Tight Junction Dynamics in the Frog Urinary Bladder

In a previous study in frog skin (Castro et al., J. Memb. Biol. 134:15-29, 1993), it was shown that TJs experimentally disrupted by a selective deposition of BaSO₄ could be re-sealed upon addition of Ca²⁺to the apical solution; in the absence of apical Ca²⁺, the normal Ca²⁺ activity of the Na₂SO₄-Ringer's bathing the basolateral side was not able to induce TJ resealing. We now show that apical Ca²⁺also activates the TJ sealing mechanism in frog urinary bladders. Three known procedures were utilized to increase TJ permeability, all in the absence of apical Ca²⁺: (i) exposure to high positive transepithelial clamping potentials; (ii) exposure of the apical surface to hypertonic solutions; and (iii) selective deposition of BaSO₄ in the TJs. The resealing of the TJs was promoted by raising the concentration of Ca²⁺ in the apical solution. This effect of Ca²⁺ is not impaired by the presence of Ca²⁺ channel blockers (nifedipine, verapamil, Mn²⁺ or Cd²⁺) in the apical solution, indicating that junction resealing does not depend on Ca²⁺ entering the cells through the apical membrane. TJ resealing that occurs in response to raised apical Ca²⁺ most likely results from a direct effect of Ca²⁺, entering the disrupted TJs from the apical solution and reaching the zonula adhaerens Ca²⁺ receptors (E-cadherins). Protein kinase C (PKC) must play a significant role in the control of TJ assembly in this tight epithelia since the PKC inhibitor (H7) and the activator (diC8) markedly affect TJ recovery after disruption by apical hypertonicity. H7 treated tissues show marked recuperation of conductance even in the absence of apical Ca²⁺. In contrast, diC8 prevents tissue recuperation which normally occurs after addition of Ca²⁺ to the apical solution.