Proximal Tubular Reabsorption of Bicarbonate
Bicarbonate is freely filtered across the glomerular membrane. bicarbonate reabsorption in the proximal tubule depends on the activity in the luminal membrane of a secondary active Na+, H+ antiporter, a primary active H+-ATPase. Also important is the presence of CA in the proximal ICF and in the proximal luminal membrane. Intracellular CA facilitates the hydration of intracellular CO2 to H2CO3 which then dissociates rapidly into H+ and HCO3- ions. Na+ moving down its concentration gradient from tubular fluid into the cell provides the energy for the secondary active secretion of H+ from the cell into the lumen. ATP provides the energy for the primary active secretion of H+ from the cell into the lumen.
In the lumen H+ combines with filtered HCO3- to form H2CO3 which then is dehydrated to form H2O and CO2. This reaction is facilitated by the presence of CA located in the brush border of the luminal membrane. All cell membranes are highly permeable to CO2 and the CO2 formed in the lumen rapidly enters the cell. There it enters the cellular CO2 pool and its rapid hydration to H2CO3 is facilitated by intracellular CA. The intracellular dissociation of H2CO3 provides additional H+ for the secretory process. The intracellular HCO3- formed by this sequence of reactions moves into the peritubular fluid where it is electrically balanced by a reabsorbed cation, primarily Na+. The major pathway is via a basolateral membrane Na+ - 3HCO3- cotransporter and additional Na+ is provided by the basolateral Na+,K+- ATPase.
In this system for each H+ ion secreted an HCO3- is reabsorbed and the secreted H+ ion appears in the urine as water and the urine pH is not changed. The amount of H2O added to the tubular fluid in this process is insignificant relative to the filtered load. About 80 to 90 % of the filtered HCO3- is reabsorbed in the proximal tubule by this mechanism. Because of the rapidity of the reactions catalyzed by CA and the high permeability to CO2 of the luminal membrane, HCO3- is reabsorbed preferentially over Cl- early in the proximal tubule.