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

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 1.46+/-0.30 mV when the activity of the Na-K pump was blocked by 2 x 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 ATPase increased the leakage whereas potentiation of the pump activity decreased it.