Renal Acid-base Regulation Research Articles

Demystifying normal-anion-gap metabolic acidosis: pathophysiology, aetiology, evaluation and diagnosis.

Normal-anion-gap metabolic acidosis (NAGMA) is a common but often under-recognised and poorly understood condition, especially by less-experienced clinicians. In adults, NAGMA might be an initial clue to a more significant underlying pathology, such as autoimmune diseases, hypergammaglobulinemia or drug toxicities. However, identifying the aetiology can be challenging due to the diverse processes involved in the development of acidosis. A better understanding of the pathophysiology of NAGMA can help treating physicians suspect and evaluate the condition early and reach the correct diagnosis. This article provides an overview of renal acid-base regulation, discusses the pathophysiological processes involved in developing NAGMA and provides a framework for evaluation to reach an accurate diagnosis.

Sex differences in renal acid-base regulation

Transport along the renal tubule differs between female and male rodents, possibly as a consequence of varying abundance and expression of transporters along the nephron. For several aspects such as renal Na+ transport and blood pressure regulation, this has been comprehensively studied; however, data on sex differences in renal acid-base regulation are less well understood. We hypothesized that there will be sex differences in transporter expression profiles in response to acid-base challenges. We used female and male C57Bl/6 mice (n=10 per sex and treatment), which were challenged by acid (NH4Cl, 0.28 mM) and base (NaHCO3, 0.28 mM) in drinking water for 8 days. Blood and urine samples were collected at baseline and at the end of the experimental period before kidneys were harvested under isoflurane anesthesia. Data were analyzed by 2-way ANOVA followed by Tukey multiple comparisons test. In response to NH4Cl challenge, urine pH significantly decreased in both sexes without sex differences (male baseline: 6.4±0.1 vs. NH4Cl: 5.7±0.1; female baseline: 6.2±0.2 vs NH4Cl: 5.6±0.1). In contrast to the urinary response, blood pH was not significantly affected in male mice (male baseline: 7.29±0.01 vs. NH4Cl: 7.26±0.02); however, significantly decreased in female mice (female baseline: 7.28±0.01 vs NH4Cl: 7.22±0.02). Despite differences in acid-base physiology, no differences were observed in the abundance of NHE3, pS552-NHE3, Npt2a, NKCC2, NCC, pT58-NCC or pendrin between sexes. In response to NaHCO3 challenge, urine pH significantly increased in both sexes; however, the increase was significantly smaller in female vs. male mice (male baseline: 6.3±0.1 vs. NaHCO3: 7.6±0.1; female baseline: 6.3±0.1 vs NaHCO3: 7.0±0.3). In contrast to the urinary response, blood pH significantly increased in male mice (male baseline: 7.28±0.01 vs. NaHCO3: 7.40±0.01); however, was not significantly affected in female mice (female baseline: 7.29±0.01 vs NaHCO3: 7.31±0.01). These differences in females compared to males were associated with a significantly greater abundance of Npt2a (~1.5-fold), pendrin (~1.4-fold) and H-ATPase (~1.5-fold). No differences were observed in the abundance of NHE3, pS552-NHE3, NKCC2, NCC, pT58-NCC, between sexes. In summary, our study demonstrates that acid-base regulation shows very specific sex differences: (i) female mice seem to be more prone to a NH4Cl disturbance compared with male mice, (ii) acid challenges under these conditions do not result in major changes in the abundance of the tested proteins, (iii) male mice are more prone to a NaHCO3 disturbance possibly because of (iv) lower abundances of Npt2a, pendrin and H-ATPase. This work was supported by a VA Merit Review Award IBX004968A (to Dr. Rieg) and a Pilot Project from the USF Microbiomes Institute (to T.R. and J.D.R). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

A review of renal tubular acidosis.

