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 of N-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 (approximately 40 microns) 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 (L-NAME), 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 approximately 46 mmHg) then severe (PaO2 approximately 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 (PaO2 approximately 32 mmHg). Following return to normoxia, a suffusion of either 8-SPT (10 microM), MK-801 (10 microM), TTX (1 microM), 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 approximately 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.