Treatment Guidelines for Hyponatremia: Stay the Course

International guidelines designed to minimize the risk of complications that can occur when correcting severe hyponatremia have been widely accepted for a decade. On the basis of the results of a recent large retrospective study of patients hospitalized with hyponatremia, it has been suggested that hyponatremia guidelines have gone too far in limiting the rate of rise of the serum sodium concentration; the need for therapeutic caution and frequent monitoring of the serum sodium concentration has been questioned. These assertions are reminiscent of a controversy that began many years ago. After reviewing the history of that controversy, the evidence supporting the guidelines, and the validity of data challenging them, we conclude that current safeguards should not be abandoned. To do so would be akin to discarding your umbrella because you remained dry in a rainstorm. The authors of this review, who represent 20 medical centers in nine countries, have all contributed significantly to the literature on the subject. We urge clinicians to continue to treat severe hyponatremia cautiously and to wait for better evidence before adopting less stringent therapeutic limits.

Use of Urine Electrolytes and Urine Osmolality in the Clinical Diagnosis of Fluid, Electrolytes, and Acid-Base Disorders

We discuss the use of urine electrolytes and urine osmolality in the clinical diagnosis of patients with fluid, electrolytes, and acid-base disorders, emphasizing their physiological basis, their utility, and the caveats and limitations in their use. While our focus is on information obtained from measurements in the urine, clinical diagnosis in these patients must integrate information obtained from the history, the physical examination, and other laboratory data.

Renal potassium physiology: integration of the renal response to dietary potassium depletion

We summarize the current understanding of the physiology of the renal handling of potassium (K⁺), and present an integrative view of the renal response to K⁺ depletion caused by dietary K⁺ restriction. This renal response involves contributions from different nephron segments, and aims to diminish the rate of excretion of K⁺ as a result of: decreasing the rate of electrogenic (and increasing the rate of electroneutral) reabsorption of sodium in the aldosterone-sensitive distal nephron (ASDN), decreasing the abundance of renal outer medullary K⁺ channels in the luminal membrane of principal cells in the ASDN, decreasing the flow rate in the ASDN, and increasing the reabsorption of K⁺ in the cortical and medullary collecting ducts. The implications of this physiology for the association between K⁺ depletion and hypertension, and K⁺ depletion and formation of calcium kidney stones are discussed.

Approach to the Treatment of Diabetic Ketoacidosis

Diabetic ketoacidosis (DKA), a common cause of severe metabolic acidosis, remains a life-threatening condition due to complications of both the disease and its treatment. This Acid-Base and Electrolyte Teaching Case discusses DKA management, emphasizing complications of treatment. Because cerebral edema is the most common cause of mortality and morbidity, especially in children with DKA, we emphasize its pathophysiology and implications for therapy. The risk for cerebral edema may be minimized by avoiding a bolus of insulin, excessive saline resuscitation, and a decrease in effective plasma osmolality early in treatment. A goal of fluid therapy is to lower muscle venous Pco₂ to ensure effective removal of hydrogen ions by bicarbonate buffer in muscle and diminish the binding of hydrogen ions to intracellular proteins in vital organs (such as the brain). In patients with DKA and a relatively low plasma potassium level, insulin administration may cause hypokalemia and cardiac arrhythmias. It is suggested in these cases to temporarily delay insulin administration and first administer potassium chloride intravenously to bring the plasma potassium level close to 4mmol/L. Sodium bicarbonate administration in adult patients should be individualized. We suggest it be considered in a subset of patients with moderately severe acidemia (pH<7.20 and plasma bicarbonate level < 12mmol/L) who are at risk for worsening acidemia, particularly if hemodynamically unstable. Sodium bicarbonate should not be administered to children with DKA, except if acidemia is very severe and hemodynamic instability is refractory to saline administration.

Optimal Dose and Method of Administration of Intravenous Insulin in the Management of Emergency Hyperkalemia: A Systematic Review

BACKGROUND AND OBJECTIVES

Hyperkalemia is a common electrolyte disorder that can result in fatal cardiac arrhythmias. Despite the importance of insulin as a lifesaving intervention in the treatment of hyperkalemia in an emergency setting, there is no consensus on the dose or the method (bolus or infusion) of its administration. Our aim was to review data in the literature to determine the optimal dose and route of administration of insulin in the management of emergency hyperkalemia.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We searched several databases from their date of inception through February 2015 for eligible articles published in any language. We included any study that reported on the use of insulin in the management of hyperkalemia.

