Multi-xenobiotic-resistance a possible explanation for the insensitivity of bivalves towards cyanobacterial toxins.

Filterfeeders, such as bivalves, are highly affected during toxic cyanobacterial blooms, as they are non-selective and may use the cyanobacteria as main nutrition source. The freshwater mussel Dreissena polymorpha, living in lakes and rivers coexisting with cyanobacteria, was exposed to 100 microg L(-1) microcystin-LR (MC-LR) for up to three days. MC-LR concentration in mussel tissue and surrounding media was quantified by HPLC-PDA during uptake and depuration phase, revealing an immediate, continuous uptake, and release of non-metabolized toxin, and occurrence of reincorporation. The involvement of multi-xenobiotic-resistance protein (P-glycoprotein, P-gp) on the excretion of MC-LR was evidenced by efflux and accumulation version of the Rhodamine Assay as well as on P-gp gene expression. P-gp expression was enhanced after 1 h exposure but no changes were detected after longer (72 h) exposure. P-gp enzyme activity showed a significant increase with exposure time, supporting the hypothesis that P-gp is involved in the excretion of MC-LR. Induction of biotransformation enzyme such as pi-class glutathione S-transferase (piGST) and antioxidant enzyme catalase (CAT) was immediately inhibited and returned to control values only after more than 72 h expose time. Heat shock protein 70 (hsp70) and protein phosphatase 2A (PP2A) gene expression was not changed due to the treatment with cyanobacterial toxin MC-LR.