Load dependence of HCO3 and H2O reabsorption in the early proximal tubule of the Munich-Wistar rat.

Studies were undertaken in Munich-Wistar rats to evaluate the influence of variations in the filtered load of bicarbonate (FLHCO3) and water [single nephron glomerular filtration rate (SNGFR)] on the pattern of reabsorption along the accessible proximal tubule. SNGFR and FLHCO3 were varied by examining animals under conditions of hydropenia, plasma and extracellular volume expansion (VE), and VE plus aortic constriction. Water and HCO-3 reabsorption rates were measured at intervals along the proximal tubule, from very early segments to late segments, and these values compared with previous measurements in euvolemic rats. The earliest accessible portion of the proximal tubule reabsorbed HCO3 and water avidly; 40-55% of FLHCO3 and 18-20% of SNGFR were reabsorbed within the first millimeter. Moreover, when FLHCO3 was increased to as high as 2,400 pmol/min, HCO3 reabsorption rate in the first millimeter of the tubule increased concomitantly, reaching values as high as 1,000 pmol X mm-1 X min-1. In a similar fashion, water reabsorption in the first millimeter increased in direct relation to increases in SNGFR, reaching values as high as 13 nl X mm-1 X min-1 at SNGFR values of 70 nl/min. These results indicate that the early proximal tubule has much higher HCO3 and water reabsorptive rates and a stronger load dependence than has been found in later segments of the proximal tubule. The early proximal tubule thus appears to play a critical role in the maintenance of glomerulotubular balance.