• Acid-base homeostasis and pH regulation are critical for both normal physiology and cell
metabolism and function.
• Renal regulation of metabolic component of acid base homeostasis - has two
components:
1)reabsorption of virtually all of the filtered HCO3 2 and
2)production of newbicarbonate to replace that consumed by normal or pathologic acids.
• NORMAL - arterial pH between 7.36 and 7.44;
• intracellular pH is usually approximately 7.2
NET ACID PRODUCTION
• Both acid and alkali are generated from diet.
• Lipid and carbohydrate metabolism results in production of carbon dioxide (CO2),
a volatile acid, at the rate of approximately 15,000 mmol/day.
• Protein metabolism yields amino acids, which can be metabolized to form
nonvolatile acid and alkali.
• Amino acids such as lysine and arginine yield acid on metabolism, whereas the
amino acids glutamate and aspartate and organic anions such as acetate and
citrate generate alkali.
• Sulfur-containing amino acids (methionine, cysteine) are metabolized
to sulfuric acid (H2SO4), and organophosphates are metabolized to
phosphoric acid (H3PO4).
• In general, animal foods are high in proteins and organophosphates
and provide a net acid diet; plant foods are higher in organic anions
and provide a net alkaline load.
• Typical high–animal protein Western diets and endogenous metabolism produce acid,
typically on the order of 1 mEq/kg body wt per day or approximately 70 mEq/d for a
70-kg person
• vegetarian diets with high fruit and vegetable content are not acid producing and may
produce a net alkali load
• Endogenous acid production may be regulated, at least under certain circumstances ;
for instance, lactic acid and ketoacid production are decreased by a low pH.
• Also, hepatic production of HCO3 2 in the metabolism of proteins and amino acids is
altered by systemic acid-base balance.
BUFFER SYSTEMS IN REGULATION OF PH
• Intracellular and extracellular buffer systems minimize the change in pH
during variations but do not remove acid or alkali from the body
• The most important buffer system is bicarbonate ion and carbon dioxide
(HCO3 −-CO2).
• Addition of acid (HA) leads to conversion of HCO3− to CO2 according to the
reaction HA + NaHCO3 → NaA + H2O + CO2
• other buffers such as plasma proteins and phosphate ions also participate
• In metabolic acidosis, the skeleton becomes a major buffer source as acidinduced dissolution of bone apatite releases alkaline Ca2+ salts and HCO3− into
the ECF.
• With chronic metabolic acidosis, this can result in osteomalacia and osteoporosis.
• ICF compartment, pH is maintained by intracellular buffers such as hemoglobin,
cellular proteins, organophosphate complexes, and HCO3 − as well as by the H+HCO3 − mechanisms
RESPIRATORY SYSTEM IN REGULATION OF PH
• As serum [HCO3 −] is much greater than that of other buffers,
changes in the HCO3 −-CO2 buffer pair easily titrate other buffer
systems and thus set pH.
• The Henderson-Hasselbalch equation explains how the lungs and
kidneys function in concert:
• Alveolar ventilation is controlled by chemoreceptor cells located in
the medulla oblongata (and to a lesser extent, those in the carotid
bodies), which are sensitive to pH and P
CO2
• Chemoreceptors respond to a decrease in cerebral interstitial pH by
increasing ventilation and hence, lowering PCO2. Therefore, small
increases in plasma CO2, which decrease pH, will result in stimulation of
ventilation
• However, in contrast to the rapid response to changes in CO ,
2
the response to nonvolatile acids or a change in plasma
HCO is slower, because the central chemoreceptors are
3
relatively insulated by the blood-brain barrier.
• Hence, acute changes in plasma HCO have a slower effect
on cerebral interstitial pH and hence, stimulation of central
chemoreceptors. (reason for 12- to 24-hour delay in the maximal
ventilatory response to metabolic acid-base disturbances)
3
• Maximal ventilation response cannot usually reduce PCO2 ,10–12
mmHg
• Limitations of resp, compensation are
lung disease, fluid overload, and central nervous system derangements.
Also, decreases in PO2 may limit the extent to which decreased
ventilation can raise PCO2.
