Transepithelial Na+ transport and the intracellular fluids: a computer study

Area of Science:

• Physiology
• Biophysics
• Computational Biology

Background:

• The Koefoed-Johnsen and Ussing model provides a framework for understanding epithelial transport.
• Previous models have limitations in simulating complex epithelial behaviors under various experimental conditions.

Purpose of the Study:

• To simulate tight epithelia using the Lew, Ferreira, and Moura model under three distinct experimental conditions.
• To investigate the roles of apical chloride permeability, intracellular sodium concentration, and serosal potassium concentration in epithelial function.
• To refine computational models of epithelial transport by incorporating feedback mechanisms and junctional permeability.

Main Methods:

• Computer simulations utilizing the rheogenic nonlinear model of Lew, Ferreira, and Moura.
• Analysis of transitions between short-circuited and open-circuited states.
• Evaluation of the model's ability to simulate effects of altered serosal potassium concentration.
• Incorporation of negative feedback between apical sodium permeability and intracellular sodium concentration.
• Investigation of osmotically induced permeability changes in apical intercellular junctions.

Main Results:

• Apical chloride permeability is a critical parameter for cell volume regulation analysis.
• Intracellular sodium concentration is significantly influenced by transepithelial clamping voltage.
• The basic model requires modification to account for negative feedback between apical sodium permeability and intracellular sodium concentration to match experimental observations.
• Osmotically induced permeability changes in apical intercellular junctions play a role in NaCl conservation.
• Basolateral sodium entry is highly dependent on transepithelial potential, intracellular sodium, and serosal potassium concentrations.

Conclusions:

• Accurate modeling of tight epithelia necessitates precise definition of apical chloride permeability.
• Modified models incorporating negative feedback mechanisms better represent epithelial responses to altered ion concentrations.
• Intercellular junctional permeability is a key factor in maintaining NaCl homeostasis.
• The relative importance of sodium entry pathways (apical vs. basolateral) is dynamically regulated by electrochemical gradients.