The amount of acetylcholine released from innervated longitudinal muscle of the guinea-pig ileum evoked by continuous high frequency stimulation reached a peak value of 952.3 pmol · g-1 followed by a gradual decrease and formation of a new equilibrium state (568.4 pmol · g-1· min-1), which probably corresponded to the rate of synthesis. Hemicholinium-3 at a concentration of 10-6m did not inhibit the instantaneous release of acetylcholine neither from the stimulated nor from the non-stimulated organ, but completely prevented the establishment of a new equilibrium state. Although 50-70% of the acetylcholine content was still present in the tissues treated with hemicholinium-3, both the stimulation-induced and resting release was abolished. It is concluded that this part of the acetylcholine content is not available for release. When calculating the net synthesis of acetylcholine in hemicholinium-3 treated organs, 37.1 nmol · g-1 acetylcholine was missing, i.e. the synthesis was not only blocked but decomposition of acetylcholine occurred. The amount of decomposed ('lost') acetylcholine was identical both in stimulated and non-stimulated strips. It is suggested that backward reaction of choline acetyltransferase is responsible for this phenomenon.
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