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Long-term regulation of collecting duct water permeability: freeze-fracture analysis of isolated perfused tubules.

Terminal inner medullary collecting duct (IMCD) segments from water-restricted rats have high osmotic permeabilities despite the absence of vasopressin (AVP). We performed freeze-fracture analysis of individual IMCD segments from such animals following measurement of their water permeability (Pf). IMCD segments from control rats did not have a high Pf in the absence of AVP (88 +/- 15 microns/s) and had a low incidence of E-face intramembrane particle (IMP) clusters (9.6 +/- 2.7 clusters/100 microns 2). Segments exposed to 0.1 nM AVP in vitro had enhanced Pf (1,060 +/- 210 microns/s) and cluster incidence (122 +/- 33 clusters/100 microns 2). IMCD segments isolated from rats dehydrated for 48 h and perfused without AVP exposure had an elevated Pf (605 +/- 71 microns/s) and a high incidence of clusters (166 +/- 36 clusters/100 microns 2). There also was an increase in the number of single particles between clusters in tubules from dehydrated rats (2.5-fold) and in AVP-treated tubules (3.6-fold). These findings indicate that IMP clusters are associated with high water permeability in tubules from dehydrated animals independent of continued AVP exposure. The increased incidence of particles between clusters suggests that water channels may also occur outside of cluster domains.