Evidence that ADH-stimulated intramembrane particle aggregates are transferred from cytoplasmic to luminal membranes in toad bladder epithelial cells.

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Abstract
In freeze-fracture (FF) preparations of ADH[antidiuretic hormone]-stimulated toad urinary bladder, characteristic intramembrane particle (IMP) aggregates are seen on the protoplasmic (P) face of the luminal membrane of granular cells while complementary parallel grooves are found on the exoplasmic (E) face. These IMP aggregates specifically correlate with ADH-induced changes in water permability. Tubular cytoplasmic structures whose membranes contain IMP aggregates which look identical to the IMP aggregates in the luminal membrane have aslo been described in granular cells from unstimulated and ADH-stimulated bladders. The diameter of these cytoplasmic structures (0.11 .+-. 0.004 .mu.m) corresponds to that of tubular invaginations of the luminal membrane seen in thin sections of ADH-treated bladders (0.13 .+-. 0.005 .mu.m). Continuity between the membranes of these cytoplasmic structures (which are not granules) and the luminal membrane was directly observed in favorable cross-fractures. In FF preparations of the luminal membrane, these apparent fusion events are seen as round, ice-filled invaginations (0.13 .+-. 0.01 .mu.m diameter), of which about half have the characteristic ADH-associated aggregates near the point of membrane fusion. They are less numerous than, but linearly related to, the number of aggregates counted in the same preparations (n = 78, r [correlation coefficient] = 0.71, P < 0.01). The IMP aggregates seen in luminal membrane after ADH stimulation are apparently transferred performed by fusion of cytoplasmic with luminal membrane.