Antidiuretic hormone-dependent membrane capacitance and water permeability in the toad urinary bladder

Area of Science:

• Cell Biology
• Membrane Physiology
• Renal Physiology

Background:

• Antidiuretic hormone (ADH) plays a key role in regulating water reabsorption in the kidney.
• The apical membrane of epithelial cells is critical for controlling water permeability.
• Understanding the biophysical mechanisms of ADH action is essential for comprehending water balance.

Purpose of the Study:

• To investigate the effect of antidiuretic hormone (ADH) on the electrical capacitance of the toad bladder apical membrane.
• To determine how osmotic gradients influence ADH-mediated changes in apical membrane capacitance.
• To explore the role of membrane dynamics and vesicle fusion in ADH-stimulated water transport.

Main Methods:

• Measurement of transepithelial voltage in response to constant-current pulses to determine electrical capacitance.
• Utilized toad bladders with modified solutions to isolate apical membrane resistance and reduce basolateral resistance.
• Applied osmotic gradients and inhibitors (colchicine, cytochalasin B) to assess their impact on ADH-induced capacitance changes.

Main Results:

• ADH increased apical membrane capacitance by 28% without an osmotic gradient and by 8% with a hypotonic gradient.
• ADH-dependent capacitance changes mirrored the time course of water flow decline when an osmotic gradient was imposed.
• Inhibitors of cytoskeletal function and hypertonic solutions blocked the ADH-induced capacitance increase.

Conclusions:

• The study demonstrates that ADH significantly alters apical membrane capacitance in the toad bladder.
• Osmotic gradients modulate the ADH-induced capacitance increase, suggesting a dynamic interplay between water flow and membrane properties.
• Results support a model where ADH stimulates water transport via the fusion of cytoplasmic vesicles with the apical plasma membrane.