RESULTS

We identified eleven studies. In seven studies, 10 units of regular insulin was administered (bolus in five studies, infusion in two studies), in one study 12 units of regular insulin was infused over 30 minutes, and in three studies 20 units of regular insulin was infused over 60 minutes. The majority of included studies were biased. There was no statistically significant difference in mean decrease in serum potassium (K+) concentration at 60 minutes between studies in which insulin was administered as an infusion of 20 units over 60 minutes and studies in which 10 units of insulin was administered as a bolus (0.79±0.25 mmol/L versus 0.78±0.25 mmol/L, P = 0.98) or studies in which 10 units of insulin was administered as an infusion (0.79±0.25 mmol/L versus 0.39±0.09 mmol/L, P = 0.1). Almost one fifth of the study population experienced an episode of hypoglycemia.

CONCLUSION

The limited data available in the literature shows no statistically significant difference between the different regimens of insulin used to acutely lower serum K+ concentration. Accordingly, 10 units of short acting insulin given intravenously may be used in cases of hyperkalemia. Alternatively, 20 units of short acting insulin may be given as a continuous intravenous infusion over 60 minutes in patients with severe hyperkalemia (i.e., serum K+ concentration > 6.5 mmol/L) and those with marked EKG changes related to hyperkalemia (e.g., prolonged PR interval, wide QRS complex) as an alternative to 10 units of short acting insulin. Because the risk of hypoglycemia is increased with using large insulin doses, sufficient glucose (60 grams with the administration of 20 units of insulin and 50 grams with the administration of 10 units) should be given to prevent hypoglycemia, and plasma glucose should be frequently monitored.

Integration of the response to a dietary potassium load: a paleolithic perspective

Our purpose is to integrate new insights in potassium (K(+)) physiology to understand K(+) homeostasis and illustrate some of their clinical implications. Since control mechanisms that are essential for survival were likely developed in Paleolithic times, we think the physiology of K(+) homeostasis can be better revealed when viewed from what was required to avoid threats and achieve balance in Paleolithic times. Three issues will be highlighted. First, we shall consider the integrative physiology of the gastrointestinal tract and the role of lactic acid released from enterocytes following absorption of sugars (fruit and berries) to cause a shift of this K(+) load into the liver. Second, we shall discuss the integrative physiology of WNK kinases and modulation of delivery of bicarbonate to the distal nephron to switch the aldosterone response from sodium chloride retention to K(+) secretion when faced with a K(+) load. Third, we shall emphasize the role of intra-renal recycling of urea in achieving K(+) homeostasis when the diet contains protein and K(+).

Non-natriuretic doses of furosemide: potential use for decreasing the workload of the renal outer medulla with minimal magnesium wasting in the rat

BACKGROUND/AIMS

Since furosemide (FS) inhibits active Na(+) reabsorption by medullary thick ascending limb (mTAL) in the renal outer medulla, it may decrease its work during periods of low O2 supply to deep in the renal outer medulla. This study was designed to demonstrate that there may be a dose of FS would reduce its metabolic work while preventing the excessive loss of magnesium (Mg(2+)). Mg(2+) is important because the ATP needed to perform work must have bound Mg(2+) to it.

METHODS

Rats were injected intraperitoneally with a range of doses of FS. The measured outcomes were urine flow rate and parameters of functions of the mTAL (i.e. urine and renal papillary osmolality and urinary excretion of Na(+), Cl(-), K(+) and Mg(2+), and concentrations of Mg(2+) in serum).

RESULTS

The urine flow rate increased significantly starting at 2.4 mg FS/kg. The renal papillary osmolality decreased at ≥0.4 mg FS/kg, and the large detectable natriuresis started at 1.6 mg FS/kg. At this latter dose, the urinary excretion of Mg(2+) rose significantly.

CONCLUSION

In rats, the non-natriuretic dose of FS may reduce the work of the mTAL. The earliest indicator of reduced work in the mTAL appears to be a decrease in urine osmolality rather than a rise in urine flow rate. Higher doses of FS should be avoided, as they induce high rates of Mg(2+) excretion, which can deplete the body of this essential electrolyte.

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.

Is there escape from renal actions of vasopressin in rats with a hyponatremia for greater than 48 hours?

