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Vol. 303, Issue 1, 179-187, October 2002
-Nitro-L-arginine
Hypertensive Rat
Department of Pharmacology and Toxicology, Michigan State
University, East Lansing, Michigan
Arterial hyperresponsiveness to serotonin (5-hydroxytryptamine, 5-HT)
is observed in experimental models and human forms of hypertension.
Presently, we test the hypothesis that the 5-HT2B receptor
is up-regulated and necessary for maintaining elevated blood pressure
in a rat made hypertensive by the nitric-oxide synthase
inhibitor N
-nitro-L-arginine
(LNNA; 0.5 g/l). After 2 weeks of treatment, thoracic aorta were
removed from LNNA hypertensive (systolic blood pressure = 189 ± 5 mm Hg) and sham normotensive rats (121 ± 1 mm Hg), denuded,
and mounted into isolated tissue baths for measurement of isometric
contraction. In sham tissues, 5-HT-induced contraction was mediated by
the 5-HT2A receptor as evidence by a parallel rightward
shift in response to 5-HT by the 5-HT2A/2C receptor antagonist ketanserin (10 nM) and lack of shift by the
5-HT2B receptor antagonist
6-methyl-1,2,3,4-tetrahydro-1-[3,4-dimethoxyphenyl)methyl]-9H-pyrido[3,4-b]indole hydrochloride (LY272015) (10 nM). In contrast, LY272015 produced a
4-fold rightward shift to 5-HT in aorta from LNNA hypertensive rats,
and blockade by ketanserin did not occur at low concentrations of 5-HT.
Maximal contraction to the 5-HT2B receptor agonist
1-[5-(2-thienylmethoxy)-1H-3-indolyl]propan-2-amine hydrochloride was significantly greater in LNNA hypertensive rats (percentage of phenylephrine contraction in sham = 7 ± 4, and in LNNA = 61 ± 7%). 5-HT2B receptor protein
was present in aortic homogenates from sham and LNNA rats, but the
density of 5-HT2B receptor protein in LNNA homogenates was
300% that in sham. Importantly, the 5-HT2B receptor
antagonist LY272015 reduced blood pressure of the LNNA hypertensive but
not the sham normotensive rats. Thus, these data suggest that the
up-regulated 5-HT2B receptor in the LNNA hypertensive rats
is physiologically activated to maintain elevated blood pressure.
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