Photosensitized oxidation of human red blood cells: Cation effects on volume changes and relevance to blood vessel occlusion

Phthalocyanines, second-generation sensitizers for photodynamic therapy, cause photohemolysis of human red blood cells. The mechanism of the processes leading to photohemolysis was studied using the change in volume of photosensitized human red blood cells, as an endpoint. The rate of increase in red blood cells volume was linearly dependent on the light fluence, following an initial lag region, and on the incubation temperature after photosensitization. The Arrhenius plot was not linear between 0°C and 40°C, indicating the involvement of more than one process. The cation present in the incubation medium was a determinant in the rate of volume increase. In the alkaline metal series, this rate increased from Li⁺ to Rb⁺. N-methylglucamine, a larger organic cation, and sucrose, in the extracellular milieu, caused no volume increase. These findings indicate selective impairment of membrane functions of red blood cells after phthalocyanine photosensitization, possibly due to the opening of selective cation channels. Photosensitization experiments in vivo in chicken comb showed blood vessels filled up with enlarged erythrocytes to the point of occlusion, thus supporting the biological relevance of the in vitro study.