RENAL REGULATION OF PH
• Two components: reabsorption of virtually all of the filtered HCO32 and production of new
HCO32 to replace that consumed by normal or pathologic acids
• As HCO32 is freely filtered at the glomerulus, approximately 4.5 mol HCO32 is normally
filtered per day (HCO3 2 concentration of 25 mM/L 3GFR of 0.120 L/min 31440 min/d).
• Virtually all of this filtered HCO32 is reabsorbed, with the urine normally essentially free of
HCO3 2.
• 70-80 % of this filtered HCO3 2 is reabsorbed in the proximal tubule; the rest is reabsorbed
along more distal segments of the nephron
NAE
• The net acid excretion of the kidneys is quantitatively equivalent to the amount
of HCO3 2 generation by the kidneys.
• Generation of new HCO3 2 by the kidneys is usually approximately 1 mEq/kg body
wt per day (or about 70 mEq/d) and replaces that HCO3 2 that has been
consumed by usual endogenous acid production (also about 70 mEq/d)
• NAE has three components, titratable acids, ammonium (NH4+), and bicarbonate,
and is calculated as
• Under basal conditions, approximately 40% of NAE is in the form of titratable acids( weak
buffer acids) and 60% is in the form of ammonia (NH3); urinary bicarbonate concentrations
and excretion are essentially zero
• The most important titratable buffer is phosphate (HPO42− ↔H2PO4−) because it has a
favorable pKa of 6.80 and there is a relatively high rate of urinary excretion.
• Loss of alkali in the urine in the form of HCO3 2 decreases the amount of net acid excretion
or new HCO3 2 generation.
• Loss of organic anions, such as citrate, in the urine represents the loss of potential alkali or
HCO3
Proximal Tubule HCO3
2 Reabsorption
• 70%–80% of the approximately 4500
mEq/d filtered HCO3 2 is reabsorbed
here.
• Most (probably .70%) of this HCO3 2
reabsorption occurs by proton
secretion at the apical membrane by
sodium-hydrogen exchanger NHE3
• This protein exchanges one Na+ ion for one H+ ion, driven by the lumen to
cell Na1 gradient (approximately 140 mEq/L in the lumen and 15–20 mEq/L
in the cell).
• The low intracellular Na+ is maintained by the basolateral Na/K-ATPase.
• An apical H+-ATPase and possibly, NHE8 under some circumstances
account for the remaining portion of proximal tubule H+ secretion and
HCO3 2 reabsorption( most of its action on distal nephron)
• In lumen, secreted H+ reacts with luminal HCO3 2 to generate CO2 and
H2O, considered to be freely permeable across the proximal tubule and
reabsorbed
• This reaction is relatively slow unless catalyzed, which occurs normally, by
carbonic anhydrase
• There are multiple isoforms of CA, but membrane-bound CAIV and
cytosolic CAII are most important
• Mutations in CAII are known to cause a form of mixed proximal and distal
RTA with osteopetrosis
BICARBONATE EXIT TO INTERSTITIUM
• The HCO3 2 generated within the proximal tubule cell by apical H1 secretion exits across
the basolateral membrane.
• Most of this HCO3 2 exit occurs by sodium HCO3 2 cotransport, NBCe1-A, or SLC4A4
• Studies suggest that 3 HCO3 2 Eq or 1 HCO3 2 Eq and 1 CO3 -2 Eq are transported with
each Na+
• Mutations in this (NBC-e1) protein cause proximal renal tubular acidosis
• Chloride bicarbonate exchange may also be present on the basolateral membrane of the
proximal tubule but is not the main mechanism of HCO32 reabsorption.
• The proximal tubule is a leaky epithelium and so unable to generate
large transepithelial solute or electrical gradients; the minimal luminal
pH and HCO3 2 obtained at the end of the proximal tubule are
approximately pH 6.5–6.8 and 5 mM, respectively.
Regulation of HCO3 2 Reabsorption in the
Proximal Tubule
• The signals for changes in bicarb reabsorption are often thought to be
pH per se, either intracellular or extracellular.
A variety of pH sensors have also been proposed , most notably
including
• nonreceptor tyrosine kinase Pyk2,
• endothelin B receptor (activated by endogenous renal endothelin)
• and CO2 activation of ErbB1/2, ERK, and
• Apical angiotensin I receptor
VOLUME STATUS AND ALKALOSIS
• ECF volume – important determinant of proximal HCO3 2
reabsorption.
