The effects of cytotoxic substances from cyanobacteria on ionic transport processes in tilapia (Oreochromis mossambicus) were examined. Inhibitory effects on ionic transport including whole-body Ca2+ fluxes and P-type ATPases of the gill were found. The compounds tested were (1) purified microcystin-LR (MC-LR), a heptapeptide hepatotoxin produced by the cyanobacterium Microcystis aeruginosa, (2) extracts from M. aeruginosa strain PCC 7820, a strain producing MC-LR and other microcystin variants, and (3) extracts of M. aeruginosa CYA 43, a strain producing toxins including small quantities of MC-LR. Whole-body Ca2+ influx was inhibited by a 24 h exposure to extracts of M. aeruginosa CYA 43 and 7820, but not by exposure to an equivalent amount (90 mg l-1) of purified MC-LR. Shorter exposure times (4 h) were ineffective. Fish exposed to extracts from M. aeruginosa CYA 43 showed significant plasma hypocalcaemia. Both strains of M. aeruginosa inhibited Ca2+ uptake by basolateral plasma membrane vesicles (BLMVs), endoplasmic reticulum (ER) and mitochondria, as well as BLMV K+-dependent p-nitrophenol phosphatase (pNPPase) activity. The hydrophobic fractions of the cyanobacterial extracts were the most potent, inhibiting BLMV, ER and mitochondrial Ca2+ uptake by up to 99 %, but they were less inhibitory of BLMV K+-dependent pNPPase activity. Purified MC-LR was without effect on these preparations. In conclusion, cytotoxic substances from cyanobacteria have the potential to disrupt normal physiological processes dependent upon Ca2+ transport processes in tilapia gills.

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