This is a low capacity, high gradient system which accounts for the excretion of the daily fixed acid load of 70 mmols/day. The maximal capacity of this system is as much as 700 mmols/day but this is still low compared to the capacity of the proximal tubular mechanism to secrete H+. It can however decrease the pH down to a limiting pH of about 4.5 : this represents a thousand-fold (ie 3 pH units) gradient for H+ across the distal tubular cell. The maximal capacity of 700 mmols/day takes about 5 days to reach.
The processes involved are:-
H+ is produced from CO2 and H2O (as in the proximal tubular cells) and actively transported into the distal tubular lumen via a H+-ATPase pump. Titratable acidity represents the H+ which is buffered mostly by phosphate which is present in significant concentration. Creatinine (pKa approx 5.0) may also contribute to TA. At the minimum urinary pH, it will account for some of the titratable acidity. If ketoacids are present, they also contribute to titratable acidity. In severe diabetic ketoacidosis, beta-hydroxybutyrate (pKa 4.8) is the major component of TA.
The TA can be measured in the urine from the amount of sodium hydroxide needed to titrate the urine pH back to 7.4 hence the term ‘titratable acidity’.
As discussed previously, ammonium is predominantly produced by proximal tubular cells. This is advantageous as the proximal cells have access to a high blood flow in the peritubular capillaries and to all of the filtrate and these are the two sources of the glutamine from which the ammonium is produced.
The medullary cycling maintains high medullary interstitial concentrations of ammonium and low concentrations of ammonium in the distal tubule fluid. The lower the urine pH, the more the amount of ammonium that is transferred from the medullary interstitium into the fluid in the lumen of the medullary collecting duct as it passes through the medulla to the renal pelvis. [Note: The medullary collecting duct is different from the distal convoluted tubule.]
The net effect of this is that the majority of the ammonium in the final urine was transferred from the medulla across the distal part of the tubule even though it was produced in the proximal tubule. [Simplistically but erroneously it is sometimes said that the ammonium in the urine is produced in the distal tubule cells.]
Ammonium is not measured as part of the titratable acidity because the high pK of ammonium means no H+ is removed from NH4+ during titration to a pH of 7.4. Ammonium excretion in severe acidosis can reach 300 mmol/day in humans.
Ammonium excretion is extremely important in increasing acid excretion in systemic acidosis. The titratable acidity is mostly due to phosphate buffering and the amount of phosphate present is limited by the amount filtered (and thus the plasma concentration of phosphate). This cannot increase significantly in the presence of acidosis (though of course some additional phosphate could be released from bone) unless other anions with a suitable pKa are present. Ketoanions can contribute to a significant increase in titratable acidity but only in ketoacidosis when large amounts are present.
In comparison, the amount of ammonium excretion can and does increase markedly in acidosis. The ammonium excretion increases as urine pH falls and also this effect is markedly augmented in acidosis. Formation of ammonium prevents further fall in pH as the pKa of the reaction is so high.
A low urine pH itself cannot directly account for excretion of a significant amount of acid: for example, at the limiting urine pH of about 4.4, [H+] is a negligible 0.04 mmol/l. This is several orders of magnitude lower than H+ accounted for by titratable acidity and ammonium excretion. (ie 0.04 mmol/l is insignificant in a net renal acid excretion of 70 mmols or more per day)
On a typical Western diet all of the filtered load of bicarbonare is reabsorbed. The sites and percentages of filtered bicarbonate involved are:
The decrease in volume of the filtrate as further water is removed in the Loop of Henle causes an increase in [HCO3-] in the remaining fluid. The process of HCO3- reabsorption in the thick ascending limb of the Loop of Henle is very similar to that in the proximal tubule (ie apical Na+-H+ antiport and basolateral Na+-HCO3- symport and Na+-K+ ATPase). Bicarbonate reabsorption here is stimulated by the presence of luminal frusemide. The cells in this part of the tubule contain carbonic anhydrase.
Any small amount of bicarbonate which enters the distal tubule can also be reabsorbed. The distal tubule has only a limited capacity to reabsorb bicacarbonate so if the filtered load is high and a large amount is delivered distally then there will be net bicarbonate excretion.
The process of bicarbonate reabsorption in the distal tubule is somewhat different from in the proximal tubule:
The net effect of the excretion of one H+ is the return of one HCO3- and one Na+ to the blood stream. The HCO3- effectively replaces the acid anion which is excreted in the urine.
The net acid excretion in the urine is equal to the sum of the TA and [NH4+] minus [HCO3] (if present in the urine). The [H+] accounts for only a very small amount of the H+ excretion and is not usually considered in the equation (as mentioned earlier).
In metabolic alkalosis, the increased bicarbonate level will result in increased filtration of bicarbonate provided the GFR has not decreased. The kidney is normally extremely efficient at excreting excess bicarbonate but this capacity can be impaired in certain circumstances. (See Section 7.2 and 7.3)
The discussion above has described the mechanisms involved in renal acid excretion and mentioned some factors which regulate acid excretion.
The major factors which regulate renal bicarbonate reabsorption and acid excretion are:
Volume depletion is associated with Na+ retention and this also enhances HCO3 reabsorption. Conversely, ECF volume expansion results in renal Na+ excretion and secondary decrease in HCO3 reabsorption.
An increase in arterial pCO2 results in increased renal H+ secretion and increased bicarbonate reabsorption. The converse also applies. Hypercapnia results in an intracellular acidosis and this results in enhanced H+ secretion. The cellular processes involved have not been clearly delineated. This renal bicarbonate retention is the renal compensation for a chronic respiratory acidosis.
