Role of non-quantal acetylcholine release in surplus polarization of mouse diaphragm fibres at the endplate zone.

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1. In mouse diaphragm, with intact cholinesterase (ChE), the mean value of the resting membrane potential was significantly higher (-84.8 +/- 0.3 mV; mean +/- S.E.M.) at the endplate zone than in the extrajunctional area of the muscle fibres (-82.5 +/- 0.3 mV) at 22 degrees C. 2. This hyperpolarization of about 2-3 mV at the endplate zone was abolished within 5 min by 1 x 10(-6) M ouabain, indicating that it might be caused by an electrogenic Na(+)-K+ pump. (+)-Tubocurarine (TC; 1 x 10(-5) M) had no effect on this hyperpolarization after bath application for 10-20 min. 3. Short-term denervation (4 h), a slight increase of Mg2+ in the bath of from 1 to 4 mM and application of a Ca(2+)-free solution for 60 min also led to the disappearance of the surplus polarization. All of these factors are known to eliminate TC-induced hyperpolarization in anti-ChE-treated muscles (H-effect), which is considered to be a correlate of non-quantal acetylcholine (ACh) leakage. 4. The time courses of the decline of the H-effect and surplus polarization after denervation were identical. 5. In short-term denervated muscles with intact ChE, the surplus polarization was restored by 5 x 10(-8) M ACh, which simulates the H-effect in anti-ChE-treated muscles. The presence of 1 x 10(-6) M ouabain either prevented or abolished the effect of the bath-applied ACh.(ABSTRACT TRUNCATED AT 250 WORDS)

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