Phenytoin teratogenicity: Hypoxia marker and effects on embryonic heart rhythm suggest an hERG-related mechanism

BACKGROUND The antiepileptic drug phenytoin (PHT) is a human and animal teratogen. The teratogenicity has been linked to PHT‐induced embryonic cardiac arrhythmia and hypoxic damage during a period when regulation of embryonic heart rhythm is highly dependent on a specific K+ ion current (IKr). PHT has been shown to inhibit IKr. The aims of this study were to investigate whether teratogenic doses cause embryonic hypoxia during and after the IKr susceptible period and to further characterize PHT effects on embryonic heart rhythm. METHODS Pregnant C57BL mice were administered the hypoxia marker pimonidazole followed by PHT or saline (controls) on GD 10 or GD 15. The embryos were fixed and sectioned, and the immunostained sections were analyzed with a computer assisted image analysis. Effects of PHT (0–250 μM) on heart rhythm in GD 10 embryos cultured in vitro were videotaped and then analyzed by using a digitalization technique. RESULTS PHT dose‐dependently increased the hypoxia staining (6‐ and 11‐fold after maternal dosing of 100 and 150 mg/kg, respectively) during the period IKr is expressed and functional (GD 10). In contrast, there were no differences between the PHT doses in hypoxia staining, and much less pronounced hypoxia after this period (GD 15). With increasing PHT concentrations, increased length of the interval (bradycardia) and large variations in length between individual heartbeats (arrhythmia) were recorded. CONCLUSIONS PHT induced bradycardia/arrhythmia and severe embryonic hypoxia during the IKr susceptible period, supporting the idea of an IKr‐arrhythmia‐hypoxia–related teratogenic mechanism. Birth Defects Research (Part A), 2005. © 2005 Wiley‐Liss, Inc.