Effect of acute hypercapnia on renal bicarbonate reabsorption in the dog

The increased plasma bicarbonate concentration seen in hypercapnia implies that tubular bicarbonate reabsorption must be increased in the presence of an elevated Paco2. In contrast to early reports, more recent experimental data in acute hypercapnia have been interpreted to show that the observed increment in tubular reabsorption of bicarbonate factored for glomerular filtration rate (THCO3/GFR) is largely related to the concurrent changes in renal sodium reabsorption and to the increment in the filtered load of bicarbonate, and that acute hypercapnia per se causes little or no change in the tubular handling of bicarbonate. We reexamined this question by observing the changes in renal function occurring in the presence of a moderate elevation of plasma bicarbonate concentration in two groups of dogs. In group I, the elevation occurred as a result of acute hypercapnia during the administration of an "isometric" solution; in group II, it was caused by the infusion of identical amounts of an isotonic solution with the same concentration of sodium as in group I, but a higher bicarbonate concentration, in the presence of eucapnia. A subset of group II provided controls for the decrease in renal perfusion pressure that occurred spontaneously in group I. With increasing filtered loads of bicarbonate, fractional excretion (FE) of HCO3 increased in group II, whereas it dropped markedly in group I. Furthermore, the relative reabsorption rate of HCO3 compared with that of Cl (assessed by changes in fractional reabsorption (FR) of HCO3/Cl) decreased in group II, whereas it increased in group I. Although FENa also decreased in group II, the opposite changes in FR(HCO3/Cl) could not be attributed solely to concurrent changes in sodium handling, indicating that in the presence of acute hypercapnia there is a preferential reabsorption of bicarbonate that tends to perpetuate the increase in plasma bicarbonate concentration. By contrast, THCO3/GFR rose in both groups. The data are interpreted to reveal that acute hypercapnia, although causing a drop in renal perfusion pressure and in natriuresis, also has an additional specific effect on raising preferential bicarbonate reabsorption. This effect can be detected best by monitoring changes in the anionic composition of tubular reabsorbate, whereas it may not be unveiled by following changes in THCO3/GFR. Changes in THCO3/GFR may not yield useful information regarding the integrated response of the kidney to acid-base perturbations, and the conclusions of previous studies based on changes in this parameter must be carefully reexamined.