Efeitos da microcistina-LR (Microcystis aeruginosa) sobre parâmetros cardio-respiratórios de tilápia-do-nilo (Oreochromis niloticus)

Abstract

Cyanobacteria, in favorable conditions can form large superficial masses of green color called blooms. Some genera of cyanobacteria are producing toxins, and during the bloom, there may be risks of contamination of water bodies and thus of those organisms that depends on this. Among the toxins produced by cyanobacteria, stands the microcystin. This type of toxin has pronounced effects on the liver, although it may show its effects in several organs. Moreover, it is the cyanotoxin which is more widely distributed in various types of ecosystems. The aim of this study was to examine the cardiorespiratory responses of Nile tilapia, Oreochromis niloticus, in control situation and 48 h after intraperitoneal injection of microcystin-LR (MC-LR - 100 μg.Kg-1 body weight) submitted to gradual hypoxia. Exposure to MC-LR decreased metabolic rate ( VO2 ) and significantly increased the critical O2 tension (PcO2), reducing the ability to maintain constant the 2 VO . The gill ventilation (Vg) increased significantly and progressively in both experimental groups in order to maintain 2 VO during hypoxia. Both groups showed the same pattern of respiratory response, while increases in G V& were characterized by increases in tidal volume (VT), while respiratory frequency (fR) remained little changed. However the magnitude of this response was markedly lower in the group exposed to MC-LR, showing that the toxin somehow interfered in the homeostasis of the animal preventing it from developing a respiratory response normal. The MC-LR also reduced the ability to extract O2 during acute hypoxia, probably due to its action injuring the gill tissues of infected animals. Heart rate (fH) was significantly reduced by microcystin-LR in all the experimental concentrations of O2. The control group developed bradycardia only in tension of 10 mmHg, while the group exposed to MC-LR has exhibited such a response to 20 mmHg. The direct effect of MC-LR on the cardiac tissue was probably the cause of the decrease in heart rate in the exposed group compared to the control and relative to their normoxic values. Together, the results of this study showed that exposure to MC-LR reduces the chances of survival of the species studied in hypoxic environments.