The action of acetylcholine upon heart rate changes markedly with development in bullfrogs.

Dose-response curves for the action of acetylcholine (ACh) and its antagonist, atropine, upon heart rate in situ were made for three larval stages and adults of the bullfrog, Rana catesbeiana. The absolute magnitude of heart rate inhibition at any ACh concentration progressively increased throughout larval development. Unexpectedly, metamorphosis to the adult resulted in a sharp decrease in cholinergic cardiac sensitivity down to levels similar to the earliest larvae. Thus, cholinergic sensitivity of heart rate, reflecting properties of the cardiac pacemaker, is greatest immediately before metamorphosis and lowest in early larvae and adults.