Molecular mechanisms for the regulation of water transport in amphibian epithelia by antidiuretic hormone

Classic laboratory studies performed in the 1950's and 1960's focused on the structures and biochemical actions of multiple neurohypophysial peptides that possessed profound effects on the overall water balance of many vertebrates [reviewed in 1]. These data emphasized the importance of antidiuretic hormones (ADH) in the evolution of vertebrates and provided a framework for our current understanding of their roles in renal function. In a similar fashion, the recent discovery of the family of aquaporin water channel proteins has supplied new tools to examine the molecular aspects of ADH-elicited water transport in epithelial physiology. Data concerning the structure and distribution of aquaporin water channels have been further advanced by recent research defining several of the most basic aspects of hormonally regulated transport in many cell types. Together, these findings have provided many new insights into the apical membrane events set in motion by the binding of ADH to the basolateral membrane of epithelial cells.The purpose of this review is to highlight the role of the ADH-elicited water permeability response in the terrestrial adaptation of amphibians and their evolutionary offspring, mammals including humans. To accomplish this, we have divided the review into three sections. The first section summarizes published data from studies in comparative physiology involving the evolution and distribution of the ADH-elicited water permeability response in various amphibian species. The second section provides an overview of the vesicle mediated water permeability response in toad bladder and mammals. In this selective review, we have emphasized several common features present in ADH-mediated insertion and removal of water channel containing vesicles and the insulin-induced vesicular translocation of a facilitative glucose transport protein. The third section re-examines two important questions about the evolution of the ADH-elicited water permeability response in light of recent data reviewed in the prior section. Taken together, these data illustrate that future research in this area will provide new insights into the origins of the ADH-elicited water permeability response through use of probes specific for various aquaporin water channel proteins