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Claudins define paracellular permeability to small molecules by forming ion-selective pores within the tight junction. We recently demonstrated that claudin-2 channels are gated and open and close on a submillisecond timescale. To determine if and how the ensemble behavior of this unique class of entirely extracellular gated ion channels could define global epithelial barrier function, we have developed an in silico model of local claudin-2 behavior. This model considers the complex anastomosing ultrastructure of tight junction strands and can be scaled to show that local behavior defines global epithelial barrier function of epithelial monolayers expressing different levels of claudin-2. This is the first mathematical model to describe global epithelial barrier function in terms of the dynamic behavior of single tight junction channels and establishes a framework to consider gating kinetics as a means to regulate barrier function., (© 2017 New York Academy of Sciences.)

A principal role of tight junctions is to seal the apical intercellular space and limit paracellular flux of ions and molecules. Despite the fact that tight junctions form heavily cross-linked structures, functional studies have fostered the hypothesis that the tight junction barrier is dynamic and defined by opening and closing events. However, it has been impossible to directly measure tight junction barrier function with sufficient resolution to detect such events. Nevertheless, recent electrophysiological and sieving studies have provided tremendous insight into the presence of at least two pathways of trans-tight junction flux: a high-capacity ion-selective "pore" pathway and a low-capacity "leak" pathway that allows the passage of macromolecules. Furthermore, it is now known that the tight junction molecular structure is highly dynamic and that dynamics are correlated with barrier function. Taken together, these data support a dynamic model of tight junction conductance and suggest that regulation of tight junction openings and closings may provide sensitive means of barrier regulation., (© 2012 New York Academy of Sciences.)

Here, we address three issues in intact ventricular myocytes that specifically relate to the role of Na/Ca exchange (NCX) current under physiological conditions. First, we revisit the issue of NCX stoichiometry in light of some recent findings that the stoichiometry of the NCX may not be fixed at 3Na: 1Ca. We discuss some data that strongly favor the 3:1 stoichiometry, at least under physiological conditions. Second, we address the controversy over the role of allosteric Ca regulation in intact myocytes. We show that outward and inward I(NCX) can be activated dynamically by changing [Ca](i) over the physiological range and that outward I(NCX) can be activated quite rapidly with sarcoplasmic reticulum Ca release. These data are well described using an instantaneous equation for NCX current that includes an allosteric activation factor with K(mCaAct) = 125 nM. Finally, we consider the effect on NCX current of submembrane elevations in [Ca](i) (that are far greater than are measured in the bulk cytoplasm). Taken together with a NCX stoichiometry of 3, these findings have allowed us to make some predictions of the role of I(NCX) during an AP. Our simulations suggest that NCX current is outward for less than approximately 10 ms at the beginning of the action potential.

A decrease in the peak systolic [Ca](i) and slow decay of the Ca(i) transient are common features of the end-stage failing human ventricular myocyte and may underlie the contractile abnormalities observed in congestive heart failure. The role of the Na/Ca exchanger has been a great area of interest given the changes observed at the molecular level. Results from these experiments have been inconsistent, however, and therefore cellular-based experiments may be required to characterize the role of the Na/Ca exchanger in failing human myocardium. We review recent data that suggest an increased ability of the Na/Ca exchanger to transport Ca into the cytoplasm in failing human myocytes. We hypothesize that this increased Ca influx can explain the slowed decay and impaired relaxation of failing human ventricular myocytes.