The effect of peritubular protein upon fluid reabsorption in rabbit proximal convoluted tubules perfused in vitro.

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Abstract
The effects of varying the peritubular protein concentration was investigated upon the rate of fluid reabsorption of proximal convoluted tubule segments of the rabbit kidney, isolated and perfused in vitro. Moderately distended tubules (11) bathed with rabbit serum (60 g/l protein) and perfused with an ultrafiltrate of serum (.apprx. 0 g/l) protein, reabsorbed fluid at a rate of 1.0 .+-. 0.07 nl/mm per min. When the protein of the bathing solution was reduced by replacing the serum with ultrafiltrate there was little change in fluid reabsorption rate. A further 11 moderately distended tubules, bathed with physiological saline containing bovine serum albumin (60 g/l) and perfused with saline, reabsorbed fluid at 1.0 .+-. 0.06 nl/mm per min. There was little change in fluid reabsorption when the protein concentration was reduced to either 20 or 0 g/l; these tubules responded to 10-5 M-ouabain in the bath, or a temperature of 25.degree. C, by reducing their absorption rate to 0.13 .+-. 0.05 nl/mm per min-1. Nine minimally distended tubules reabsorbed fluid at 0.8 ml/mm per min when bathed with physiological saline containing bovine serum albumin (60 g/l) and perfused with saline. Under these circumstances reabsorption rate fell by 26% when protein was removed from the bath. A study of the pressure-diameter relationship was made for 8 tubules typical of those used in this laboratory. The probable range of lumen hydrostatic pressures was discussed for the distended and undistended states. The effect of peritubular protein concentration upon fluid reabsorption in proximal convoluted tubule segments perfused in vitro may be dependent upon some property of the tubule wall that is changed when distension occurs.