Abstract
Epithelial tight junction forms the barrier with ion moving pathway. Paracellular ion permeability is modified by expression pattern of claudins, a component of tight junction, but the mechanism of its regulation and physiological implication is unknown. In renal epithelial A6 cells, hypotonicity induces Na+ transport via the transcellular pathway. However, the effect of hypotonicity on paracellular ion permeability is unknown. To study the effect of hypotonicity on paracellular ion permeability, we measured currents in A6 cells. Basolateral but not apical hypotonicity markedly increased the paracellular conductance (Gp) and current (Ip) from apical to basolateral side, indicating that the basolateral hypotonicity increases the paracellular permeability to more Na+ than Cl+. Furthermore, replacement of NaCl with sucrose in the basolateral solution did not increase Gp or Ip. Furthermore, under the condition that Na+ replacement with NMDG, basolateral hypotonicity increased Gp but not Ip. Under the condition that Cl+ replacement with gluconate, basolateral hypotonicity did not increase Gp or Ip. These observations indicate that basolateral hypotonicity and paracellular Cl+ movement increase paracellular permeability and induce more conductance for Na+ than Cl+ and NMDG. Based on these observations, we conclude that the hypotonicity-induced changes of paracellular ion conductances stimulate Na+ reabsorption in the renal A6 cells via not only transcellular but also paracellular pathway. [J Physiol Sci. 2006;56 Suppl:S187]
