Dynamics of apical membrane responses to ADH in amphibian bladder

The dynamic insertion and retrieval of membrane at the apical surface plays an important role in the action of antidiuretic hormone (ADH). The addition of membrane with water channels is a crucial event in initiating the water permeability response. ADH-stimulated bladders display distinctive differentiations in the apical membrane that represent sites where intracellular vesicles carrying intramembrane particle aggregates have fused with the apical surface. In the absence of an osmotic gradient these fusion sites appear to be relatively stable structures, but in the presence of an osmotic gradient there seems to be continuous addition and retrieval of membrane during sustained exposure to ADH. It is now clear that a dynamic feedback process is present, such that the water permeability of the apical membrane is adjusted by retrieval or addition of membrane depending on the magnitude of the transepithelial osmotic gradient. Removal of ADH leads to a striking retrieval of apical membrane, and intact aggregates have been demonstrated in the membrane of the vesicles that form in the apical cytoplasm after reversal of the response. Structure-function analysis has provided unique information, demonstrating that membrane dynamics is central to the mechanism whereby ADH regulates osmotic permeability in the toad urinary bladder.