Sequential ultrastructural and biochemical changes induced by microcystin-LR in isolated perfused rat livers.

The cyanobacterial hepatotoxin, microcystin-LR (MCLR), is a potent protein phosphatase inhibitor that disrupts actin microfilament, cytokeratin intermediate filament, and microtubule networks in hepatocytes. To determine ultrastructural and biochemical changes that develop concurrently with microcystin-induced cytoskeletal disorganization, isolated rat livers were perfused with MCLR at 0.1 to 5.0 micrograms/ml for 5 to 40 min. Lactate dehydrogenase, alanine aminotransferase, and aspartate aminotransferase changed over time, but trends for toxin-treated and control livers did not differ. The earliest toxin-induced ultrastructural changes, observed in livers perfused at 0.1 microgram/ml for 15-20 min or at 0.3 microgram/ml for 5-10 min, were loss of hepatocyte microvilli in the space of Disse, widening of sinusoidal fenestrae, disruption of sinusoidal endothelium, dilation of bile canaliculi with loss of microvilli, and widening of hepatocyte intercellular spaces. Lesions progressed with increasing toxin concentrations and exposure times. In livers perfused with MCLR at 0.5 microgram/ml for 10-20 min, hepatocytes had plasma membrane blebs and concentric whorls of rough endoplasmic reticulum, and there was marked disassociation of hepatocytes resulting in disrupted hepatic cords. At toxin concentrations of 2.0 or 5.0 micrograms/ml for 10-20 min, there was mild dilation of mitochondrial cristae, cytoplasmic vacuolization or invagination of plasma membranes, redistribution of organelles, and sometimes nuclear degenerative change. Some hepatocytes exhibited clusters of plasma membrane blebs radiating from round cytoplasmic structures, which may be composed primarily of condensed microfilaments.