To review the current scientific literature on renal tubular acidosis (RTA) in people and small animals, focusing on diseases in veterinary medicine that result in secondary RTA. Scientific reviews and original research publications on people and small animals focusing on RTA. RTA is characterized by defective renal acid-base regulation that results in normal anion gap hyperchloremic metabolic acidosis. Renal acid-base regulation includes the reabsorption and regeneration of bicarbonate in the renal proximal tubule and collecting ducts and the process of ammoniagenesis. RTA occurs as a primary genetic disorder or secondary to disease conditions. Based on pathophysiology, RTA is classified as distal or type 1 RTA, proximal or type 2 RTA, type 3 RTA or carbonic anhydrase II mutation, and type 4 or hyperkalemic RTA. Fanconi syndrome comprises proximal RTA with additional defects in proximal tubular function. Extensive research elucidating the genetic basis of RTA in people exists. RTA is a genetic disorder in the Basenji breed of dogs, where the mutation is known. Secondary RTA in human and veterinary medicine is the sequela of diseases that include immune-mediated, toxic, and infectious causes. Diagnosis and characterization of RTA include the measurement of urine pH and the evaluation of renal handling of substances that should affect acid or bicarbonate excretion. Commonality exists between human and veterinary medicine among the types of RTA. Many genetic defects causing primary RTA are identified in people, but those in companion animals other than in the Basenji are unknown. Critically ill veterinary patients are often admitted to the ICU for diseases associated with secondary RTA, or they may develop RTA while hospitalized. Recognition and treatment of RTA may reverse tubular dysfunction and promote recovery by correcting metabolic acidosis.

Pendrin: linking acid base to blood pressure.

Pendrin (SLC26A4) is an anion exchanger from the SLC26 transporter family which is mutated in human patients affected by Pendred syndrome, an autosomal recessive disease characterized by sensoneurinal deafness and hypothyroidism. Pendrin is also expressed in the kidney where it mediates the exchange of internal HCO3- for external Cl- at the apical surface of renal type B and non-A non-B-intercalated cells. Studies using pendrin knockout mice have first revealed that pendrin is essential for renal base excretion. However, subsequent studies have demonstrated that pendrin also controls chloride absorption by the distal nephron and that this mechanism is critical for renal NaCl balance. Furthermore, pendrin has been shown to control vascular volume and ultimately blood pressure. This review summarizes the current knowledge about how pendrin is linking renal acid-base regulation to blood pressure control.

Section-specific H+ fluxes in renal tubules of fasted and fed goldfish

A recent study demonstrated that in response to a feeding-induced metabolic acidosis, goldfish (Carassius auratus) adjust epithelial protein and/or mRNA expression in their kidney tubules for multiple transporters known to be relevant for acid-base regulation. These include Na+/H+ exchanger (NHE), V-type H+-ATPase (V-ATPase), cytoplasmic carbonic anhydrase, HCO3 - transporters and Rhesus proteins. Consequently, renal acid output in the form of protons and NH4 + increases. However, little is known about the mechanistic details of renal acid-base regulation in C. auratus and teleost fishes in general. The present study applied the scanning ion-selective electrode technique (SIET) to measure proton flux in proximal, distal and connecting tubules of goldfish. We detected increased H+ efflux into the extracellular fluid from the tubule in fed animals, resulting from paracellular back-flux of H+ through the tight junction. By applying inhibitors for selected acid-base regulatory epithelial transporters, we found that cytosolic carbonic anhydrase and HCO3 - transporters were important in mediating H+ flux in all three tubule segments of fed goldfish. Contrastingly, V-ATPase seemed to play a role in H+ flux only in proximal and distal tubules, and NHE in proximal and connecting tubules. We developed working models for transport of acid-base relevant equivalents (H+, HCO3 -, NH3/NH4 +) for each tubule segment in C. auratus kidney. While the proximal tubule appears to play a major role in both H+ secretion and HCO3 - reabsorption, the distal and connecting tubules seem to mainly serve for HCO3 - reabsorption and NH3/NH4 + secretion.

