Cortical interstitium as a site for solute polarization during tubular absorption.

The possibility that significant concentration differences could exist between the interstitial fluid and capillary plasma was investigated by modeling the renal cortex as the following three compartments: tubular lumen, interstitium, and capillary lumen. A simple analysis of this system suggests that for the interstitium surrounding a proximal tubule, the concentration in the interstitium of a solute like glucose could be well over 1 mM greater than in the peritubular capillary if the solute permeability of the peritubular capillary were like that measured in other organs (i.e., less than 10 micron/s). The effect of varying capillary permeability on the interstitial concentrations of several solutes was examined using a modification of a model of the proximal tubule, and results were found to be similar to those obtained with the simpler analysis for glucose. This model was also used to see if placing an osmotic difference between the tubule lumen and capillary could cause significant solute polarization within the interstitium, as might occur in an experiment to measure the osmotic water permeability (Pf) of the proximal tubule in vivo. The results show that an apparent Pf calculated from the difference between the osmolalities of tubular perfusate and peritubular plasma is likely to underestimate the true Pf of the proximal tubule. Even for a high value of capillary permeability (10 micron/s, which allows relatively rapid diffusion between capillary and interstitium), the model predicts that the apparent Pf may underestimate the true value by half. Thus the analyses presented suggest that if the permeability of renal peritubular capillaries is similar to that measured in other organs the composition of the interstitium may be significantly different from capillary plasma, a situation that would have great impact on our view of the mechanism of volume absorption in the proximal tubule in vivo.