Escape from the renal actions of vasopressin is said to occur in rats with chronic hyponatremia. Our objective was to provide specific evidence to test this hypothesis. Hence the osmolality in the excised renal papilla and in simultaneously voided urine (U_Osm) was measured in rats with and without hyponatremia. To induce hyponatremia, rats were fed low-electrolyte chow for 6 days. In the first 3 days, water was provided ad lib. On days 4 to 6, a long acting vasopressin preparation (dDAVP) was given every 8 hours to induce water retention. The hyponatremic rats drank 21 mL 5% sucrose on day 4 and 6 mL on day 5. On the morning of day 6, these rats were given 10 mL of 5% glucose in water (D5W) by the intraperitoneal route at 09:00 hour and at 11:00 hour. Analyses were performed in blood, urine, and the excised renal papilla at 13:00 hour on day 6. The concentration of Na⁺ in plasma (P_Na) in rats without intraperitoneal D5W was 140±1 mEq/L (n=7) whereas it was 112±3 mEq/L in the hyponatremic group (n=12). The hyponatremic rats had a higher osmolality in the excised papillary (1,915±117 mOsm/kg H₂O) than the U_Osm (1,528±176 mOsm/kg H₂O, P<0.05). One explanation for this difference is that the rats escaped from the renal action of vasopressin. Nevertheless, based on a quantitative analysis, other possibilities will be considered.

Importance of Residual Water Permeability on the Excretion of Water during Water Diuresis in Rats

When the concentration of sodium (Na(+)) in arterial plasma (P(Na)) declines sufficiently to inhibit the release of vasopressin, water will be excreted promptly when the vast majority of aquaporin 2 water channels (AQP2) have been removed from luminal membranes of late distal nephron segments. In this setting, the volume of filtrate delivered distally sets the upper limit on the magnitude of the water diuresis. Since there is an unknown volume of water reabsorbed in the late distal nephron, our objective was to provide a quantitative assessment of this parameter. Accordingly, rats were given a large oral water load, while minimizing non-osmotic stimuli for the release of vasopressin. The composition of plasma and urine were measured. The renal papilla was excised during the water diuresis to assess the osmotic driving force for water reabsorption in the inner medullary collecting duct. During water diuresis, the concentration of creatinine in the urine was 13-fold higher than in plasma, which implies that ~8% of filtered water was excreted. The papillary interstitial osmolality was 600 mOsm/L > the urine osmolality. Since 17% of filtered water is delivered to the earliest distal convoluted tubule micropuncture site, we conclude that half of the water delivered to the late distal nephron is reabsorbed downstream during water diuresis. The enormous osmotic driving force for the reabsorption of water in the inner medullary collecting duct may play a role in this reabsorption of water. Possible clinical implications are illustrated in the discussion of a case example.

Occult risk factor for the development of cerebral edema in children with diabetic ketoacidosis: possible role for stomach emptying

The incidence of cerebral edema during therapy of diabetic ketoacidosis (DKA) in children remains unacceptably high-this suggests that current treatment may not be ideal and that important risk factors for the development of cerebral edema have not been recognized. We suggest that there are two major sources for an occult generation of osmole-free water in these patients: first, fluid with a low concentration of electrolytes that was retained in the lumen of the stomach when the patient arrived in hospital; second, infusion of glucose in water at a time when this solution can be converted into water with little glucose. In a retrospective chart review of 30 patients who were admitted with a diagnosis of DKA and a blood sugar > 900 mg/dL (50 mmol/L), there were clues to suggest that some of the retained fluid in the stomach was absorbed. To minimize the likelihood of creating a dangerous degree of cerebral edema in patients with DKA, it is important to define the likely composition of fluid retained in the stomach on admission, to look for signs of absorption of some of this fluid during therapy, and to be especially vigilant once fat-derived brain fuels have disappeared, because this is the time when glucose oxidation in the brain should increase markedly, generating osmole-free water.

Antenatal Bartter's syndrome: why is this not a lethal condition?

There are four themes in this teaching exercise for Professor McCance. The first challenge was to explain how a premature infant with Bartter's syndrome could survive despite having such a severe degree of renal salt wasting. Second, the medical team wanted to know why there was such a dramatic decrease in the natriuresis in response to therapy, despite the presence of a permanent molecular defect that affected the loop of Henle. Third, Professor McCance was asked why this patient seemed to have a second rare disease, AQP2 deficiency type of nephrogenic diabetes insipidus. The fourth challenge was to develop a diagnostic test to help the parents of this baby titrate the dose of indomethacin to ensure an effective dose while minimizing the likelihood of developing nephrotoxicity. The missing links in this interesting story emerge during a discussion between the medical team and its mentor.

