Orlando, FL - An unexpected finding in pig coronary arteries suggests a mechanism that might explain why, after so much work showing estrogen has beneficial cardiovascular effects, it was actually harmful in controlled randomized trials in postmenopausal women.

Researchers at the Medical College of Georgia report that when the activity of nitric-oxide synthase (NOS) that produces nitric oxide is blocked, the administration of estrogen causes not vasodilation, but vasoconstriction. Theoretically, since NO production is reduced in older women, giving postmenopausal women estrogen might in this context be expected to have detrimental cardiovascular effects such as those seen in trials like the Heart Estrogen/Progestin Replacement Study (HERS) and the Women's Health Initiative (WHI), said lead author Dr Richard White (Medical College of Georgia, Augusta).

Their study was presented recently at the American Heart Association Second International Conference on Women, Heart Disease, and Stroke.

White and colleagues have been studying basic mechanisms related to estrogen for about 10 years, he told heartwire. Until about 2002, when the WHI findings were released, it had looked as though estrogen was a "cardiovascular magic bullet," he said, preserving cardiac function, lowering blood pressure, and reducing LDL cholesterol. The results in clinical trials were hard to reconcile with the previous work, White said. They went back to the basic mechanisms to see whether they could find anything that might explain these contradictory findings.

In their own lab, they have been working with pig arteries that are similar to human coronary arteries. It had been previously reported and was fairly well known, White said, that estrogen had beneficial effects on vasodilation and blood-pressure lowering by acting to stimulate nitric oxide production in the artery wall. In one experiment, White blocked the action of nitric-oxide synthase, the enzyme that produces NO, then added estrogen, with the expectation that nothing would happen, just to confirm that NO was an important mediator of vasodilation in response to estrogen.

"But when I did this, I actually got a contraction," White said. "It didn't just blunt the effect of estrogen, it turned into a contraction. I didn't believe it—it was totally unexpected."

They repeated the experiment, with the same outcome. They found that the response was mediated through the production of superoxide in lieu of nitric oxide when the activity of nitric-oxide synthase was blocked. Further investigation into the literature on NOS showed that it is "sort of a quirky enzyme," he said, apparently requiring the presence of a certain biochemical environment to produce NO. Among these biochemical cofactors are two called L-arginine and tetrahydrobiopterin, both of which are known to downregulate as a woman ages, just as estrogen does.

These findings then could explain the "Jekyll-and-Hyde" nature of estrogen before and after menopause, he said. "In premenopausal women, when estrogen is given it does what it always does, which is to stimulate NOS to produce nitric oxide and you get positive effects. But as a woman ages and becomes postmenopausal, and those biochemical cofactors are no longer present, estrogen does harmful things—it produces superoxide."

"Antioxidants help prevent aging," he added, "but now in older women, you can almost think of estrogen as inducing oxidation and enhancing the aging process."

In WHI, for example, women who took estrogen were found to have more dementia than those not on treatment. Since NOS is also produced by vessels in the brain and other organs, there is reason to believe that the same dual effect of estrogen would be true in these other systems, he pointed out, although of course this would have to be confirmed.

However, their findings do suggest a way—perhaps by supplementing some of these essential cofactors as well—that the beneficial effects of estrogen seen in premenopausal women might be reproduced in postmenopausal women.

"One of the things this means is that menopause is a good thing, a sort of revolutionary endocrinology idea," White pointed out. "Menopause is adaptive because a woman is not supposed to have as much estrogen when she gets older because it can kill her."

White and colleagues recently received a $1.2-million, four-year grant from the National Heart, Lung, and Blood Institute to pursue their findings, a release from the Medical College of Georgia notes. They plan this time to block the effects of L-arginine and tetrahydrobiopterin to confirm their role in the interaction between estrogen and NOS.

Dr David Herrington (Wake Forest University Baptist Medical Center, Winston-Salem, NC), principal investigator of the HERS trial and a WHI investigator, reviewed the abstract for heartwire. "From what I had to review, it sounds as if they have made some observations that might help explain the lack of a cardioprotective effect of estrogen in older women," he said. "In fact, their observations are consistent with earlier work showing no effect of estrogen on endothelial function in older women."

However, he cautioned that while in vitro experiments in porcine arteries may suggest interesting new hypotheses, "ultimately, these ideas will need to be confirmed or refuted in women before we can draw definitive conclusions about their implications for clinical use of HRT."