Permanent Photodynamic Cholecystokinin 1 Receptor Activation: Dimer-to-Monomer Conversion

The G protein-coupled cholecystokinin 1 receptor (CCK1R) is activated permanently by type II photodynamic action (i.e., by singlet oxygen) in the freshly isolated rat pancreatic acini, in contrast to reversible activation by CCK. But how CCK1R is photodynamically activated is not known. Therefore, in the present work, we subjected membrane proteins extracted from isolated rat pancreatic acini to photodynamic action with photosensitiser sulphonated aluminium phthalocyanine (SALPC), and used reducing gel electrophoresis and Western blot to detect possible changes in CCK1R oligomerization status. Photodynamic action (SALPC 1 µM, light 36.7 mW cm^- 2 × 10 min) was found to convert dimeric CCK1R nearly quantitatively to monomers. Such conversion was dependent on both irradiance (8.51-36.7 mW cm^- 2) and irradiation time (1-20 min). Minimum effective irradiance was found to be 11.1 mW cm^- 2 (× 10 min, with SALPC 1 µM), and brief photodynamic action (SALPC 1 µM, 36.7 mW cm^- 2 × 1 min) was effective. Whilst CCK stimulation of purified membrane proteins alone had no effect on CCK1R dimer/monomer balance, sub-threshold photodynamic action (SALPC 100 nM, 36.7 mW cm^- 2 × 10 min) plus CCK revealed a bell-shaped CCK dose response curve for CCK1R monomerization, which was remarkably similar to the dose response curve for CCK-stimulated amylase secretion in isolated rat pancreatic acini. These two lines of evidence together suggest that during photodynamic CCK1R activation, CCK1R is permanently monomerized, thus providing a unique approach for permanent G protein-coupled receptor (GPCR) activation which has not been achieved before.