Hyperlipidaemia induced by a high-cholesterol diet leads to the deterioration of guanosine-3',5'-cyclic monophosphate/protein kinase G-dependent cardioprotection in rats

Background and purpose: Hyperlipidaemia interferes with cardioprotective mechanisms, but the cause of this phenomenon is largely unknown, although hyperlipidaemia impairs the cardioprotective NO-cGMP system. However, it is not known if natriuretic peptide-cGMP-protein kinase G (PKG) signalling is affected by hyperlipidaemia. Therefore, we investigated the cardioprotective efficacy of cGMP-elevating agents in hearts from normal and hyperlipidaemic rats. Experimental approach: Male Wistar rats were rendered hyperlipidaemic by feeding with 2% cholesterol-enriched chow for 12 weeks. Hearts isolated from normal and hyperlipidaemic rats were perfused (Langendorff mode) and subjected to 30 min occlusion of the left main coronary artery, followed by 120 min reperfusion. 8-Br-cGMP (CG, 10 nM), B-type natriuretic peptide-32 (BNP, 10 nM), S-nitroso-N-acetyl-penicillamine (SNAP, 1 mu M) were perfused from 10 min prior to coronary occlusion until the 15th min of reperfusion. Infarct size (% of ischaemic risk zone) was determined by triphenyltetrazolium staining. Key results: Treatment with CG, SNAP or BNP decreased infarct size significantly in normal hearts from its control value of 41.6 +/- 2.9% to 15.5 +/- 2.4%, 23.3 +/- 3.0% and 25.3 +/- 4.6%, respectively (P < 0.05). Protection by BNP was abolished by co-perfusion of PKG inhibitors KT5823 (600 nM) or Rp-8pCPT-PET-cGMPs (1 mu M), confirming its PKG dependence. In hearts from hyperlipidaemic rats, CG, SNAP or BNP failed to decrease infarct size. Hyperlipidaemia did not alter basal myocardial PKG content, but decreased its activity as assessed by phosphorylation of cardiac troponin I. Conclusions and implications: This is the first demonstration that defects in the cardioprotective cGMP-PKG system could be a critical biochemical anomaly in hyperlipidaemia.