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 resealed 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.