DISTAL RENAL TUBULAR ACIDOSIS (dRTA)
There are 4 forms of renal acidosis:
- distal (type 1)
-proximal (type 2)
- Mixed (type 3)
- distal generalized hyperkalemia (type 4)
Distal renal tubular acidosis (dRTA) or Type 1 Renal tubular acidosis (RTA) is the classical form of RTA, being the first described. Distal RTA is characterized by a failure of acid secretion by the alpha intercalated cells of the cortical collecting duct of the distal nephron. This failure of acid secretion may be due to a number of causes, and it leads to an inability to acidify the urine to a pH of less than 5.3.
DRTA belongs to the group of renal genetic diseases with a very low incidence in any population. There are no widely accepted statistics on the rate of RTA. The disease, however, does occur at any age and afflicts men more so than women.
The transmission can be autosomal dominant or autosomal recessive:
- Autosomal recessive dRTA appears in the first months of life and progresses with nephrocalcinosis and early or late hearing loss. Autosomal recessive dRTA is associated with mutations in genes ATP6V1B1, ATP6V0A4 and SLC4A1, which encode subunits a4 and B1 of V-ATPase and the AE1 bicarbonate/chloride exchanger respectively.
- Autosomal dominant dRTA is less severe and appears during adolescence or adulthood and may or may not develop nephrocalcinosis. , autosomal dominant dRTA is only related to mutations in AE1.
ACID-BASE BALANCE IN THE KIDNEY
The kidney maintains and controls the acid-base balance of blood through three mechanisms: filtration and reabsorption of bicarbonate, acid (or alkali) excretion and synthesis of ammonium and bicarbonate. In the kidney, two connected biochemical processes take place: bicarbonate reabsorption and the synthesis, secretion, recycling and urinary excretion of ammonium.
The presence of multiple transport systems in the different segments of the nephron tubules makes it possible to recover all the bicarbonate (HCO3-) filtered (4320mmol/day) in the glomerulus. In the first tubular segments of the nephron, the proximal tubules reabsorb approximately 80% of bicarbonate. In this tubular segment, bicarbonate reabsorption occurs through the Na+/HCO3-(NBCe1) cotransporter; this absorption is connected the secretion of acid in urine by the Na+/H+ (NHE3) exchanger. In the proximal tubules, circulating glutamine is reabsorbed from which ammonium and bicarbonate are simultaneously synthesised.
The reabsorption of 15% of the bicarbonate occurs in the thick ascending loop of Henle and only about 5% of the bicarbonate is recovered in the distal tubules of the nephron. Lastly, kidney performs the excretion of the acid load in the urine: diacid phosphate H2PO4- (titratable acid) and ammonium sulphate.
THE IMPORTANCE OF URINARY ACIDIFICATION
Urinary acidification, together with citrate excretion, is essential in the removal of organic and inorganic salts in soluble form. The urinary buffers are phosphates, but ammonium/ammonia acts as a buffer to a greater extent.
The intake of an acid load such as in a high-protein meal, causes the kidneys to produce a more acidic urine (pH<5.5); it also decreases the rate of bicarbonate excretion and increases phosphate and ammonium excretion.
TRANSPORT MECHANISMS WHICH PARTICIPATE IN ACID-BASE HOMEOSTASIS IN ALPHA INTERCALATED CELLS
The secretion of H+ hydrogen ions in urine is carried out in the alpha-intercalated cells of cortical and medullary collecting ducts (Figure 1). H+ ATPase, V-ATPase, catalyses the passage of H+ from the cytoplasm to the tubular lumen. Anhydrase carbonic CA2 produces H+ hydrogen ions and simultaneously, bicarbonate is reabsorbed through the Cl-/HCO3- exchanger, corresponding to the AE1 isoform. The ammonium excretion mechanism takes place in two stages: firstly, there is uptake from the interstitium to the cytoplasm via HCN2 voltage-activated ammonium channels and Rhcg ammonia channels. HCN2 channels are constitutive, they may uptake ammonium and/or sodium and are not regulated by metabolic acidosis. By contrast, Rhcg ammonia channels are located both in apical membranes and in basolateral membranes, and their destination to the membranes is regulated by metabolic acidosis.
