DEPOSITION OF BASO4 IN THE TIGHT JUNCTIONS OF AMPHIBIAN EPITHELIA CAUSES THEIR OPENING - APICAL CA2+ REVERSES THIS EFFECT

Selective deposition of BaSO4 in the tight junctions (TJs) of frog ski ns led to profound and reversible functional alterations of these stru ctures, as revealed by changes of tissue conductance (G), clamping cur rent (I), and fluxes of extracellular markers (sulfate (J(SO4)) and su crose (J(SUC))). Experiments were performed with nominally Ca2+-free s imple salt solutions on the apical side (usually KCl) and Na2SO4-Ringe r on the inner side of skins. The deposition of BaSO4 in the TJs was o btained by diffusion and/or migration through the paracellular path of Ba2+ from the apical solution and SO42- from the inner solution. A br ief presence (2 to 6 min) of apical Ba2+ (Ba2+ pulse) is followed (i.e ., when Ba2+ is removed from the apical fluid) by a large increase of G, I, J(SO4) and J(SUC), above pre-Ba2+ levels. These attain a steady state within 15 to 30 min (overshoot phase), characterizing a conspicu ous increase of the paracellular permeability. During the overshoot ph ase, a second Ba2+ pulse blocks the paracellular route while apical Ba 2+ is present, leading to a new and larger overshoot when the Ba2+ pul se is terminated. Addition of apical Ca2+ triggers the resealing of th e TJs, resulting in a full recovery of G, I, J(SO4) and J(SUC). This C a2+-induced recovery persists when apical Ca2+ is removed. The presenc e of a normal Ca2+ concentration in the inner bathing Ringer does not induce the recovery process. Tissues remain viable after being submitt ed to the Ba2+ treatment and the subsequent overshoot. Experiments per formed in the urinary bladder of Rana catesbeiana and skins and urinar y bladders of Bufo marinus indicate that Ba2+ effect can also be elici ted in these tissues. The above results seem to report general propert ies of the TJs. Incidentally, they warn about the use of Ba2+ as an io n channel blocker in epithelial membranes in association with SO42--co ntaining solutions on the contralateral side.