Atropine sulphate enhances neuromuscular transmission in the rat

The effect of atropine (0.001-10 mumol.l-1) on neuromuscular transmission in the rat hemidiaphragm preparation was investigated by analysing its effects on directly and indirectly-elicited twitch, tetanic, post-tetanic twitch responses and on the phenomenon of post-tetanic twitch potentiation. The effect of atropine on contractions produced by endogenous acetylcholine (ACh) or exogenous ACh (added directly into organ bath containing muscle) was studied in rat ileum. The results showed that atropine in low concentrations (1 mumol.l-1 or less), enhanced the indirectly-elicited twitch, tetanic and post-tetanic twitch responses in the rat diaphragm preparation. The mean EC50 value of atropine-induced increase in twitch tension was 0.08 +/- 0.01 mumol.l-1 (mean +/- s.e. mean, n = 6). Atropine had little effect on directly-elicited twitch tension, but in high concentrations (10 mumol.l-1 or more), it reduced the directly, and indirectly-elicited twitch contractions and produced a neuromuscular block in the rat diaphragm preparation. Atropine increased the contraction produced, in rat ileum, by endogenous ACh, i.e. ACh released from the phrenic nerve stimulated at 50 Hz for 20 s duration (control contraction: 1.3 +/- 0.1 g, contraction in atropine: 1.7 +/- 0.2 g). In contrast, atropine significantly reduced the contraction produced by exogenous ACh in the same preparation (control contraction: 3.0 +/- 0.5 g, atropine: 2.0 +/- 0.1 g), suggesting that a different mechanism may be involved in the latter effect of atropine. It was concluded that atropine, in low concentration, enhanced neuromuscular transmission, possibly via a presynaptic mechanism. In high concentration, atropine may reduce and then block transmission, possibly via pre- and postsynaptic mechanisms.