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Department of Pharmacology, Southern Illinois University School of Medicine, Springfield, Illinois 62794-9629
Nitric oxide (NO)
is a major transmitter in mediating cerebral neurogenic vasodilation in
several species. Recent findings have suggested that acetylcholine,
which is costored with NO in cerebral perivascular nerves, plays a role
in modulating NO release, presumably by acting on muscarinic (M)
receptors on nitric oxidergic nerve terminals. The present study was
designed using an in vitro tissue bath technique to pharmacologically
characterize the presynaptic muscarinic-receptor subtype(s) that
mediate modulation of NO release and therefore neurogenic vasodilation
and to investigate further the possible mechanisms involved in this
presynaptic modulation in porcine basilar arteries. Transmural nerve
stimulation (TNS) elicited a frequency-dependent,
tetrodotoxin-sensitive relaxation. The relaxation was abolished by
nitro-L-arginine (30 µM) and
was completely reversed by
L-arginine and
L-citrulline, but not by their D-enantiomers. Atropine
(0.01-1 µM), pirenzepine (an
M1-receptor antagonist,
0.01-1 µM), and methoctramine (an
M2-receptor antagonist, 0.01-1 µM), but not 4-DAMP (an
M3-receptor antagonist) or
tropicamide (an M4-receptor
antagonist) at concentrations as high as 10 mM, significantly increased
the TNS-elicited relaxation. This relaxation, on the other hand, was
significantly attenuated by arecaidine but-2-ynyl ester tosylate (an
M2-receptor agonist, 0.1 µM) but was not affected by McN-A-343 (an
M1-receptor agonist, 1 µM). Double-labeling immunohistochemical study demonstrated that
perivascular M2
receptor-immunoreactive fibers were completely coincident with NADPH
diaphorase fibers. Furthermore, the muscarinic receptor-mediated modulation of TNS-elicited relaxation was completely prevented by
-conotoxin GVIA (0.1 µM), a specific N-type
Ca2+ channel inhibitor, but was
still observed in the presence of tetraethylammonium (1 mM),
8-bromo-cAMP (0.5 mM), and pertussis toxin. It is concluded that the
presynaptic M2 receptors on
porcine cerebral perivascular nitric oxidergic nerves mediate
inhibition of NO release. The inhibition is due primarily to a
decreased Ca2+ influx through
N-type Ca2+ channels.
nitric oxide; presynaptic muscarinic receptor; transmural nerve stimulation; M2 receptor-immunoreactive fibers; N-type calcium channel; cerebral blood vessels; porcine
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