Photofrin II sensitized modifications of ion transport across the plasma membrane of an epithelial cell line: I. Electrical measurements at the whole-cell level.

Photofrin II is a photosensitizer frequently applied in photodynamic therapy. Light-induced tumor cell inactivation observed in the presence of this substance has been suggested to start with modifications at the level of cellular membranes. In the present study electrophysiological techniques are applied in order to investigate the action of photofrin II on functional properties of the plasma membrane of opossum kidney (OK) cells (as an epithelial model system) and of fibroblasts. Illumination of the cells in the presence of photofrin II (or Zn-phthalocyanine) leads to comparatively fast depolarization of the membrane potential. It is caused by a strong change of the membrane conductance which proceeds in two phases. Both phases contribute to a loss of ion selectivity of the plasma membrane between K+ and Na+. In the first phase, specific pathways for K+, which determine the resting potential under physiological conditions, are inactivated. The second phase is distinguished by a marked increase of a nonselective conductance. The increase of the latter - after light-induced initiation - continues in the dark. The conclusions are derived from light-induced, time-dependent changes of the membrane conductance and of the shape of the current-voltage relationship detected under different experimental conditions.