• Decreasing ecf causes increased reabsorption of not only sodium but
also, HCO3 2, both in large part through increased Na-H exchange.
• Increases in volume status inhibit reabsorption of sodium and HCO3
and also causes increased backleak of HCO3 2 into the tubule lumen.
• In metabolic alkalosis, volume contraction (and the associated hormonal changes
and frequent fall in GFR) and high filtered loads of Bicarb increase proximal
tubule HCO3 2 reabsorption
• chronic volume contraction is associated with an adaptive increase in the activity
of the proximal tubule apical membrane Na+-H+ antiporter NHE3
• whereas increases in peritubular HCO3 2 and increased intracellular pH may be
inhibiting HCO3 2 reabsorption—the net effect is maintenance of metabolic
alkalosis until the volume status is corrected.
Hormonal effects on PCT
• On an acute basis, for instance, adrenergic agonists and angiotensin II stimulate HCO3 2
reabsorption
• Parathyroid hormone acting through cAMP inhibits but hypercalcemia stimulates
proximal HCO3 2 reabsorption
• chronic acid loads or acidosis - intrarenal endothelin-1 acting through the endothelin B
receptor has been identified as a crucial element in the upregulation of Na1/H1
exchange
• Glucocorticoids also are important in the development of proximal HCO3 2 transport
Distal Tubule Acidification
• Thick ascending limb (TAL) reabsorbs a significant amount of HCO3 2, approximately 15% of
the filtered load, predominantly through an apical Na1/H1 exchanger
• Majority of apical membrane H+ secretion is mediated by the Na+-H+ antiporter NHE3.
• As in the proximal tubule, the low intracellular Na+ concentration maintained by the
basolateral Na+,K+-ATPase provides the primary driving force for the antiporter.
• Base efflux across the basolateral membrane is mediated by a Cl−-HCO3 − exchanger (AE2)
and K+-HCO3 − cotransport likely mediated by the K+-Cl− cotransporter KCC4
• reabsorb the remaining 5% of filtered HCO3−
• distal nephron must secrete a quantity of H+ equal to that generated
systemically by metabolism to maintain acid-base balance
• Main segments appear to be in the collecting duct
Segments of the collecting duct –
• the cortical collecting duct (CCD),
• the outer medullary collecting duct, and
• the inner medullary collecting duct
The principal
cell reabsorbs Na+ and secretes K+
Two histologically
distinct cell types in
the CCD
two types of Intercalated cells: the
acid-secreting α-IC cell and the
base-secreting β-IC cell. Both IC cell
types are rich in carbonic anhydrase
II.
α- intercalated cell action
• Two transporters secrete H+: a vacuolar H+ATPase and an H+-K+-ATPase
• Active H+ secretion by the apical membrane
generates intracellular base that must exit
the basolateral membrane.
• Secrete H+ into the lumen and to reabsorb
K+
• activity of the H+-K+-ATPase increases in K+
depletion and thus provides a mechanism by
which K+ depletion enhances both collecting
duct H+ secretion and K+ absorption
• The HCO3 −-secreting β-IC cell is a mirror image of the α-IC cell
• The Cl−- HCO3− exchanger is distinct from the basolateral Cl−-HCO3− exchanger present in the αIC cell and functions as an anion exchanger or Cl− channel in the luminal membrane of epithelial
cells
• The SLC26A4 protein (pendrin) is a family member that mediates apical Cl−-HCO3 − exchange in
the β-IC cell of the kidney.
• The Na+-driven Cl−-HCO3 − exchanger (NDCBE) colocalizes with pendrin on the apical membrane
and together may explain a component of electroneutral NaCl reabsorption in the collecting duct
that is thiazide sensitive
• Principal cells mediate
electrogenic Na+ reabsorption
that results in a net negative
luminal charge.
• The greater this negative charge,
the lesser is the electrochemical
gradient for electrogenic proton
secretion and therefore the
greater the rate of net proton
secretion
Ammonia Metabolism
• Ammonia excretion accounts for approximately 60% of total NAE, and in
chronic metabolic acidosis, almost the entire increase in NAE is caused by
increased NH3 metabolism
• PCT is responsible for both ammonia production and luminal secretion.
