Vasopressin-induced changes in the three-dimensional structure of toad bladder apical surface.

The apical plasma membrane of toad bladder granular cells undergoes a rapid and dramatic increase in water permeability in response to vasopressin stimulation. Previous studies have shown that this permeability increase is accompanied by characteristic changes in the morphology of this membrane and that these changes may be involved in the hormonal response. In this report, we have used the technique of rapid freezing and freeze drying to obtain high resolution stereo images of the surface of the granular cell apical plasma membrane before and during vasopressin stimulation. Using this approach, we confirmed that vasopressin induces a ridge-to-villus transformation of the cell surface even in the absence of osmotic water flow, but now show that this transformation occurs at least in part via a retraction of segments of preexisting ridges, rather than by the growth of new microvilli from the apical cell surface. This is also demonstrated by the finding that vasopressin induces the ridge-to-villus transformation of the cell surface even in the presence of cytochalasin D. In addition, the rapid-freeze, freeze-dry technique reveals that the surface glycocalyx of the epithelial cells consists of a complex, three-dimensional network of filaments that is heterogeneous among different cells. Finally, vasopressin-induced tubular invaginations of the apical plasma membrane were visualized in stereomicrographs, and the number and size of such invaginations were altered in the presence of cytochalasin D. These may represent surface images of vasopressin-induced exo- and endocytotic events that are related to membrane permeability changes.