Comparative effects of atropine and oxotremorine on neuromuscular transmission at the rat hemidiaphragm preparation

Abstract

The effects of atropine (0.001-10 microM) and oxotremorine (0.51-51.3 microM) on neuromuscular transmission and muscle contraction were studied by analysing their effects on the twitch, tetanic and post-tetanic twitch responses and on the phenomenon of post-tetanic twitch potentiation (PTP) in a rat phrenic nerve-diaphragm preparation. In addition, the effects of atropine (0.1 microM) and oxotremorine (0.6 microM) were studied on the contractions produced in rat ileum by superfusate from repetitively stimulated phrenic nerve diaphragm preparation. The results showed that atropine had a dual action at the neuromuscular junction; at low concentrations (0.001-1.0 microM) enhanced, whereas at high concentration (greater than 1.0 microM) decreased the twitch, tetanic and post-tetanic twitch responses in the rat diaphragm preparation. Oxotremorine, on the other hand, always reduced these responses and also reduced the contractions produced by diaphragm perfusate and by exogenous acetylcholine (ACh) in the rat ileum. Since atropine enhanced the contractions, in rat diaphragm, to electrical stimulation, and those in rat ileum, to superfusate of stimulated diaphragm preparation, the results may suggest that atropine has a presynaptic action at the rat neuromuscular junction. The presence of more than one type of presynaptic muscarinic autoreceptors is discussed in relation to the results obtained.