The rapid component of the delayed rectifying
potassium ion current (IKr), plays an important role in cardiac repolarization. In rats, potent IKr channel blocking drugs cause similar stage-specific malformations (such as orofacial clefts and digital reductions) on gestational days (GDs) 10-14 as after periods of embryonic
oxygen deprivation (
hypoxia). The idea of a
hypoxia-related teratogenic mechanism is supported by studies using rat embryos cultured in vitro. These studies show that the embryonic heart reacts with concentration-dependent
bradycardia,
arrhythmia, and
cardiac arrest when exposed to IKr blockers on GDs 10-14. The main purpose of this study was to investigate whether previously shown teratogenic doses on GD 11 and 13 of the selective IKr blocker
almokalant (ALM) induce
hypoxia in rat embryos in vivo by using the
hypoxia marker
pimonidazole hydrochloride (PIM). Rats were orally dosed with
almokalant or tap water on GD 11 (150 micromol/kg), 13 (50 micromol/kg), or 16 (800 micromol/kg), followed by PIM intravenously 30 min later. Two hours after the PIM dose, the embryonic heart activity was videotaped and analysed, and the embryos were fixed, sectioned, and immunostained. Computer-assisted image analysis showed a two- and threefold increase in
hypoxia staining in embryos exposed to teratogenic doses of ALM on GDs 11 and 13. Embryonic
arrhythmia was observed in
almokalant groups on these GDs, but not in controls. In contrast, dosing on GD 16, with a much higher dose (800 micromol/kg), caused neither
hypoxia nor any effects on heart rhythm. The results support the IKr-related
arrhythmia-
hypoxia hypothesis, by showing that the potent IKr-blocking drug,
almokalant, (1) causes severe embryonic
hypoxia and
arrhythmia at stages (GDs 11 and 13) when developmental toxicity could be induced and IKr is functional and (2) does not cause
hypoxia or affect heart rhythm at a developmental stage when IKr is suppressed (GD 16) and potent IKr blockers do not induce developmental toxicity.