicotinamide adenine dinucleotide phosphate oxidase. While aldosterone activates the renal outer medullary potassium channel (ROMK), this channel is inhibited by Ang II. The key kinase responsible for this effect is c-Src, which phosphorylates ROMK and leaves WNK4 unphosphorylated to further inhibit ROMK.

SUMMARY

The effects of Ang II on NCC, ENaC, and ROMK help explain the renal response to hypovolemia which is to conserve both sodium and potassium. Pathophysiologically, Ang-II-induced activation of NCC appears to contribute to salt-sensitive hypertension.

Aldosterone does not require angiotensin II to activate NCC through a WNK4-SPAK-dependent pathway

We and others have recently shown that angiotensin II can activate the sodium chloride cotransporter (NCC) through a WNK4–SPAK-dependent pathway. Because WNK4 was previously shown to be a negative regulator of NCC, it has been postulated that angiotensin II converts WNK4 to a positive regulator. Here, we ask whether aldosterone requires angiotensin II to activate NCC and if their effects are additive. To do so, we infused vehicle or aldosterone in adrenalectomized rats that also received the angiotensin receptor blocker losartan. In the presence of losartan, aldosterone was still capable of increasing total and phosphorylated NCC twofold to threefold. The kinases WNK4 and SPAK also increased with aldosterone and losartan. A dose-dependent relationship between aldosterone and NCC, SPAK, and WNK4 was identified, suggesting that these are aldosterone-sensitive proteins. As more functional evidence of increased NCC activity, we showed that rats receiving aldosterone and losartan had a significantly greater natriuretic response to hydrochlorothiazide than rats receiving losartan only. To study whether angiotensin II could have an additive effect, rats receiving aldosterone with losartan were compared with rats receiving aldosterone only. Rats receiving aldosterone only retained more sodium and had twofold to fourfold increase in phosphorylated NCC. Together, our results demonstrate that aldosterone does not require angiotensin II to activate NCC and that WNK4 appears to act as a positive regulator in this pathway. The additive effect of angiotensin II may favor electroneutral sodium reabsorption during hypovolemia and may contribute to hypertension in diseases with an activated renin–angiotensin–aldosterone system.

Intrarenal urea recycling leads to a higher rate of renal excretion of potassium: an hypothesis with clinical implications

PURPOSE OF REVIEW

This review aims to illustrate why urea recycling may play an important role in potassium (K⁺) excretion and to emphasize its potential clinical implications.

RECENT FINDINGS

A quantitative analysis of the process of intrarenal urea recycling reveals that the amount of urea delivered to the distal convoluted tubule is about two-fold larger than the quantity of urea excreted in the urine. As the number of osmoles delivered to the late cortical distal nephron (CCD) determines its flow rate when aquaporin 2 water channels have been inserted in the luminal membrane of principal cells, urea recycling may play an important role in regulating the rate of excretion of K⁺ when the distal delivery of electrolytes is not very high.

SUMMARY

Urea recycling aids the excretion of K⁺; this is especially important in patients with disorders or those who are taking drugs that lead to a less lumen-negative voltage in the CCD. As a large quantity of urea is reabsorbed daily in the inner medullary collecting duct, the assumption made in the calculation of the transtubular K concentration gradient that there is no appreciable reabsorption of osmoles downstream CCD is not valid.

Potassium excretion during antinatriuresis: perspective from a distal nephron model

