Effect of temperature on nonelectrolyte permeation across the toad urinary bladder

Area of Science:

• Membrane transport
• Biophysics
• Physiology

Background:

• The toad urinary bladder is a model system for studying epithelial transport.
• Understanding nonelectrolyte permeability is crucial for drug delivery and physiological studies.

Purpose of the Study:

• To quantify the permeability of the toad urinary bladder to various nonelectrolytes.
• To elucidate the relationship between molecular properties and membrane permeability.
• To investigate the temperature dependence of permeation and its activation energies.

Main Methods:

• Radioactive tracer flux measurements were used to determine permeability coefficients (P).
• Unstirred layers were corrected for accurate P calculations.
• Permeability was correlated with olive oil/water partition coefficients (Koil) and molecular volume.
• Apparent activation energies (Eα) were determined using Arrhenius plots over a temperature range (2-32°C).

Main Results:

• Permeability coefficients (P) were proportional to olive oil/water partition coefficients (PαKoil^0.3).
• Hydroxyl groups reduced P 500-fold; methylene groups increased P 4-fold.
• Branched solutes and small molecules showed higher permeability than predicted.
• Small molecule permeation suggests diffusion through organized lipid structures; large polar solutes use shunt pathways.
• Activation energies (Eα) increased with methylene groups and decreased with hydroxyl groups.

Conclusions:

• Molecular structure, including branching and functional groups, significantly modulates nonelectrolyte permeability in the toad bladder.
• Permeation mechanisms differ for small and large molecules, involving lipid diffusion and extracellular shunts.
• Temperature-dependent studies reveal distinct activation energies for different molecular structures, providing insights into the transport barriers.