Endogenous myocardial norepinephrine is not essential for ischemic preconditioning in rabbit heart

To determine whether endogenous cardiac catecholamines mediate ischemic preconditioning (PC) in the rabbit heart, myocardial catecholamines were depleted by reserpine (5 mg/kg, 18-24 h pre-PC) or surgical sympathectomy (2 wk pre-PC). In vivo hearts were subjected to 30 min of regional ischemia and 3 h of reperfusion. PC involved either one or four cycles of 5-min ischemia and 10-min reperfusion before the 30-min ischemic period. Right ventricular norepinephrine content (pmol/mg protein), 51.4 +/- 11.1 in untreated rabbits, was reduced to 0.6 +/- 0.2 and 1.8 +/- 0.5 by surgical sympathectomy and reserpine, respectively. Infarct size (IS) was measured by tetrazolium and expressed as percentage of the risk zone. In untreated animals exposed solely to 30 min of regional ischemia IS was 35.5 +/- 1.6% and was unchanged by reserpine (43.3 +/- 5.4%) or surgical sympathectomy (33.4 +/- 3.5%). Compared with infarction in the respective non-PC controls, IS in untreated (7.4 +/- 1.5%, P < 0.0001) and surgically sympathectomized (11.2 +/- 1.5%, P < 0.0001) animals was significantly diminished by a single cycle of PC, but the latter exerted less protection in reserpinized animals (27.6 +/- 3.5%. P < 0.0025). Four cycles of PC, however, reduced IS to 10.3 +/- 1.2% in reserpinized animals. Therefore, despite comparable depression of myocardial norepinephrine content, surgical and chemical sympathectomy had different effects on the level of protection afforded by ischemic PC. These data demonstrate that endogenous myocardial catecholamines are not essential for protection from PC in the rabbit.