Interstitial washout and hydrolysis of acetylcholine in the perfused heart.

The efflux of acetylcholine, of radioactively labelled acetylcholine and choline, into the venous effluent of the perfused chicken heart was studied to determine the kinetics of both interstitial washout and hydrolysis of acetylcholine. Stimulation of both cervical vagus nerves (e.g., for 5 s at 40 Hz) caused a release of acetylcholine, which appeared partially unhydrolyzed in the venous effluent, and reduced force of contraction and heart rate. For comparison, labelled acetylcholine or choline was infused for 5 s into the heart and again the venous efflux of either substance was determined. It was found that the kinetics of efflux of acetylcholine or choline from the interstitial space were of first order. The mean half times were 16.2 s (after infusion of acetylcholine) and 17.9 s (after nerve stimulation) for acetylcholine and 17.9 s (after infusion of choline) for choline. In the interstitial space, radioactivity (sum of [3H]-acetylcholine and [3H]-choline formed from [3H]-acetylcholine) released by nerve stimulation declined mono-exponentially with a rate constant of 0.069 s-1 and a half time of 10 s (due to washout), whereas the concentration of unhydrolyzed [3H]-acetylcholine decreased in multi-exponential fashion due to both washout and hydrolysis. The interstitial concentration of [3H]-acetylcholine reached the 50% level after 2.5 s. In conclusion, the long persistence of unhydrolyzed acetylcholine in the interstitial space of the heart appears to be due to an apparently low rate of hydrolysis. This, in turn, is responsible for the importance of diffusion and washout of acetylcholine from the interstitial space as significant factors of synaptic removal of acetylcholine. Moreover, the results support the notion that the sustained interstitial concentration of acetylcholine determines the long duration of cardiac responses to vagal stimulation.