Renal excretion of Na(+) and K(+) must be regulated independently within the distal nephron, but is complicated by the fact that changing excretion of one solute requires adjustments in the transport of both. It is long known that hypovolemia increases Na(+) reabsorption while impairing K(+) excretion, even when distal Na(+) delivery is little changed. Renewed interest in this micropuncture observation came with identification of the molecular defects underlying familial hyperkalemic hypertension (FHH), which also increases distal Na(+) reabsorption and impairs K(+) excretion. In this work, a mathematical model of the distal nephron (Weinstein AM. Am J Physiol Renal Physiol 295: F1353-F1364, 2008), including the distal convoluted tubule (DCT), connecting segment (CNT), and collecting duct (CD), is used to examine renal K(+) excretion during antinatriuresis. Within the model, Na(+) avidity is represented as the modulation of DCT NaCl reabsorption, and the K(+) secretion signal is an aldosterone-like effect on principal cells of the CNT and CD. The first model prediction is that changes in DCT NaCl reabsorption are not mediated by NaCl cotransporter density alone, but require additional adjustments of both peritubular Na-K-ATPase and KCl cotransport. A second observation is that the CNT response to increased DCT Na(+) reabsorption should not only stabilize CD K(+) delivery but also compensate for the compromise of K(+) excretion downstream, as low Na(+) delivery increases CD K(+) reabsorption. Such anticipatory regulation is seen with the aldosterone response of hypovolemia, while the FHH phenotype manifests enhanced DCT NaCl transport but a blunted aldosterone effect. The model emphasizes the need for two distinct signals to the distal nephron, regulating Na(+) excretion and K(+) excretion, in contrast to a single switch apportioning NaCl reabsorption and Na(+)-for-K(+) exchange.

Through a glass darkly: salt transport by the distal tubule

The distal convoluted tubule (DCT) plays a central role in blood pressure and potassium homeostasis, as evidenced by diseases that occur when its function is modified. The paper by van der Lubbe and colleagues makes clear that angiotensin II itself increases the activity and abundance of the thiazide-sensitive Na-Cl cotransporter (NCC), independent of changes in circulating aldosterone. This Commentary provides additional perspective on that work.

Effects of potassium on expression of renal sodium transporters in salt-sensitive hypertensive rats induced by uninephrectomy

Dietary potassium is an important modulator of systemic blood pressure (BP). The purpose of this study was to determine whether dietary potassium is associated with an altered abundance of major renal sodium transporters that may contribute to the modulation of systemic BP. A unilateral nephrectomy (uNx) was performed in male Sprague-Dawley rats, and the rats were fed a normal-salt diet (0.3% NaCl) for 4 wk. Thereafter, the rats were fed a high-salt (HS) diet (3% NaCl) for the entire experimental period. The potassium-repleted (HS+KCl) group was given a mixed solution of 1% KCl as a substitute for drinking water. We examined the changes in the abundance of major renal sodium transporters and the expression of mRNA of With-No-Lysine (WNK) kinases sequentially at 1 and 3 wk. The systolic BP of the HS+KCl group was decreased compared with the HS group (140.3 ± 2.97 vs. 150.9 ± 4.04 mmHg at 1 wk; 180.3 ± 1.76 vs. 207.7 ± 6.21 mmHg at 3 wk). The protein abundances of type 3 Na(+)/H(+) exchanger (NHE3) and Na(+)-Cl(-) cotransporter (NCC) in the HS+KCl group were significantly decreased (53 and 45% of the HS group at 1 wk, respectively; 19 and 8% of HS group at 3 wk). WNK4 mRNA expression was significantly increased in the HS+KCl group (1.4-fold of control at 1 wk and 1.9-fold of control at 3 wk). The downregulation of NHE3 and NCC may contribute to the BP-attenuating effect of dietary potassium associated with increased urinary sodium excretion.

The WNKs: atypical protein kinases with pleiotropic actions

WNKs are serine/threonine kinases that comprise a unique branch of the kinome. They are so-named owing to the unusual placement of an essential catalytic lysine. WNKs have now been identified in diverse organisms. In humans and other mammals, four genes encode WNKs. WNKs are widely expressed at the message level, although data on protein expression is more limited. Soon after the WNKs were identified, mutations in genes encoding WNK1 and -4 were determined to cause the human disease familial hyperkalemic hypertension (also known as pseudohypoaldosteronism II, or Gordon's Syndrome). For this reason, a major focus of investigation has been to dissect the role of WNK kinases in renal regulation of ion transport. More recently, a different mutation in WNK1 was identified as the cause of hereditary sensory and autonomic neuropathy type II, an early-onset autosomal disease of peripheral sensory nerves. Thus the WNKs represent an important family of potential targets for the treatment of human disease, and further elucidation of their physiological actions outside of the kidney and brain is necessary. In this review, we describe the gene structure and mechanisms regulating expression and activity of the WNKs. Subsequently, we outline substrates and targets of WNKs as well as effects of WNKs on cellular physiology, both in the kidney and elsewhere. Next, consequences of these effects on integrated physiological function are outlined. Finally, we discuss the known and putative pathophysiological relevance of the WNKs.

