TRANSCELLULAR WATER-FLOW MODULATES WATER CHANNEL EXOCYTOSIS AND ENDOCYTOSIS IN KIDNEY COLLECTING TUBULE

The regulation of osmotic water permeability (P(f)) by vasopressin (VP) in kidney collecting tubule involves the exocytic-endocytic trafficking of vesicles containing water channels between an intracellular compartment and apical plasma membrane. To examine effects of transcellular water flow on vesicle movement, P(f) was measured with 1-s time resolution in the isolated perfused rabbit cortical collecting tubule in response to addition and removal of VP (250-mu-U/ml) in the presence of bath > lumen (B > L), lumen > bath (L > B), and lumen = bath (L = B) osmolalities. With VP addition, P(f) increased from 12 to 240-270 x 10(-4) cm/s (37-degrees-C) in 10 min. At 1 min, P(f) was approximately 70 x 10(-4) cm/s for B > L, L > B, and L = B conditions. At later times, P(f) increased fastest for L > B and slowest for B > L osmolalities; at 5 min, P(f) was 250 x 10(-4) cm/s (L > B) and 158 x 10(-4) cm/s (B > L). With VP removal, P(f) returned to pre-VP levels at the fastest rate for B > L and the slowest rate for L > B osmolalities; at 30 min, P(f) was 65 x 10(-4) cm/s (B > L) and 183 x 10(-4) cm/s (L > B). For a series of somotic gradients of different magnitudes and directions, the rates of P(f) increase and decrease were dependent upon the magnitude of transcellular volume flow; control studies showed that paracellular water flux, asymmetric transcellular water pathways, or changes in cell volume could not account for the data. VP-dependent endocytosis was measured by apical uptake of rhodamine-dextran; in paired studies where the same tubule was used for + and - gradients, B > L and L > B osmolalities gave 168% and 82% of uptake measured with no gradient. In contrast, endocytosis in proximal tubule was not dependent on gradient direction. These data provide evidence that transcellular volume flow modulates the vasopressin-dependent cycling of vesicles containing water channels, suggesting a novel driving mechanism to aid or oppose the targeted, hormonally directed movement of subcellular membranes.