ROLE OF NONQUANTAL ACETYLCHOLINE-RELEASE IN SURPLUS POLARIZATION OF MOUSE DIAPHRAGM FIBERS AT THE END-PLATE ZONE

1. In mouse diaphragm, with intact cholinesterase (ChE), the mean valu e of the resting membrane potential was significantly higher (-84.8 +/ - 0.3 mV; mean +/- S.E.M.) at the endplate zone than in the extrajunct ional 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 abol ished within 5 min by 1x10(-6)M ouabain, indicating that it might be c aused by an electrogenic Na+-K+ pump. (+)-Tubocurarine (TC; 1x10(-5) M ) had no effect on this hyperpolarization after bath application for 1 0-20 min. 3. Short-term denervation (4 h), a slight increase of Mg2+ i n the bath of from 1 to 4 mM and application of a Ca2+-free solution f or 60 min also led to the disappearance of the surplus polarization. A ll of these factors are known to eliminate TC-induced hyperpolarizatio n in anti-ChE-treated muscles (II-effect), which is considered to be a correlate of non-quantal acetylcholine (ACh) leakage. 4. The time cou rses of the decline of the II-effect and surplus polarization after de nervation were identical. 5. In short-term denervated muscles with int act ChE, the surplus polarization was restored by 5 x 10(-8) M ACh, wh ich simulates the H-effect in anti-ChE-treated muscles. The presence o f 1 x 10(-6) M ouabain either prevented or abolished the effect of the bath-applied ACh. 6. The surplus hyperpolarization is probably caused by a small continuous leakage of ACh from the nerve terminal, which m ight escape hydrolysis by acetycholinesterase (AChE)and activate the e lectrogenic Na+-K+ pump at the subsynaptic muscle membrane.