The document discusses metabolic acidosis, focusing on renal regulation of acid-base homeostasis and various types of renal tubular acidosis (RTA), including diagnostic approaches and treatment options. Key points include the roles of bicarbonate reabsorption, diagnostic algorithms for differentiating types of acid-base disorders, and specific details about type 1, type 2, and type 4 RTA. The takeaway emphasizes the importance of thorough patient assessment and appropriate therapy based on the underlying causes of acidosis.

1. Acid-base disorders
metabolic acidosis
October 22, 2013

2. Outline of the lecture
• Renal regulation of acid-base homeostasis
• Diagnostic approach in patients with acidosis
• Renal tubular acidosis (RTA)
– Distal renal tubular acidosis (dRTA), type 1
– Proximal renal tubular acidosis (pRTA), type 2
– dRTA with hyperkalemia, type 4
• Diagnostic algorithm in patients with RTA
• Take home message

3. Maintanance of acid-base homeostasis
•
Aim: maintain arterial pH 7.35-7.45
•
Metabolism: production of acids
(Manz et al. 2004)
•
Systems involved in pH regulation:
– Extracellular and intracellular buffering
– Lung: excretion of CO2
– Kidney
Bidani et al. 2002

4. Renal regulation of acid-base
homeostasis
• Bicarbonate reabsorption (~4000
mmol/1.73 m2/24 hrs)
• Acids and ammonium (NH4+)
excretion
Net acid excretion (NAE)
NAE = NH4 + TA – HCO3
+
_
TA: titratable acid
L. Lee Hamm et al.
2008

5. Renal proximal tubule (PT)
NBC1
• Reabsorption of ~ 80 % HCO3
_
• Different transport rate and
mechanism in S1, S2 and S3
(highest in S1)
• NHE 8 on the apical membrane
(Goyal et al. 2003, 2005)
3 Na+ Citrate 2NHE 3,
amiloride
sensitive
H+-ATP-ase
L. Lee Hamm et al.
2008

6. Renal ammonium generation and
transport
RhCG (apical)
NH4+ generation in PT cells
RhBG
(basolateral)
(Knepper 2008)
L. Lee Hamm et al.
2008

7. Role of Rhesus factor proteins in renal
ammonium excretion and male fertility
Rh-factor family of proteins:
homology to ammonia (NH3)
-transport proteins in
bacteria, fungi, plants,
invertebrates
Rhbc -/- mouse model (Biver et al.
2008) :
- reduced body weight
- decreased urinary ammonium excretion
- reduced urine acidification capacity after
acid load
- reduced male fertility due to impaired
ammonia secretion in epididymus
Non-erythroid members
(RhCG and RhBG) are
expressed in the kidney
Rhbg -/- mouse model: no
acid-base abnormalities
(Chambrey et al. 2005)
(Knepper 2008)

8. Regulation of PT acid-base transport
Acidosis:
insertion of NHE3 and H+-ATP-ase
function of NBC1 (alkalosis: opposite
effects)
secretion of endothelin – 1
secretion of cortisol
K+:
PTH:
_
increases HCO3 reabsorption
_
acute effect: decreases HCO3 via
cAMP
PKA phosphorylation of
NHE 3
inhibition
chronic acidosis:
excretion
ATII:
_
PTH
net acid
increases HCO3 reabsorption
L. Lee Hamm et al.
2008

9. Thick ascending loop of Henle (TAL)
• Reabsorption of ~ 20 % HCO3
_
_
• Acidosis: increases HCO3
reabsorption, alkalosis: no effect
•Loop diuretics: increases HCO3
reabsorption
_
L. Lee Hamm et al.
2008

10. Cortical collecting duct (CCD)
intercalated cells
intercalated cells
L. Lee Hamm et al.
2008

11. Regulation of CCD acid-base transport
•
•
•
Acute acidosis:
H+
secretion,
_
HCO3 reabsorption in type A IC
_
Chronic acidosis:
HCO3 secretion in type B IC (some type B cells
transform into type A cells), role of hensin (Schwartz et al. 2005)
_
Na CL :
AE1 in type A IC,
_
HCO3 secretion in type B IC
•
K+: K + / H+ ATPase (K + reabsorption, H+ excretion), increase
H+ATPase insertion to the apical membrane
•
Mineralocorticoids: rapid nongenomic stimulation of H+ATPase
(independent of Na +) (toevoegen genomic effects)
•
ET-1: in acidosis increase of ET-1 in renal interstitium,
and Na +/ H+ exchange
•
PTH: stimulates distal nephron acidification
_
HCO3 secretion

