Bisphenol A (BPA) is an industrial chemical widely present in plastics. It is one of the worst contaminants in groundwater in the USA, with water levels at 0.14 microg/L compared to 0.0047 in Germany. BPA is found in fish, feeding bottles, beverage containers and the milk of nursing mothers. Furthermore, BPA levels are 5 times higher in amniotic fluid than in maternal plasma. Urinary testing shows that adults are exposed to 7 micrograms of BPA per day, and 6-month old infants are exposed to 66 micrograms of BPA per day.

Bisphenols harm humans by interfering with normal hormonal function. They are called endocrine disruptors (EDs). BPA shows estrogenic activity and may interfere with reproduction. BPA can interfere with thyroid hormone levels, and laboratory animal embryos exposed to BPA had abnormal development and increased mortality rates. Sodium/potassium pump activity is needed for the action of BPA.

The authors chose to study Xenopus* and zebrafish embryo organ development when exposed to BPA. At higher doses, it was found that zebrafish embryos exposed to BPA during development had abnormalities of the movement sensing mechanism of the inner ear (otolith**). Earlier exposure produced more severe abnormalities. As well, abnormalities of the balance mechanism (semicircular canals) were seen.

Xenopus embryos were also studied in BPA solutions to determine if there were abnormalities of otolith development; it was found that there was reduced development of Xenopus otoliths and a severe reduction of development of semicircular canals.

Researchers were able to demonstrate that BPA was able to act on estrogen receptors, however the otolith abnormalities were not mediated primarily by estrogen receptors. Still, modulation of estrogen receptors can alter the BPA effect on the otolith. Ouabain does reverse the BPA effect. Most bisphenols have an effect on otolith development, especially BPA and BPE. The effects found in aquatic species are, probably, not relevant for humans.

CONCLUSION: BPA alters otolith development in zebrafish and in Xenopus frogs early in embryo development. The BPA does not cause this through estrogen-receptor mechanisms, but may be altering the sodium/potassium pump activity.

NOTE: *Xenopus is an aquatic frog.

**Otoliths are tiny structures in the inner ear which are responsible for balance. They are sensitive to gravity and to horizontal movements.

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PMID: 21269433.

Summary #659.