Sodium transport effects on the basolateral membrane in toad urinary bladder.

Area of Science:

• Physiology
• Membrane Transport
• Epithelial Biology

Background:

• Amiloride is a known inhibitor of sodium (Na) transport in epithelial tissues.
• Understanding the electrical behavior of the toad urinary bladder epithelium is crucial for studying ion transport mechanisms.

Purpose of the Study:

• To investigate the effects of amiloride on apical and basolateral membrane potentials and resistances in the toad urinary bladder.
• To determine the relationship between changes in membrane potentials/resistances and Na transport inhibition.
• To elucidate the electrical behavior of the basolateral membrane in response to Na transport inhibition.

Main Methods:

• Measurements of apical (Vmc) and basolateral (Vcs) membrane potentials and resistances (Ra, Rb) in toad urinary bladder epithelium.
• Application of amiloride to inhibit Na transport.
• Use of external current (Ie) to voltage clamp the apical membrane (Vmc=0) and assess transcellular current pathways.
• Analysis of an equivalent electrical circuit model of the epithelium.

Main Results:

• Amiloride decreased Vmc and Vcs and increased Ra and Rb.
• A secondary decrease in Rb occurred at longer amiloride exposure times, increasing the Ra/Rb ratio.
• Voltage clamping experiments indicated that the depolarization of Vcs and increased Rb were due to a decrease in the Na pump's current output, suggesting the basolateral membrane is not purely ohmic.
• Under basolateral voltage clamp, the apical membrane behaved as a passive, ohmic element.

Conclusions:

• Amiloride-induced changes in toad urinary bladder epithelium involve complex alterations in membrane potentials and resistances.
• The basolateral membrane's electrical behavior is influenced by the rheogenic Na pump activity and is not simply resistive.
• The apical membrane exhibits passive electrical properties when Na transport is inhibited under specific voltage clamp conditions.