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Abstract
The cyclic peptide toxins microcystins and nodularins are the most common and abundant
cyanotoxins present in diverse water systems. They have been the cause of human and
animal health hazards and even death. Over 60 microcystin variants have been reported
so far. We report here the results of our study on comparative toxicity evaluation
of three most predominant microcystins, MC-LR, MC-RR and MC-YR in mice. The mice were
administered one LD(50) dose of MC-LR, RR and YR (43, 235.4 and 110.6 micro g/kg body
weight, respectively), and biochemical and histological variables were determined
at 30 min post-treatment and mean time to death (MTD). Significant increase in liver
body weight index was induced by all three variants. There was marginal increase in
serum levels of hepatic enzymes viz. AST, ALT and gamma-GT at 30 min post-treatment
but 3-4 fold increase was observed at MTD. In contrast, enhanced LDH leakage, DNA
fragmentation and depletion of hepatic glutathione was observed at 30 min post treatment
in all three variants. There was no change in levels of serum protein, albumin and
albumin/globulin ratio. Liver histology showed time dependent severe pathological
lesions like congestion, haemorrhage, portal mononuclear cell infiltration and obliteration
of chromatin material. Lung lesions were predominantly in bronchi and parenchyma.
Though qualitatively lesions were identical in all three microcystin variants, degree
of liver and lung lesions varied quantitatively with the toxin. The breathing pattern
and respiratory frequency of the mice after i.p. administration of the toxin showed
uniform pattern for 90 min followed by abrupt change in the respiratory pattern and
instantaneous death. Based on biochemical and histological studies, MC-LR was found
to be the most potent toxin followed by MC-YR and MC-RR.