Regulation of paracellular Na⁺ and Cl⁻ conductances by hydrostatic pressure

The effect of hydrostatic pressure on the paracellular ion conductance (Gp) composed of the Na⁺ conductance (G_Na) and the Cl⁻ conductance (G_Cl) has been Investigated. Gp, G_Na and G_Cl were time‐dependently increased after applying an osmotic gradient generated by NaCl with basolateral hypotonicity. Hydrostatic pressure (1–4 cm H₂O) applied from the basolateral side enhanced the osmotic gradient‐induced increase in Gp, G_Na and G_Cl in a magnitude‐dependent manner, while the hydrostatic pressure applied from the apical side diminished the osmotic gradient‐induced increase in Gp, G_Na and G_Cl. How the hydrostatic pressure influences Gp, G_Na and G_Cl under an isosmotic condition was also investigated. Gp, G_Na and G_Cl were stably constant under a condition with basolateral application of sucrose canceling the NaCl‐generated osmotic gradient (an isotonic condition). Even under this stable condition, the basolaterally applied hydrostatic pressure drastically elevated Gp, G_Na and G_Cl, while apically applied hydrostatic pressure had little effect on Gp, G_Na or G_Cl. Taken together, these observations suggest that certain factors controlled by the basolateral osmolality and the basolaterally applied hydrostatic pressure mainly regulate the Gp, G_Na and G_Cl.