Changes in the countercurrent system in the renal papilla: Diuresis increases pH and HCO₃^- gradients between collecting duct and vasa recta

This study was designed to elucidate the acid-base balance local to the collecting duct urine (CD) and vasa recta blood (VR) in the rat renal papilla in diuresis. The pH changes were measured in both a furosemide-induced and a volume-load-induced diuresis, whereas the PCO₂ (i.e., CO₂ tension) and HCO₃^- concentration were measured only in a furosemide-induced diuresis. In an antidiuresis, the pH of the VR was more acidic than that of the systemic arterial blood (ΔpH = 0.44-0.73). Additionally, the pH of the ascending VR was significantly lower than that of the descending VR (ΔpH = 0.14-0.16). In diuresis, the pH of the CD decreased (ΔpH = 0.81-0.97), while the pH of the descending and the ascending VR increased; however, the increase was only significant in the ascending VR (ΔpH = 0.23-0.30). Consequently, the significant difference in the pH gradient between the descending and the ascending VR was eliminated. The PCO₂ values in the CD and the ascending VR were not different from those in antidiuresis, while the HCO₃^- concentration in the CD and the ascending VR, respectively, decreased and increased significantly. Thus, in diuresis, the decrease in the pH of the CD and the increase in the pH of the ascending VR result, respectively, from the decrease and the increase in the HCO₃^- concentration, with no changes in the PCO₂ values.