Diphtheria toxin at low pH depolarizes the membrane, increases the membrane conductance and induces a new type of ion channel in Vero cells.

Receptor-dependent translocation of diphtheria toxin across the surface membrane of Vero cells was studied using patch clamp techniques. Translocation was induced by exposing cells with surface-bound toxin to low pH. Whole cell current and voltage clamp recordings showed that toxin translocation was associated with membrane depolarization and increased membrane conductance. The conductance increase was voltage independent, with a reversal potential of approximately 15 mV. This value was unaffected by changing the Cl- gradient across the membrane and microfluorometric measurements showed that the cytosolic Ca2+ concentration was only marginally elevated by the translocation. The conductance increase is thus mainly due to monovalent cations. Exposing outside-out and cell-attached patches with bound toxin to low pH induced a new type of ion channel in the membrane. The channel current was inward at negative membrane potentials and the single channel conductance was approximately 30 pS. This value is about three times larger than for receptor-independent channels induced by diphtheria toxin or toxin fragments in artificial lipid membranes.