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Abstract

In the first experimental series proximal convolutions of the rat kidney were perfused with a modified Ringer solution and the isotonic fluid absorption was measured. In a second series the tubule was perfused with equilibrium solution which contained ³⁶Cl and the chloride permeability was determined. By the recollection method each individual tubule was perfused twice either at constant luminal diameter but different perfusion rates (10:30 or 6:16 nl/min) or at constant perfusion rates but different luminal diameters (20:30 μ). The perfusate was recollected at two different sites which were at least 500 μ distant from the infusion site.

The isotonic fluid absorption as well as the ³⁶Cl permeability was unchanged when the tubule was distended from 20–30 μ. Both, however, increased about 20% when the perfusion rate was increased 3-fold.

The data led to the following conclusions: 1. It is unlikely that there is a flow reactor type dependence of proximal tubular transport on flow rate. 2. The tubular distension cannot be responsible for the glomerulo-tubular balance. 3. It is more advantageous to relate permeability data of the rat nephron to tubular length. 4. In microperfusion experiments non steady sampling does not affect transepithelial fluxes per unit tubular length, provided that the pump delivery is constant.