Water Diuresis Promotes Urine Acidification with Decreases in HCO3- Concentration

It is known that the reabsorption of filtered HCO3-depends on the tubular flowrate in the proximal uniferous tubule. In animal experiments, studies have demonstratedthat, when diuresis was induced, the pH of the collecting duct urine decreased, resulting from the decrease in HCO3-concentration with no change in the pCO2 level.The present study was designed to examine changes in urine acidification in humansunder a water diuretic state. Urine pH, pCO2 levels, and HCO3-concentration weremeasured before and after the drinking of water. In 120 minutes after hydration, theurine pH level fell about 0.5 pH unit from that before the intake of water. The urine HCO3-concentration coincidentally decreased significantly. However, pCO2 values remainedunchanged. These results indicated that also in humans, water diuresis promotesurine acidification, resulting from a decrease in HCO3-concentration with nochange in the pCO2 level, which may contribute to the prevention of the loss of plasmaHCO3-into the urine when the urine flow rate increases.