The importance of nitric oxide bioavailability and endothelial mechanisms for cardioprotection by pharmacological intervention during myocardial ischaemia and reperfusion
Sammanfattning: Myocardial ischaemia initiates complex severe cellular changes and, after a critical time period, myocardial cell death. Despite that restitution of coronary blood flow is an absolute pre-requisite for tissue survival, myocardial reperfusion per se will also increase irreversible damage in the jeopardised myocardium. Endothelial dysfunction is an early event during ischaemia and reperfusion, which is characterised by an impairment of endothelium-dependent relaxations, mainly due to reduced nitric oxide (NO) bioavailability and increased production of endothelin-1 (ET-1). Certain ET-I receptor antagonists and calcium channels blockers may in addition to their classical actions, affect NO bioavailability. The aim of the studies was to investigate the involvement and mechanisms of the endothelial factors NO and ET- I in the pathophysiology of myocardial ischaemia-reperfusion damage. Of particular interest was the importance of NO bioavailability for the cardioprotective effects of pharmacological intervention by an ETA receptor antagonist and a short-acting calcium antagonist. 1. Administration of the selective ETA receptor antagonist LU 135252 i.v. 10 min before ischaemia reduced infarct size (IS) and attenuated the increase in myocardial ET-like immunoreactivity in anaesthetised pigs subjected to 45 min of ischaemia and 4 h of reperfusion. These cardioprotective effects were abolished in the presence of NO synthase (NOS) inhibition by L-NNA and restored by addition of the NO precursor L-arginine. NOS inhibition did not affect IS per se. 2. Administration of ET- I i.v. during three 5 min infusion periods separated by 5 min before ischaemia reduced IS in anaesthetised rats subjected to 30 min of ischaemia followed by 2 h of reperfusion. This preconditioning (PC)-like cardioprotective effect was blocked either by the ETA receptor antagonist LU 135252, the nonselective ATP sensitive K+ channels (KATP) antagonist glibenclamide or the selective mithochondrial KATP channel antagonist 5-hydroxydecanoic acid. 3. Intracoronary administration of short-acting calcium antagonist clevidipine during the last 10 min of ischaemia and the first 5 min of reperfusion reduced IS and preserved endothelium-dependent vasodilation induced by substance P in anaesthetised pigs. The cardioprotective effect of clevidipine was abolished when it was administered together with the NOS inhibitor L-NMMA. Addition of L-arginine restored cardioprotection induced by clevidipine. The cardioprotective effect of clevidipine was also abolished, when it was given together with the bradykinin B2 receptor antagonist HOE 140. Addition of the NO donor S-nitroso-N-acetyl-D, L-penicillamine (SNAP) restored cardioprotection. 4. Intracoronary administration of the NO donor SNAP in a narrow concentration range for a brief period before ischaemia reduced IS, attenuated myeloperoxidase activity as index of neutrophil accumulation in the ischaemic/reperfused myocardium and improved endothelium-dependent coronary vasodilation induced by substance P in anaesthetised pigs. In conclusion, the cardioprotective effect of the ETA receptor antagonist during ischaemia and reperfusion is mediated via a mechanism related to no; under certain conditions exogenous ET-1 may also exert cardioprotection in vivo via the ETA receptor and activation of mitochondrial KATP channels; the cardioprotective effect of the short-acting calcium antagonist clevidipine during late ischaemia and early reperfusion is mediated via bradykinin- and NO-related mechanisms; NO exerts a classic PC-like cardioprotection against ischaemiareperfusion injury in vivo in a narrow concentration range.
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