Uncovering the basis of a severe degree of acidemia in a patient with diabetic ketoacidosis

In this teaching exercise, the goal is to demonstrate how an application of principles of physiology can reveal the basis for a severe degree of acidaemia (pH 6.81, bicarbonate <3 mmol/l (P(HCO(3))), PCO(2) 8 mmHg), why it was tolerated for a long period of time, and the issues for its therapy in an 8-year-old female with diabetic ketoacidosis. The relatively low value for the anion gap in plasma (19 mEq/l) suggested that its cause was both a direct and an indirect loss of NaHCO(3). Professor McCance suggested that ileus due to hypokalaemia might cause this direct loss of NaHCO(3), and that an excessive excretion of ketoacid anions without NH(4)(+) in the urine accounted for the indirect loss of NaHCO(3). In addition, he suspected that another factor also contributing to the severity of the acidaemia was a low input of alkali. He was also able to explain why there was a 16-h delay before there was a rise in the P(HCO(3)) once therapy began. The missing links in this interesting story, including a possible basis for the hypokalaemia, emerge during the discussion between the medical team and Professor McCance.

A Brain Protein–Centered View of H+Buffering

In the traditional approach to buffering of H(+) during metabolic acidosis, the sole focus is on lowering the H(+) concentration, but this overlooks several important points. First, increased binding of H(+) to proteins changes their charge, shape, and possibly function. Second, organs in which buffering of H(+) occurs is not assessed even though it would be advantageous to spare brain proteins in this process. Third, only the arterial and not the capillary PCO(2) of individual organs is considered. This article provides a "brain protein-centered" view, which leads to different conclusions concerning the way H(+) are removed physiologically.

Physiology of acid-base balance: links with kidney stone prevention

Two processes permit the urine pH and the medullary interstitial pH to remain in an "ideal range" to minimize the risk of forming kidney stones. First, a medullary shunt for NH(3) maintains the urine pH near 6.0 to minimize uric acid precipitation when distal H(+) secretion is high. Second, excreting dietary alkali excreting alkali as a family of organic anions--including citrate--rather than as bicarbonate maintains the urine pH near 6.0 while urinary citrate chelates ionized calcium, which minimizes CaHPO(4) precipitation. In patients with idiopathic hypercalciuria and recurrent calcium oxalate stones, the initial nidus is a calcium phosphate precipitate on the basolateral membrane of the thin limb of the loop of Henle (Randall's plaque). Formation of this precipitate requires medullary alkalinization; K(+) -depletion and augmented medullary H(+)/K(+) -ATPase may be predisposing factors.

A hyperglycaemic hyperosmolar state in a young child: diagnostic insights from a quantitative analysis

This teaching exercise demonstrates how the application of principles of physiology can identify the cause of a severe degree of hyperglycaemia (plasma glucose concentration 80 mmol/l) in a very young patient with newly diagnosed diabetes mellitus, determine whether the patient has diabetic ketoacidosis, and highlight the potential risks for this patient on admission and during initial therapy. A consultation with Professor McCance was sought to determine whether this patient had an unusual degree of 'insulin resistance'. There were also uncertainties regarding the acid-base diagnosis. The patient did not appear to have an important degree of metabolic acidosis as judged from his pH of 7.39 and plasma bicarbonate concentration of 20 mmol/l in arterial blood; hence the diagnostic impression was that he had a hyperglycaemic hyperosmolar state. However, his plasma anion gap was significantly elevated, and remained so for 60 h, despite the administration of insulin. Issues in management concerning the basis for this severe degree of hyperglycaemia and how to minimize the risk of developing cerebral oedema are addressed. The missing links in this interesting story emerge during a discussion between the medical team and their mentor, Professor McCance.

Studies to identify the basis for an alkaline urine pH in patients with calcium hydrogen phosphate kidney stones

BACKGROUND

Patients with CaHPO(4) kidney stones belong to a diagnostic category that has a high urine pH as its common feature. Our objective was to provide a new clinical approach to examine the basis for this high pH.

METHODS

The study group consisted of 26 CaHPO(4) stone formers and 28 normal volunteers. Urine was collected q2h plus an overnight sample to identify patients with a urine pH > 6.5 for 12/24 h. Urine ammonium (U(NH4)), sulphate (U(SO4)) and citrate were measured and diet net alkali was calculated.

RESULTS

Of the 26 patients, 13 had persistently alkaline urine. In 7/13, U(NH4) (68 +/- 13 mEq/day) and U(SO4) (57 +/- 7 mEq/day) were both high. In 6/13 patients, U(NH4) was the usual 31 +/- 3 mEq/day; in 4/6, U(NH4)/U(SO4) was 0.9 +/- 0.1; the cause of the alkaline urine pH seemed to be a dietary alkali load because the rise in urine pH was episodic and coincided with a high net diet alkali load and peak citrate excretion rates. The remaining two patients had a high U(NH4)/U(SO4) (2.2 and 1.6). Citrate excretion was very low in the male, but not in the female patient.

