It has been reported that smooth muscles pre-treated with Bay K 8644 become sensitive to visible light. Intensities of 500 lx were sufficient to inhibit total mechanical activity in gastric fundus preparations from guinea pig and rats (Golenhofen K, & Noack T, Pflügers Arch 408, (1987). Later, this inhibitory effect of Bay K and light radiation was reported to be evoked by NO which is an integral part of the Bay K molecule.

5-aminolaevulinic acid (ALA) is clinically used to marker cancer cells during photodynamic therapy and to induce apoptosis in those cells. However, the apoptotic effects are limited to the upper cell layers. Recently, we have demonstrated that cell proliferation und apoptosis are linked to a balanced intracellular calcium level. Since both substances interfere with the calcium systems, the combined effects of both substances were investigated systematically in gastric fundus and antrum, bladder and the portal vein. The effects of ALA at concentrations of 0.3 mM were below 10% of the max. acetylcholine response. Application of light (up to 3 kLx) induced contractions especially in gastric antrum. These contractions persisted over the next minutes. The additional application of Bay K up to 1 mM (-enantiomer) induced inhibitions (light on) or strong contractions (light off) in all preparations. When Bay K was used in a 10-fold less concentration, the inhibitory effects during light exposure were absent and additional contractile tone was induced. The reported double-photo activation could be useful for cancer treatment, since first studies also showed, that proliferation of smooth muscle was strong inhibited during Bay K treatment and light radiation.