Cardioprotective mechanisms by inhibition of the HMG-CoA reductase pathway and stimulation of peroxisome proliferator-activated receptors in myocardial ischaemia-reperfusion

Sammanfattning: Acute myocardial ischaemia causes metabolic changes and results in a rapid decrease in the energy available to the cell. This leads to cell injury that, depending on the length of the ischaemic time, is reversible or irreversible. Restoration of coronary flow to the jeopardized myocardial tissue, referred to as reperfusion, is generally a prerequisite for tissue survival, but may also per se contribute to the injury. Endothelial dysfunction, characterized by an impairment of endothelium-dependent relaxation, due to reduced bioavailability of endothelial nitric oxide (NO), is an early event in the pathophysiology of myocardial ischaemia-reperfusion injury. Endothelial dysfunction is also important in other cardiovascular disorders such as atherosclerosis, hypertension, and diabetes. It has been suggested that drugs used in the treatment of these disorders also exert cardioprotective effects. Inhibitors of 3 hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase (statins) and agonists of peroxisome proliferatoractivated receptor (PPAR) -alpha and -gamma may protect the myocardiurn against ischaemiareperfusion injury in addition to their respective lipid-lowering and insulin-sensitizing effects.The aim of the present thesis was to investigate the cardioprotective mechanisms of drugs used for treatment of hyperlipidaemia and insulin resistance. Of particular interest was to study the mechanisms related to NO bioavailability. 1. Oral pretreatment with the HMG-CoA reductase inhibitors rosuvastatin and pravastatin for 5 days, without affecting serum cholesterol, reduced infarct size (IS) in pigs subjected to 45 min of ischaemia followed by 4 h of reperfusion. Myeloperoxidase (MPO) activity, an index of neutrophil infiltration, was attenuated in the ischaemic/reperfased myocardiurn by pretreatment with rosuvastatin. The rosuvastatin-induced cardio protection and attenuation of MPO activity in the ischaemic/reperfused myocardium were abolished by NO synthase (NOS) inhibition. The expression of endothelial NOS (eNOS) and inducible (iNOS) proteins in the myocardiurn were not altered by rosuvastatin. 2. Intraperitoneal (i.p.) administration of rosuvastatin for 48 h reduced IS in rats subjected to 30 min of ischae mia followed by 120 min of reperfusion. Rosuvastatin also increased the cytosol/membrane ratio of RhoA protein in the myocardiurn from rats not subjected to ischaemia-reperfusion. Addition of the isoprenoid geranylgeranyl pyrophosphate (GGPP) abolished both the rosuvastatin-induced cardioprotection and the inhibition of RhoA translocation. 3. Intravenous (i.v.) administration of the PPAR-alpha agonist WY- 14643 (WY) 3 0 min before the onset of ischaemia reduced IS in rats subjected to 30 min of ischaemia followed by 120 min of reperfusion. WY also in creased eNOS, but not iNOS, protein expression and decreased endothelin- I mRNA levels in the ischaemic myocardium. Co-administration of WY with the NOS-inhibitor L-NNA abolished the cardioprotective effect of WY, but did not alter the PPAR-alpha-agonist-induced changes in eNOS protein or endothelin-1 mRNA in the ischaemic myocardium. 4. The PPAR-gamma agonist rosiglitazone increased the recovery of the left ventricular function and the coronary flow following global ischaemia in the isolated hearts from wild-type mice, but suppressed the recovery of myocardial function in eNOS knockout mice. Addition of the NOS inhibitor L-NNA significantly blunted the improvement in the recovery of the left ventricular function induced by rosiglitazone in the wild-type mice. Rosiglitazone induced eNOS phosphorylation at Ser1177 and Thr495 without affecting total eNOS expression. There was no effect of L-NNA on the expression of phosphorylated eNOS. In conclusion, endothelial dysfunction characterized by decreased NO bioavailability, is an important component in the pathophysiology of myocardial ischaemia-reperfusion injury. Cardioprotection induced by inhibition of HMGCoA reductase and PPAR-alpha and -gamma stimulation is mediated via a mechanism involving maintenance of NO bioavailability. Thus, maintained bioavailability of NO is a key cardioprotective mechanism of the lipid-lowering statins and fibrates, and the insulin sensitizing thiazolidinediones.