Abstract : In the present study, we investigated the efficacy of pharmacological postconditioning induced by 17beta-estradiol and the phytoestrogen, genistein, against myocardial infarction induced by increasing durations of coronary artery occlusion (CAO). Anesthetized rabbits underwent either 20-min (protocol A) or 30-min (protocol B) CAO, followed by 4 h of coronary artery reperfusion (CAR). Before CAR, they randomly received an intravenous injection of either vehicle (control), 100 or 1000 microg/kg genistein (Geni(100) and Geni(1000), respectively), or 100 microg/kg 17beta-estradiol (17beta-E(100)). In protocol A, infarct size was significantly reduced in Geni(100) (n = 6), Geni(1000) (n = 6), and 17beta-E(100) (n = 6) versus control (n = 9) (6 +/- 2, 15 +/- 4, and 11 +/- 3 versus 35 +/- 5%, respectively). In protocol B, none of these drugs reduced infarct size versus control. Western blots demonstrated an increase of Akt phosphorylation in the Geni(100) and 17beta-E(100) hearts submitted to 20-min CAO but not to 30-min CAO. The selective GSK3beta inhibitor SB 216763 (0.2 mg/kg) [3-(2,4)-dichlorophenyl)-4(1-methyl-1H-indol-3-yl)-1H-pyrrole-2,5-dione] did not exhibit cardioprotection at this dose, but its administration restored the cardioprotective effect of genistein and 17beta-estradiol with 30-min CAO. Administration of 5-hydroxydecanoate (5 mg/kg) abolished the cardioprotective effects of Geni(100) and 17beta-E(100) alone with 20-min CAO and also those observed when combined to SB 216763 with 30-min CAO. Thus, pharmacological postconditioning with genistein and 17beta-estradiol is limited by a "ceiling effect of protection" along with a loss of Akt phosphorylation. However, this ceiling effect is reversed by concomitant inhibition of GSK3beta by SB 216763 through opening of mitochondrial ATP-dependent potassium channels.