Transcellular water flow modulates water channel exocytosis and endocytosis in kidney collecting tubule

Abstract

The regulation of osmotic water permeability (Pf) 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, Pf was measured with 1-s time resolution in the isolated perfused rabbit cortical collecting tubule in response to addition and removal of VP (250 μU/ml) in the presence of bath > lumen (B > L), lumen > bath (L > B), and lumen = bath (L = B) osmolalities. With VP addition, Pf increased from 12 to 240-270 × 10-4 cm/s (37°C) in 10 min. At 1 min, Pf was ∼ 70 × 10-4 cm/s for B > L, L > B, and L = B conditions. At later times, Pf increased fastest for L > B and slowest for B > L osmolalities; at 5 min, Pf was 250 × 10-4 cm/s (L > B) and 158 × 10-4 cm/s (B > L). With VP removal, Pf returned to pre-VP levels at the fastest rate for B > L and the slowest rate for L > B osmolalities; at 30 min, Pf was 65 × 10-4 cm/s (B > L) and 183 × 10-4 cm/s (L > B). For a series of osmotic gradients of different magnitudes and directions, the rates of Pf 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.