Mechanism of glomerulotubular balance. I. Effect of aortic constriction and elevated ureteropelvic pressure on glomerular filtration rate, fractional reabsorption, transit time, and tubular size in the proximal tubule of the rat.

Abstract

The mechanism responsible for the maintenance of glomerulotubular balance was examined in rats whose filtration rates had been reduced to the same extent by either aortic constriction or elevated ureteral pressure. During aortic constriction both proximal tubular fluid to plasma inulin ratio and the transit time were constant. The constancy of the inulin ratio establishes that tubular reabsorption was diminishing in direct proportion to reductions in glomerular filtration rate (GFR), whereas the constancy of the transit time establishes that tubular volume was diminishing in direct proportion to reductions in GFR. This combination of findings means that reabsorptive rate diminished in proportion to the decrease in tubular volume. After elevation of ureteral pressure, tubular size increased, transit time was prolonged, and glomerulotubular balance disrupted. Nevertheless, despite the fall in glomerular filtration rate, proximal reabsorption did not diminish but actually increased in direct proportion to the magnitude of tubular dilation. It was concluded that proximal tubular reabsorption is a function of tubular volume. Glomerulotubular balance is maintained when tubular volume falls in proportion to GFR. Glomerulotubular balance is disrupted when tubular volume increases as GFR falls.