Human cerebral arteriovenous vasoactive exchange during alterations in arterial blood gases

Abstract

Evidence from animal studies indicates that various vasoactive factors, including release of norepinephrine, endothelin, adrenomedullin, C-natriuretic peptide (CNP), and nitric oxide (NO), may play a role in arterial blood gas-induced alterations in CBF. For the first time, we directly quantified exchange of these vasoactive factors across the human brain. Using the Fick principle and transcranial Doppler ultrasonography, we measured CBF in 12 healthy humans at rest and during hypercapnia (4 and 8% CO 2), hypocapnia (voluntary hyperventilation), and hypoxia (12 and 10% O 2). At each level, blood was sampled simultaneously from the internal jugular vein and radial artery. With the exception of CNP and NO, the simultaneous quantification of norepinephrine, endothelin, or adrenomedullin showed no cerebral uptake or release during changes in arterial blood gases. Hypercapnia, but not hypocapnia, increased CBF and caused a net cerebral release of nitrite (a marker of NO), which was reflected by an increase in the venous-arterial difference for nitrite: 57 Ϯ 18 and 150 Ϯ 36 mol/l at 4% and 8% CO 2, respectively (both P Ͻ 0.05). Release of cerebral CNP was also observed during changes in CO 2 (hypercapnia vs. hypocapnia, P Ͻ 0.05). During hypoxia, there was a net cerebral uptake of nitrite, which was reflected by a decreased venous-arterial difference for nitrite: Ϫ96 Ϯ 14 mol/l at 10% O 2 (P Ͻ 0.05). These data indicate that there is a differential exchange of NO across the brain during hypercapnia and hypoxia and that CNP may play a complementary role in CO 2-induced CBF changes. cerebral circulation; nitric oxide; endothelium RESPIRATORY-INDUCED CHANGES in arterial PCO 2 (Pa CO 2 ) and, to a lesser extent, PO 2 (Pa O 2 ) play a major role in cerebral blood flow (CBF) regulation (9). Elevations in Pa CO 2 (hypercapnia) lead to vasodilation of cerebral arterioles in the downstream bed and a subsequent increase in CBF, whereas a reduction in Pa CO 2 (hypocapnia) leads to vasoconstriction and a subsequent decrease in CBF (28, 65). A fall in Pa O 2 (hypoxia) below a certain threshold (Ͻ40 Torr) also produces cerebral vasodilation (19). These arterial blood gas-induced changes in CBF are mediated through changes in cerebrovascular resistance via alterations in vascular smooth muscle diameter in response to changes in the level of intracellular calcium and/or calcium sensitivity (for review see Ref. 22). Cerebrovascular tone is influenced by several chemical substances, including nitric oxide (NO). NO, synthesized from the terminal guanidino nitrogen atom(s) of L-arginine in a two-step reaction that is catalyzed by the enzyme NO synthase (NOS) (48), is an important vasoactive factor that dilates the cerebral vasculature. Under normal conditions, two isoforms of NOS are expressed in the brain, endothelium-derived NOS (eNOS) and neuronal-derived NOS (nNOS) (61), although evidence for the role of eNOS vs. nNOS in the control of CBF is controversial (3). Vasodilation evoked by NO is initiated by an increase in cGMP (7).Pharmac...

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