CHAPTER 15 



Photodynamic Action and Its Pathological Effects 



N. T. Clare 

 Ruakura Animal Research Station, Private Bag, Hamilton, New Zealand 



Introduction. Nature and mechanism of photodynamic action: Definition — Mech- 

 anism of photodynamic action — Alternative hypotheses — Other factors influencing photo- 

 dynamic action — Nature of photodynamic agents — Nature of the substrate in photody- 

 namic action — Effects of photodynamic action on the skin — Distinction between sunburn 

 and photodynamic action. Investigation of photosensitivity diseases. Photosensitivity 

 in animals: Syndrome of phot osensitizat ion — Classification of diseases involving photo- 

 sensitivity — Diseases of Type I — Diseases of Type II — Diseases of Tijpe III — Photo- 

 sensitivity of uncertain etiology. Photodynamic action in human ailments. Addendum. 

 References. 



INTRODUCTION 



The recognition of photodynamic action as a distinct branch of the 

 biological effects of radiation has arisen from the observation by Raab 

 (1900) that the kiUing of infusoria by acridine was greatly accelerated 

 when the organisms were exposed to light in the presence of the dye. 

 Raab's experiments were soon extended by Tappeiner and his associates, 

 and the phenomenon was recognized as an oxidation reaction in which 

 the necessary energy was supplied from radiation absorbed by the dyes 

 introduced into the system. Jodlbauer and Busck (1905) showed that 

 pruritus and edema were produced in mice and rats exposed to sunlight 

 after injection with fluorescein dyes. These experiments gave a clue to 

 the hitherto mysterious role of sunlight in certain skin diseases of ani- 

 mals, and Busck (1905) was apparently the first to follow this lead in his 

 search for a fluorescent pigment in buckwheat as a cause of fagopyrism. 

 Over the period 1930-1940 the systematic studies of Blum and his col- 

 laborators added considerably to our understanding of the fundamental 

 mechanism of photodynamic action. The theoretical aspects of the sub- 

 ject and the existing knowledge of diseases caused by light were compre- 

 hensively surveyed in a monograph by Blum (194 la). 



Since the pubHcation of Blum's book there has been comparatively 

 little further elucidation of the principles underlying photodynamic 

 action, but much more is now known of the etiology of certain of the 

 photosensitivity diseases, particularly those occurring in domestic ani- 



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