Role of nitric oxide, adenosine,N-methyl-d-aspartate receptors, and neuronal activation in hypoxia-induced pial arteriolar dilation in rats

In this study, we tested the hypothesis that nitric oxide (NO) and adenosine (ADO) are the principal mediators of severe hypoxia-induced vasodilation. In addition, we examined whether activation ofN-methyl-d-aspartate (NMDA) receptors and/or perivascular nerves plays a role. A closed cranial window and intravital microscopy system was used to monitor diameter changes in pial arterioles ( 40 μm) in anesthetized rats. The relative contributions of ADO, NMDA, NO, and neuronal activation to hypoxic cerebrovasodilation were assessed using the blockers 8-sulfophenyltheophylline (8-SPT), MK-801, nitro-l-arginine methylester (LNAME), and tetrodotoxin (TTX). Two experimental series were studied. In the first, we tested the effects of NOS inhibition, via topical L-NAME (1 mM), on moderate (PaO₂ ≈ 46 mmHg)then severe (PaO₂ ≈ 34 mmHg) hypoxia-induced dilation. To confirm that L-NAME was affecting specifically NO-dependent responses, we also examined, in each experiment, the vasodilatory responses to topical applications of NOS-dependent (adenosine diphosphate (ADP); acetylcholine (ACh)(and -independent (sodium nitroprusside (SNP)) agents, in the presence of L-NAME or, in controls, the presence of D-NAME or no added analogue. In the second series, topical suffusions of ADP, ADO, and NMDA were sequentially applied, followed by 5 min exposure to severe hypoxia (PaO, ≈ 32 mmHg). Following return to normoxia, a suffusion of either 8-SPT (10 μM), MK-801 (10 μM), TTX (1 μM), or 8-SPT + MK-801 was initiated (or, in controls, application of a drug-free suffusate was maintained), and the above sequence repeated. In control, TTX, and 8-SPT + MK-801 experiments, baseline conditions were then restored and hypercapnia (PaCO₂ = 70–85 mmHg) was imposed. In the series 1 control groups, moderate and severe hypoxia elicited ≈ 20% and 35–40% increases in diameter, respectively. L-NAME attenuated ADP- and ACh-induced dilations, did not alter the arteriolar responses to SNP or moderate hypoxia, but prevented further dilation upon imposition of severe hypoxia. This suggested that 45–50% of the severe hypoxia response was NO-dependent. In series 2, 8-SPT blocked the adenosine response and reduced severe hypoxia-induced dilation by 46%. MK-801 predictably blocked NMDA-induced relaxation and reduced the hypoxic response by 42%. When combined, 8-SPT and MK-801 affected hypoxic vasodilation additively. After TTX, the ADP and ADO responses were normal, but NMDA and hypoxia responses were completely blocked. Hypercapnia-induced dilation was unaffected by TTX or 8-SPT + MK-801. The results imply that severe hypoxia-induced release of NO and ADO, and the accompanying pial arteriolar dilation, are wholly dependent on the capacity to generate action potentials in perivascular nerves. The similarity of the L-NAME and MK-801 effects on hypoxic cerebrovasodilation suggests that the NO-dependency, to a large degree, derives from NMDA receptor activation.