We present a comparative study in frog urinary bladders (FUB) and A6 cell monolayers (A6CM) on the effect of La3+ on tight junction (TJ) dynamics. These tissues react similarly to changes of basolateral Ca2+ (Ca2+ bl), while responding differently to the action of La3+ bl. In FUB, La3+ bl shows a Ca2+-antagonistic effect that promotes TJ opening in the presence of a normal Ca2+ bl concentration. In A6CM, in contrast, La3+ bl always shows a clear Ca2+-agonistic effect. The fact that a concentration of La3+ bl one fifth of the normal Ca2+ bl leads in FUB to TJ opening and in A6CM to a complete recovery of the TJ seal indicates a high affinity of La3+ for the Ca2+-binding sites in both tissues. In FUB, apical La3+ (La3+ ap) exhibits, differently from its basolateral effect, an evident Ca2+-agonistic effect, suggesting a dual effect of La3+, depending on which side of the bladder La3+ is applied. In A6CM La3+ ap has a Ca2+-agonistic effect similar to La3+ bl. The effects of La3+ bl in FUB and in A6CM are consistent, according to our previous publications, with La3+ acting antagonistically or agonistically, respectively, on the Ca2+ binding sites of zonula adhaerens. Despite the fact that the effect of La3+ ap is clear in both tissues, its site of action is yet to be determined. Protonation of the Ca2+-binding sites causes a decrease of its agonistic effect on A6CM, consistent with a negatively charged binding site. In A6CM La3+ apparently replaces Ca2+, mimicking the effect of Ca2+ triggering the cascade of events leading to TJ closure. In FUB, La3+ interacts with the binding sites, dislodging Ca2+, with a high affinity, but this interaction is inadequate to initiate or sustain the process of junction closing. Possibly, the difference between the two preparations resides in subtle conformation differences of the outer segment of E-cadherin molecules.
The Journal of Membrane Biology – Springer Journals
Published: Jan 1, 2004
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