Nitric oxide/cGMP signalling mediates the cardioprotective action of adrenomedullin in reperfused myocardium

We demonstrated previously that adrenomedullin (AM), when given during early reperfusion, limited infarct size in rat heart. The present study was undertaken to provide direct evidence of the NO-dependency of AM’s cardioprotective action by assessing NO biosynthesis and involvement of the soluble guanylyl cyclase (sGC) pathway. Perfused hearts from male CD-1 mice were subjected to 30-min left coronary occlusion and 60-min reperfusion. Infarct size was determined by tetrazolium staining. AM 10 nM was administered from 20 min after coronary occlusion until 10 min after reperfusion. Coronary effluent was analysed for NO₂ ⁻ and NO₃ ⁻, and myocardial samples were analysed for NO₂ ⁻, NO₃ ⁻, nitroso-adducts and cGMP concentration. To examine the role of NO/sGC signalling in the infarct-limiting action of AM, further hearts received the sGC inhibitor ODQ 2 μM. AM treatment stimulated NO synthesis, indicated by increased NO₂ ⁻ efflux in coronary effluent throughout reperfusion (summarised as area under curve, AM 29.2 ± 3.9 vs. control 14.4 ± 2.8 μmol min² mL⁻¹, P < 0.05). AM limited infarct size (35.4 ± 2.7 vs. 12.2 ± 2.3%, P < 0.01), associated with a 2.45-fold increase (P < 0.05) in myocardial cGMP concentration at 10 min after reperfusion. ODQ abolished the infarct size-limiting effect of AM (28.9 ± 4.3%). These data provide the first evidence that AM increases NO bioavailability in intact murine myocardium and confirm that the NO/sGC/cGMP pathway is central to the cytoprotective action of AM against ischaemia–reperfusion injury.