CONCLUSIONS

There are heterogeneous causes for a persistently high urine pH. Two of the patients had a possible molecular basis: the lesion could be a low proximal convoluted tubule cell pH in the male and an increased entry of NH(3) into the late distal nephron in the female.

Properties Permitting the Renal Cortex to Be the Oxygen Sensor for the Release of Erythropoietin: Clinical Implications

The PO2 at this site where erythropoietin release is regulated should vary only when the hemoglobin concentration changes in capillary blood. The kidney cortex is an ideal location for this O2 sensor for four reasons. First, it extracts a small proportion of the oxygen that is delivered in each liter of blood; this makes the PO2 signal easier to recognize. Second, there is a constant ratio of the work performed (consumption of O2) to the renal blood flow rate (delivery of O2). Third, the high renal blood flow rate improves diffusion of O2 from capillaries to this O2 receptor. Fourth, a high renal cortical PCO2 prevents an additional shift of the O2:hemoglobin dissociation curve by other factors from being a confounding variable. This suggests that the GFR and the renal blood flow rate should be examined in patients with unexplained anemia or erythrocytosis.

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 patient with a severe degree of metabolic acidosis: a deductive analysis

This teaching exercise demonstrates how principles of physiology might help in identifying the cause of a particularly severe case of metabolic acidosis and making appropriate decisions about therapy. The patient's plasma pH was 7.00 and their plasma bicarbonate concentration was 2 mmol/l. Because the time course of the patient's illness was believed to be <24 h, this suggested that a large quantity of acid had been added to the body in this short time period, but the medical team managing the case could not identify any acid that could have been produced rapidly by endogenous processes, or was ingested by the patient. Moreover, there was a question about how such a very low arterial PCO(2) (8 mmHg) could be sustained. Even once the diagnosis was made, there were issues to resolve concerning therapy. These included questions about how much sodium bicarbonate to administer, and what dangers might arise during this therapy. The missing links in this interesting story emerge during a discussion between the medical team and their imaginary mentor, Professor McCance.

An improved approach to the patient with metabolic acidosis: a need for four amendments

Clinicians should identify life-threatening issues in patients with metabolic acidosis. These threats may be present before therapy begins and/or anticipated after therapy commences. By adding four amendments, short-comings in the commonly used clinical approaches for the diagnosis of metabolic acidosis can be overcome. First, a definition of metabolic acidosis should consider not only the concentration of bicarbonate but also the content of bicarbonate in the extra cellular fluid compartment. The latter requires a quantitative estimate of the ECF volume, which can be obtained using the hematocrit and/or the total protein concentration in plasma. Second, to determine if the basis for metabolic acidosis was the addition of acids or the loss of NaHCO 3 , one must hunt for new anions, not only in plasma, but also in the urine. Third, it is important to measure the venous as well as the arterial PCO2 to assess the capacity to buffer H+ while minimizing H + binding to intracellular proteins. Fourth, to assess the role of the kidney in a patient with metabolic acidosis, the urine osmolal gap and the concentration of creatinine in the urine should be measured to provide an estimate of the rate of excretion of ammonium.

Requirements for a high rate of potassium excretion in rats consuming a low electrolyte diet

Control mechanisms for potassium (K(+)) excretion in humans developed in Palaeolithic times when diets were sodium poor and episodically K(+) rich. Nevertheless, our understanding of the regulation of K(+) excretion comes from experiments in rats with large sodium and K(+) intakes. Our objective was to identify how K(+) excretion was regulated when rats consumed a low NaCl diet to reflect Palaeolithic conditions. Rats that were given mineralocorticoids plus either NaCl, mannitol, or NaHCO(3) had a small kaliuresis. In contrast, KCl load induced a large kaliuresis and a near-maximal luminal [K(+)] in the terminal cortical collecting duct ([K(+)](CCD)). The time course of events was important. The rise in the [K(+)](CCD) was prompt, but the initial kaliuresis was only modest. Over the next 4 h, kaliuresis increased markedly due solely to a higher calculated distal flow rate, which appeared to be due to diminished reabsorption of NaCl in the loop of Henle; of note, the measured papillary [K(+)] rose. In summary, the increase in the [K(+)](CCD) in rats given KCl is likely to be due to an increase in the number of luminal K(+) channels rather than to mechanisms that are known to induce a lumen-negative voltage in cortical distal nephron segments. The higher distal flow rate might be due to a higher interstitial [K(+)], which inhibited NaCl reabsorption in the loop of Henle. Thus, to understand which of the potential control mechanisms are operating, one must look very closely at the conditions imposed by the experimental setting.

