In the preceding publication we have reported that, contrary to the prevailing opinion in the literature, the tight-junction tightness of Necturus gall bladder epithelium is not up-regulated by cAMP-mediated or by Ca2+-mediated stimulation. This conclusion was based on our observation that the stimulant-induced increase in transepithelial resistance (R,) occurred only when the lateral intercellular spaces were allowed to collapse, which suggested that the increase reflected primarily or exclusively the increasing resistance of the lateral spaces (R(lis)) rather than the postulated increase in tight-junction resistance (R(j)). An alternative explanation could have been that the constancy of R(t) after space dilatation reflected an increase R(j) that was masked by a concomitant fall in apical and basolateral cell membrane resistances R(a) and R(bl). To decide between those possibilities we have performed impedance measurements with transepithelial and intracellular microelectrodes on Necturus gall bladder epithelium. Applying previously developed analysis procedures, the measurements readily showed that elevation of intracellular Ca2+ concentration increased R(lis) but left R(j) as well as R(a) and R(bl) quasi constant. Experiments with forskolin, theophylline or isobutylxanthine, on the other hand, were less clear. These stimulants activated an apical Cl- conductance, which drastically reduced R(a) and apparently caused low-frequency polarization effects that could not be accounted for by the classical epithelial equivalent circuit. After elimination of the polarization phenomena by uni- or bilateral substitution of Cl- by isethionate or sulphate, however, we were able to demonstrate that R(j) remains constant under cAMP-mediated stimulation irrespective of whether the lateral spaces are kept open or are allowed to collapse. We conclude that the tight-junction resistance of Necturus gall bladder epithelium is not controlled by intracellular Ca2+ or by cAMP-mediated stimulation.
