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Department of Anesthesiology and Critical Care Medicine and Department of Pediatrics, Johns Hopkins Medical Institutions, Baltimore, Maryland 21205
Glutamine is
purported to inhibit recycling of citrulline to arginine and to limit
nitric oxide release in vitro. However, vasoactive effects of glutamine
have not been clearly demonstrated in vivo. During hyperammonemia,
impaired cerebrovascular reactivity to CO2 is related to
glutamine accumulation. We tested the hypotheses that 1)
glutamine infusion in the absence of hyperammonemia impairs cerebrovascular CO2 reactivity and 2) arginine
infusion preserves CO2 reactivity during glutamine infusion
and during hyperammonemia. Pentobarbital sodium-anesthetized rats were
equipped with a closed cranial window for measuring pial arteriolar
diameter. Intravenous infusion of 3 mmol · kg
1 · h
1
of L-glutamine for 6 h produced threefold increases in
plasma and cerebrospinal fluid concentrations. Dilation to hypercapnia was reduced by 45% compared with that of a time control group at 6 h
but not at 3 h of glutamine infusion. Coinfusion of 2 mmol · kg
1 · h
1
of L-arginine with glutamine maintained the hypercapnic
vasodilation at the control value. Infusion of ammonium acetate at a
rate known to produce threefold increases in cortical tissue glutamine
concentration resulted in no significant hypercapnic vasodilation.
Coinfusion of arginine with ammonium acetate maintained hypercapnic
vasodilation at 60% of the control value. Arginine infusion did not
augment hypercapnic vasodilation in a control group. We conclude that glutamine modulates cerebrovascular CO2 reactivity in vivo.
Glutamine probably acts by limiting arginine availability because the
vascular inhibitory effect required >3 h to develop and because
arginine infusion counteracted the vascular effect of both endogenously and exogenously produced increases in glutamine.
ammonia; carbon dioxide; cerebral blood vessels; nitric oxide; rat
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