Effects of a Hepatic Toxin from the Cyanophyte Microcystis Aeruginosa

Abstract

Microcystis aeruginosa is commonly involved in freshwater blooms and one of its toxins (microcystin) causes liver damage in birds and mammals. This study determined the site of action of microcystin and characterized the hepatic damage at the ultrastructural level. Histological changes in centrilobular regions of liver tissue were noted after i.p. administration in mice. Hepatic sinusoids expanded, parenchymal cords disintegrated, and cells lysed resulting in extensive tissue damage and subsequent death within one hour. Ultrastructural studies showed that sinusoidal epithelium and hepatocyte plasma membranes ruptured with the release of cellular components that pooled with blood. Mitochondria appeared swollen but there were no obvious distortions of organelles. Extensive vesiculation of membrane fragments was observed. Hepatic damage caused hemorrhaging into the liver where blood and cell debris accumulated and produced a significant increase in liver weight.

Primary cultures of hepatocyte microexplants from pre- and postnatal mice and rats were exposed to microcystin but in vitro effects were not observed. In vivo experiments demonstrated that young animals were not sensitive to the toxin but developed sensitivity as they matured.

Microcystin was shown to be very specific in its site of action since it had no antibiotic activity against green algae, yeast, or bacteria and was non-toxic to certain Zooplankton, crayfish, amphibians and teleosts. Electrical or mechanical activity in isolated nerve, nerve-muscle and cardiac preparations also was not effected. The toxins employed in this study came from cultured cells and naturally occurring blooms and grave identical results in all experiments.