Pulses of Cell Ca2+ and the Dynamics of Tight Junction Opening and Closing

A mathematical modeling of tight junction (TJ) dynamics was elaborated in a previous study (Kassab, F., Marques, R.P., Lacaz-Vieira, F. 2002. Modeling tight junction dynamics and oscillations. J. Gen. Physiol. 120:237–247) to better understand the dynamics of TJ opening and closing, as well as oscillations of TJ permeability that are observed in response to changes of extracellular Ca²⁺ levels. In this model, TJs were assumed to be specifically controlled by the Ca²⁺ concentration levels at the extracellular Ca²⁺ binding sites of zonula adhaerens. Despite the fact that the model predicts all aspects of TJ dynamics, we cannot rule out the likelihood that changes of intracellular Ca²⁺ concentration (Ca²⁺ _cell), which might result from changes of extracellular Ca²⁺ concentration (Ca²⁺ _extl), contribute to the observed results. In order to address this aspect of TJ regulation, fast Ca²⁺-switch experiments were performed in which changes of Ca²⁺ _cell were induced using the Ca²⁺ ionophore A23187 or thapsigargin, a specific inhibitor of the sarco-endoplasmic reticulum Ca²⁺-ATPase. The results indicate that the ionophore or thapsigargin per se do not affect basal tissue electrical conductance (G), showing that the sealing of TJs is not affected by a rise in Ca²⁺ _cell. When TJs were kept in a dynamic state, as partially open structures or in oscillation, conditions in which the junctions are very sensitive to disturbances that affect their regulation, a rise of Ca²⁺ _cell never led to a decline of G, indicating that a rise of Ca²⁺ _cell does not trigger per se TJ closure. On the contrary, always the first response to a rise of Ca²⁺ _cell is an increase of G that, in most cases, is a transient response. Despite these observations we cannot assure that a rise of Ca²⁺ _cell is without effect on the TJs, since an increase of Ca²⁺ _cell not only causes a transient increase of G but, in addition, during oscillations a rise of Ca²⁺ _cell induced by the Ca²⁺ ionophore transiently halted the oscillatory pattern of TJs. The main conclusion of this study is that TJ closure that is observed when basolateral Ca²⁺ concentration (Ca²⁺ _bl) is increased after TJs were opened by Ca²⁺ _bl removal cannot be ascribed to a rise of Ca²⁺ _cell and might be a consequence of Ca²⁺ binding to extracellular Ca²⁺ sites.