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

We previously reported that 17beta-estradiol (betaE2) inhibits the rise in Ca(2+) and Na(+) 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 Ca(2+) rise. Male mouse ventricular myocytes were studied. Flow cytometry was used with fluo-3 for Ca(2+) measurement. Dead cells were excluded from analysis by propidium iodide fluorescence. betaE2 reduced the increase in Ca(2+) during MI even in the presence of the ER blocker tamoxifen. A similar effect on Ca(2+) was produced by its non-estrogenic isomer, betaE2-estradiol. Other hormones (estrone and estriol) with a phenolic structure also inhibited Ca(2+) overload during MI, but testosterone without the structure did not. The betaE2 effect was attenuated by inhibition of Na(+)-Ca(2+) exchanger (KB-R7943) or Na(+)-K(+) ATPase (low K(+) or ouabain), but not by block of L-type Ca(2+) channel (nifedipine). Tiron (4,5-dihydroxy-1,3-benzenedisulfonic acid), a superoxide scavenger, decreased the rise in Ca(2+) and abolished the betaE2 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.