Photodynamic action inhibits compound 48/80-induced exocytosis in rat peritoneal mast cells

Photostimulation of sulfonated aluminum phthalocyanine (SALPC)-loaded mast cells (20,000 lux, 2 min) itself caused neither exocytosis nor [Ca2+]i increase in isolated rat peritoneal mast cells. This result is incompatible with that reported in other cell types such as pancreatic acinar cells. Stimulation with 50 micrograms/ml compound 48/80, a direct G-protein activator, induced massive exocytosis which was easily detectable under conventional microscope. The fluorescent granules stained with sulforhodamine B were found to be numerous on the perimetry of mast cells, confirming occurrence of exocytosis. The stimulation also increased [Ca2+]i and cell volume before initiation of exocytosis. Pretreatment of the cells with photodynamic action with 5 microM SALPC inhibited the compound 48/80-induced exocytosis, but the [Ca2+]i increase and the increase of cell volume were unaffected. NaN3 at 0.5 mM could relieve the photodynamic action-induced inhibition of exocytosis. These results indicate that, unlikely to other secretory or contractile cells, photodynamic action with SALPC does not directly affect exocytotic machinery but modulates some functional proteins involved in signal transduction process which may be posterior to G-protein activation in mast cells. Singlet oxygen may be involved in the photodynamic action-induced modulation. A possible target protein can be a protein in the cell membrane which binds with a protein of a granular membrane during the course of exocytosis.