CALCIUM SITE-SPECIFICITY - EARLY CA2-RELATED TIGHT JUNCTION EVENTS()

The molecular mechanisms by which Ca2+ and metal ions interact with th e binding sites that modulate the tight junctions (TJs) have not been fully described. Metal ions were used as probes of these sites in the frog urinary bladder. Basolateral Ca2+ withdrawal induces the opening of the TJs, a process that is abruptly terminated when Ca2+ is readmit ted, and is followed by a complete recovery of the TJ seal. Mg2+ and B a were incapable of keeping the TJ sealed or of inducing TJ recovery. In addition, Mg2+ causes a reversible concentration-dependent inhibiti on of. the Ca2+-induced TJ recovery. The effects of extracellular Ca2 manipulation on the TJs apparently is not mediated by changes of cyto solic Ca2+ concentration. The transition elements, Mn2+ and Cd2+, act as Ca2+ agonists. In the absence of Ca2+, they prevent TJ opening and almost immediately halt the process of TJ opening caused by Ca2+ withd rawal. In addition, Mn2+ promotes an almost complete recovery of the T J seal. Cd2+, in spite of stabilizing the TJs in tile closed state and halting TJ opening, does not promote TJ recovery, an effect that appa rently results from a superimposed toxic effect that is markedly atten uated by the presence of Ca2+. The interruption of TJ opening caused b y Ca2+, Cd2+, or Mn2+, and the stability they confer to the closed TJs , might result from the interaction of these ions with E-cadherin. Add ition of La3+ (2 mu M) to the basolateral Ca2+-containing solution cau ses an increase of TJ permeability that fully reverses when La3+ is re moved. This effect of La3+, observed in the presence of Ca2+ (1 mM), i ndicates a high La3+ affinity for the Ca2+-binding sites. This ability of La3+ to open TJs in the presence of Ca2+ is a relevant aspect that must be considered when using La3+ in the evaluation of TJ permeabili ty of epithelial and endothelial membranes, particularly when used dur ing in vivo perfusion or in the absence of fixatives.