Using the methods described in the preceding papier (Levine et al., 1984) for measuring the magnitude of the water-permeable barriers in series with the luminal membrane, we correct measured values of Pd(w) in bladders stimulated with low doses of antidiuretic hormone (ADH) or 8-bromo cyclic AMP to obtain their true values in the luminal membrane. Simultaneously, we also determine Pt. We thus are able to calculate Pf/, Pd(w) for the hormone-induced water permeation pathway in the luminal membrane. Our finding is that Pf/, Pd(w) ≈ 17. Two channel models consistent both with this value and the impermeability of the ADH-induced water permeation pathway to small nonelectrolytes are: (a) a long (≈50 Å), small-radius (≈2 Å) pore through which 17 water molecules pass in single-file array, and (6) a shower-head-like structure in which the stem is long and of large radius (≈20 Å) and the cap has numerous short, small-radius (≈2 Å) pores. A third possibility is that whereas the selective permeability to H2O results from small-radius (≈2 Å) pores, the large value of Pf/Pd(w) arises from their location in the walls of long tubular vesicles (~2 µM in length and 0.1 pm in diameter) that are functionally part of the luminal membrane after having fused with it. Aggregate-containing tubular vesicles of these dimensions have been reported to fuse with the luminal membrane in response to ADH stimulation and have been implicated in the ADH-induced hydroosmotic response.
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