12. Diagnostic approach in patients with acidosis
•
Step 1: obtain arterial (capillary) blood gas _
_
analysis and plasma HCO3 , Na +, K +, CL
•
Step 2: distinguish simple from mixed type acidbase disorders
•
Step 3: calculate blood Anion Gap_ (AG):
_
–
–
–
AG = (Na+ + K +) - (CL + HCO3 )
AG: unmeasured anions in plasma (albumin,
and globulins):
•
1 g/dl albumin
2.5 - 4 meq/L AG
Ca2+, Mg2+, Li+ (intoxication)
– High IgG (cationic)
•
AG
AG
Step 4: calculate urine Anion Gap (AG):
–
AG = (Na+ + K +) - CL
_
Emmet et al. 2002

13. Clinical causes of high and normal AG acidosis
High AG acidosis
Ketoacidosis
Normal AG acidosis
Gastrointestinal loss of HCO3
_
negative urine AG: U (Na+ + K +) < U Cl diarrhea
Diabetic (acetoacetate)
Alcoholic ( -hydroxybutyrate)
Starvation
Renal tubular acidosis
hyperchloremic, positive urine AG :
U (Na+ + K+) > Cl Proximal tubular acidosis (pRTA)
Lactic acid acidosis
Distal tubular acidosis (dRTA) (low K +)
Acetazolamide, topiramate (inhibitor of CA)
Generalized distal renal tubular defect (high K +)
Toxins
Ethylene glycol, propylene glycol, methylalcohol,
salicylate
Miscellaneous
NH4+Cl ingestion
Sulfur ingestion

14. Distal renal tubular acidosis: type 1 dRTA
•
Dysfunction of
acid urine
•
Clinical diagnosis: pH urine > 5.5 when arterial pH < 7.34, normal AG,
normal GFR
•
NH4+Cl loading (100 mg/kg): failure to reduce urine pH < 5.3 during the
following 6 hrs (Wrong and Davis 1959)
•
Other features: hypokalemia, metabolic bone disease, nephrocalcinosis,
nephrolithiasis
•
In adults: mostly secondary, associated with autoimmune disease
(Sjögren syndrome) (Wrong et al. 1993)
•
Children: inherited forms
intercallated cells ( IC or type A IC): failure to produce

15. Inherited forms of type 1 dRTA
Autosomal dominant
dRTA (Karet et al. 2009)
Autosomal recessive dRTA
(Karet et al. 2009)
• Mutations in 3 genes:
• Mutations in AE1 ( SLC4A1
gene) (most common mutation:
R588 (arginin) in 6th
transmembrane domain
SLC4A1 gene (other mutations than in
AD dRTA); in some kindreds in
combination with hemolytic anemia
Mutations in H+ ATPase subunits
CAII
CO2 + H2O
AR dRTA with deafness: mutations in
B1-subunit (ATP6V1B1 gene) mutations:
mostly loss of function. In inner ear:
expression in cochlea and
endolymphatic sac (high K+ 150 mM and
pH 7.4 due to H+ ATPase)
H+ + HCO3 -
• Dominant-negative mechanism:
depending on mutation’s sort :
mutated AE1 prevents
expression of wild-type AE1 on
basolateral membrane, ER
retention (Quilty et al. 2002, Toye et
al. 2002)
• Expression on the apical
membrane (R901X, G609R)
AR dRTA with late onset hearing loss:
(Devonald et al. 2003).
Mutations in a4-subunit ATP6V0A4
gene) (expression kidney, cochlea)
CAII deficiency (McMahon et
Other genes?
al. 2001)
Combination dRTA and osteopetrosis
•More severe disease in these
individuals (Rungroj et al. 2004)

16. Proximal renal tubular acidosis: type 2 pRTA
•
Defect in PT capacity to reabsorb HCO3_
(Rodriguez Soriano 1967, 2002)
•
Normal distal capacity to acidify urine
(urine pH < 5.5 when plasma HCO3_ < 15
meq/L)
•
Patients maintain normal acid-base
balance with HCO3_ supplements
•
Hypokalemia not always present (reduced PT
fluid reabsorption, hyperaldosteronism, increased fluid and
alcali delivery to distal nephron; mostly absent in metabolic
acidosis and aggravated by base suppletion)
•
Hypercalciuria/nephrocalcinosis: absent
or less severe (Lemann et al. 2000)
Rodriguez Soriano et al. 1972

