Remote preconditioning by infrarenal occlusion of the aorta protects the heart from infarction: a newly identified non-neuronal but PKC-dependent pathway.

BACKGROUND: Ischemic preconditioning is a powerful mechanism in reducing infarct size of the heart. Protection can be performed either by an ischemic stimulus of the heart itself or by ischemia of an organ distant to the heart. To address the question whether this remote preconditioning is transduced by neuronal or humoral factors an in situ model of infrarenal occlusion of the aorta (IOA) in the rat was developed. Furthermore, the signal transduction pathways of classical and remote preconditioning regarding protein kinase C, which is one of the key enzymes in classical preconditioning, were compared. METHODS AND
RESULTS: Controls (30 min regional ischemia followed by 2 h of reperfusion) had an infarct size of 62+/-5% whereas classical preconditioning reduced it to 10+/-3% of the risk zone (P< or =0.001). Fifteen minutes IOA without reperfusion of the aorta had no influence on infarct size (52+/-4%). When, however, IOA was performed for 15, 10, or 5 min, respectively, followed by a 10-min reperfusion period the size of myocardial infarction decreased significantly. This decrease was dependent on the duration of IOA (18+/-3%, 37+/-8%, 42+/-2%, respectively; P< or =0.001 for the time-dependent linear trend in decrease of infarct size). Fifteen minutes IOA showed the strongest protection which was comparable to classical preconditioning (18+/-3%, P< or =0.001 vs. control). Blockade of the nervous pathway by 20 mg/kg hexamethonium could not inhibit the protection afforded by IOA (14+/-4%). Using chelerythrine, a selective protein kinase C-inhibitor, at a dose of 5 mg/kg body weight, protection from remote (68+/-4%, P< or =0.001 vs. 15 min IOA followed by 10 min of reperfusion without chelerythrine) as well as from classical preconditioning (56+/-5%, P< or =0.001) was completely blocked.
CONCLUSION: Protection of the heart by remote preconditioning using IOA is as powerful as classical preconditioning. Both protection methods share protein kinase C as a common element in their signal transduction pathways. Since hexamethonium could not block the protection from IOA and a reperfusion period has to be necessarily interspaced between the IOA and the infarct inducing ischemia of the heart, a neuronal signal transmission from the remote area to the heart can be excluded with certainty. A humoral factor must be responsible for the remote protection. Interestingly the production of the protecting factor is dependent on the duration of the ischemia of the lower limb. The protecting substance, which must be upstream of protein kinase C, remains to be identified.