The effect of atropine on the frog sartorius neuromuscular junction.

1. The effect of atropine sulphate and of (+)-tubocurarine chloride (TC) on the amplitude and time course of end-plate potentials (e.p.p.s) and miniature end-plate potentials (m.e.p.p.s) was studied in the sartorius muscle of the frog.2. Atropine sulphate reduces the amplitude of intracellularly recorded e.p.p.s and m.e.p.p.s, in concentrations 100 times higher than TC (6 x 10(-5)M for 50% reduction of amplitude compared with 6 x 10(-7)M for TC).3. Atropine sulphate causes a marked shortening of both e.p.p.s and m.e.p.p.s: when the amplitude of e.p.p.s or m.e.p.p.s is reduced by 50%, their rise-time and half-decay time are both shortened by 40%. The corresponding shortening produced by TC is 15%.4. E.p.p.s prolonged by prostigmine 10(-6) g/ml. undergo a larger shortening (30%) in TC, while atropine-induced shortening related to the corresponding drop of amplitude is the same whether prostigmine is used or not.5. On repeated applications after recovery of amplitude and time course, TC loses its shortening effect on e.p.p.s while the atropine shortening effect remains unchanged.6. Atropine sulphate shortens the rise-time but not the falling phase of brief depolarizations produced by electrophoretic applications of acetylcholine (ACh) to the muscle fibre surface in the end-plate region. It also reduces their amplitude, in the same way that it reduces the amplitude of e.p.p.s.7. Atropine sulphate in concentrations which markedly reduce the amplitude and time course of e.p.p.s has no effect on their quantum content.8. Atropine sulphate at a concentration of 10(-4)M does not change the amplitude and time course of an electrotonic potential produced by a rectangular current pulse passed through the end-plate region of a muscle fibre.9. It is suggested that either enhanced removal of ACh or a spatial gradient of effectiveness of the blocking drugs, or both these mechanisms, participate in shortening the e.p.p. by atropine sulphate and TC.