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Laboratory for Research in Neonatal Physiology, Department of Physiology, University of Tennessee Health Science Center, Memphis, Tennessee 38163
The hypothesis that glutamate dilates
pial arterioles of newborn pigs through the production of carbon
monoxide (CO) was addressed. Anesthesized newborn pigs were equipped
with cranial windows to measure pial arteriolar responses to stimuli.
Heme oxygenase (HO) inhibitors added topically inhibited dilation to
glutamate and to specific glutamate receptor agonists. The initial
dilation to glutamate (10
5 M) was 22% from baseline
without an inhibitor and decreased to 9% with the HO inhibitor
chromium mesoporphyrin (CrMP). Inhibition of dilation upon HO
inhibition was similar when specific glutamate receptor agonists were
employed.
RS-
-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid
caused 24% dilation from the baseline without an inhibitor, and the
dilation was decreased to 1% with tin protoporphyrin (SnPP). (RS)-2-amino-3-(3-hydroxy-5-t-butylisoxazol-4-yl)propionic acid (kainate receptors) caused dilation of 18% from baseline without an
inhibitor, but only 2% when tin mesoporphyrin was applied. 1-Aminocyclopropanecarboxylic acid
(N-methyl-D-aspartate receptors) dilated pial
arterioles 33% from baseline in control, but only to 2% in the
presence of SnPP. Neither copper mesoporphyrin, which does not inhibit
HO, nor light-inactivated CrMP affected the dilations. Furthermore,
cerebral microvessels removed from the brain produced CO (stable
isotope dilution gas chromatography-mass spectrometry), and this
production was dose dependently increased by glutamate and inhibited by
metal porphyrin HO inhibitors. These data suggest that dilation of
newborn pig pial arterioles to glutamate and specific glutamate
receptor agonists involves vascular production of CO. Additional
cerebral sources of CO also could be stimulated by glutamate and
contribute to the dilation.
heme oxygenase; cerebral blood flow; excitatory amino acids
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