ESR1 and ischemia: In this study, we discovered that in male mice: 1) Gper1 activation is essential for the cardioprotective action of acute E2 on the perfused heart subjected to I/R, and that Esr1 and Esr2 are dispensable to this effect; and 2) although both PI-3K/Akt and MEK1/2/ERK1/2 pathways are initially (prior ischemia) triggered by E2-Gper1 activation, only the MEK1/2/ERK1/2 pathway transcends in mediating I/R cardioprotection via GSK3-β phosphorylation and regulation of the mPTP opening.