Release of endogenous acetylcholine from a vascularly perfused rat stomach in vitro; inhibition by M3 muscarinic autoreceptors and alpha-2 adrenoceptors.

We measured endogenous overflow of acetylcholine (ACh) from a vascularly perfused rat stomach in vitro with modified Krebs-Ringer solution containing 100 microM physostigmine. Evoked ACh overflow by vagal stimulation at 2.5 Hz for 2 min was abolished by tetrodotoxin (3 x 10(-7) M) or Ca++ removal and reduced by hexamethonium (10(-4) M). The evoked overflow was inhibited by oxotremorine (10(-7) to 10(-5) M), but not attenuated by 4-(N-[3-chlorophenyl]carbamoyloxy)-2-butynyltrimethylammonium chloride (10(-6) and 10(-5) M). The evoked ACh overflow was enhanced by atropine (10(-9) to 10(-6) M), 4-diphenylacetoxy-N-methylpiperidine (10(-8) to 10(-6) M), methoctramine (10(-8) to 10(-6) M) and pirenzepine (10(-8) to 10(-6) M) in a concentration-dependent manner with the following potency; atropine > 4-diphenylacetoxy-N-methylpiperidine > methoctramine > pirenzepine. In the presence of 10(-6) M atropine, clonidine (10(-8) and 10(-7) M), but not phenylephrine (10(-7) and 10(-6) M), concentration-dependently decreased the evoked overflow of ACh at 1 Hz. Electrical stimulation of periarterial gastric sympathetic nerves (5 and 10 Hz for 2 min) frequency-dependently inhibited the evoked ACh overflow at 1 Hz in the presence of atropine and sympathetic inhibition mediated at 5 Hz was abolished by 10(-6) M rauwolscine, but not by 10(-6) M prazosin. These results indicate that the ACh release from gastric parasympathetic neurons is inhibited by M3 muscarinic autoreceptors and alpha-2 adrenoceptors.