Peritubular control of proximal tubular fluid reabsorption in the rat kidney

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Abstract
Micropuncture techniques were employed to study the effect of variations in the rate of peritubular transfer of tubular epithelial reabsorbate on net proximal tubular reabsorption of fluid. During control periods, proximal tubular reabsorptive capacity, measured during stationary microperfusion, was linearly related to filtration fraction. Absolute fluid reabsorption (free flow) varied directly with the square of the tubular luminal radius (r2) and GFR [glomerular filtration rate], and fractional reabsorption with passage time. During partial renal venous occlusion tubular reabsorptive capacity decreased as a function of reductions in renal plasma flow. The relationship between absolute reabsorption and r2 was lost but balance between absolute reabsorption and GFR was sustained. It is suggested that capillary absorption partly determines the rate of epithelial transport either by altering active Na transport and/or Na backflux across the cell membrane, or by varying the backflux of solute and water through the epithelium.

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