Urea transport in toad skin (Bufo marinus)

Area of Science:

• Physiology
• Comparative Physiology
• Membrane Transport

Background:

• Urea transport across biological membranes is crucial for osmoregulation and waste excretion.
• The skin of the toad Bufo marinus serves as a model for studying epithelial transport processes.

Purpose of the Study:

• To investigate the characteristics of urea transport across the skin of the toad Bufo marinus.
• To determine if urea transport is carrier-mediated and how it is affected by adaptation to hypertonic conditions and various pharmacological agents.

Main Methods:

• Short-circuit current (SCC) measurements were used to assess transepithelial transport.
• Urea transport (Jm-->s(urea)) was quantified in toads adapted to hypertonic NaCl.
• The effects of amiloride, phloretin, ouabain, NaCN, vasopressin, and 2-pyrazinoylguanidine on urea and sodium transport were evaluated.

Main Results:

• Transepithelial urea transport (Jm-->s(urea)) was enhanced and showed saturation kinetics in toads adapted to 150 mM NaCl, indicating carrier-mediated transport.
• Amiloride inhibited Jm-->s(urea) in a dose-dependent manner, but with significantly lower potency than its effect on SCC.
• Phloretin, ouabain, NaCN, vasopressin, and 2-pyrazinoylguanidine did not significantly affect Jm-->s(urea), suggesting a distinct transport mechanism from active sodium transport.

Conclusions:

• The skin of Bufo marinus possesses a carrier-mediated system for urea transport from mucosa to serosa.
• This urea transport system is upregulated in hypertonic conditions and operates independently of active sodium transport pathways.
• Pharmacological agents known to affect urea transport in other tissues or sodium transport in toad skin had minimal impact on Jm-->s(urea).