Effect of potassium propionate on free and bound acetylcholine in frog muscle.

Frog sartorius muscles were homogenized under various conditions which allowed, by means of mass spectrometry, the measurement of total ACh, and different ACh compartments in the tissue: 'bound', 'free-1' and 'free-2' ACh. Bound ACh presumably corresponded to the vesicular compartment, and the free-1 and free-2 fractions to the cytoplasmic compartments of ACh. Stimulation of ACh release by La3+ ions for 60 min caused a decrease of both bound and free-2 ACh, but at 20 min bound ACh was reduced much more than free-2 ACh. Stimulation of ACh release by isotonic potassium propionate (KPr) solution for only 5 min caused a decrease of bound ACh, in contrast to free-1 and free-2 ACh which were not significantly changed. When muscles after 5 min stimulation in KPr were allowed to recover in normal Ringer, free-1 ACh did not change, but free-2 and bound ACh increased; after 180 min in Ringer bound ACh had recovered to control values. When ACh synthesis was prevented by hemicholinium-3 during recovery of the muscles in Ringer, bound ACh increased at the expense of free-2 ACh. In deuterium labeling experiments, in which the Ringer contained choline-d9, much more ACh-d9 was formed in stimulated than in unstimulated muscles. It appeared that almost all newly formed ACh was ACh-d9, since no significant synthesis of unlabeled ACh (ACh-d0) took place. Yet again, the amount of bound ACh-d0 significantly increased, apparently at the expense of preformed free-2 ACh-d0.(ABSTRACT TRUNCATED AT 250 WORDS)