WNK4 diverts the thiazide-sensitive NaCl cotransporter to the lysosome and stimulates AP-3 interaction

With-no-lysine kinase 4 (WNK4) inhibits electroneutral sodium chloride reabsorption by attenuating the cell surface expression of the thiazide-sensitive NaCl cotransporter (NCC). The underlying mechanism for this effect remains poorly understood. Here, we explore how WNK4 affects the trafficking of NCC through its interactions with intracellular sorting machinery. An analysis of NCC cell surface lifetime showed that WNK4 did not alter the net rate of cotransporter internalization. In contrast, direct measurements of forward trafficking revealed that WNK4 attenuated the rate of NCC surface delivery, inhibiting the anterograde movement of cotransporters traveling to the plasma membrane from the trans-Golgi network. The response was paralleled by a dramatic reduction in NCC protein abundance, an effect that was sensitive to the lysosomal protease inhibitor leupeptin, insensitive to proteasome inhibition, and attenuated by endogenous WNK4 knockdown. Subcellular localization studies performed in the presence of leupeptin revealed that WNK4 enhanced the accumulation of NCC in lysosomes. Moreover, NCC immunoprecipitated with endogenous AP-3 complexes, and WNK4 increased the fraction of cotransporters that associate with this adaptor, which facilitates cargo transport to lysosomes. WNK4 expression also increased LAMP-2-positive lysosomal content, indicating that the kinase may act by a general AP-3-dependent mechanism to promote cargo delivery into the lysosomal pathway. Taken together, these findings indicate that WNK4 inhibits NCC activity by diverting the cotransporter to the lysosome for degradation by way of an AP-3 transport carrier.

Sodium-bicarbonated mineral water decreases aldosterone levels without affecting urinary excretion of bone minerals

AIM To assess in healthy postmenopausal women the influence of consuming sodium-bicarbonated mineral water on postprandial evolution of serum aldosterone and urinary electrolyte excretion.

METHODS

Eighteen postmenopausal women consumed 500 ml of two sodium-bicarbonated mineral waters (sodium-bicarbonated mineral water 1 and sodium-bicarbonated mineral water 2) and a low-mineral water with a standard meal. Postprandial blood samples were taken at 60, 120, 240, 360 and 420 min and aldosterone concentrations were measured. Postprandial urinary minerals were determined.

RESULTS

Urinary and total mineral excretion and urinary mineral concentrations did not differ except for sodium concentration, which was significantly higher with sodium-bicarbonated mineral water 1 than with low-mineral water (P = 0.005). There was a time effect (P = 0.003) on the aldosterone concentration. At 120 min, aldosterone concentrations were lower with sodium-bicarbonated mineral water 1 (P = 0.021) and sodium-bicarbonated mineral water 2 (P = 0.030) compared with low-mineral water.

CONCLUSION

Drinking a sodium-rich bicarbonated mineral water with a meal increases urinary sodium concentration excretion without changes in the excretion of potassium and bone minerals.

Control of potassium excretion: a Paleolithic perspective

PURPOSE OF REVIEW

Regulation of potassium (K) excretion was examined in an experimental setting that reflects the dietary conditions for humans in Paleolithic times (high, episodic intake of K with organic anions; low intake of NaCl), because this is when major control mechanisms were likely to have developed.

RECENT FINDINGS

The major control of K secretion in this setting is to regulate the number of luminal K channels in the cortical collecting duct. Following a KCl load, the K concentration in the medullary interstitial compartment rose; the likely source of this medullary K was its absorption by the H/K-ATPase in the inner medullary collecting duct. As a result of the higher medullary K concentration, the absorption of Na and Cl was inhibited in the loop of Henle, and this led to an increased distal delivery of a sufficient quantity of Na to raise K excretion markedly, while avoiding a large natriuresis. In addition, because K in the diet was accompanied by 'future' bicarbonate, a role for bicarbonate in the control of K secretion via 'selecting' whether aldosterone would be a NaCl-conserving or a kaliuretic hormone is discussed.