Recurrent uric acid stones

A 46-year-old female had a history of recurrent uric acid stone formation, but the reason why uric acid precipitated in her urine was not obvious, because the rate of urate excretion was not high, urine volume was not low, and the pH in her 24-h urine was not low enough. In his discussion of the case, Professor McCance provided new insights into the pathophysiology of uric acid stone formation. He illustrated that measuring the pH in a 24-h urine might obscure the fact that the urine pH was low enough to cause uric acid to precipitate during most of the day. Because he found a low rate of excretion of NH(4)(+) relative to that of sulphate anions, as well as a high rate of citrate excretion, he speculated that the low urine pH would be due to a more alkaline pH in proximal convoluted tubule cells. He went on to suspect that there was a problem in our understanding of the function of renal medullary NH(3) shunt pathway, and he suggested that its major function might be to ensure a urine pH close to 6.0 throughout the day, to minimize the likelihood of forming uric acid kidney stones.

Dogmas and controversies in the handling of nitrogenous wastes: Excretion of nitrogenous wastes in human subjects

Two major nitrogenous waste products, urea and ammonium(NH4+), are produced in humans when proteins are oxidized, and in this manuscript their excretions are examined from two perspectives. First, the specific physiology of each nitrogenous waste is reviewed and the current dogmas summarized. Second, their excretions are considered in the context of integrative physiology, i.e. the need to ensure that the urine composition is appropriate to minimize the risk of kidney stone formation. After the latter analysis, weak links in our understanding of the overall physiology become apparent and a conundrum is defined. The conundrum for the excretion of urea focuses on the fact that urea is not an effective osmole in the medullary-collecting duct when vasopressin acts. As a result, it appears that urinary urea cannot prevent a large decline in the urine flow rate and thereby minimize the risk of forming kidney stones in electrolyte-poor urine. The conundrum for the excretion of NH4+ is: high rates of NH4+excretion require a low urine pH, yet a pH ∼6.0 must be maintained in order to reduce the risk of precipitating uric acid in the urine. Possible ways of resolving these conundrums require novel physiological interpretations.

A conceptual approach to the patient with metabolic acidosis. Application to a patient with diabetic ketoacidosis

We shall illustrate that management of patients with an acid-base disorder could be improved if the acid-base analysis was based on a better understanding of basic concepts of physiology. Three concepts of acid-base physiology and their clinical implications are emphasized in a patient with diabetic ketoacidosis. First, when an acid is produced from neutral precursors in the body, there is a net increase in the number of hydrogen ions (H(+)) and new anions. The corollary is that H(+) will be removed when the accompanying anion is metabolized to a neutral end-product or is excreted in the urine with H(+) or ammonium (NH(4)(+)). Second, buffering of H(+) is beneficial if H(+) are removed by bicarbonate rather than being able to bind to proteins. This latter function depends on having a low tissue PCO(2), due to a combination of hyperventilation plus an adequate blood flow rate to vital organs. Third, the kidneys add new bicarbonate to the body when NH(4)(+) is excreted with chloride ions.

Bartter's, Gitelman's, and Gordon's syndromes. From physiology to molecular biology and back, yet still some unanswered questions

The molecular basis of many of the inherited disorders of potassium homeostasis has become much clearer in the last two decades. Despite these new insights into the physiology of renal potassium handling, a number of questions remain to be answered. The examples we use to illustrate these issues are Gordon's syndrome, Bartter's syndrome, and Gitelman's syndrome. Our objective is to integrate these new insights into an understanding of the pathophysiology of renal potassium handling. We also propose different ways to think about some of the unresolved issues in this area.

Studies on the pathophysiology of the low urine pH in patients with uric acid stones

BACKGROUND

A very low urine pH is the major risk factor for uric acid stone formation.

METHODS

A subgroup of patients with a history of uric acid stones and a persistently low urine pH (<5.5 for at least 12 h/day) were selected for detailed study. Based on their relative ammonium (NH(+)(4)) and sulfate (SO(2-)(4)) excretions, patients were divided into two groups.

RESULTS

The first group (N = 2) excreted 173 and 139% more NH(+)(4) than SO(2-)(4). Their daily urinary unmeasured anion excretion was higher than their calculated net diet alkali input (38 and 61 vs. 24 and 49 mEq, respectively). In the second group (N = 12), NH(+)(4) excretion was 69 +/- 5% that of SO(2-)(4). In 2 of 12, decreased renal ammoniagenesis was suspected due to a plasma potassium of 5.3 mmol/L and/or a lower GFR (65 and 59 L/day); these patients had an extremely low citrate excretion (3 and 1 mEq/day). In contrast, citrate excretion was not low in the remaining 10 patients (10.4 +/- 1.3 mEq/day).

