Endogenous catecholamines released during myocardial ischemia have been considered both to aggravate cell injury and exacerbate arrhythmias and to exert a protective action on the post-ischemic contractile function. The present work was addressed to look for evidence to explain this controversy. The effects of cardiac catecholamine depletion and of α- and β-adrenoceptor (AR) blockade on the post-ischemic contractile dysfunction, as well as its possible relationship with cardiac oxidative stress, were studied in isolated and perfused rat hearts submitted to 20 min of ischemia and 30 min of reperfusion (stunning). Catecholamine depletion improves the contractile recovery in the stunned heart. This mechanical effect was associated with decreased levels of lipid peroxidation. A similar enhancement of the contractile function during reperfusion was detected after the simultaneous blockade of α₁- and β-ARs with prazosin plus propranolol. To ascertain which specific AR pathway was involved in the effects of catecholamines on the stunned heart, selective AR blockers, prazosin (α₁-blocker), atenolol (β₁-blocker), ICI 118,551 (β₂-blocker) and selective inhibitors of G_i-PI3K pathway (pertussis toxin and wortmannin) were alternatively combined. The results indicate that catecholamines released during ischemia exert a dual action on the contractile behavior of the stunned heart: a deleterious effect, related to the activation of the β₂-AR-G_i-PI3K-pathway, which was counteracted by a beneficial effect, triggered by the stimulation of α₁-AR. Neither the depression nor the enhancement of the post-ischemic contractile recovery were related with the increase in ROS formation induced by endogenous catecholamines.Facultad de Ciencias MédicasCentro de Investigaciones Cardiovasculares