Extracellular Ca2+ controls outward rectification by apical cation channels in toad urinary bladder: patch-clamp and...

Area of Science:

• Physiology
• Ion Channels
• Epithelial Transport

Background:

• The toad urinary bladder is a model for studying epithelial ion transport.
• Vasopressin-sensitive apical K+ channels play a role in regulating water and ion balance.

Purpose of the Study:

• To characterize the biophysical and pharmacological properties of an outward rectifying cation channel in toad urinary bladder epithelial cells.
• To determine the role of divalent cations in the outward rectification of this channel.

Main Methods:

• Patch-clamp technique on isolated toad urinary bladder epithelial cells.
• Single-channel and whole-bladder electrophysiological recordings.
• Bi-ionic potential measurements to assess cation permeability.

Main Results:

• An outward rectifying cation channel with conductances of 24 pS (0 to +60 mV) and 157 pS (-60 to -100 mV) for K+ was identified.
• Channel conductance decreased with increasing dehydration energy of monovalent cations (Rb+ = K+ > Na+ > Li+).
• Outward rectification resulted from block by extracellular divalent cations, particularly Ca2+, which inhibited inward but not outward currents.

Conclusions:

• The identified cation channel is likely the single-channel correlate of the vasopressin-sensitive apical K+ conductive pathway in the toad urinary bladder.
• Divalent cations, especially Ca2+, play a crucial role in the outward rectification mechanism of this channel.
• The channel shares properties with other apical conductances in amphibian epithelia, suggesting a conserved transport mechanism.