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Recent micropuncture studies (1-3) have shown that reductions in glomerular filtration rate (GFR) induced by aortic constriction are accompanied by proportionate changes in the rate of proximal tubular reabsorption, thus maintaining constant fractional reabsorption (i.e., glomerulotubular balance). We have shown, however, that similar reductions in GFR, induced by elevating ureteral pressure, are associated with increased fractional reabsorption in the proximal convoluted tubule (3). An analysis of the effects of aortic constriction and increased ureteral pressure on the relations among GFR, fractional reabsorption, tubular velocity, and tubular size led to the conclusion that tubular volume was the critical factor governing the proximal reabsorptive rate (3). In our previous studies, however, tubular volume and reabsorptive rate were estimated indirectly from an analysis of tubular fluid to plasma ratios of inulin [(TF/P)In] and the transit time. The present experiments were designed to investigate in a more direct fashion the effects of altering tubular dimensions on the intrinsic reabsorptive capacity of the tubular epithelium. The stopped-flow microperfusion technique of Gertz (4), in which the rate of reabsorption of a drop of isotonic saline placed between two columns of oil in an isolated segment of the tubule is measured, was used * to estimate the intrinsic reabsorptive capacity of the tubular epithelium. This method has the great advantage of permitting an assessment of tubular reabsorption independent of glomerular filtration and velocity flow. In the first series of experiments the effect of renal hypoperfusion, induced by constriction of the aorta above the renal arteries, on the rate of proxi-mal reabsorption was examined. In the second series of experiments the role of tubular geometry in glomerulotubular balance was evaluated more precisely by measuring the rate of tubular reab-sorption before and after dilatation of the proxi-mal tubules. Graded dilatation was achieved by applying hydrostatic pressure to the renal pelvis. In a third series of experiments, increased ureteral pressure was superimposed on aortic constriction to exclude the possibility that the experimental procedure per se rather than tubular size was determining reabsorption. Methods Unilaterally nephrectomized Sprague-Dawley rats were prepared for micropuncture studies as described in the preceding paper (3). Glomerular filtration rate (GFR) was measured with inulin-"C. Transit times through the visible portions of the proximal tubules were measured according to the method of Steinhausen (5). Proximal tubular reabsorptive rate was determined by the technique of Gertz (4). A proximal convolution on the surface of the kidney was punctured through …

Mechanism of glomerulotubular balance. II. Regulation of proximal tubular reabsorption by tubular volume, as studied by stopped-flow microperfusion.