We also examined whether Gper1 expression is required for acute E2-induced improvement of heart viability by determining the myocardial infarct size in sham or hearts subjected to I/R. 3A. Hearts were sectioned and analyzed at the end of the perfusion or reperfusion periods. As expected, acute E2 treatment reduced myocardial infarct size significantly in WT animals. Supporting the functional data, acute E2 treatment had the same protective action (as in the WT animals) when hearts from Esr1and Esr2 knockouts were used but lost its protective effect in the absence of Gper1 as hearts from Gper1-/- animals displayed a robust infarcted area regardless of E2 treatment.