CONCLUSIONS

Patients in group 1 needed a higher NH(+)(4) excretion possibly because of a H+ load from excessive renal excretion of organic anions. We speculate that an alkaline proximal tubular cell pH could be the basis for the low NH(+)(4) and high citrate excretions in 10 of 12 patients in group 2. Dietary factors and/or a molecular lesion may contribute to their pathophysiology.

A patient with partial central diabetes insipidus: clarifying pathophysiology and designing treatment

Studies were undertaken in a 32-year-old man who developed polyuria (4 L/d) a few days after a basal skull fracture; the condition persisted 1 year after the accident. The other major features were thirst, a plasma sodium of 143 mmol/L, 24-hour urine osmolality of 221 mOsm/kg H(2)O, and levels of vasopressin in plasma that were less than 0.5 pg/mL on 20 separate occasions. The 24-hour urine volume implied that the diagnosis was partial rather than complete central diabetes insipidus; however, several random urine samples had a much higher osmolality. An infusion of hypertonic saline led to the release of vasopressin and the excretion of concentrated urine. We propose that the basis for the lesion may be the transection of some, but not all, of the fibers connecting the osmostat and vasopressin release center. This partial transection could permit vasopressin to be secreted in response to a larger rise in plasma sodium concentration. This pathophysiologic analysis provided the basis for therapy to minimize the degree of polyuria.

Effect of NaHCO3 on cardiac energy metabolism and contractile function during hypoxemia

OBJECTIVE

To examine the impact of administration of NaHCO3 on contractility and energy metabolism of the myocardium during hypoxemia.

METHODS

Regional myocardial hypoxia was induced in the left anterior descending (LAD) artery myocardium in anesthetized, open-chest dogs, using a perfusion circuit between the right atrium and the LAD artery, and a membrane oxygenator. The rate of flow in LAD artery was maintained constant with the use of a roller pump. During hypoxia, eight dogs were administered isotonic NaHCO3 in the circuit and six other dogs received equimolar NaCl. Myocardial contractile function was assessed using sonomicrometry for measurement of percentage of systolic shortening and preload recruitable stroke work. Oxygen consumption and the rate of appearance of lactate were measured. Clamp-frozen tissue samples were obtained at the end of the experiment from the hypoxic LAD myocardium and the nonhypoxic circumflex myocardium for measurement of tissue lactate level.

RESULTS

During hypoxia, there was a significant decrease in oxygen consumption by the LAD myocardium (35 +/- 7 micromol/min in the NaCl group and 40 +/- 7 micromol/min in the NaHCO3 group during hypoxia vs. 131 +/- 11 micromol/min during aerobic perfusion). There was also a significant decrease in myocardial contractility as measured by percentage of systolic shortening (14 +/- 3% to -8 +/- 3%); NaHCO3 infusion during hypoxia did not improve myocardial contractility (-7 +/- 2%). Similar results were obtained with measurements of preload recruitable stroke work. The rate of production of lactate during hypoxia was substantially lower than expected, based on the calculated oxygen deficit, and was not significantly increased by the administration of NaHCO3 (33 +/- 9 micromol/min in the NaCl group and 51 +/- 5 micromol/min in the NaHCO3 group). Tissue lactate was not statistically different in the hypoxic myocardium supplied by the LAD artery and the nonhypoxic myocardium supplied by the circumflex artery in either group.

CONCLUSION

The response of the myocardium to hypoxia is to decrease its mechanical work and metabolic demand. The infusion of NaHCO3 did not enhance myocardial contractile function or flux in glycolysis during hypoxia. We speculate that this diminished mechanical work and metabolic demand may represent an adaptive response to preserve cellular integrity until oxygen delivery is restored.

Studies on the pathogenesis of hypokalemia in Gitelman's syndrome: role of bicarbonaturia and hypomagnesemia

OBJECTIVE

Hypokalemia and renal potassium (K) wasting are hallmarks of the group of disorders called Bartter's syndrome. The presence of hypomagnesemia and a low rate of excretion of calcium are currently used to characterize a subgroup of these patients as having Gitelman's syndrome (GS) in which the molecular lesion is a defect in the thiazide-sensitive NaCl cotransporter in the distal convoluted tubule. This study was undertaken to examine whether bicarbonaturia or hypomagnesemia exacerbates the kaliuresis in patients with GS.

