#Dynamic duo free#

( d ) Leak pathway permeability can be assessed by directly measuring macromolecular flux of tracers across the epithelium. The permeability of each organic cation can be determined and used to assess size selectivity of the pore pathway.

In practice, this makes a minimal contribution to measurements made across intact, charge-selective tight junctions.) ( c) The bi-ionic substitution approach replaces Na + on one side of the monolayer with organic cations of various sizes.

#Dynamic duo free#

(Due to the higher mobility in free solution of Cl − relative to Na +, the dilution potential of a non-charge-selective pathway will be slightly negative. For example, a permeability ratio close to 1.0 indicates no charge selectivity and does not alter the equilibrium potential across the monolayer. By inducing a transepithelial electrochemical gradient, e.g., by altering, iso-osmotically, apical or basolateral NaCl concentrations, one can establish a new equilibrium potential based on the relative paracellular permeabilities of Na + and Cl −. ( b) Charge selectivity can be measured by the dilution potential technique. Like all electrophysiological measures of tight junction function, TER reflects tight junction permeability best when ion conductance across the junction is far greater than across apical and basolateral membranes, such as in leaky epithelia. Increased permeability of either pathway reduces TER. These small ions do not discriminate between pore and leak pathways, and therefore TER cannot be used to measure tight junction size or charge selectivity. The most common ions in physiological solutions, Na + and Cl −, carry this current. This is typically done by applying a transepithelial current, by measuring the generated potential, and by using Ohm's law to calculate the resistance to current flow. ( a) Transepithelial electrical resistance (TER) measures the flux of all ions across the epithelium. However, these measure population averages no tools are presently available to measure local tight junction permeability on a submicrometer scale. The permeability of these pathways can be measured using several complementary methods. The tight junction is composed of at least two functionally distinct pathways: a high-capacity, charge-selective pore pathway that allows passage of small ions and uncharged molecules and a low-capacity leak pathway that allows flux of larger ions and molecules, regardless of charge. This review presents these data in a historical context and proposes an integrated model in which dynamic regulation of tight junction protein interactions determines barrier function.

Many tight junction components have been identified over the past 20 years, and recent progress has provided new insights into the proteins and interactions that regulate structure and function. The tight junction barrier is not absolute but is selectively permeable and is able to discriminate between solutes on the basis of size and charge.

This function is accomplished in vertebrates by the zonula occludens, or tight junction. Although the plasma membranes of these barrier-forming cells prevent flux of most hydrophilic solutes, the paracellular, or shunt, pathway between cells must also be sealed. In more complex species, the diversity of cell types capable of forming barriers increases dramatically.

In simple organisms this allows distinct cell types to interface with the external environment. Tissue barriers that restrict passage of liquids, ions, and larger solutes are essential for the development of multicellular organisms.