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 select ive deposition of BaSO4 could be resealed upon addition of Ca2+ to the apical solution; in the absence of apical Ca2+, the normal Ca2+ activ ity of the Na2SO4-Ringer's bathing the basolateral side was not able t o 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 apic al Ca2+: (i) exposure to high positive transepithelial clamping potent ials; (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 so lution. This effect of Ca2+ is not impaired by the presence of Ca2+ ch annel blockers (nifedipine, verapamil, Mn2+ or Cd2+) in the apical sol ution, indicating that junction resealing does not depend on Ca2+ ente ring the cells through the apical membrane. TJ resealing that occurs i n response to raised apical Ca2+ most likely results from a direct eff ect of Ca2+, entering the disrupted TJs from the apical solution and r eaching the zonula adhaerens Ca2+ receptors (E-cadherins), Protein kin ase C (PKC) must play a significant role in the control of TJ assembly in this tight epithelia since the PKC inhibitor (H7) and the activato r (diC8) markedly affect TJ recovery after disruption by apical hypert onicity. H7 treated tissues show marked recuperation of conductance ev en in the absence of apical Ca2+. In contrast, diC8 prevents tissue re cuperation which normally occurs after addition of Ca2+ to the apical solution.