Chick embryos as an alternative experimental animal for cardiovascular investigations: stable recording of electrocardiogram of chick embryos in ovo on the 16th day of incubation.

Recording of electrocardiogram (ECG) tracings in developing chick embryos often fails because of spontaneous motion of the embryos in the egg shell. We attempted to record ECG of chick embryos in ovo. When we injected a mixture of 450 mg/ml urethane and 45 mg/ml alpha-chloralose into the air sac of fertile eggs at volumes of 0.1 to 0.3 ml, the spontaneous motor activity of chick embryos was decreased and stable ECG tracings could be obtained from at least 10 min after the injection. The P, QRS, and T waves were noted in the electrograms, and the QT interval was positively correlated to the RR interval. The heart rate (HR) could be analyzed for the RR interval in fertile eggs after the 8th day of incubation. The HR of the 16-day fertile embryos was linearly increased with incubation temperature in the range from 31 to 41 degrees C. Using this system, cardiac effects of some drugs were examined. Isoprenaline and acetylcholine increased and decreased the HR in a dose-dependent manner, respectively, and these effects were inhibited by respective antagonists, propranolol and atropine. These ECG responses of chick embryos were similar to those of mammals or humans. In conclusion, stable ECG tracings could be obtained from chick embryos anesthetized by urethane and alpha-chloralose in ovo and this method may be applicable for the investigation of the developing heart and the evaluation of cardiovascular drugs.