Photofrin II sensitized modifications of ion transport across the plasma membrane of an epithelial cell line: II. Analysis at the level of membrane patches

In the first part of this study, photofrin II sensitized membrane modificat ions of OK-cells were investigated at the level of macroscopic membrane cur rents. In this second part, the inside-out configuration of the patch-clamp technique is applied to analyze the phenomena at the microscopic level. It is shown that the characteristic single channel fluctuations of the electr ic current disappear after the start of illumination of membrane patches in the presence of photofrin II. This holds for all three types of ion channe ls investigated: the large-conductance Ca2+-dependent K+ channel (maxi-K-Ca ), a K+ channel of small conductance (sK), and a stretch-activated nonselec tive cation channel (SA-cat). Part of the experiments show a transient acti vation of the channels (indicated by an increase of the probability in the open-channel state) before the channels are converted into a closed noncond uctive state. Inactivation of all three channel types proceeds by a continu ous reduction of their open probability, while the single channel conductan ce values are not affected. The process of photodynamically induced channel inactivation is followed by a pronounced increase of the leak conductance of the plasma membrane. The latter process - after light-induced initiation - is found to continue in the dark. The ionic pathways underlying the leak conductance also allow permeation of Ca2+ ions. The resulting Ca2+-flux ma y contribute to the photodynamically induced increase of the intracellular Ca2+ concentration observed in various cell lines.