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1 Center for Perinatal Biology,
To test the hypothesis that variations in
cerebrovascular reactivity to 5-HT among arteries of different size or
type, during maturation, or during acclimatization to high altitude
involve differences in serotonergic receptor subtype, we determined
relative agonist potency orders and antagonist affinities in common
carotid (Com), main branch middle cerebral (Main), and
second branch middle cerebral (2BR) arteries from term fetal lambs and
nonpregnant adult sheep acclimatized at sea level or at an altitude of
3,820 m for
110 days. In normoxic adult Com segments, agonist
potency order was 5-hydroxytryptamine (5-HT) > 5-carboxamidotryptamine (5-CT)
8-hydroxy-2(di-n-propylamino)tetraline
(8-OH-DPAT); sumatriptan (Suma) produced no contractile response; and
antagonist dissociation constant
(pKb) values were
9.4 and 9.5 for ketanserin against 5-HT and 5-CT, 7.5 for GR-127935
against 5-HT, and 7.2 for SB-206553 against 5-HT. In normoxic adult
Main segments, agonist potency order was 5-HT > 5-CT
Suma
DPAT,
and pKb values were 9.1 and 9.2 for ketanserin against 5-HT and 5-CT and 7.4 and 8.5 for GR-127935 against 5-HT and Suma, respectively. In the 2BR segments from normoxic
adults, agonist potency order was 5-CT > 5-HT > Suma > DPAT and
pKb values were 7.4 and 7.2 for
ketanserin against 5-HT and 5-CT and 10.0 and 8.7 for GR-127935 against
5-HT and Suma, respectively. Compared with normoxic adults, none of
these values were significantly different in hypoxic adults and in
fetuses only the pKb values for
ketanserin against 5-HT in the 2BR segments (8.8) were greater. From
these results we propose that the ratio of
5-HT2 to
5-HT1 receptors is greatest in the
Com and decreases progressively to its smallest values in 2BR or
smaller segments. Because this gradient appears stable and relatively
resistant to the effects of maturation and chronic hypoxia, changes in
reactivity associated with these perturbations may involve alterations
in receptor density and/or coupling efficiency for 5-HT in
ovine cranial arteries.
5-carboxamidotryptamine; 5-hydroxytryptamine types 1 and 2; 8-hydroxy-2(di-n-propylamino)tetraline; cerebral arteries; cerebrovascular circulation; GR-127935; hypoxia; ketanserin; maturation; methiothepin; SB-206553; sumatriptan; sheep
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