Osteomalacia Induced by Renal Tubular Acidosis Type 1

Renal Tubular Acidosis, a group of disorders characterized by defective renal acid-base regulation, can impair mineralization of bone matrix in adults known as osteomalacia. RTA is classified into 3 major forms, such as proximal RTA or type 2; distal RTA or type 1; and hyperkalemic RTA or type 4. Among all type of RTA, proximal RTA or type 2 is known have association with Fanconi syndrome and bone involvement. However, distal RTA or type 1 can also cause osteomalacia. Hereby we report a case of 22 years old Asian woman who wasfirstly diagnosed with distal type RTA several years ago then started developing bone involvement recently. She was complaining with low back pain due to fracture on left medial side of inferior pubic ramus and endplate fracture on right side superior L4 and BMD examination showed low mineral density. She was diagnosed with osteomalacia induced by distal type RTA due to the loss of calcium salts from bone and hypophosphatemia.Keywords: Osteomalacia; Renal Tubular Acidosis; Hypokalemia; Hypophosphatemia

Urinary Strong Ion Difference as a Marker of Renal Dysfunction. A Retrospective Analysis.

IntroductionThe kidneys play a crucial role in the regulation of electrolytes and acid-base homeostasis. Urinary Strong Ion Difference (SIDu = NaU + KU—ClU) represents an important aspect of renal acid-base regulation. We evaluated the role of SIDu as a marker of renal dysfunction in critically ill patients.Materials and MethodsPatients admitted to the Medical Intensive Care Unit with a diagnosis of AKI for whom concomitant urinary samples available for SIDu calculation were retrospectively reviewed and staged according to KDIGO criteria for 3 days from inclusion. Patients were classified as Recovered (R-AKI) or Persistent-AKI (P-AKI) whether they exited KDIGO criteria within the 3-day observation period or not. A control group with normal renal function and normal serum acid-base and electrolytes was prospectively recruited in order to identify reference SIDu values.ResultsOne-hundred-and-forty-three patients with a diagnosis of AKI were included: 77 with R-AKI, and 66 with P-AKI. Thirty-six controls were recruited. Patients with P-AKI had more severe renal dysfunction and higher mortality than patients with R-AKI (SCr 2.23(IQR:1.68–3.45) and 1.81(IQR1.5–2.5) mg/dl respectively, p<0.001; 24-h UO 1297(950) and 2100(1094) ml respectively, p = 0.003); 30-d mortality, 39% and 13% respectively; p<0.001). SIDu significantly differed between groups, with rising values from controls to P-AKI groups (16.4(12), 30(24) and 47.3(21.5) mEq/l respectively, p<0.001).DiscussionSIDu may be a simple and inexpensive tool in AKI patients’ evaluation. Further research is needed to evaluate the ability of SIDu to identify patients with renal dysfunction before derangements in serum creatinine or urine output are observed.

Effect of dietary cation–anion difference on ruminal metabolism, total apparent digestibility, blood and renal acid–base regulation in lactating dairy cows

The present study aimed to evaluate the effect of dietary cation–anion difference (DCAD) on ruminal fermentation, total apparent digestibility, blood and renal metabolism of lactating dairy cows. Sixteen Holstein cows were distributed in four contemporary 4×4 Latin Square designs, which consisted of four periods of 21 days and four treatments according to DCAD: +290; +192; +98 and −71 milliequivalent (mEq)/kg dry matter (DM). Ruminal pH and concentrations of acetic and butyric acid increased linearly according to the increase of DCAD. Similarly, NDF total apparent digestibility linearly increased by 6.38% when DCAD increased from −71 to 290 mEq/kg DM [Y=65.90 (SE=2.37)+0.0167 (SE=0.0068)×DCAD (mEq/kg DM)]. Blood pH was also increased according to DCAD, which resulted in reduction of serum concentrations of Na, K and ionic calcium (iCa). To maintain the blood acid–base homeostasis, renal metabolism played an important role in controlling serum concentrations of Na and K, since the Na and K urinary excretion increased linearly by 89.69% and 46.06%, respectively, from −71 to 290 mEq/kg DM. Changes in acid–base balance of biological fluids may directly affect the mineral composition of milk, as milk concentrations of Na, K, iCa and chlorides were reduced according to blood pH increased. Thus, it can be concluded that the increase of DCAD raises the pH of ruminal fluid, NDF total apparent digestibility, and blood pH, and decreases the milk concentration of cationic minerals, as well as the efficiency of Na utilization to milk production.

Renal acid-base regulation: new insights from animal models.

