Anti-oxidant effects of estrogen reduce [Ca2+]i during metabolic inhibition

We previously reported that 17β-estradiol (βE2) inhibits the rise in [Ca2+]i and [Na+]i during metabolic inhibition (MI) in mouse cardiomyocytes, but the mechanism has not yet been clarified. Estrogen has been reported to have anti-oxidant properties. We, therefore, have investigated whether interaction with the estrogen receptor (ER) is involved, or whether estrogen reduces free-radical-induced impairment of Na+-K+ ATPase in cardiac myocytes, and whether this effect reduces [Ca2+]i rise. Male mouse ventricular myocytes were studied. Flow cytometry was used with fluo-3 for [Ca2+]i measurement. Dead cells were excluded from analysis by propidium iodide fluorescence. βE2 reduced the increase in [Ca2+]i during MI even in the presence of the ER blocker tamoxifen. A similar effect on [Ca2+]i was produced by its non-estrogenic isomer, βE2-estradiol. Other hormones (estrone and estriol) with a phenolic structure also inhibited Ca2+ overload during MI, but testosterone without the structure did not. The βE2 effect was attenuated by inhibition of Na+-Ca2+ exchanger (KB-R7943) or Na+-K+ ATPase (low K+ or ouabain), but not by block of L-type Ca2+ channel (nifedipine). Tiron (4,5-dihydroxy-1,3-benzenedisulfonic acid), a superoxide scavenger, decreased the rise in [Ca2+]i and abolished the βE2 effect during MI. We conclude that the acute cardioprotective effect of estrogen during MI may be mediated by an ER-independent anti-oxidant action, which results in improved function of Na+-K+ ATPase.