Fluid reabsorption by Necturus proximal tubule perfused with solutions of normal and reduced osmolarity

Fluid absorption in Necturus proximal tubule was studied when the kidneys were perfused with solutions of different osmolarities. The rate of fluid absorption was inversely proportional to the perfusion fluid osmolarity, while Na uptake remained constant. No difference was detected between the collected and injected luminal fluid, i.e. reabsorption was isotonic at normal and reduced osmolarities. The transtubular osmotic permeability remained fairly constant under the different perfusion osmolarities. Using our experimental results to test various models based on osmotic equilibration across the tubule wall we show that none of these provides an adequate mechanism for fluid absorption in this epithelium.