Acetylcholine overflow during infusion of a high potassium —Low sodium solution into the perfused chicken heart in the absence and presence of physostigmine

Summary

• 1. The effect of infusion of a modified Tyrode's solution (“high K⁺-low Na⁺ solution”) into the isolated chicken heart on the content of acetylcholine in the tissue and the overflow of acetylcholine were compared to those evoked by vagal stimulation.

• 2. The release of acetylcholine was measured over 15-min periods of either stimulation of the vagus nerves (40 V, 1 ms) at 20 Hz or of infusion of the high K⁺-low Na⁺ solution (108 mM K⁺, 44 mM Na⁺).

• 3. Both stimuli caused a maximum overflow of acetylcholine in the first few minutes whether or not 10⁻⁶ M physostigmine was present. The overflow was maintained during the vagal stimulation at a constant rate of at least 35% the initial rate, whereas the overflow evoked by the high K⁺-low Na⁺ solution ceased after 5 min. Under the latter conditions, the size of the acetylcholine store available for release (overflow in the presence of physostigmine) was 41% of the total store.

• 4. Physostigmine (10⁻⁶ M) caused a 2–3-fold increase of the overflow of acetylcholine evoked either by nerve stimulation or by infusion of the high K⁺-low Na⁺ solution. Reduction of [Ca²⁺]_o from 1.8 to 0.2 mM strongly reduced the overflow caused by either kind of stimulation.

• 5. The sum of the overflow evoked by the high K⁺-low Na⁺ solution and the cardiac content of acetylcholine after the stimulation did not exceed the cardiac content before stimulation.

• 6. In conclusion, infusion of a high K⁺-low Na⁺ solution caused an overflow of acetylcholine in the absence and presence of physostigmine that was similar to that evoked by nerve stimulation with respect to Ca²⁺-dependency and hydrolysis of the released acetylcholine. The rate of synthesis of acetylcholine maintaining a steady state release was high during nerve stimulation, but was blocked during infusion of the high K⁺-low Na⁺ solution, presumably by block of the high affinity uptake of choline.