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 receptor agonist methacholine and the partial and putatively M2-selective agonist pilocarpine on endogenous acetylcholine release from guinea-pig trachea by use of high-performance liquid chromatography with electrochemical detection. Atropine-induced increases in acetylcholine release were used to monitor the system. Electrical field stimulation (8 V, 30 Hz, 0.5 ms for 5 min)-induced acetylcholine release in the presence of neostigmine, with or without preincubation with choline to maximally enhance acetylcholine output, was increased to about 225% by 0.3 microM atropine, indicating functional autoinhibition. However, methacholine (10 microM) did not affect the acetylcholine release, whereas it was enhanced to 166% by 30 microM pilocarpine. When electrical field stimulation was applied at lower intensity (8 V, 16 Hz, 0.1 ms for 5 min) and in the absence of neostigmine, and increase by 0.3 microM atropine (to 177%) but a decrease of the acetylcholine release by 10 microM methacholine (to 65%) and 30 microM pilocarpine (to 63%) were observed. These results clearly demonstrate (i) that inhibition of evoked endogenous acetylcholine release from prejunctional nerve terminals in guinea-pig trachea can only be demonstrated under conditions of low junctional concentrations of acetylcholine, and (ii) that pilocarpine, as a partial muscarinic agonist, behaves as an antagonist under high junctional concentrations of the neurotransmitter.
ten Berge
R E
RE
Department of Molecular Pharmacology, University of Groningen, The Netherlands.
Weening
E C
EC
Roffel
A F
AF
Zaagsma
J
J
D000109:Acetylcholine / Q000378:metabolism*; D000818:Animals; D001285:Atropine / Q000494:pharmacology; D002851:Chromatography, High Pressure Liquid; D004558:Electric Stimulation; D006168:Guinea Pigs; D016210:Methacholine Chloride / Q000494:pharmacology*; D018721:Muscarinic Agonists / Q000494:pharmacology*; D018727:Muscarinic Antagonists / Q000494:pharmacology; D009411:Nerve Endings / Q000378:metabolism; D009469:Neuromuscular Junction / Q000187:drug effects / Q000378:metabolism; D010862:Pilocarpine / Q000494:pharmacology*; D011976:Receptors, Muscarinic / Q000502:physiology; D014132:Trachea / Q000187:drug effects*; D014630:Vagus Nerve / Q000502:physiology
D018721:Muscarinic Agonists; D018727:Muscarinic Antagonists; D011976:Receptors, Muscarinic; D010862:Pilocarpine; D016210:Methacholine Chloride; D001285:Atropine; D000109:Acetylcholine