Intraperitoneal exposure of whitefish to microcystin-LR induces rapid liver injury followed by regeneration and resilience to subsequent exposures

•The study presents pathological changes in whitefish during prolonged MC-LR exposure.•After early, severe injury, the damaged liver parenchyma of the fish regenerated.•Endoplasmic reticulum, cytoskeleton, and chromatin were the main targets for MC-LR.•MC-LR did not accumulate over time of exposure despite weekly-repeated injections.•Down-regulation of oatp1d expression may serve as a defence mechanism against MC-LR.

To date, there has been no systematic approach comprehensively describing the sequence of pathological changes in fish during prolonged exposure to microcystin-LR (MC-LR). Towards this aim, juvenile whitefish individuals received an intraperitoneal injection with pure MC-LR, and the injection was repeated every week to maintain continuous exposure for 28 days. During the exposure period, growth and condition of the fish were assessed based on biometric measurements. Additionally, selected biochemical markers were analysed in the fishes' blood, and their livers were carefully examined for morphological, ultrastructural, and molecular changes. The higher dose of MC-LR (100 μg·kg− 1) caused severe liver injury at the beginning of the exposure period, whereas the lower dose (10 μg·kg− 1) caused less, probably reversible injury, and its effects began to be observed later in the exposure period. These marked changes were accompanied by substantial MC-LR uptake by the liver. However, starting on the 7th day of exposure, cell debris began to be removed by phagocytes, then by 14th day, proliferation of liver cells had markedly increased, which led to reconstruction of the liver parenchyma at the end of the treatment. Surprisingly, despite weekly-repeated intraperitoneal injections, MC-LR did not accumulate over time of exposure which suggests its limited uptake in the later phase of exposure. In support, mRNA expression of the membrane transport protein oatp1d was decreased at the same time as the regenerative processes were observed. Our study shows that closing of active membrane transport may serve as one defence mechanism against further MC-LR intoxication.