Microcystin-LR (MCLR) is a potent cyclic heptapeptide hepatotoxin produced by the blue-green algae, Microcystis aeruginosa. Toxic blooms of this cyanobacteria have been reported throughout the temperate world. In spite of the potential economic loss and health hazard posed by this toxin, few studies on the development of an antidote have been conducted. Thus, a number of biologically active compounds were tested in mice for effectiveness in preventing the toxicity of a lethal dose of MCLR (100 pg kg-I). Efficacy was evaluated based upon the percentage of surviving mice, time to death and serum lactate dehydrogenase activity 45 min after treatment with the toxin. The biologically active compounds were separated into groups based upon proposed mechanisms of action. Enzyme induction by phenobarbital but not by 2,3,7,8tetrachlorodibenzo-pdioxin (TCDD) resulted in partial protection against toxicity. Calcium channel blockers, free-radical scavengers and water-soluble antioxidants produced little protection against toxicity. The membrane-active antioxidants vitamin E and silymarin, as well as glutathione and the monoethyl ester of glutathione, produced significant protection from lethality. Rifampin and cyclosporin-A, both immunosuppressive and membrane-active agents, which also block the bile acid uptake system of hepatocytes, produced complete protection from the toxicity of MCLR. Thus, lipophilic antioxidants provide partial protection against MCLR toxicity while cyclosporin-A and rifampin are highly effective and potentially useful antidotes. The toxicity of MCLR may depend upon stimulation of the immune system and may be mediated by membrane alterations .
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MATERIALS AND METHODS
Animals and treatment
Female NIH non-Swiss outbred mice, weighing 20-24 g (Amitech, Omaha, NE), were used in all studies. Animals were housed under conditions of controlled temperature (24 "C) and lighting (12-h light-dark cycle) and were allowed free access to Purina Rodent Chow (Ralston Purina Co., St. Louis, MO) and tapwater. All mice were allowed to acclimatize for 3-5 days before the initiation of experiments.
Microcystin-LR was provided by Dr Wayne W. Carmichael, who isolated the toxin from cultured Microcystis aeruginosa strain 7820. The toxin had a purity of >%YO, as verified by HPLC. The MCLR was stored at -80ยฐC as a lyophilized powder. At the time of use, it was dissolved in 20% methanol in distilled