AE1 Cl-/HCO3- EXCHANGER
The SLC4A1 gene encodes the AE1 exchanger, a dimeric glycoprotein with 12-14 transmembrane domains (Figure 2). There are three genes in the AE1 family and in the tissues in which AE1 is expressed, AEI it participates in the regulation of pH, cell volume and the transcellular transport of acid and base in epithelial cells.
AE1 presents a specific isoform of erythrocytes and a specific short isoform of the kidney. In erythrocytes, AE1, in addition to exchanging chloride for the bicarbonate of the plasma, has a structural role in interacting with cytoskeletal proteins that contribute to AE1 traffic and its stability in the plasma membrane. As such, AE1 plays a central role in respiration by transporting and removing CO2 via the lungs and in acid-base homeostasis in the kidney. In the kidney, AE1 performs bicarbonate reabsorption into the interstitial space and blood vessels. There is a group of mutations in AE1 that cause deformations in the erythrocyte and whose inheritance is autosomal dominant: inherited spherocytic anaemia, Southeast Asian ovalocytosis and other stomatocytosis with normal kidney function. There are other series of AE1 mutations that generate dRTA associated with erythrocyte problems. AE1 mutations can be consulted at: www.ensembl.org and www.hgmd.org
Vacuolar H- ATPase (V-ATPase) belongs to an H+ hydrogen-ion pump family and is located in a variety of membranes: endosomes, lysosomes, secretory vesicles and in the plasma membranes of eukaryotes. V-ATPase is a multimeric enzyme complex that consists of 14 subunits (Figure 3); it has two domains: one in the cytoplasm (V1) and the other in the membrane (V0). V1 is the catalytic domain and it has 8 subunits (A-H). Domain V0 comprises 6 subunits (a, c, c”, d, e, and Ac45 in mammals) and translocates H+ through the membrane. There are three copies of subunits A and B that alternate in a ring-shaped arrangement (Figure 3). The catalytic sites are in subunit A1 and the interface between subunits A-B regulates the activity of the enzyme. Subunit 'a' in V0 allows access to hemichannels through which H+ hydrogen ions are exported to the luminal space. There are four isoforms of subunit 'a' (a1-a4) and they have a 47-61% identity in humans. Subunit 'a' also participates in traffiking of V-ATPase in mammal cells.
In alpha-intercalated cells of the collecting duct, V-ATPase is located on the apical membranes and secretes H+ in urine (Figure 1). Subunits B1 and a4 of V-ATPase are specific alpha-intercalated cells of the collecting duct. Defects in these subunits lead to “distal renal tubular acidosis” or dRTA. As the B1 subunit is also expressed in the ciliary cells of the inner ear, mutations in subunit B1 produce dRTA with deafness.
The gene ATP6V1B1 encodes B1 subunit and comprises 14 exons, which produce a protein consisting of 513 amino acids. The gene ATP6V0A4 has 24 exons of which 20 encode the 840 amino acids of a4 subunit.
There are other transport systems in alpha-intercalated cells of the distal nephron which are also involved in acid-base homeostasis, such as carbonic anhydrase II, the KCC4 potassium/chloride cotransporter, Rhcg and the HCN2 ammonium channel (Figure 1). H-K-ATPase present in the apical membrane of alpha-intercalated cells does not seem to participate in secretion, but rather in reabsorption of K+ in hypokalemia.
Figure 1 illustrates transporters, ion channels and V-ATPase in alpha-intercalated cells of the collecting duct. It is important to highlight that, traduction, and destination to the membrane of many transporters and ion channels depend on metabolic conditions.
Collecting duct microperfusion trials and knockout mouse models have helped to elucidate transport pathways involved in acid-base homeostasis in alpha-intercalated cells. For example, the mouse not expressing KCC4 develops sensorineural deafness, as well as dRTA. There is another Cl/bicarbonate exchanger which also operates as a Cl-channel, Slc26a7, activated by hypertonicity. Mouse Slc26a7
/ develops dRTA. It is noteworthy that mice that do not express the ammonia channel (Rhcg /) have problems in excreting only in metabolic acidosis, suchas in incomplete dRTA. The ammonium channel HCN2 is a constitutive ion channel involved in baseline ammonium excretion but it does not appear to be regulated by metabolic acidosis.
