Inhibitory effects of hypoxia and adenosine on N-methyl-d-aspartate-induced pial arteriolar dilation in piglets

Our previous studies have indicated that oxygen radicals, produced during reoxygenation following short-term arterial hypoxia, lead to sustained suppression of cerebral arteriolar responses to N-methyl-

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-aspartate (NMDA). However, whether arteriolar dilator responses to NMDA are reduced during arterial hypoxia has never been examined. In this study, we determined whether hypoxia or hypoxia-related metabolites such as adenosine or nitric oxide (NO) will reduce NMDA-induced arteriolar dilation. We have also determined the location of NMDA receptor- and brain nitric oxide synthase (bNOS)-positive neurons in the cerebral cortex. In anesthetized piglets, pial arteriolar diameters were determined using intravital microscopy. Baseline arteriolar diameters were 100 μm. Topical application of NMDA at concentrations of 10⁻⁵, 5×10⁻⁵ and 10⁻⁴ M resulted in dose-dependent vasodilation (9±2, 18±2 and 29±2% above baseline, respectively, n=21). Administration of theophylline (20 mg/kg, i.v.) had no effect on NMDA-dependent vasodilation, but it did block dilation to hypoxia (inhalation of 8.5% O₂). In theophylline-treated animals, NMDA responses were completely abolished during hypoxia (28±2 vs. 2±1%, respectively to 10⁻⁴ M, n=7) while sodium nitroprusside (SNP, 10⁻⁴ M) still dilated pial arterioles normally. NMDA-induced vasodilation was not modified after application and removal of adenosine (10⁻⁴ M; n=5) or SNP (10⁻⁵ M; n=4), or when SNP (10⁻⁷ M) was coapplied with NMDA (n=6). Conversely, coapplication of adenosine (10⁻⁶ M) attenuated NMDA responses (31±5 vs. 20±3%, n=7). We also found that NMDA receptor- and bNOS-containing neurons were located predominantly in layers II/III of the cortex. Proximity of these neurons to the cortical surface is consistent with diffusion of NO to pial arterioles as the mechanism of dilation to NMDA. We conclude that NMDA-induced cerebral arteriolar dilation is inhibited by hypoxia alone and by exogenous adenosine, but not by NO.