Effect of divalent cations on acetylcholine release from cardiac parasympathetic nerve endings

The influence of several divalent cations on acetylcholine (ACh) release from vagus nerve terminals was studied in quiescent guinea-pig auricles. ACh release was induced by stimulus trains subthreshold for excitation of atrial cells. The ACh released evoked a hyperpolarization of the atrial cell membrane. Changes in ACh sensitivity of the atrial cell membrane were tested by application of ACh-containing solutions. ACh release increased with the external Ca concentration. External Sr maintained the ACh release in 'Ca-free' media. However, Sr ions were less effective than Ca ions. External Mg acted as a weak inhibitor of the stimulus evoked ACh release, whereas Co and Mn ions exerted a strong inhibition.