PATIENT RISK FACTORS
This condition may be inherited or acquired. In the majority of cases, this form of RTA results from the presence of another disease such as Sjogren’s syndrome, hypergammaglobulinaemia, chronic active hepatitis and systemic Lupus Erythematosus.
Because renal excretion is the primary means of eliminating acid from the body, there is consequently a tendency towards acidemia. This leads to the clinical features of dRTA.
Normal anion gap metabolic acidosis/acidemia
- Urinary stone formation (related to alkaline urine, hypercalciuria, and low urinary citrate).
- Nephrocalcinosis(deposition of calcium in the substance of the kidney)
- Bone demineralisation (causing rickets in children and osteomalacia in adults)
- dRTA commonly leads to sodium loss and volume contraction, which causes a compensatory increase in blood levels of aldosterone. Aldosterone causes increased resorption of sodium and loss of potassium in the collecting duct of the kidney, so these increased aldosterone levels cause the hypokalemia which is a common symptom of dRTA.
The symptoms and sequelae of dRTA are variable and range from being completely asymptomatic, to loin pain and hematuria from kidney stones, to failure to thrive and severe rickets in childhood forms as well as possible renal failure and even death. In particular in children affected by dRTA there is stunted growth, vomiting, constipation, loss of appetite, polydipsia and polyuria, nephrocalcinosis, weakness and muscle paralysis due to hypokalaemia.
To diagnose dRTA in the clinic, it is necessary to determine plasma creatinine and fractional sodium, potassium and chloride excretion, calciuria and citraturia. Acidosis is generally observed in blood (pH<7.35) as well as a marked decrease in the concentration of bicarbonate and CO2 (<15mEq/l). In dRTA, urine pH is higher than 6 in the presence of systemic metabolic acidosis.
For cases in which diagnosis is uncertain, as in incomplete dRTA, it is advisable to perform acidification tests. These tests involve the administration of NH4Cl to determine pH, titratable acidity and urinary ammonium excretion. Due to complications of this test in children, acidification capacity can be evaluated by determining the maximum urinary pCO2 (UpCO2) with the intake of sodium bicarbonate (4mEq/kg). The pCO2 urinary test can be performed with sodium bicarbonate or acetazolamide stimuli or both, in this case administered at half the usual dose. Another test is furosemide with fludrocortisone. Diagnostic tests confirm the inability to excrete acid loads by observing a urinary pH higher than 5.5.
Ultrasound studies in patients with dRTA show the presence of calcium deposits in the renal tissue (nephrocalcinosis) and/or urinary tract stones (nephrolithiasis).
Chronic acidosis and intercurrent secondary problems (vomiting, polyuria, dehydration, rejection of dose, etc..) affect growth and, consequently, there is a decrease in the size and weight of the patients.
dRTA is accompanied by hyperchloraemia as a result of decreased HCO3- in blood. In dRTA, hypokalemia is observed ([K]<3.5mEq/l), along with hypercalciuria and hypocitraturia. Hypercalciuria occurs when there is urinary calcium excretion greater than 4mg/kg/day in both adults and in children. It is necessary to consider that the urinary calcium/creatinine quotient in infants varies with age. Normal values according to age are: 0-6 months <0.8 mg/mg, 6 to 12 months <0.6 mg/mg, 1 to 2 years <0.5 mg/mg. In adults, hypocitraturia is considered a value below 300 mg/day for both sexes, and/or a citrate/creatinine rate value below 250 mg/g. In children hypocitraturia is considered a value below 8 mg/kg/day and/or a citrate/creatinine quotient below 400 mg/g.53. It is important to highlight that calcium excretion in urine is high in infants and decreases progressively with age. As such, hypocitraturia is most relevant in the development of nephrocalcinosis and urolithiasis, in which primarily calcium phosphate salts are deposited. It is also noteworthy that dRTA cases have been found without no hipercalciuria.
This is relatively straightforward. It involves correction of the acidemia with oral sodium bicarbonate, sodium citrate or potassium citrate. This will correct the acidemia and reverse bone demineralisation. Hypokalemia and urinary stone formation and nephrocalcinosis can be treated with potassium citrate tablets which not only replace potassium but also inhibit calcium excretion and thus do not exacerbate stone disease as sodium bicarbonate or citrate may do.