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Ion‐selective modification of paracellular conductance by the water fluxes

Osmolality has various actions on epithelial transports. To study the effects of osmolality on paracellular ion conductance (Gp), we established a method estimating Gp by measuring the transepithelial potential (Vt) with abolishment of transcellular ion conductance using a modified Ussing chamber in renal epithelia. Gp changed proportionally to the ionic strength of bathing solutions and was independent of Vt. Based on these characteristics, we estimated the paracellular ion conductances of Na+ (GNa) and Cl− (GCl) using equilibrium potentials for Na+ and Cl− between apical and basolateral solutions. Secretory directed water flux generated by osmotic gradients associated with NaCl gradients between apical and basolateral solutions increased the Na+‐selective Gp. Secretory directed water flux without NaCl gradients itself had little effects on Gp, but produced precondition generating Gp elevation observed with NaCl gradients. In contrast, absorptive directed water flux increased Gp with no discrimination between GNa and GCl. These observations indicate that Gp is composed of Na+ selective and nonselective components, which are selectively regulated by the water fluxes in a direction‐dependent manner.