Vasopressin increases the permeability of the toad urinary bladder to water and urea. Freeze-fracture electron microscopy studies link the increased water permeability to aggregates of 80A diameter particles, which may be the water transport sites, within the luminal membrane. We have proposed a new model of the luminal membrane, in which water moves across the membrane via aggregate-rich tubular vesicles which fuse to the luminal membrane after vasopressin stimulation. We will test our proposal using transport experiments and mathematical models, and determine whether aggregate permeability can be modulated by cell pH and prostaglandins. Data from other laboratories suggest that water crosses the frog bladder by a different path than it crosses the toad bladder. We will study transport across frog bladder using our own methodology to resolve this problem. In addition, we will examine the urea transport system in the light of our proposal, determine whether urea and water share a common aqueous pathway having a low reflection coefficient, and examine the role of series barriers to urea diffusion in the cell. We have recently developed a series of photoaffinity labels which we will use to define luminal membrane proteins associated with vasopressin's action, to relate them to the water transport system, and eventually to isolate both these proteins and segments of luminal membrane which contain them. We plan to insert aggregate-rich membrane into lipid bilayers, and thereby to determine aggregates' transport characteristics uncomplicated by their placement within the cell.
