ALPHA-ADRENOCEPTOR STIMULATION WITH EXOGENOUS NOREPINEPHRINE OR RELEASE OF ENDOGENOUS CATECHOLAMINES MIMICS ISCHEMIC PRECONDITIONING

Background Brief episodes of ischemia induced by proximal coronary artery occlusion can precondition the myocardium. Whether other stressful stimuli have the potential to protect the myocardium from subsequent ischemia remains controversial. Methods and Results To study the hypothesis that transient alpha-adrenoceptor stimulation mimics preconditioning, for 5 minutes we administered 0.25 mu g.kg(-1).min(-1) norepinephrine or saline 10 minutes before a 30-minute coronary occlusion and 4 hours of reperfusion in an in vivo rabbit model. The area of necrosis (AN) and area of risk (AR) were measured. We found that norepinephrine pretreatment caused a reduction in infarct size when compared with controls (AN/AR, 0.17+/-0.04 versus 0.31+/-0.04; P<.02). Ischemic preconditioning also reduced infarct size (AN/AR, 0.22+/-0.03). The protection observed with norepinephrine treatment was entirely eliminated by pretreatment with cu-adrenergic blockade using prazosin (AN/AR, 0.42+/-0.06). Tyramine, an agent that causes release of endogenous catecholamines, was administered (1.5 mg/kg IV) 10 minutes before coronary occlusion in another group of rabbits. Tyramine pretreatment resulted in a smaller infarct size than in untreated controls (AN/AR, 0.16+/-0.04 versus 0.41+/-0.07; P<.01). Both norepinephrine and tyramine caused an increase in systemic arterial pressure during infusion; tyramine also increased heart rate, In rabbits pretreated with prazosin, heart rate and systemic pressure during the norepinephrine infusion were similar to baseline values. During coronary occlusion, the degree of ischemia was similar in all groups. Conclusions Exposure of the heart to either transient exogenous norepinephrine or endogenous release of norepinephrine and/or other catecholamines by tyramine can mimic the effects of ischemic preconditioning in rabbits.