Non-quantal release of transmitter at mouse neuromuscular junction and its dependence on the activity of Na+−K+ ATP-ase

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Abstract
The postjunctional membrane of mouse diaphragm fibres was hyperpolarized by 1–2 mV during local curarization of the end-plate zone in the presence of anticholinesterase. In a solution containing 5 mM K+, the mean hyperpolarization was 1.0±0.35 mV. Hyperpolarization was 1.43±0.30 mV when the activity of the Na−K pump was blocked by 2×10−5 M ouabain and 1.82±0.43 mV when it was blocked by bathing the muscle in a K+-free medium. Reactivation of membrane ATP-ase by addition of potassium after a period in K+-free medium reduced the hyperpolarization to zero, when measurements were made 10–20 min after the readdition. It is concluded, that spontaneous non-quantal leakage of acetylcholine occurs at the mouse neuromuscular junction, as it does in the frog /2/. Conditions which block the Na+−K+-dependent ATP-ase increased the leakage whereas potentiation of the pump activity decreased it.