By Janet Raloff, Science News
Women may want to reconsider that popular style accessory, certain hard plastic water bottles available in fashion-coordinating colors. New animal studies link the chemical bisphenol A, which leaches from such polycarbonate plastics and food can linings, with heart arrhythmias in females and permanent damage to a gene important for reproduction. Other recent research suggests that human exposure to BPA is much higher than previously thought.
In animals, fetal exposures to BPA can be especially risky, sometimes fostering brain, behavioral or reproductive problems (SN: 9/29/07, p. 202). Canada and some states are moving to ban polycarbonate plastic in baby bottles for that reason. But the new heart data suggest that even adult exposures to BPA might cause harm.
In one new study, researchers treated mice with BPA during the middle of their pregnancies. All female offspring of the treated mice suffered an irreversible genetic change in one of the “master regulatory genes” of fertility, Hugh Taylor of the Yale School of Medicine reported in June in Washington, D.C., at the annual meeting of the Endocrine Society.
This gene, HOXA10, orchestrates the activity of “hundreds — if not thousands — of downstream genes,” Taylor says. Through the genes it controls, HOXA10 helps synchronize the timing of uterine changes and ovulation. Without that synchrony, “you won’t get pregnancies,” he explains.
The HOXA10 gene lost a methyl group (a carbon bound to three hydrogen atoms), permanently altering its activity and rendering uterine tissue hypersensitive to the effects of estrogen.
That’s probably not good, Taylor says, because “many diseases we see in adults owe their origins to fetal exposures” — when genes become inappropriately modified.
In another study presented at the endocrine meeting, Scott Belcher of the University of Cincinnati and his colleagues reported that BPA boosted “pro-arrhythmic activity” in isolated muscle cells from mice and rats.
Arrhythmias, or heartbeat irregularities, are blamed for a higher mortality rate after heart attacks in premenopausal women compared with men, Belcher says.
During pregnancy, vulnerability to heart arrhythmias rises with higher estrogen levels. Belcher’s team found that in these cells, BPA’s effect on arrhythmia risk was nearly identical to estrogen’s.
In whole rat hearts exposed to BPA or estrogen, pockets of cells refused to beat in concert with others, his group showed, setting up potentially life-threatening arrhythmic events. The problem escalated dramatically when female hearts were exposed to both estrogen and BPA.
Belcher’s group traced the effect to a certain type of estrogen receptor called the beta form, which is more active (and more abundant) in women. The scientists linked this receptor’s activity to a leading cause of arrhythmias — a leak of calcium from a part of heart cells known as the sarcoplasmic reticulum.
These data suggest that at estrogen levels typically found in premenopausal women, the addition of BPA would probably spike vulnerability to arrhythmias, Belcher says. In postmenopausal women, where estrogen levels are naturally low, it’s possible that BPA might also boost arrhythmia risk. “We’re doing studies in whole animals to address that,” Belcher says.
Although a broad host of animal studies have linked BPA to adverse health effects, comparable human data do not exist. Any human risks would depend on how much BPA actually gets into the body.
To probe one BPA source, Karin Michels of the Harvard School of Public Health recruited 77 undergraduates to consume all their cold drinks for a week out of stainless steel bottles. The next week, the participants drank from polycarbonate alternatives. Michels’ team sent students’ urine samples to a lab at the Centers for Disease Control and Prevention to assay BPA levels.
Even in the first week, when drinking from steel bottles, most students showed measurable levels of BPA. Those concentrations rose by 69 percent in the second, polycarbonate week to 2.0 micrograms per gram of creatinine, a waste product in urine, the researchers report online May 12 in Environmental Health Perspectives.