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Adaptation of inner medullary collecting duct to dehydration involves a paracellular pathway.

Prolonged fluid restriction in rats is accompanied by functional modifications of the terminal part of the inner medullary collecting duct (IMCD) revealed by a sustained increase in arginine vasopressin (AVP)-independent transepithelial osmotic water permeability (PTE) in vitro. The cellular basis of this adaptation was explored in isolated and perfused terminal IMCDs of Sprague-Dawley rats using video and fluorescence microscopy. Basolateral membrane osmotic water permeability (Posm), transcellular Posm, and PTE were measured in quick sequence in every tubule. They were expressed per unit area of basolateral membrane corrected for infoldings, based on previous stereological studies and assuming no major change in membrane surface area between hydrated and dehydrated animals. Compared with IMCDs of rats with a high water intake, IMCDs of rats deprived of fluid for 36 h displayed a significantly higher basal PTE (24.9 +/- 5.1 vs. 6.1 +/- 0.6 microns/s), a similar basolateral Posm, and a higher transcellular Posm, implying a higher permeability of the apical membrane, despite the absence of exogenous AVP. However, when IMCDs of thirsted rats were exposed to AVP in vitro, their transcellular Posm (36.0 +/- 2.4 microns/s) was significantly smaller than their PTE determined simultaneously (51.8 +/- 7.1 microns/s), suggesting that part of the water flow may follow a paracellular route. A change in paracellular pathways was supported by higher apparent permeabilities to [14C]sucrose (0.85 +/- 0.27 vs. 0.28 +/- 0.04 x 10(-5) cm/s) and to [methoxy-3H]inulin (0.25 +/- 0.04 vs. 0.14 +/- 0.03 x 10(-5) cm/s) in IMCDs of thirsted rats. The nonelectrolyte permeabilities were affected neither by AVP nor by urea-rich bathing solutions. We conclude that in vivo factors related to dehydration produce a conditioning effect on terminal IMCD, which includes stabilization of the apical membrane in a state of high Posm and opening up of paracellular pathways revealed by a higher permeability to water and nonelectrolytes. The role of these adaptive phenomena remains unclear but may pertain to the sudden transitions between antidiuresis and diuresis.