Localization of barriers to water flow in toad urinary bladder

Area of Science:

• Cell biology
• Renal physiology
• Membrane transport

Background:

• Vasopressin (ADH) is known to increase water permeability in toad bladder granular cells.
• Recent studies suggest a
• postluminal
• regulatory site in addition to the luminal membrane.
• Freeze-fracture electron microscopy shows particle aggregates on the luminal membrane, believed to mediate water transport.

Purpose of the Study:

• To differentiate between changes in luminal membrane aggregate permeability and a postluminal barrier.
• To investigate the effect of specific experimental protocols on water permeability regulation in toad bladders.

Main Methods:

• Paired toad bladders were used to study three protocols altering vasopressin-stimulated water flow.
• Amphotericin B was used to create water-permeant channels in the luminal membrane to estimate postluminal barrier permeability.
• Protocols included diminishing bladder filling volume, increasing PCO2, and repeated vasopressin stimulation.

Main Results:

• Diminishing bladder filling volume decreased water flow elicited by amphotericin B, indicating reduced postluminal barrier permeability.
• Increasing PCO2 and repeated vasopressin stimulation did not alter amphotericin B-elicited flow, suggesting no change in postluminal barriers.
• These results dissociate aggregate frequency from changes in water permeability.

Conclusions:

• Reduced bladder filling volume specifically impacts a postluminal water barrier in toad bladder granular cells.
• Increased PCO2 and repeated vasopressin stimulation do not alter this postluminal barrier.
• Water permeability regulation in toad bladders involves distinct luminal and postluminal components.