The effects of endogenous and exogenous catecholamines on hypoxic cardiac performance in red‐bellied piranhas

Catecholamines protect the fish heart during hypoxia. However, the humoral adrenergic stress response may only be invoked in extremis. We investigated the hypothesis that endogenous (e.g., neuronal) myocardial catecholamines may also impact cardiac performance during hypoxia in a hypoxia‐tolerant tropical fish, the red‐bellied piranha ( Pygocentrus nattereri ). First, we measured endogenous tissue catecholamines and in vitro catecholamine release from piranha myocardium using ultraperformance liquid chromatography. Ventricle homogenates contained detectable levels of both adrenaline (7.27 ng/g) and noradrenaline (14.48 ng/g), but only noradrenaline was released from ventricular tissue incubated in Ringer’s solution. Noradrenaline released in this assay was not affected by hypoxia but was promoted by the catecholamine releasing agent tyramine. Our second series of experiments explored cardiac contractile performance in vitro using tyramine, exogenous noradrenaline or adrenaline, and propranolol (a β‐adrenoceptor antagonist). In ventricular strip preparations, β‐adrenergic blockade with propranolol had no effects on twitch force or contraction kinetics in either normoxia or hypoxia, confirming that spontaneous endogenous catecholamine release did not impact cardiac performance. However, in the absence of propranolol, tyramine mimicked the positive inotropic effect of noradrenaline (10 µM) during hypoxia, although adrenaline was capable of generating larger effects. Our results suggest that, although it is not spontaneously released, inducible endogenous noradrenaline release may have a significant β‐adrenoceptor‐dependent impact on hypoxic performance in the fish heart.