There is indirect evidence that the plasmalemma-integrated eukaryotic porin (the voltage-dependent anion-selective channel, VDAC) functions as the outwardly rectifying chloride channel (ORCC). The channel, which is believed to play a role in cell volume regulation, appears to be relevant for cystic fibrosis (CF) therapy, in that it may function as an alternative Cl(-) channel. In the present study we showed first that Gd(3+) altered the voltage dependence of human type-1 porin incorporated into artificial planar lipid bilayers. Next, using a light-scattering approach on transformed normal or CF human B-lymphocytes in hypotonic Ringer solution, we found slightly differing regulatory volume decrease (RVD) curves for the cell lines under study. Addition of 15-60 microM GdCl3 in hypotonic Ringer increased light scattering, pointing to cell swelling beyond normal values. RVD was not observed in those experiments. A corresponding effect was seen in isotonic Ringer containing GdCl3. In either osmotic situation Gd(3+)-induced cell swelling was abolished by monoclonal mouse anti-human type-1 porin antibodies. Agonist and antibody effects were dose dependent. Finally, videocamera-monitored control experiments with adherent HeLa cells verified the direct effect of the agonist on cell swelling in hypo- or isotonic situations and its prevention by the antibodies. We conclude that GdCl3 opens plasmalemma-integrated porin channels, allowing ions to following their gradients, resulting in cell swelling. Since respiratory epithelium expresses porin channels in the apical membrane, the use of gadolinium to activate ORCC may represent a new therapeutic approach in CF.