Potassium has a role in bicarbonate reabsorption. Low intracellular K+ levels result in increased HCO3 reabsorption in the kidney. Chloride deficiency is extremely important in the maintenance of a metabolic alkalosis because it prevents excretion of the excess HCO3 (ie now the bicarbonate instead of chloride is reabsorbed with Na+ to maintain electroneutrality). (See discussion in Section 7.3)
Aldosterone at normal levels has no role in renal regulation of acid-base balance. Aldosterone delpetion or excess does have indirect effects. High aldosterone levels result in increased Na+ reabsorption and increased urinary excretion of H+ and K+ resulting in a metabolic alkalosis. Conversely, it might be thought that hypoaldosteronism would be associated with a metabolic acidosis but this is very uncommon but may occur if there is coexistent significant interstitial renal disease.
Phosphate is the major component of titratable acidity. The amount of phosphate present in the distal tubule does not vary greatly. Consequently, changes in phosphate excretion do not have a significant regulatory role in response to an acid load.
It has recently been established that a reduction in GFR is a very important mechanism responsible for the maintenance of a metabolic alkalosis. The filtered load of bicarbonate is reduced proportionately with a reduction in GFR.
The kidney responds to an acid load by increasing tubular production and urinary excretion of NH4+. The mechanism involves an acidosis-stimulated enhancement of glutamine utilisation by the kidney resulting in increased production of NH4+ and HCO3- by the tubule cells. This is very important in increasing renal acid excretion during a chronic metabolic acidosis. There is a lag period: the increase in ammonium excretion takes several days to reach its maximum following an acute acid load. Ammonium excretion can increase up to about 300 mmol/day in a chronic metabolic acidosis so this is important in renal acid-base regulation in this situation. Ammonium excretion increases with decreases in urine pH and this relationship is markedly enhanced with acidosis.
There are different views on the true role of NH4+ excretion in urine. How can the renal excretion of ammonium which has a pK of 9.2 represent H+ excretion from the body?
One school says the production of ammonium from glutamine in the tubule cells results in production of alpha-ketoglutarate which is then metabolised in the tubule cell to ‘new’ bicarbonate which is returned to the blood. The net effect is the return of one bicarbonate for each ammonium excreted in the urine. By this analysis, the excretion of ammonium is equivalent to the excretion of acid from the body as one plasma H+ would be neutralised by one renal bicarbonate ion for each ammonium excreted. Thus an increase in ammonium excretion as occurs in metabolic acidosis is an appropriate response to excrete more acid.
The other school says this is not correct. The argument is that metabolism of alpha-ketogluarate in the proximal tubule cells to produce this ‘new’ HCO3- merely represents regeneration of the HCO3 that was neutralised by the H+ produced when alpha-ketoglutarate was metabolised to glutamate in the liver originally so there can be no direct effect on net H+ excretion. The key to understanding is said to lie in considering the role of the liver. Consider the following:
Every day protein turnover results in amino acid degradation which results in production of HCO3- and NH4+. For a typical 100g/day protein diet, this is a net production of 1,000mmol/day of HCO3- and 1,000mmol/day of NH4+. (These are produced in equal amounts by neutral amino acids as each contains one carboxylic acid group and one amino group.) The high pK of the ammonium means it cannot dissociate to produce one H+ to neutralise the HCO3- so consequently amino acid metabolism is powerfully alkalinising to the body. The body now has two major problems:
The solution is to react the two together and get rid of both at once. This process is hepatic urea synthesis (Krebs-Henseleit cycle). The cycle consumes significant energy but solves both problems. Indeed, the cycle in effect acts as a ATP-dependent pump that transfers H+ from the very weak acid NH4+ to HCO3-. The overall reaction in urea synthesis is:
The body has two ways in which it can remove NH4+:
The key thing here is that the acid-base implications of these 2 mechanisms are different.
For each ammonium converted to urea in the liver one bicarbonate is consumed. For each ammonium excreted in the urine, there is one bicarbonate that is not neutralised by it (during urea synthesis) in the liver. So overall, urinary excretion of ammonium is equivalent to net bicarbonate production -but by the liver! Indeed in a metabolic acidosis, an increase in urinary ammonium excretion results in an exactly equivalent net amount of hepatic bicarbonate (produced from amino acid degradation) available to the body. So the true role of renal ammonium excretion is to serve as an alternative route for nitrogen elinination that has a different acid-base effect from urea production.
The role of glutamine is to act as the non-toxic transport molecule to carry NH4+ to the kidney. The bicarbonates consumed in the production of glutamine and then released again with renal metabolism of ketoglutarate are not important as there is no net gain of bicarbonate.
Overall: renal NH4+ excretion results indirectly in an equivalent amount of net hepatic HCO3 production.
Other points are:
Finally: The role of urine pH in situations of increased acid secretion is worth noting. The urine pH can fall to a minimum value of 4.4 to 4.6 but as mentioned previously this itself represents only a negligible amount of free H+.
As pH falls, the 3 factors involved in increased H+ excretion are:
1. Increased ammonium excretion (increases steadily with decrease in urine pH and this effect is augmented in acidosis) [This is the major and regulatory factor because it can be increased significantly].
2. Increased titratable acidity:
(As discussed also in section 2.5.4, increases in TA are limited and are not as important as increases in ammonium excretion)
3. Bicarbonate reabsorption is complete at low urinary pH so none is lost in the urine (Such loss would antagonise the effects of an increased TA or ammonium excretion on acid excretion.)