Ca2+ regulation of tight-junction permeability and structure in Necturus gallbladder

Area of Science:

• Cell Biology
• Epithelial Physiology
• Biophysics

Background:

• Tight junctions regulate paracellular transport.
• Calcium ions (Ca2+) are critical for maintaining epithelial barrier integrity.
• The Necturus gallbladder serves as a model for studying epithelial transport.

Purpose of the Study:

• To investigate the influence of Ca2+ on tight-junction permeability.
• To correlate electrophysiological changes with tight-junction morphology.
• To elucidate the mechanisms by which Ca2+ and ionophores affect epithelial resistance.

Main Methods:

• Exposure of Necturus gallbladder to varying Ca2+ concentrations and the Ca2+ ionophore A23187.
• Measurement of electrophysiological parameters (transepithelial resistance, membrane potential) in an Ussing-type chamber.
• Analysis of tight-junction ultrastructure using freeze-fracture electron microscopy.

Main Results:

• Ca2+-free conditions fragmented tight junctions and decreased transepithelial resistance.
• A23187 induced an initial drop then a rise in transepithelial resistance, with decreased NaCl diffusion potentials.
• Increased junctional strand number correlated with increased junctional depth.
• Changes in cell membrane potential and resistance ratio indicated alterations in both transcellular and paracellular pathways.

Conclusions:

• Ca2+ plays a crucial role in maintaining tight-junction structure and function.
• The ionophore A23187 exerts complex effects on epithelial permeability by modulating both cellular and paracellular pathways.
• Tight-junction morphology, specifically the number and depth of junctional strands, is dynamically regulated by Ca2+ levels.