Membrane structural specialization of the toad urinary bladder revealed by the freeze-fracture technique. III. Location, structure and vasopressin dependence of intramembrane particle arrays.

Examination of the toad urinary bladder by freeze-fracture electron microscopy reveals intramembrane particle arrays at a number of membrane sites. An array in which particles are aggregated into closely apposed parallel rows is found in the granular cell luminal membrane of dehydrated toads fixed in situ. These aggregates are structurally indistinguishable from those previously associated with vasopressin exposure in vitro. Aggregates are not found in granular cell luminal membrane in the case of hydrated toads fixed in situ. However, structurally similar arrays are found at low frequency in the membrane of cytoplasmic vacuoles in granular cells and in the plasma membrane of basal cells in both hydrated and dehydrated toads. Aggregates are also present at these sites in control and vasopressin-treated bladders from in vitro experiments. Particle arrays characteristic of gap junctions, desmosomes and hemidesmosomes also occur in the plasma membrane of basal cells. In addition, distinctive square arrays of particles exist in the plasma membrane of the bladder's mesothelium. Although a variety of intramembrane particle arrays exist in the toad urinary bladder, only the occurrence of organized particle aggregates in the luminal membrane of granular cells appears to be associated with vasopressin exposure.