METHODS

Six patients with most of the diagnostic features of GS were examined. To examine the role of bicarbonaturia, the transtubular K concentration gradient (TTKG) was assessed before and after an oral load of NH4Cl which caused the urine pH to be < 6. To evaluate the role of hypomagnesemia, the TTKG was examined after an infusion of enough magnesium (Mg) to achieve normal levels of Mg in plasma for close to 24 h.

RESULTS

The TTKG remained very high even when the pH of the urine was < 6.0. An infusion of Mg caused the TTKG to approach expected values for hypokalemia in 4 of 6 patients. The infusion of Mg was extended in 1 patient who had a sustained high TTKG for 24 h; the TTKG remained elevated for 96 h despite normal plasma Mg levels.

CONCLUSIONS

Bicarbonaturia does not play a critical role in maintaining the very high TTKG in these patients. The K wasting in 4 of 6 of these patients could largely be attributed to hypomagnesemia and/or Mg depletion. The plasma aldosterone level tended to be higher in patients who did not respond to the infusion of Mg. Therefore, these patients may not represent a homogeneous group with regard to the pathophysiology of their renal K wasting.

Dynamic interactions between integrative physiology and molecular medicine: The key to understand the mechanism of action of aldo sterone in the kidney

Our objective is to illustrate how an approach that integrates new insights from molecular biology and traditional physiology can lead to the development of new concepts. This dynamic interaction is illustrated by examining the steps taken to improve our understanding of the renal actions of aldosterone. We began by defining the big picture of what aldosterone does in the kidney. This led to the conclusion that aldosterone must at times become a sodium chloride-retaining hormone, while at other times it must function primarily or exclusively as a kaliuretic hormone. The second step was to define the major molecular actions of this hormone. Acting on the principal cells in the cortical collecting duct (CCD), aldosterone leads to the insertion of active epithelial sodium ion channels (ENaC) in their luminal membranes. This active ENaC, however, does not distinguish between the two major renal actions of aldosterone. Accordingly, we returned to integrative physiology and examined a possible role of renal and non-renal events. We implicated the potential importance of the delivery of bicarbonate ions to the CCD to determine which effect of aldosterone will become manifest. This, however, required that we reconsider some of the traditional views in interpretation of acid-base balance. At the clinical level, this global view can help us understand why, for example, a low dietary intake of potassium salts might predispose a person to an elevated blood pressure. Using a similar approach, it is possible to understand how the risk of the formation of kidney stones can be minimized.Key words: acid-base, hypertension, integrative physiology, kidney stones, potassium, sodium.

Dynamic interactions between integrative physiology and molecular medicine: the key to understand the mechanism of action of aldosterone in the kidney

Our objective is to illustrate how an approach that integrates new insights from molecular biology and traditional physiology can lead to the development of new concepts. This dynamic interaction is illustrated by examining the steps taken to improve our understanding of the renal actions of aldosterone. We began by defining the big picture of what aldosterone does in the kidney. This led to the conclusion that aldosterone must at times become a sodium chloride-retaining hormone, while at other times it must function primarily or exclusively as a kaliuretic hormone. The second step was to define the major molecular actions of this hormone. Acting on the principal cells in the cortical collecting duct (CCD), aldosterone leads to the insertion of active epithelial sodium ion channels (ENaC) in their luminal membranes. This active ENaC, however, does not distinguish between the two major renal actions of aldosterone. Accordingly, we returned to integrative physiology and examined a possible role of renal and non-renal events. We implicated the potential importance of the delivery of bicarbonate ions to the CCD to determine which effect of aldosterone will become manifest. This, however, required that we reconsider some of the traditional views in interpretation of acid-base balance. At the clinical level, this global view can help us understand why, for example, a low dietary intake of potassium salts might predispose a person to an elevated blood pressure. Using a similar approach, it is possible to understand how the risk of the formation of kidney stones can be minimized.

Potassium

In a logical, stepwise approach to patients presenting with hypokalaemia or hyperkalaemia the clinician must first recognise circumstances in which the dyskalaemia represents a clinical emergency because therapy then takes precedence over diagnosis. If a dyskalaemia has been present for a long time, there is an abnormal renal handling of K+. The next step to analyse is the rate of excretion of K+ and, if necessary, its two components (urine flow rate and K+ concentration in the cortical collecting duct [CCD]) analysed independently. If the K+ concentration in the CCD is not in the expected range, its basis should be defined at the ion-channel level in the CCD from clinical information that can be used to deduce the relative rates of reabsorption of Na+ and Cl- in the CCD. This analysis provides the basis for diagnosis and may indicate where non-emergency therapy should then be directed.