Because majority of biological processes are dependent on pH, maintaining systemic acid-base balance is critical. The kidney contributes to systemic acid-base regulation, by reabsorbing HCO3 (-) (both filtered by glomeruli and generated within a nephron) and acidifying urine. Abnormalities in those processes will eventually lead to a disruption in systemic acid-base balance and provoke metabolic acid-base disorders. Research over the past 30 years advanced our understanding on cellular and molecular mechanisms responsible for those processes. In particular, a variety of transgenic animal models, where target genes are deleted either globally or conditionally, provided significant insights into how specific transporters are contributing to the renal acid-base regulation. Here, we broadly overview the mechanisms of renal ion transport participating to acid-base regulation, with emphasis on data obtained from transgenic mice models.

Rh versus pH: the role of Rhesus glycoproteins in renal ammonia excretion during metabolic acidosis in a freshwater teleost fish.

Increased renal ammonia excretion in response to metabolic acidosis is thought to be a conserved response in vertebrates. We tested the hypothesis that Rhesus (Rh) glycoproteins in the kidney of the freshwater common carp, Cyprinus carpio, play a crucial role in regulating renal ammonia excretion during chronic metabolic acidosis. Exposure to water pH 4.0 (72 h) resulted in a classic metabolic acidosis with reduced plasma arterial pH and [HCO3(-)], no change in PCO2 and large changes in renal function. Urine [NH4(+)] as well as [titratable acidity-HCO3(-)] rose significantly over the acid exposure, but the profound reduction (fivefold) in urine flow rates eliminated the expected elevations in renal ammonia excretion. Low urine flow rates may be a primary strategy to conserve ions, as urinary excretion rates of Na(+), Cl(-) and Ca(2+) were significantly lower during the acid exposure relative to the control period. Interestingly, renal Rhcg1 mRNA and protein levels were elevated in acid-exposed relative to control groups, along with mRNA levels of several ion transporters, including the Na(+)/H(+) exchanger, H(+)-ATPase and Na(+)/K(+)-ATPase. Immunofluorescence microscopy showed a strong apical Rhcg1 signal in distal tubules. Taken together, these data show that renal Rh glycoproteins and associated ion transporters are responsive to metabolic acidosis, but conservation of ions through reduced urine flow rates takes primacy over renal acid-base regulation in the freshwater C. carpio. We propose that an 'acid/base-ion balance' compromise explains the variable renal responses to metabolic acidosis in freshwater teleosts.

The acid-base regulation by renal proximal tubule

The kidney plays quite an important role in the regulation of acid-base homeostasis. The dysfunction of renal acid-base regulation causes diseases such as developmental disorder, bone malformation, calcification of eye and brain, etc. In the kidney, this regulation is performed, to a considerable part, in the proximal tubule of the nephron. In the luminal side the key player is sodium-proton exchanger type 3 (NHE3), whereas sodium-bicarbonate cotransporter (NBCe1) plays the critical role in the basolateral side. In the cytoplasm there is carbonic anhydrase type 2 (CAII) that intermediates the conversion of CO2/ . Interestingly, in human, mutations have been found in NBCe1 and CAII but not in NHE3 so far. Mutations of NBCe1 lead to severe proximal renal tubular acidosis (pRTA) and other systemic manifestations. In animal model studies, however, the relative contribution of NHE3 to proximal tubule functions remains controversial. Recently, V-ATPase with renal specific subunits is suggested to have some roles in the regulation of proximal tubule functions. In this review, we will discuss the regulation of acid-base transport in the proximal tubule and the updates.

Self assessment questions: Renal tubular acidosis

1. Renal acid-base regulation: 1. An average diet generates around 70 mmol of hydrogen ion to excrete in the urine each day 2. Approximately 3,500 mmol of bicarbonate ion is filtered each day and most of it is reabsorbed by the proximal tubule 3. The two main urinary

Differential Diagnosis of Nongap Metabolic Acidosis

Nongap metabolic acidosis is a common form of both acute and chronic metabolic acidosis. Because derangements in renal acid-base regulation are a common cause of nongap metabolic acidosis, studies to evaluate renal acidification often serve as the mainstay of differential diagnosis. However, in many cases, information obtained from the history and physical examination, evaluation of the electrolyte pattern (to determine if a nongap acidosis alone or a combined nongap and high anion gap metabolic acidosis is present), and examination of the serum potassium concentration (to characterize the disorder as hyperkalemic or hypokalemic in nature) is sufficient to make a presumptive diagnosis without more sophisticated studies. If this information proves insufficient, indirect estimates or direct measurement of urinary NH(4)(+) concentration, measurement of urine pH, and assessment of urinary HCO(3)(-) excretion can help in establishing the diagnosis. This review summarizes current information concerning the pathophysiology of this electrolyte pattern and the value and limitations of all of the diagnostic studies available. It also provides a systematic and cost-effective approach to the differential diagnosis of nongap metabolic acidosis.

