Changes in total and quantal release of acetylcholine in the mouse diaphragm during activation and inhibition of membrane ATPase.

1. Acetylcholine (ACh) released from mouse diaphragm was gel filtrated and estimated by bio‐assay and compared with electrophysiologically measured quantal release, expressed either as frequency of miniature end‐plate potentials or quantum content of end‐plate potentials. 2. Activation of Na+‐K+‐dependent membrane ATPase (membrane ATPase) in Na+‐loaded muscles lowered the total amount of ACh released at rest to one tenth of the control value, but quantal release remained unchanged. 3. Inhibition of membrane ATPase by 2 X 10(‐5) M‐ouabain or by K‐free solution led to an increase in total release and to a delayed progressive increase in quantal release. When Ca2+ was removed only the total release was enhanced. 4. Depolarization of the diaphragm by 8, 11 and 14 mM‐K increased both total and quantal release only in the presence of Ca2+ in the perfusion medium. When Ca2+ was removed, no significant increase in release was observed. 5. The total and quantal release in response to 2 Hz stimulation of the preparation was increased 1.4 and 45 times, respectively. It is concluded that the total amount of ACh released at rest consists of two fractions, quantal and non‐quantal, the former representing about 1% of the total release.

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