Is atropine a pilocarpine antagonist in cases of eliminated parasympathetic innervation of the human parotid salivary gland?

1 Following denervation of the human parotid salivary gland, pilocarpine caused an intensified stimulant response on salivation and a stimulant effect of atropine on salivary secretion was revealed. However, despite its stimulant effect on salivary secretion, atropine retained its action in blocking the salivatory response to pilocarpine. 2 This dualism in the action of atropine is explained by an action on different muscarinic receptor sub-types, i.e. on some sub-types atropine behaves as an antagonist and on others as an agonist. 3 Under the particular conditions in which the studies were performed, pilocarpine neither prevented nor increased the subsequent paradoxical response to atropine. Moreover, when injected at the peak of the atropine salivatory response it caused neither addition nor synergism to the atropine response. 4 Following the simultaneous injection of both pilocarpine and atropine, atropine initially suppressed the effect of pilocarpine and then itself caused a powerful paradoxical salivation. 5 Pilocarpine injected at the end of the paradoxical secretory response to atropine caused no secretion indicating that atropine retained its antisecretory effect against pilocarpine. 6 The extent of pilocarpine secretory responses is dependent upon the presence or absence of atropine, whilst the atropine effect is independent of the presence of pilocarpine. This points to the presence of differing populations of cholinoreceptors to explain the agonist effects of pilocarpine and atropine.