Plasma lactate and stress hormones in common carp (Cyprinus carpio) and rainbow trout (Oncorhynchus mykiss) during stepwise decreasing oxygen levels

By measuring the lactate response it is possible to determine whether a teleost is able to adapt to a certain oxygen level. It is hypothesized that recovery will occur at oxygen levels above the critical oxygen level (PO2)(crit) reflected by a transient lactate increase. In contrast, continuous lactate accumulation occurs at oxygen levels below the (PO2)(crit), which will be lethal in case of prolonged exposure. Since catecholamines as well as cortisol increase the availability of glucose, it is expected that these stress hormones are involved in the activation of the anaerobic metabolism. Common carp and rainbow trout were cannulated and exposed to stepwise decreasing oxygen levels. At each oxygen level blood samples were taken at several time-points and analyzed for plasma lactate, adrenaline, noradrenaline and cortisol. The results show that both individual and inter-specific differences in lactate response occur during exposure to hypoxia. These differences can be associated with observed differences in behaviour. Whereas carp stayed quiet during the hypoxia treatment, trout displayed individually different behaviour. In contrast to the passive responders, the active responding trout did not survive as a result of continuous lactate accumulation. Interestingly, both in carp and trout a strong correlation exists between the lactate and catecholamine levels. This may indicate that these stress hormones are of importance for the metabolic changes occurring during anaerobic activation.