Abstract
Single convoluted proximal tubules of the rat kidney were lumen perfused in situ with isosmotic solutions containing C14-sucrose and H3-inulin as tracers, to evaluate whether the extracellular marker sucrose is entrained by water during proximal tubular reabsorption. Inulin was used as volume marker. The absorptive rate was varied by using as luminal perfusion fluids either a solution made up of (in mmole/l) 120 NaCl, 5 glucose, 25 NaHCO3 and altering the perfusion rate, or a solution containing 110 NaCl and 70 raffinose.J S, the net sucrose efflux is found to be a function of the net volume flow,J V, such that atJ V=0,J S is very small and at high rates ofJ V,J S is over 60-fold the value observed at lowJ V values. In addition, the transported to luminal sucrose concentrations decreased withJ V in a hyperbolic manner.
Unstirred layers affect the diffusive component ofJ S, but only to a small extent. Therefore, the large remaining dependency ofJ S withJ V must be due to drag of sucrose by water, within the paracellular pathway. This leads to the conclusion that water flows through the paracellular pathway during absorption in the rat proximal tubule, in addition to transcellular water flow. Using equations for molecular sieving and the measured value of σs for sucrose of 0.76–0.91, it is calculated that the pathway where entrainment of solute by water occurs must be 1.0–1.1 nm wide. This calculation is only tentative since σs depends on the as yet unknown relative contribution of transcellular and paracellular pathways to transepithelial water osmotic permeability.
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Whittembury, G., Malnic, G., Mello-Aires, M. et al. Solvent drag of sucrose during absorption indicates paracellular water flow in the rat kidney proximal tubule. Pflugers Arch. 412, 541–547 (1988). https://doi.org/10.1007/BF00582545
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DOI: https://doi.org/10.1007/BF00582545