Infarct size reduction by ischemic preconditioning is believed to be mediated by adenosine; however, whether adenosine is the factor responsible for the initiation of this protection remains unknown. It is possible that during preconditioning, adenosine stimulates receptors on presynaptic nerve terminals and retards the release of norepinephrine (NE) during the prolonged ischemia or that NE release during preconditioning augments adenosine production.Methods and Results
To test whether the release of NE is involved in the preconditioning phenomenon, rabbits were pretreated with reserpine (5 mg/kg sc, 24 hours before) to deplete presynaptic nerve terminals of NE stores. On the day of the experiment, the rabbits were anesthetized with ketaminexylazine and instrumented for coronary occlusion. Nonreserpinized animals were used as controls. The control group (n=7) was subjected to 30 minutes of coronary occlusion and 120 minutes of reperfusion (ischemia-reperfusion) only. The preconditioned group (n=10) received 5 minutes of preconditioning ischemia and 10 minutes of reperfusion before the prolonged ischemia-reperfusion. Of the reserpinized animals, half (n=7) received preconditioning before ischemia-reperfusion and the remaining animals (n=7) did not. At termination of the experiment, an intravenous tyramine challenge (1 mg/kg) was used to confirm NE depletion in reserpinized rabbits. The resulting infarcts were measured with tetrazolium and planimetry. With comparable hemodynamics and areas at risk, infarct size in control animals was 39.8±2.1% of the risk region. Preconditioned animals showed an expected reduction of infarct size to 14.8±2.2% of risk region (P<.05 vs control). Of the reserpinized animals, those that received reserpine alone had infarcts that were 38.5±4.5% of risk region, and those that were preconditioned had infarcts that were 41.4±3.6% of risk region, which was not significantly different than the control group.Conclusions
We conclude that preconditioning fails to protect ischemic-reperfused myocardium in reserpinized rabbit myocardium, indicating that the release ofNE during either preconditioning or prolonged ischemia is critical to preconditioning mediated protection.