Sex-related resistance to myocardial ischemia-reperfusion injury is associated with high constitutive ARC expression.

The female sex has been associated with improved myocardial salvage after ischemia and reperfusion (I/R). Estrogen, specifically 17beta-estradiol, has been demonstrated to mediate this phenomenon by limiting cardiomyocyte apoptosis. We sought to quantitatively assess the effect of sex, ovarian hormone loss, and I/R on myocardial Bax, Bcl-2, and apoptosis repressor with caspase recruitment domain (ARC) expression. Male (n = 48), female (n = 26), and oophorectomized female (n = 20) rabbits underwent 30 min of regional ischemia and 3 h of reperfusion. The myocardial area at risk and infarct size were determined using a double-staining technique and planimetry. In situ oligo ligation was used to assess apoptotic cell death. Western blot analysis was used to determine proapoptotic (Bax) and antiapoptotic (Bcl-2 and ARC) protein levels in all three ischemic groups and, additionally, in three nonischemic groups. Infarct size (43.7 +/- 3.2%) and apoptotic cell death (0.51 +/- 0.10%) were significantly attenuated in females compared with males (56.4 +/- 1.6%, P < 0.01, and 4.29 +/- 0.95%, P < 0.01) and oophorectomized females (55.7 +/- 3.4%, P < 0.05, and 4.36 +/- 0.51%, P < 0.01). Females expressed significantly higher baseline ARC levels (3.62 +/- 0.29) compared with males (1.78 +/- 0.18, P < 0.01) and oophorectomized females (1.08 +/- 0.26, P < 0.01). Males expressed a significantly higher baseline Bax-to-Bcl-2 ratio (4.32 +/- 0.99) compared with females (0.65 +/- 0.13, P < 0.01) and oophorectomized females (0.42 +/- 0.10, P < 0.01). I/R significantly reduced Bax-to-Bcl-2 ratios in males. In all other groups, ARC levels and Bax-to-Bcl-2 ratios did not significantly change. These results support the conclusion that in females, endogenous estrogen limits I/R-induced cardiomyocyte apoptosis by producing a baseline antiapoptotic profile, which is associated with estrogen-dependent high constitutive myocardial ARC expression.