Acetylcholine, bradykinin, opioids, and phenylephrine, but not adenosine, trigger preconditioning by generating free radicals and opening mitochondrial KATP channels

Abstract

It has been assumed that all Gi-coupled receptors trigger the protective action of preconditioning by means of an identical intracellular signaling pathway. To test this assumption, rabbit hearts were isolated and perfused with Krebs buffer. All hearts were subjected to a 30-minute coronary artery occlusion followed by 120 minutes of reperfusion. Risk area was measured with fluorescent particles and infarct size with triphenyltetrazolium chloride staining. Control hearts showed 29.1±2.8% infarction of the risk zone. A 5-minute infusion of acetylcholine (0.55 mmol/L) beginning 15 minutes before the 30-minute occlusion resulted in significant protection (9.2±2.7% infarction). This protection could be blocked by administration of 300 μmol/L N-2-mercaptopropionyl glycine (MPG), a free radical scavenger, or by 200 μmol/L 5-hydroxydecanoate (5-HD), a mitochondrial KATP antagonist, for 15 minutes beginning 5 minutes before the acetylcholine infusion (35.2±3.9% and 27.8±2.4% infarction, respectively). Similar protection was observed with other known triggers, ie, bradykinin (0.4 μmol/L), morphine (0.3 μmol/L), and phenylephrine (0.1 μmol/L), and in each case protection was completely abrogated by either MPG or 5-HD. In contrast, protection by adenosine or its analog N6-(2-phenylisopropyl) adenosine could not be blocked by either MPG or 5-HD. Therefore, whereas most of the tested agonists trigger protection by a pathway that requires opening of mitochondrial KATP channels and production of free radicals, the protective action of adenosine is not dependent on either of these steps. Hence, it cannot be assumed that all Gi-coupled receptors use the same signal transduction pathways to trigger preconditioning.