A study has been made of the effect of acetylcholine upon the heart rate of the early chick embryo, and it has been noted that under certain experimental conditions acetylcholine stimulates the heart rate during the period of vagal innervation, while depressing the heart rate after the nerve supply is established. Isolated chick embryo hearts freely suspended in a phosphate-buffered (bicarbonate-free) medium beat at only about one-third their in ovo rates, and acetylcholine produces biphasic effects of stimulation and inhibition of the heart rate depending upon the concentration of the drug and the age of the hearts, the maximal stimulation occurring at five days. When suspended in Krebs bicarbonate medium the hearts beat at rates of about 72 to 74% their in ovo rates, and acetylcholine produces only slowing of the heart rate. This effect of acetylcholine can be diminished or reversed when an inhibitor of endogenous acetylcholine, hemicholinium-3 is added. We have postulated that both inhibitory and stimulatory receptors for acetylcholine exist in the early chick embryo heart and that the response to added acetylcholine depends in part on levels of endogenous acetylcholine already present in the heart. The role of endogenous acetylcholine in regulating heart rate is discussed.
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