Photodynamic damage induced by bilirubin on human platelets: possible relevance to newborn pathology

Several reports have appeared showing the possibility of bilirubin-sensitized photodamage. We have extended these observations to platelets. In the presence of 300 μM bilirubin the in vitro irradiation of isolated platelets or platelet-rich plasmas with visible light induced significant lysis as determined by the release of lactate dehydrogenase (LDH). The extent of LDH release was a function of irradiation time, being about 20% after 2 h of irradiation. A loss of membrane-bound ATPase activity was also observed at earlier times, indicating that membrane damage was preliminary to the lytic effect. The release of β-thromboglobulin, induced by close cell-to-cell contact, was lower in bilirubin- and light-treated platelets with respect to controls. Our results suggest that bilirubin may act as a photodynamic agent producing some damage on human blood platelets.