Urea and Na+ permeabilities in toad urinary bladder: one or two solute pathways?

Area of Science:

• Physiology
• Renal Physiology
• Membrane Transport

Background:

• Amiloride is known to block Na+ transport and inhibit urea permeability (Pu) in toad urinary bladders.
• This dual effect raises questions about the regulation of Pu by Na+ and whether amiloride targets single or multiple solute pathways.

Purpose of the Study:

• To investigate if Na+ transport rates or cytosolic Na+ pools regulate urea permeability (Pu).
• To determine if amiloride inhibits distinct pathways for Na+ and urea or a single shared pathway.

Main Methods:

• Experiments were conducted on toad bladders to assess urea permeability (Pu) and Na+ transport under various conditions.
• Manipulations included varying mucosal Na+ concentrations, absence of mucosal Ca2+, and the use of amiloride, La3+, and Tm3+.

Main Results:

• Urea permeability (Pu) was unaffected by the absence of mucosal Na+ and was inhibited by amiloride even without apical Na+.
• Amiloride's inhibition of urea permeability (Pu) was independent of its effect on Na+ transport, with different concentration dependencies observed.
• Lanthanum (La3+) and Thulium (Tm3+) potentiated Na+ transport but blunted the ADH-induced increase in urea permeability (Pu), without affecting osmotic flow.

Conclusions:

• The inhibitory effect of amiloride on urea permeability (Pu) is not mediated by its action on Na+ transport.
• Toad bladders possess separate pathways for water, Na+, and urea.
• Amiloride reversibly blocks the specific pathways for Na+ and urea permeation.