Microcystin-LR: How it affects the cardio-respiratory responses to hypoxia in Nile tilapia, Oreochromis niloticus

The effects of microcystin on the cardio-respiratory function of Nile tilapia were analyzed 48 h after intraperitoneal injection of microcystin-LR (MC-LR - 100 mu g kg(-1) body weight). Exposure to MC-LR induced significant reduction in metabolic rate (((V) over dotO(2)) and increase in the critical O-2 tension (PCO2) in relation to the control group. Gill ventilation ((V) over dot(G)) and ventilatory tidal volume (V-T) were considerably lower in fish exposed to MC-LR, probably due to an alteration in the homeostatic mechanisms, impairing the regular respiratory response of this species to environmental hypoxia. The ability to maintain the O-2 extraction from the ventilatory current (EO2) during severe hypoxia was also significantly reduced in fish exposed to MC-LR exposure. Control fish displayed the characteristic reflex bradycardia in response to hypoxia. However, when compared to the control group, fish exposed to MC-LR presented significantly lower heart rate (f(H)) in normoxia and in all experimental hypoxic levels, probably due to a direct effect of this toxin on the cardiac tissue. (C) 2011 Elsevier Ltd. All rights reserved.