• Ammonia is synthesized in the proximal tubule predominantly from
glutamine metabolism through enzymatic processes in which
phosphoenolpyruvate carboxykinase and phosphate-dependent
glutaminase are the rate-limiting steps
• This produces two ammonium (NH4+) and two HCO3− ions from each glutamine
ion
• Preferentially secreted into the lumen and primary mechanism appears to be
NH4+ transport by the apical Na+-H+ antiporter NHE3
• Metabolic acidosis increases the mobilization of glutamine from skeletal muscle
and intestinal cells.
• Glutamine is preferentially taken up by the proximal tubular cell through the Na+and H+-dependent glutamine transporter SNAT3.
• SNAT3 expression increases several-fold in metabolic acidosis, and it
is preferentially expressed on the cell’s basolateral surface, where it is
poised for glutamine uptake and is upregulated with increase in
plasma cortisol that typically accompanies metabolic acidosis.
Most of the ammonia that leaving PCT does not reach
the distal tubule as there is transport of ammonia out of the loop
of Henle, predominantly in
the TAL and is mediated by at least three mechanisms
lumen-positive voltage provides driving
force for passive paracellular NH4 +
transport out of the TAL
apical membrane
K+ channel of the
TAL cel
furosemidesensitive Na+-K+2Cl− transporter
• loop of Henle - NH4+ ion is reabsorbed into the interstitium,
accumulates in the medulla(One fraction of this medullary
ammonium secreted back into the late PCT, enters the lumen of the
cortical and medullary collecting ducts and other fraction enters
systemic circulation)
• TAL----NH4 +is transported across the apical membrane
predominantly on the Na1-K1-2Cl cotransporter, with perhaps some
entry on K+ channels
• nonerythroid glycoproteins Rhbg and Rhcg may be involved in
collecting duct ammonia secretion
• Chronic acidosis and hypokalemia--- increase ammonia synthesis and
increases expression of both NHE3 and the loop of Henle Na+-K+-2Cl−
cotransporter
• Hyperkalemia--- suppresses ammonia synthesis and inhibit NH4 +
reabsorption from the TAL (low urinary [NH4 +] found in hyperkalemic
distal renal tubular acidosis.)
REGULATION OF RENAL ACIDIFICATION
• Decreased intracellular Ph --->---- increased availability of H+ ions for
secretion ------>> enhances Na+/H+ exchanger
• In acute acidosis ---> insertion of additional transport proteins into
the apical membrane
• during metabolic acidosis the number of α-IC cells increases and the
number of β-IC cells decreases, without a change in the total number
of IC cells
• In addition to acute regulation, Chronic acidosis or alkalosis leads to
parallel changes in the activities of the proximal tubule apical
membrane Na+-H+ antiporter and basolateral membrane Na+HCO3−-CO3 2− cotransporter
• In chronic acidosis -----> induces the production of additional
transport proteins by increased transcription and production of
mRNA(NHE3)
• Increases tubular ammonia synthesis by increasing the activities of
the enzymes involved in ammonia metabolism
Mineralocorticoids
• key regulators of distal nephron and collecting duct H+ secretion.
Two mechanisms
• Stimulates Na+ absorption in principal cells of the CCD, leads to a
more lumen-negative voltage that then stimulates H+ secretion
• direct activation of H+ secretion by mineralocorticoids(apical
membrane H+-ATPase and basolateral membrane Cl−-HCO3 −
exchanger activity)
POTASSIUM
• Hypokalemia causes increase in renal NAE
Chronic hypokalemia – mechanism is similar to chronic acidsis
• (increases the proximal tubule apical membrane Na+-H+ antiporter
and basolateral membrane Na+-HCO3 −-CO32− cotransporter
activities)
• proximal tubular ammonia production
• increase in collecting duct H+ secretion.(H+-K+-ATPase)
• K+ deficiency decreases aldosterone secretion, which can inhibit
distal acidification
• Thus, in normal individuals, the net effect of K+ deficiency is typically
a minor change in acid-base balance.
• However, in patients with nonsuppressible mineralocorticoid
secretion (e.g., hyperaldosteronism, Cushing syndrome), K+ deficiency
can greatly stimulate renal acidification and cause profound
metabolic alkalosis.