Studies on Glomerulotubular Balance During Aortic Constriction, Ureteral Obstruction and Venous Occlusion in Hydropenic and Saline-Loaded Rats

Abstract

Proximal glomerulotubular balance was examined in hydropenic rats and rats undergoing saline diuresis. Three experimental procedures were used to lower GFR: aortic constriction, ureteral obstruction and renal venous occlusion. During hydropenia all three manipulations lowered total GFR and nephron GFR, and prolonged transit-time, but did not significantly alter TF/P inulin ratios. Absolute reabsorption decreased proportionately to GFR and glomerulotubular balance was well maintained. Calculated tubular volume increased with ureteral obstruction and venous occlusion, but did not change with aortic constriction. The ratio of reabsorption to tubular volume (C/πr²) decreased in all the experiments. During saline diuresis all three experimental maneuvers lowered total GFR and nephron GFR. Aortic constriction did not change absolute reabsorption and glomerulotubular balance was disrupted. Venous occlusion reduced reabsorption but not proportionately to the fall in GFR; glomerulotubular balance occurred but was incomplete. Ureteral obstruction reduced reabsorption proportionately to GFR and balance was well maintained. C/πr² increased with aortic constriction, did not change with venous occlusion and fell with ureteral obstruction. The changes in reabsorption occurring during glomerulotubular balance were not related to changes in tubular volume. It is concluded that some factor(s) other than tubular geometry, possibly hydrostatic and oncotic pressures in the peritubular capillaries are responsible for linking tubular reabsorption to the rate of glomerular filtration.