Renal expression and localization of SLC9A3 sodium/hydrogen exchanger and its possible role in acid-base regulation in freshwater rainbow trout (Oncorhynchus mykiss)

Experiments were performed to assess the possible involvement of the Na(+)/H(+) exchanger isoform 3 (NHE3; SLC9A3) in renal acid-base regulation in adult rainbow trout (Oncorhynchus mykiss). NHE3 mRNA was expressed at high levels in the kidney relative to its paralog, NHE2. The results of in situ hybridization demonstrated an abundance of NHE3 mRNA in renal tubules. The combination of immunocytochemistry and histological staining revealed that NHE3 was confined to the apical membrane of proximal tubules, where it was colocalized with the vacuolar-type H(+)-ATPase. Levels of NHE3 protein (assessed by Western blotting) were increased during hypercapnia, likely as a result of increased transcription, as indicated by increasing levels of NHE3 mRNA (as determined by real-time PCR). Plasma cortisol concentration was increased during hypercapnia, and administration of exogenous cortisol caused a marked increase in NHE3 mRNA and protein. Thus we speculate that the elevation of plasma cortisol during hypercapnia contributes to transcriptional activation of NHE3 that ultimately promotes acid-base regulation by stimulating H(+) secretion and HCO(3)(-) reabsorption.

Adaptive redistribution of NBCe1-A and NBCe1-B in rat kidney proximal tubule and striated ducts of salivary glands during acid-base disturbances

The cellular distribution of the NH2-terminal electrogenic Na+-HCO3(-) cotransporter (NBCe1) variants NBCe1-A and NBCe1-B has been investigated in rat kidney and submandibular gland (SMG) under physiological conditions and after systemic acid-base perturbations. Moreover, the in vivo data were complemented in vitro by using an immortalized cell line derived from the S1 segment of the proximal tubule (PT) of normotensive Wistar-Kyoto rats (WKPT-0293 Cl.2). NBCe1-A was basolaterally localized in PT cells, whereas NBCe1-B exhibited intracellular and basolateral distribution. SMG showed transcript and protein expression for NBCe1-A and NBCe1-B. NBCe1-B was basolaterally localized in duct cells; NBCe1-A was found intracellularly in salivary striated ducts and apically in main duct cells. Acute metabolic acidosis significantly increased cells that showed basolateral NBCe1-A in the PT, indicating increased HCO3(-) reabsorption, and significantly decreased cells that exhibited basolateral NBCe1-B in the salivary ducts, suggesting decreased HCO3(-) secretion. Chronic acidosis had no effect on NBCe1 distribution in PT but significantly increased the percentage of cells with basolateral NBCe1-A in salivary striated duct cells, suggesting increased HCO3(-) reabsorption. In contrast, chronic alkalosis caused adaptive redistribution of NBCe1-A and NBCe1-B in renal PT, favoring decreased HCO3(-) reabsorption. In vitro, WKPT-0293 Cl.2 cells expressed key acid-base transporters. Extracellular alkalosis downregulated NBCe1-A protein. WKPT-0293 Cl.2 cells are therefore a useful model to study renal acid-base regulation in vitro. The results propose redistribution of the transporters as a potential posttranslational regulation modus during acid-base disturbances. Moreover, the data demonstrate that renal PT and salivary duct epithelia respond to acid-base disturbances by an opposite redistribution pattern for NBCe1-A and NBCe1-B, reflecting specialized functions as the HCO3(-)-reabsorbing and HCO3(-)-secreting epithelium, respectively.

