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

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Abstract
The relation of tubular reabsorption to tubular dimensions in the proximal convolution of the rat was examined during aortic constriction and during elevated ureteral pressure. Reabsorptive rate was estimated directly from the time (reabsorptive half-time) required to reabsorb 50% of a drop of saline instilled into the proximal tubule between 2 oil droplets. The shrinking-drop technique permits the examination of reabsorption independent of glomerular filtration, velocity flow, and hormonal influences. Proximal tubular volume decreased with constriction of the aorta and increased with elevation of ureteral pressure. It was demonstrated in both experimental procedures that the reabsorptive half-time remained remarkably constant over a wide range of tubular radii. During aortic constriction and elevated ureteral pressure, either alone or in combination, proximal reabsorptive changes in direct proportion to tubular volume.