Photodynamic action of sulphonated aluminium phthalocyanine (SALPC) on isolated rat pancreatic acini

Abstract

The photodynamic action of SALPC has been investigated on dispersed, perifused, acini isolated from the rat pancreas. Stimulation of secretion was assessed by measuring amylase release and membrane permeabilization determined by the leakage of cytoplasmic lactate dehydrogenase (LDH) and by the efflux of 86Rb from preloaded acini. Light alone (>570 nm, ⩽ 18,400 lux), or SALPC (⩽ 1 μM) in the absence of light, had no effect on pancreatic acini but cellularly bound SALPC when illuminated caused a dose-dependent, light intensity-dependent and temperature-dependent release of amylase. Singlet oxygen generated by photon-activation of SALPC was measured by the formation of an imidazole adduct and bleaching of the secondary substrate, RNO. Whereas illumination caused a rapid increase in photodynamically-evoked pancreatic amylase release, the efflux of 86Rb and loss of cytosolic LDH were markedly delayed in onset: similar results were obtained with monochromatic laser light (633 nm). In contrast, the muscarinic agonist bethanechol evoked a rapid increase in amylase release but with an almost immediate efflux of 86Rb. Finally, electron microscopy confirmed that the structural integrity of the pancreatic acinar cells was maintained after the photodynamic action of SALPC. It is concluded that the stimulation of amylase secretion and membrane permeabilization by SALPC is due to the generation of singlet oxygen. However, the consistent difference between the time course of amylase secretion and membrane permeabilization makes it likely that an initial stage in photodynamic drug action involves oxidation of plasma membrane protein and activation of secretagogue receptors or the G-proteins and their effector systems.