Induction of Anti-Carbonic-Anhydrase-II Antibody Causes Renal Tubular Acidosis in a Mouse Model of Sjögren’s Syndrome

Background/Aim: We recently reported that renal tubular acidosis (RTA) in Sjögren’s syndrome (SjS) is associated with high titers of an autoantibody against carbonic anhydrase (CA) II, an important enzyme in renal acid-base regulation. The purpose of this study was to determine whether a CA-II antibody could cause RTA in a mouse model of SjS. Methods: PL/J mice were immunized with human CA II to induce CA II antibody formation, whereas controls were injected with phosphate-buffered saline and adjuvant. After 6 weeks, anti-CA-II antibody titers were measured, then ammonium chloride was administered orally for 1 week to detect any acidification defect. Results: CA-II-immunized mice showed higher anti-CA-II antibody titers than control mice. Pathologically, lymphocytic and plasma cell infiltration was seen in the salivary glands and kidneys of CA-II-immunized mice, but not in controls. On acid loading, blood pH and urine pH decreased in both groups of mice, but the slope of urine pH versus blood pH was less steep in the CA-II-immunized mice, suggesting that these mice had an impaired ability to reduce their urine pH in the face of metabolic acidosis. Conclusion: CA-II-immunized mice had a urinary acidification defect, which may be similar to that seen in patients with SjS.

Salt sensitivity in humans is associated with abnormal acid-base regulation.

Metabolic acidosis has recently been observed in rat models of salt-sensitive genetic hypertension. To test the hypothesis that salt sensitivity in humans may be associated with abnormal acid-base homeostasis, we performed arterial blood gas analyses in young (20-31 years old) normotensive subjects (n = 40) who were placed on a low salt diet (20 mmol NaCl/day) for 2 weeks with either 200 mmol sodium chloride or placebo added to the low salt diet for 1 week each in a randomized, single-blind crossover order. Furthermore, a subset of the subjects (seven salt-sensitive and eight salt-resistant) received 200 mmol sodium/day as the citrate salt as a supplement to the low salt diet for a third week. During each regimen, blood pressure as well as arterial pH and bicarbonate levels were measured. Salt sensitivity was defined as a significant drop in mean arterial pressure greater than 3 mm Hg (mean of 30 readings taken during each diet, p less than 0.05) while the subject was on the low salt diet. According to this definition, 16 subjects were salt-sensitive and 24 salt-resistant. During the high sodium chloride regimen, arterial pH and bicarbonate levels were significantly lower in the salt-sensitive than in the salt-resistant group (p less than 0.0001). The increase in blood pressure caused by sodium chloride correlated inversely to the arterial pH (r = -0.57, p = 0.0002) and bicarbonate levels (r = -0.52, p = 0.0007) during the high salt diet. Sodium chloride increased mean arterial blood pressure in the salt-sensitive subjects; sodium citrate did not. Sodium citrate led to an increase in pH and bicarbonate levels in both groups. Our finding that a sodium chloride-induced rise in blood pressure is associated with lower arterial plasma pH and bicarbonate levels points to an abnormality in renal acid-base regulation in salt-sensitive subjects.

Renal Function in Anaesthesia and Surgery

This monograph will be welcomed by those who wish to bring up to date their understanding of how anesthesia, surgery, and trauma affect renal function. The author has made several valuable contributions to this field in recent years. He knows his subject. He writes well. He distinguishes the important and the practical from the esoteric and the arcane. The result is a succinct, clear presentation that is a pleasure to read. The book starts with a review of basic principles, which covers gross and microscopic anatomy, circulation, osmoregulation, excretion of water and solutes, renal acid-base regulation, and renal function tests. This section alone makes the book worthwhile for those of us who are nephrologic novices. Basic principles are followed by a systematic exposition of how anesthesia affects renal function. The author carefully dissects out direct effects of anesthetics on various aspects of renal function from indirectly mediated effects secondary to

Scanning of the apical pole of distal tubular cells under differing acid-base conditions

