title: The mode of action of vasopressin: membrane microstructure and biological transport.
abstract: Vasopressin affects a variety of cell systems. This review is focused on permeability changes induced by vasopressin in tight epithelia such as the collecting duct of the mammalian kidney and the skin and the bladder of anurans. These vasopressin effects are discussed with reference to current concepts and models of the microstructure of the plasma membrane. The transport of three major chemical species--Na, urea and water--is analyzed. In each instance, the hormone appears to activate selective membrane pathways situated at the rat-limiting barrier of the epithelium, i.e., the apical membrane. Available data suggest that two intra-cellular messengers -- cAMP and calcium -- plan a key role in the coupling between stimulus (receptor occupancy) and biological effect (permeability change). The enhancement of Na transport (natriferic effect) depends on the opening and/or the insertion of Na channels, the biophysical and biochemical characteristics of which have been investigated by fluctuation analysis and by means of several chemical blockers of Na transport, particularly the amiloride molecule and its congeners. Likewise, the finding of inhibitors and activators of urea transport, which do not cause any appreciable change in Na or water permeability, led to the notion of selective urea channels or pores. Finally, the enhancement of water transport (hydrosmotic effect) possibly results from the insertion in the apical membrane of water channels already present in vesicular cytoplasmic structures. The restructuring of the apical membrane underlying the transition from a low to a higher state of water permeability is very likely related to the appearance of intramembrane particle aggregates detectable with the freeze-fracture technique in epithelia exposed to vasopressin. The putative water channels (or pores) appear to be so narrow that trans-apical water movement is constrained to single-file diffusion. Recent data also suggest that, in addition to cAMP, microtubules and microfilaments, the calmodulin-Ca complex is a major element in the hydrosmotic effect of vasopressin.
mesh_terms: D000663:Amphibians; D000818:Animals; D001692:Biological Transport; D002462:Cell Membrane / Q000648:ultrastructure*; D007473:Ion Channels; D007685:Kidney Tubules, Collecting / Q000166:cytology; D011817:Rabbits; D012867:Skin / Q000166:cytology; D014508:Urea / Q000378:metabolism; D001743:Urinary Bladder / Q000166:cytology; D014667:Vasopressins / Q000378:metabolism*; D014867:Water / Q000378:metabolism
chemical_list: D007473:Ion Channels; D014867:Water; D014667:Vasopressins; D014508:Urea