K+ transport and capacitance of the basolateral membrane of the larval frog skin

Integument structure and function in juvenile Xenopus laevis with disrupted thyroid balance

The skin is the largest organ in the body and is a barrier between the internal and external environment. The present study evaluates how PTU, a goitrogen, that is used to treat hyperthyroidism affects the structure and electrical properties of the frog (Xenopus laevis) skin. The results are considered in the context of the two-membrane model established in the seminal work of Ussing and collegues in the 1940s and 1950s. In vitro experiments with skin from Xenopus adults revealed that PTU can act directly on skin and causes a significant increase (p<0.05, One-way ANOVA) in short circuit current (Isc) via an amiloride-insensitive mechanism. Juvenile Xenopus exposed to waterborne PTU (5 mg/L) had a significantly bigger and more active thyroid gland (p<0.01, Student's t-test) than control Xenopus. The bioelectric properties of skin taken from Xenopus juveniles treated with PTU in vivo had a lower Isc, (3.05±0.4, n=13) and Rt (288.2±39.5) than skin from control Xenopus (Isc, 4.19±1.14, n=14; Rt, 343.3±43.3). A histological assessment of skin from PTU treated Xenopus juveniles revealed the epidermis was significantly thicker (p<0.01, Student's t-test) and had a greater number of modified exocrine glands (p<0.01, Student's t-test) in the dermis compared to control skin. Modifications in skin structure are presumably the basis for its changed bioelectric properties and the study highlights a site of action for environmental chemicals which has been largely neglected.

Interrelations/cross talk between transcellular transport function and paracellular tight junctional properties in lung epithelial and endothelial barriers

In this synopsis of a symposium at EB2007, we start with an overview of noise and impedance analyses that have been applied to various epithelial barriers. Noise analysis yields specific information about ion channels and their regulation in epithelial and endothelial barriers. Impedance analysis can yield information about apical and basolateral membrane conductances and paracellular conductance of both epithelial and endothelial barriers. Using a morphologically based model, impedance analysis has been used to assess changes in apical and basolateral membrane surface areas and dimensions of the lateral intercellular space. Impedance analysis of an in vitro airway epithelial barrier under normal, nucleotide-stimulated, and cigarette smoke-exposed conditions yielded information on how activation and inhibition of secretion occur in airway epithelial cells. Similarly, impedance analysis of primary rat alveolar epithelial cell monolayer model under control and EGTA exposure conditions indicate that EGTA causes decreases in resistances of tight junctional routes as well as apical and basolateral cell membranes without causing much change in cell capacitances. In a stretch-caused injury model of alveolar epithelium, transcellular ion transport function and paracellular permeability of solute transport appear to be differentially regulated. Finally, inhibition of caveolae-mediated transcytosis in lung endothelium led to disruption of paracellular routes, increasing the physical dimension and permeability of tight junctional region. These data together demonstrate the cross talk between transcellular and paracellular transport (function and routes) of lung epithelial and endothelial barriers. Mechanistic (e.g., signaling cascades) information on such cross talk remain to be determined.

Alloxan decreases intracellular potassium content of the isolated frog skin epithelium

Alloxan has been widely used to provoke diabetes mellitus. This compound induces necrosis of the beta-pancreatic cells and the renal tubules. However, the mechanism of this action has not been fully established. There is some evidence that this drug may act by an alteration of several ionic transport mechanisms. Nevertheless, there is scant information on the effect of alloxan on these ionic transport mechanisms of the membrane in epithelial cells. We reported that this drug induces a decrease in sodium transport in the frog skin. In order to obtain information about the mechanism involved in the sodium transport diminution provoked by alloxan, in this study the function of Na+-K+ ATPase enzyme on transepithelial sodium transport altered by alloxan is explored. We measured changes in the short circuit current and in the intracellular content of sodium and potassium under conditions of maximally stimulated enzyme activity. Short circuit current was not modified by the treatment with alloxan during the period of highest activity of the enzyme, suggesting a site of action independent of this ATPase. Cell potassium was reduced in alloxan-treated epithelia, without significant changes in Na+ content. This finding points out the existence of an alteration induced by alloxan of some modulator mechanisms of the intracellular K+ concentration. The treatment of the frog skin with cesium chloride, a K+ channel blocker, prevented the decrease of Na+ transport produced by alloxan. This result suggests an action of this diabetogenic drug on the K+ channels of the frog skin epithelium.