Titratable acidity: a Pitts concept revisited

NBCe1-B/C-knockout mice exhibit an impaired respiratory response and an enhanced renal response to metabolic acidosis

The sodium-bicarbonate cotransporter (NBCe1) has three primary variants: NBCe1-A, -B and -C. NBCe1-A is expressed in renal proximal tubules in the cortical labyrinth, where it is essential for reclaiming filtered bicarbonate, such that NBCe1-A knockout mice are congenitally acidemic. NBCe1-B and -C variants are expressed in chemosensitive regions of the brainstem, while NBCe1-B is also expressed in renal proximal tubules located in the outer medulla. Although mice lacking NBCe1-B/C (KOb/c) exhibit a normal plasma pH at baseline, the distribution of NBCe1-B/C indicates that these variants could play a role in both the rapid respiratory and slower renal responses to metabolic acidosis (MAc). Therefore, in this study we used an integrative physiologic approach to investigate the response of KOb/c mice to MAc. By means of unanesthetized whole-body plethysmography and blood-gas analysis, we demonstrate that the respiratory response to MAc (increase in minute volume, decrease in pCO2) is impaired in KOb/c mice leading to a greater severity of acidemia after 1 day of MAc. Despite this respiratory impairment, the recovery of plasma pH after 3-days of MAc remained intact in KOb/c mice. Using data gathered from mice housed in metabolic cages we demonstrate a greater elevation of renal ammonium excretion and greater downregulation of the ammonia recycling enzyme glutamine synthetase in KOb/c mice on day 2 of MAc, consistent with greater renal acid-excretion. We conclude that KOb/c mice are ultimately able to defend plasma pH during MAc, but that the integrated response is disturbed such that the burden of work shifts from the respiratory system to the kidneys, delaying the recovery of pH.

Renal Tubular Acidosis

Renal tubular acidosis (RTA) comprises a group of disorders characterized by low capacity for net acid excretion and persistent hyperchloremic metabolic acidosis, despite preserved glomerular filtration rate. RTA are classified into chiefly three types (1, 2 and 4) based on pathophysiology and clinical and laboratory characteristics. Most patients have primary RTA that presents in infancy with polyuria, growth retardation, rickets and/or hypotonia. Diagnosis requires careful evaluation, including exclusion of other entities that can cause acidosis. A variety of tests, administered stepwise, are useful for the diagnosis and characterization of RTA. A genetic or acquired basis can be determined in majority of patients through focused evaluation. Management involves correction of acidosis and dyselectrolytemia; patients with proximal RTA with Fanconi syndrome and rickets require additional supplements of phosphate and vitamin D.

Western diets are not responsible for chronic acid retention: a critical analysis of organic acid and phosphate contribution

According to usual literature, the diet-dependent endogenous production of titratable acidity (TA) is contributed by sulphuric and phosphoric acids (NA) and by metabolizable acids (MAs), representing 'net-endogenous acid production' (NEAP). NEAP is mainly neutralised by diet-dependent [Formula: see text] salts of inorganic cations ([Formula: see text]), estimated in foods, faeces and urine from inorganic cation-anion difference (NB). It is claimed that urinary loss of organic acids' anions, '[Formula: see text]', induces metabolizable H⁺ ions' retention. Since '[Formula: see text]' is normally lost in urine as '[Formula: see text]' or '[Formula: see text]', no MA retention takes place. Therefore, in our approach, net acid production (NAP) reduces to endogenous sulphuric acidity only. Since in western diets (WDs) alkaline cations exceed inorganic anions (NB excess), acid excess from phosphorus is neutralized. Moreover, the renal reabsorption of ultra-filtered Pi takes place at [Formula: see text] ratios greater than '4/1', which means that the kidney operates as a dietary Pi-dependent NB generator ([Formula: see text] or [Formula: see text]). Since, in standard WDs, H₂SO₄ generation is less than '[Formula: see text]' production, the sulphuric acidity escaping the intestinal [Formula: see text] absorption is neutralized by [Formula: see text] and excreted as diet-dependent [Formula: see text], without interfering in normal A/B status. Only when extreme acidifying diets are ingested, sulphuric acidity may exceed '[Formula: see text]'. In this case, the excess of sulphuric acidity production is neutralised by the intervention of urinary [Formula: see text] excretion, whose employment is normally restricted to prevent loss of ultra-filtered NB. Finally, the whole body NA balance (NAb(W)) is calculated from the difference 'NA_abs - NA(u)', where abs = intestinal absorption and u = urinary excretion. Being 'NA_abs ≈ NA(u)', NAb(W) approximates zero, confirming WDs as non-acidifying foods.