STRUCTURAL-CHANGES INDUCED BY OSMOTIC WATER-FLOW IN RABBIT PROXIMAL TUBULE

When a transepithelial osmotic difference was imposed in perfused proximal straight tubules (270 mOsm/kg H2O in the lumen and 290 in the bath) in the absence of bath colloid, a severe vacuolation (appearance of lucent spaces) developed within the epithelium such that view of the lumen border was obscured within 5 +/- 1 min (N = 13 tubules at 23-degrees-C). This vacuolation was less severe if the bath was hypotonic to the lumen or if the magnitude of the osmotic difference was reduced. If colloid (6% wt/vol of either bovine serum albumin or 70,000 molecular wt dextran) was included in the bathing medium, vacuolation was either not observed or was minimal, but became severe upon removal of the colloid and obscured the lumen within 6 +/- 1 min (N = 8 for albumin and N = 4 for dextran at 23-degrees-C). At 38-degrees-C, vacuolation obscured the lumen within 4 +/- 1 min following the removal of albumin (N = 5). ANOVA suggests that none of the times for vacuolation to occur differed. The rate of passive volume flow due to the osmotic difference was unaffected by vacuolation (0.9 +/- 0.1 nl.min-1.mm-1 with albumin to 0.8 +/- 0.1 without albumin and vacuolated, N = 8 at 23-degrees-C, P > 0.2 using a paired t-test). Electron microscopic examination of tubules fixed after vacuolation showed lucent spaces within the cytoplasm. These results suggest that the presence of serosal colloid protected the epithelial cells from injury during rapid transepithelial water flow. The mechanism for this protective effect is not apparent, but may be related to effects of colloid in maintaining normal volume absorption in the proximal nephron.