SUMMARY

This way of examining the control of K excretion provides new insights into clinical disorders with an abnormal plasma K concentration secondary to altered K excretion, and also into the pathophysiology of calcium-containing kidney stones.

The Role of Aldosterone in Renal Sodium Transport

Aldosterone is the body's major hormone involved in volume homeostasis because of its effects on sodium reabsorption in the distal nephron. Our comprehension of the signaling pathways that this mineralocorticoid unleashes has been enhanced through the convergence of bedside physiologic observations with advances in medical genetics and molecular biology. This overview updates our current understanding of the aldosterone-initiated pathways throughout the distal nephron to promote sodium retention. Three essential features of the pathways are explored: how the mineralocorticoid gains specificity and targets gene transcription in distal tubular cells; how the key endpoints of aldosterone action in these cells-the epithelial sodium channel, the thiazide-sensitive sodium chloride cotransporter, and Na,K,ATPase-are regulated; and how 3 kinases, directly or indirectly, are activated by aldosterone and serve as critical intermediaries in regulating the sodium transporters. Remarkably, perturbations in many genes integral to aldosterone-induced pathways result in blood-pressure abnormalities. The familial disorders of hypertension and hypotension that follow from these mutated genes are presented with their molecular and physiologic consequences. The clustering of so many genetic disorders within the aldosterone-sensitive distal nephron supports the hypothesis that renal sodium regulation plays a pivotal role in long-term blood-pressure control. Identifying and characterizing other components of the pathways that modulate these sodium transporters represent the core challenges in this scientific field. It is posited that meeting these challenges will help elucidate the pathogenesis of human hypertension and provide new therapeutic options for its treatment.

Osmotic demyelination syndrome: a potentially avoidable disaster

Osmotic demyelination of the brain (ODS) is a dreaded complication that typically occurs several days after aggressive therapy for chronic hyponatraemia, but is eminently avoidable. In this teaching exercise, Professor McCance, an imaginary consultant, is asked to explain how he would have treated a 28-year-old female who had hyperkalaemia, hypoglycaemia, hypotension and hyponatraemia (118 mM) to prevent the development of ODS. He begins with a review of the physiology, including his own landmark work on chronic hyponatraemia associated with a contracted extracellular fluid volume. Adding quantitative analysis, the cause of the excessive rise in plasma sodium concentration is revealed, and a better plan for therapy is proposed.

Aminopeptidases in visceral organs during alterations in body fluid volume and osmolality

Enzymatic cleavage of some peptides in the local environment could be included among the mechanisms related to the regulation of hydrosaline balance. In order to examine this hypothesis, we measured representative aminopeptidase activities in visceral organs of rats after applying certain hydrosaline challenges. Decreased levels (about 30%) of particulate puromycin-insensitive-neutral aminopeptidase in the renal medulla and of soluble acid aminopeptidase in the lung were observed under hyperosmolality and hypovolemia. Decreased levels (more than 45%) of particulate type-I-pyroglutamyl aminopeptidase in the heart were observed under altered volemia. These results indicate that aminopeptidases at these anatomical locations might be involved in the regulation of body fluid volume and osmolality.

Mechanisms used to dispose of progressively increasing alkali load in rats

Our objective was to describe the process of alkali disposal in rats. Balance studies were performed while incremental loads of alkali were given to rats fed a low-alkali diet or their usual alkaline ash diet. Control groups received equimolar NaCl or KCl. Virtually all of the alkali was eliminated within 24 h when the dose exceeded 750 micromol. The most sensitive response to alkali input was a decline in the excretion of NH(4)(+). The next level of response was to increase the excretion of unmeasured anions; this rise was quantitatively the most important process in eliminating alkali. The maximum excretion of citrate was approximately 70% of its filtered load. An even higher alkali load augmented the excretion of 2-oxoglutarate to >400% of its filtered load. Only with the largest alkali load did bicarbonaturia become quantitatively important. We conclude that renal mechanisms eliminate alkali while minimizing bicarbonaturia. This provides a way of limiting changes in urine pH without sacrificing acid-base balance, a process that might lessen the risk of kidney stone formation.