17. Proximal renal tubular acidosis: type 2 pRTA
•
NBC1
Most commonly: part of generalized proximal
tubular dysfunction (renal Fanconi syndrome),
combined with aminoaciduria, glucosuria,
phosphaturia, LMW proteinuria
• Isolated pRTA
NBC1 mutations (SLC4A4 gene):
AR, severe metabolic acidosis (pH 7.1-7.2), bicarbonate
+10 mEq/L (Igarashi et al. 2002)
In acidotic state: urinary pH < 5.5; short stature, ocular
abnormalities (glaucoma, cataract) in all patients; basal
ganglia calcifications; abnormal dentition
Most mutants are retained in ER
3 Na+ Citrate 2-
CAII:
osteopetrosis (also distal acidification defect),
defective bone resorption by osteoclasts
NHE 3,
amiloride
sensitive
H+-ATP-ase
NHE3 mutations: not yet idenitfied in
humans; NHE3 KO mice: mild metabolic acidosis
(Schultheis et al. 1998)
Transient pRTA of infants
(Rodriguez Soriano 1967),
growth retardation, good responce to alcali therapy

18. Hyperkalemic renal tubular acidosis: type 4 dRTA
•
Aldosterone action:
–
–
Na + reabsorption
lumen negative potential
required for K+ and H+ secretion
Direct activation of distal H+ ATPase (Winter et al.
2004)
True hypoaldosteronism: hyporeninemic
hypoaldosteronism (diabetes, amiloidosis, TIN
due to NSAID), adrenal dysfunction, ACE
inhibition, ATII receptor antagonists, inhibition
of aldosterone synthesis by heparin (Kutyrina et al.
1987)
Functional hypoaldosteronism: antagonists
of MR (spironolactone), ENaC blockers
(amiloride, triamterene, trimethoprim),
cyclosporine therapy (interference with
basolateral Na +/ K+ ATPase, NKCC2 and
distal K+ channels (Karet 2009)
Karet 2009

19. Mendelian forms of type 4 dRTA
•
Pseudohypoaldosteronism type 1 (PHA 1)
– Renal Na+ waisting, hyperkalemia, hyponatremia and metabolic acidosis
– Elevated renin and aldosterone levels
– Mineralocorticoid receptor (MR) mutations:
• AD, haploinsufficiency (mutant RNA degraded)
–
•
(Geller et al. 2009)
ENaC mutations
• AR, loss of function (alpha, beta, gamma subunits), extremely rare (Geller 2009)
Pseudohypoaldosteronism type 2 (PHA 2) (Gordon syndrome)
– Hyperkalemic hypertension associated with (mild) acidosis
– Impaired removal of distal NaCl cotransporter in DCT and increased
expression of ENaC and decreased expression of ROMK in CD
– Mutations in WNK 1 and 4 (with no lysine [K] kinase), regulating NCC,
ROMK, ENaC (Kahle et al. 2009)

20. Diagnostic algorithm in patients with RTA
Hyperchloremic normal AG
metabolic acidosis
Mesure urine AG
(Na+K)-CL
Positive (CL < Na+K)
pH< 5.5
Nl of laag K+
pH> 5.5
Nl of laag K+
pRTA
Increased FE
Bicarb (10-15%)
dRTA
Check for renal
stones and
nephrocalcinosis
Negative (Cl> Na+K)
pH < 5.5
Hoog K+
Fractional excretion of
bicarbonate
Type 4 RTA
Decreased (< 5%)
= gastro-intestinaal losses of
bicarbonate (urine pH <6.5)
Fractional excretion of bicarbonate (%) =
{[HCO3]u / [HCO3]p} x {Pcr / Ucr} X 100

21. Treatment of RTA
•
Correction of acidosis:
– Na or K citrate or bicarbonate supplements
• pRTA 10-15 meq/kg/day;
• dRTA 2-4 meq/kg/day in 4 doses)
– Further treatment depending on the cause

22. Take home message
•
In patients with acidosis:
– Anamnesis (diarrhea, medication use, intoxication, family history)
– Clinical examination (growth parameters, blood pressure, exclude autoimmune
disease, bone disease)
– Arterial (capillary) blood gas analysis simultaneously with blood and urine
electrolytes, renal function, bicarbonate, albumin prior to alkali supplements
– Follow steps 1-4
determine sort of metabolic acidosis (high AG or normal AG,
renal or extrarenal)
•
Renal RTA
– primary or secondary (medications, autoimmune disorder, other renal disease)
– distal or proximal; low/normal K+ or high K+
– MAKE DIAGNOSIS (DNA analysis, genetic counselling)
– FOLLOW PATIENT (acidosis might be transient!)

23. Fons Sapientiae by Jef Claerhout