Sodium flux in the apical membrane of the toad skin: aspects of its regulation and the importance of the ionic strength of the outer solution upon the reversibility of amiloride inhibition

Lanthanum Effect on the Dynamics of Tight Junction Opening and Closing

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+ (Ca(2+)bl), while responding differently to the action of La3+(bl). In FUB, La(3+)bl shows a Ca(2+)-antagonistic effect that promotes TJ opening in the presence of a normal Ca(2+)bl concentration. In A6CM, in contrast, La(3+)bl always shows a clear Ca(2+)-agonistic effect. The fact that a concentration of La(3+)bl one fifth of the normal Ca(2+)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 Ca(2+)-binding sites in both tissues. In FUB, apical La3+ (La(3+)ap) exhibits, differently from its basolateral effect, an evident Ca(2+)-agonistic effect, suggesting a dual effect of La3+, depending on which side of the bladder La3+ is applied. In A6CM La(3+)ap has a Ca(2+)-agonistic effect similar to La(3+)bl. The effects of La(3+)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 La(3+)ap is clear in both tissues, its site of action is yet to be determined. Protonation of the Ca(2+)-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.

Pulses of cell Ca(2+) and the dynamics of tight junction opening and closing

A mathematical modeling of tight junction (TJ) dynamics was elaborated in a previous study 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 Ca(2+) levels. In this model, TJs were assumed to be specifically controlled by the Ca(2+) concentration levels at the extracellular Ca(2+) 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 Ca(2+) concentration (Ca(2+) (cell)), which might result from changes \ of extracellular Ca(2+) concentration (Ca(2+) (extl)), contribute to the observed results. In order to address this aspect of TJ regulation, fast Ca(2+)-switch experiments were performed in which changes of Ca(2+) (cell) were induced using the Ca(2+) ionophore A23187 or thapsigargin, a specific inhibitor of the sarco-endoplasmic reticulum Ca(2+)-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 Ca(2+) (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 Ca(2+) (cell) never led to a decline of G, indicating that a rise of Ca(2+) (cell) does not trigger per se TJ closure. On the contrary, always the first response to a rise of Ca(2+) (cell) is an increase of G that, in most cases, is a transient response. Despite these observations we cannot assure that a rise of Ca(2+) (cell) is without effect on the TJs, since an increase of Ca(2+) (cell) not only causes a transient increase of G but, in addition, during oscillations a rise of Ca(2+) (cell) induced by the Ca(2+) ionophore transiently halted the oscillatory pattern of TJs. The main conclusion of this study is that TJ closure that is observed when basolateral Ca(2+) concentration (Ca(2+) (bl)) is increased after TJs were opened by Ca(2+) (bl) removal cannot be ascribed to a rise of Ca(2+) (cell) and might be a consequence of Ca(2+) binding to extracellular Ca(2+) sites.

Modeling tight junction dynamics and oscillations

Tight junction (TJ) permeability responds to changes of extracellular Ca(2+) concentration. This can be gauged through changes of the transepithelial electrical conductance (G) determined in the absence of apical Na(+). The early events of TJ dynamics were evaluated by the fast Ca(2+) switch assay (FCSA) (Lacaz-Vieira, 2000), which consists of opening the TJs by removing basal calcium (Ca(2+)(bl)) and closing by returning Ca(2+)(bl) to normal values. Oscillations of TJ permeability were observed when Ca(2+)(bl) is removed in the presence of apical calcium (Ca(2+)(ap)) and were interpreted as resulting from oscillations of a feedback control loop which involves: (a) a sensor (the Ca(2+) binding sites of zonula adhaerens), (b) a control unit (the cell signaling machinery), and (c) an effector (the TJs). A mathematical model to explain the dynamical behavior of the TJs and oscillations was developed. The extracellular route (ER), which comprises the paracellular space in series with the submucosal interstitial fluid, was modeled as a continuous aqueous medium having the TJ as a controlled barrier located at its apical end. The ER was approximated as a linear array of cells. The most apical cell is separated from the apical solution by the TJ and this cell bears the Ca(2+) binding sites of zonula adhaerens that control the TJs. According to the model, the control unit receives information from the Ca(2+) binding sites and delivers a signal that regulates the TJ barrier. Ca(2+) moves along the ER according to one-dimensional diffusion following Fick's second law. Across the TJ, Ca(2+) diffusion follows Fick's first law. Our first approach was to simulate the experimental results in a semiquantitative way. The model tested against experiment results performed in the frog urinary bladder adequately predicts the responses obtained in different experimental conditions, such as: (a) TJ opening and closing in a FCSA, (b) opening by the presence of apical Ca(2+) and attainment of a new steady-state, (c) the escape phase which follows the halt of TJ opening induced by apical Ca(2+), (d) the oscillations of TJ permeability, and (e) the effect of Ca(2+)(ap) concentration on the frequency of oscillations.