Scanning of the apical pole of distal tubular cells under differing acid-base conditions. The apical pole of cells of cortical distal tubule was studied by scanning electron microscopy (SEM). In normal rats, as well as in pigeon, wall lizard, grass snake and triton, the apical poles are schematically either smooth with short and few microvilli, or rough with folds and leaflets, these two typical features being connected with intermediary states. A relief pattern of folds and leaflets corresponds to the surface of the dark cells as they appear on transmission electron microscopy. At least in rats, the repartition of these cells corresponds exactly to what is observed in the different segments of the cortical tubule beyond the loop of Henle. During bicarbonate loading and, to an even greater degree during respiratory acidosis, the relief pattern of the apical pole becomes rough in all cells. Microvilli are denser and longer and the leaflets are more pleated and profound. The apical pole of every cell assumes, at least, the morphologic appearance of intermediary types. As seen on transmission electron microscopy, such changes of the urinary surface of the cells correspond to modifications of the following: the nucleus, which is now central; the mitochondria, which are numerous and spread throughout the cytoplasm, especially between the nucleus and the apical pole; and the micro vesicles and free ribosomes, which become very abundant. These observations support the conclusion drawn from our previous findings on light microscopy: clear and dark cells are not two different types of cells but differing functional forms of a unique kind of cell; the change from light to dark cells is related to renal acid-base regulation, most likely to changes of bicarbonate reabsorption. Thus, a definite morpho-physiologic correlation within the kidney is established.

The renal response of children to acute ammonium chloride acidosis.

Extract: The present observations were undertaken to establish the response of children to the acute administration of ammonium chloride, in order to provide a basis for identifying and interpreting abnormalities in the mechanisms involved in renal acid base regulation. Fifty-eight normal children aged 4 to 13 years were studied before and for 5 hours following administration of ammonium chloride. Group A, consisting of 23 children, received 75 mEq/m2 body surface area or 2.75 mEq/kg body weight. Group B, consisting of 35 children, received 150 mEq/m2, or 5.45 mEq/kg. Values for serum electrolytes, urea, creatinine, osmolality and acid-base parameters in children in Groups A and B are shown in tables I and III. Urinary pH and rates of excretion of titratable acid and ammonium are shown in tables II and IV. The relations between blood tCO2 and urinary concentration of H+, rate of excretion of titratable acid, and rate of excretion of ammonium for all subjects are shown in figures 1-3. Progressively lower values of pH were observed over the entire range of tCO2 studied. During control periods, an inverse correlation was present between tCO2 and rates of excretion of both titratable acid and ammonium. At levels of tCO2 below 22.5 mmoles/l, the rate of excretion of TA and ammonium became constant, with mean rates of excretion of 52 μEq/min/1.73 m2 (range, ± 2 SD, 33 to 71), and 73 μEq/min/1.73 m2 (46 to 100), respectively. These data are interpreted as demonstrating the importance of the blood tCO2 or, more precisely, the concentratibn of bicarbonate in glomerular filtrate in urinary acidification mechanisms as well as in control of excretion of bicarbonate. Assessment of renal acidifying mechanisms requires administration of sufficient ammonium chloride or other acidifying substance to depress blood tCO2 to levels well below the renal threshold. The physiologic validity of the hydrogen ion clearance index must be questioned, since correcting rates of excretion of hydrogen ion for concentration of total CO2 in blood presumes that there is a fixed relation between these two measurements. In figure 4, urinary excretion of total hydrogen ion is plotted as a function of blood tCO2. Hydrogen ion excretion, negligible at tCO2 levels of 26 or more, increases steadily as tCO2 falls to values below the threshold. The rate of excretion then reaches maximal levels, despite further reduction in blood tCO2. Thus, the hydrogen ion clearance index is valid only over the range of blood tCO2 in which there is a linear relationship with rate of excretion of hydrogen ion. Renal acidification mechanisms should be evaluated with blood tCO2 several millimoles below the threshold, assuring a maximal response and permitting direct comparison with normals. In subjects with a normal bicarbonate threshold, blood tCO2 should be depressed at least to 17–18 mmoles/l in infants and 20–21 mmoles/l in older children. If an abnormal bicarbonate threshold is present the dose must be adjusted accordingly. Speculation: These studies suggest that both the concentration of bicarbonate in glomerular filtrate and the level of the bicarbonate threshold play major roles in the regulation of renal excretion of hydrogen ion.