Hemodynamic effects of acetylcholine in the chick embryo and differences from those in the rat embryo

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Abstract
It has been reported that acetylcholine induces cardiac anomalies in the chick embryo. Thus, we studied hemodynamic effects of this drug in the chick embryo and also compared them with those in the rat embryo since we found that the effect of caffeine was different between the chick and rat embryos. Acetylcholine was given at doses of 5, 0.5, and 0.05 μg into the vitelline vein in chick embryos at Hamburger–Hamilton stage 21 and at a dose of 0.5 μg into the placenta in rat embryos at gestational day 12. In the chick embryo, heart rate was reduced to 91, 88, and 87% of control at the end of injection of 0.05, 0.5, and 5 μg, respectively, then returned to the baseline level. Vitelline arterial blood pressure was 110% of control with 0.05 μg, 134% with 0.5 μg, and 142% with 5 μg at 1 min after injection. The dorsal aortic blood flow decreased with time after injection, but it was increased only by a 5 μg dose at the end of injection. The vascular resistance increased in a dose‐dependent manner. In the rat embryo, the change of heart rate was qualitatively similar to that of the chick embryo. The blood pressure did not change significantly. The blood flow velocity at the outflow tract decreased at the end of injection, which indicated the decrease in cardiac output, along with slowing of heart rate, then returned to the control level thereafter. We conclude that acetylcholine has a vasoconstrictive effect at this stage of the chick embryo and its teratogenicity may not be explained by an hemodynamic effect based on a one‐to‐one relation of cause and effect and that there is species difference in hemodynamic effects but significance of this result is to be clarified.