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K Yokotani and Y Osumi
Department of Pharmacology, Kochi Medical School, Japan.
We measured endogenous noradrenaline (NA) overflow from a vascularly perfused rat stomach in vitro. The stomach was perfused with Krebs- Ringer solution containing 10 microM pargyline. Periarterial nerves, which contain postganglionic sympathetic nerves, around the left gastric artery were stimulated for 1 min with square-wave pulses of 2 msec duration, 2.5 to 5.0 Hz, supramaximal intensity (10 mA). Oxotremorine (10(-8) to 10(-6) M) concentration-dependently inhibited the periarterial nerve stimulation-evoked NA overflow under the presence of 10(-6) M phentolamine. Bilateral vagus nerve stimulation (5 Hz, 2 msec duration, 10 mA, for 1 min) reduced the evoked NA overflow. Oxotremorine (10(-7) M)-induced inhibition of NA overflow was attenuated by atropine, methoctramine (muscarinic M2 receptor antagonist), 4-diphenylacetoxy-N-methylpiperidine (M3 receptor antagonist) and pirenzepine (M1 receptor antagonist) with the following potency; atropine > methoctramine > 4-diphenylacetoxy-N- methylpiperidine >> pirenzepine. The oxotremorine-induced inhibition was attenuated by N-ethylmaleimide (3 x 10(-5) M for 50 min), but was not affected by pertussis toxin pretreatment (10 micrograms/rat, for 4 days). However, this pretreatment with pertussis toxin abolished completely negative chronotropic and inotropic effects of oxotremorine in rat atria. These results suggest that NA release from gastric sympathetic nerve terminals is inhibited by activation of muscarinic M2 receptor, and this receptor-mediated inhibitory mechanisms are insensitive to pertussis toxin.
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