Calcium site specificity. Early Ca2+-related tight junction events

The molecular mechanisms by which Ca2+ and metal ions interact with the 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 readmitted, and is followed by a complete recovery of the TJ seal. Mg2+ and Ba2+ were incapable of keeping the TJ sealed or of inducing TJ recovery. In addition, Mg2+ causes a reversible concentration-dependent inhibition of the Ca2+-induced TJ recovery. The effects of extracellular Ca2+ manipulation on the TJs apparently is not mediated by changes of cytosolic 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+ withdrawal. In addition, Mn2+ promotes an almost complete recovery of the TJ seal. Cd2+, in spite of stabilizing the TJs in the closed state and halting TJ opening, does not promote TJ recovery, an effect that apparently results from a superimposed toxic effect that is markedly attenuated by the presence of Ca2+. The interruption of TJ opening caused by Ca2+, Cd2+, or Mn2+, and the stability they confer to the closed TJs, might result from the interaction of these ions with E-cadherin. Addition of La3+ (2 microM) to the basolateral Ca2+-containing solution causes an increase of TJ permeability that fully reverses when La3+ is removed. This effect of La3+, observed in the presence of Ca2+ (1 mM), indicates 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 permeability of epithelial and endothelial membranes, particularly when used during in vivo perfusion or in the absence of fixatives.

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.

Effect of oxytocin on transepithelial transport of water and Na+ in distinct ventral regions of frog skin (Rana catesbeiana)

Thoracic, abdominal, and pelvic fragments of ventral skin of Rana catesbeiana were analysed regarding the effect of oxytocin on: (1) transepithelial water transport; (2) short-circuit current; (3) skin conductance and electrical potential difference; (4) Na+ conductance, the electromotive force of the Na+ transport mechanism, and shunt conductance; (5) short-circuit current responses to fast Na+ by K+ replacement in the outer compartment, and (6) epithelial microstructure. Unstimulated water and Na+ permeabilities were low along the ventral skin. Hydrosmotic and natriferic responses to oxytocin increased from thorax to pelvis. Unstimulated Na+ conductance was greater in pelvis than in abdomen, the other electrical parameters being essentially similar in both skin fragments. Contribution of shunt conductance to total skin conductance was higher in abdominal than in pelvic skin. Oxytocininduced increases of total skin conductance, Na+ conductance, and shunt conductance in pelvis were significantly larger than in abdomen. An oscillatory behaviour of the short-circuit current was observed only in oxytocin-treated pelvic skins. Decrease of epithelial thickness and increase of mitochondria-rich cell number were observed from thorax to pelvis. Oxytocin-induced increases of interspaces were more conspicuous in pelvis and abdomen than in thorax.

Roles of external and cellular Cl- ions on the activation of an apical electrodiffusional Cl- pathway in toad skin

This study is concerned with the short-circuit current, Isc, responses of the Cl(-)-transporting cells of toad skin submitted to sudden changes of the external Cl- concentration, [Cl]o. Sudden changes of [Cl]o, carried out under apical membrane depolarization, allowed comparison of the roles of [Cl]o and [Cl]cell on the activation of the apical Cl- pathways. Equilibration of short-circuited skins symmetrically in K-Ringer's solutions of different Cl- concentrations permitted adjustment of [Cl]cell to different levels. For a given Cl- concentration (in the range of 11.7 to 117 mM) on both sides of a depolarized apical membrane, this structure exhibits a high Cl- permeability, P(Cl)apical. On the other hand, for the same range of [Cl]cell but with [Cl]o = 0, P(Cl)apical is reduced to negligible values. These observations indicate that when the apical membrane is depolarized P(Cl)apical is modulated by [Cl]o; in the absence of external Cl- ions, intracellular Cl- is not sufficient to activate P(Cl)apical. Computer simulation shows that the fast Cl- currents induced across the apical membrane by sudden shifts of [Cl]o from a control equilibrium value strictly follow the laws of electrodiffusion. For each experimental group, the computer-generated Isc versus [( Cl]cell - [Cl]o) curve which best fits the experimental data can only be obtained by a unique pair of P(Cl)apical and Rb (resistance of the basolateral membrane), thus allowing the calculation of these parameters. The electrodiffusional behavior of the net Cl- flux across the apical membrane supports the channel nature of the apical Cl- pathways in the Cl(-)-transporting cells. Cl- ions contribute significantly to the overall conductance of the basolateral membrane even in the presence of a high K concentration in the internal solution.

Comparative roles of voltage and Cl ions upon activation of a Cl conductive pathway in toad skin

(1) Combined use of external Cl concentration pulses and apical membrane depolarization permitted to compare the roles of apical voltage and Cl ions upon the activation of a skin Cl conductance, GCl, which is assumed to reflect activation of the permeability of a Cl pathway. (2) Apical membrane depolarization induced by skin hyperpolarization, or by short-circuiting skins with high K Ringer's on the inner side, failed to activate GCl in the absence of external Cl, GCl remaining negligible. Under apical membrane depolarization, a step elevation of [Cl]0 slowly activated GCl as characterized by a sigmoidal current response of slow onset concomitant to a slow conductance increase. External Cl removal had the reverse effect, slowly inactivating GCl. (3) With the apical membrane in the normal polarized state, a step increase of [Cl]0 slowly activated GCl to submaximal values. This indicates that the interaction of Cl ions with the apical membrane partially activates GCl in the absence of apical membrane depolarization. (4) Activation of GCl was interpreted on the basis of a direct effect of Cl ions upon the apical membrane, having been attributed to the apical membrane voltage an indirect role. Voltage would affect the Cl distribution across the apical membrane, and, as a result, the Cl concentration at a proposed regulatory site which modulates the apical membrane permeability to Cl ions.