DIFFERENCES IN THE PREJUNCTIONAL EFFECTS OF METHACHOLINE AND PILOCARPINE ON THE RELEASE OF ENDOGENOUS ACETYLCHOLINE FROM GUINEA-PIG TRACHEA

We investigated the effects of the full muscarinic acetylcholine recep tor agonist methacholine and the partial and putatively M(2)-selective agonist pilocarpine on endogenous acetylcholine release from guinea-p ig trachea by use of high-performance liquid chromatography with elect rochemical detection. Atropine-induced increases in acetylcholine rele ase were used to monitor the system. Electrical field stimulation (8 V , 30 Hz, 0.5 ms for 5 min)-induced acetylcholine release in the presen ce of neostigmine, with or without preincubation with choline to maxim ally enhance acetylcholine output, was increased to about 225% by 0.3 mu M atropine, indicating functional autoinhibition. However, methacho line (10 mu M) did not affect the acetylcholine release, whereas it wa s enhanced to 166% by 30 mu M pilocarpine. When electrical field stimu lation was applied at lower intensity (8 V, 16 Hz, 0.1 ms for 5 min) a nd in the absence of neostigmine, an increase by 0.3 mu M atropine (to 177%) but a decrease of the acetylcholine release by 10 mu M methacho line (to 65%) and 30 mu M pilocarpine (to 63%) were observed. These re sults clearly demonstrate (i) that inhibition of evoked endogenous ace tylcholine release from prejunctional nerve terminals in guinea-pig tr achea can only be demonstrated under conditions of low junctional conc entrations of acetylcholine, and (ii) that pilocarpine, as a partial m uscarinic agonist, behaves as an antagonist under high junctional conc entrations of the neurotransmitter.