Inhibitory effect and mechanism of scopolamine on respiration

In conscious rabbits, dogs and anaesthetized dogs, the respiratory frequency (FR), tidal volume (Vt) and minute ventilation (Vm) were determined in order to confirm whether the respiratory effect of scopolamine is stimulatory or inhibitory. Arterial blood samples were taken intermittently and analyzed for Po₂, Pco₂ and pH. When scopolamine was injected icv, ia and iv, the decrease of FR, Vt, Vm and Po₂ and the increase of Pco₂ were observed in two animals. When the drug was given in clinical doses used for treating respiratory failure, the effect was seen still observable. The results indicate that scopolamine inhibits respiration. Moreover, it was observed that excitatory effect of pilocarpine on respiration can be antagonized by scopolamine, but the inhibitory effect of 6β-acetoxy nortropane can not be antagonized, on the contrary, can be coordinated. The above results suggest that respiratory inhibition induced by scopolamine may be related to blocking M₁-R of the respiratory center.