Transcellular Water Flow Modulates Water Channel Exocytosis
نویسنده
چکیده
Introduction 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 AU/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 240270 X locm/s (370C) in 10 min. At 1 min, Pf was 70 X lo-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 X 10'cm/s (L > B) and 158 X 10cm/s (B > L). With VP removal, Pf returned to pre-VP levels at the fastest rate for B > L and the slowest rate forL > B osmolalities; at 30 min, Pf was 65 X 1locm/s (B > L) and 183 X i0-4 cm/s (L > B). For a series ofosmotic 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. (J. Clin. Invest. 1991. 88:423-429.)
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