Oestrogen-mediated cardioprotection following ischaemia and reperfusion is mimicked by an oestrogen receptor (ER)α agonist and unaffected by an ERβ antagonist

Oestrogen protects the heart from ischaemic injury. The current study aims to characterise two novel oestrogen receptor (ER) ligands, an ERα agonist ERA-45 and an ERβ antagonist ERB-88, and then use them to investigate the roles of ERα and ERβ in mediating the cardioprotection by E from ischaemia–reperfusion injury in the rat. The ER ligands were characterised by gene transactivation assay using ER-transfected Chinese hamster ovary (CHO) cells and in bioavailability studies in vivo. Female rats (n=48) were ovariectomised and implanted with 17β-oestradiol (17βE(2)) releasing or placebo pellets. ERA-45, ERB-88 or vehicle was administered for 5 days prior to ischaemia–reperfusion studies. Necrosis, neutrophil infiltration (myeloperoxidase activity) and oxidant stress production (electron paramagnetic resonance) from the area-at-risk were measured to assess reperfusion injury. The ERα agonist ERA-45 showed more than 35-fold selectivity for ERα compared with ERβ gene transactivation. In vitro, the ERβ antagonist ERB-88 inhibited transactivation by 17βE(2) via ERβ with 46-fold selectivity relative to inhibition via ERα. In vivo, 17βE(2) significantly reduced neutrophil infiltration, oxidant stress and necrosis following ischaemia and reperfusion. Cardioprotection by 17βE(2) was not inhibited by ERB-88 but was completely reproduced by ERA-45. In conclusion, protection of the rat heart after ischaemia–reperfusion by 17βE(2) is achieved through the reduction of cardiomyocyte death, neutrophil infiltration and oxygen-free radical availability.The results of this study indicate that these effects are primarily mediated via activation of ERα.