Toad bladder compliance and water permeability in response to stretch

Area of Science:

• Physiology
• Cell Biology
• Biophysics

Background:

• The toad urinary bladder is a model system for studying epithelial water and solute transport.
• Understanding the mechanical properties of the bladder wall is crucial for interpreting transport studies.

Purpose of the Study:

• To investigate the pressure/volume relationship and mechanical properties of the toad urinary bladder.
• To determine the effects of stretching on bladder permeability and cellular integrity.
• To assess the influence of vasopressin on bladder weight loss and permeability.

Main Methods:

• Measurement of pressure/volume relationships in toad urinary bladders.
• Calculation of bladder wall compliance at different intraluminal pressures.
• Gravimetric determination of fluid loss in the presence and absence of vasopressin.
• Measurement of [14C]mannitol permeability across the bladder wall at varying tensions.
• Electron microscopy to examine cellular ultrastructure under stretch.

Main Results:

• Bladder wall compliance was not affected by vasopressin or atropine.
• Bladder weight loss (indicating fluid transport) increased significantly with vasopressin, particularly at higher capacities.
• Permeability to [14C]mannitol increased with increasing bladder wall tension.
• Electron microscopy revealed microvilli flattening and apical membrane rupture in some cells, but intact tight junctions.

Conclusions:

• Bladder stretching increases permeability to small solutes like mannitol and can cause localized apical membrane damage.
• Despite cellular damage, the bladder's responsiveness to vasopressin for water transport remains intact.
• These findings highlight the complex interplay between mechanical stress, cellular integrity, and hormonal regulation of epithelial transport.