Role of Endothelial Nitric Oxide Synthase in Hypoxia-Induced Pial Artery Dilation

Abstract

Nitric oxide (NO) contributes to hypoxia-induced pial artery dilation, at least in part, through the formation of cGMP and the subsequent release of methionine enkephalin and leucine enkephalin in the newborn pig. In separate studies, these opioids also were observed to elicit NO-dependent pial artery dilation, whereas light/dye endothelial injury reduced hypoxic pial dilation. The current study was designed to investigate the role of the endothelial isoform of NO synthase in hypoxic pial dilation, associated opioid release, and opioid dilation in piglets equipped with a closed cranial window. N-iminoethyl-l-ornithine (l-NIO) (10⁻⁶ mol/L), an antagonist that may have greater endothelial NO synthase inhibitory selectivity, had no effect on dilation elicited by hypoxia (Po₂ ≈ 35 mm Hg) (24 ± 2 versus 24 ± 2% in the absence and presence of l-NIO, respectively, n = 8). Hypoxic dilation was accompanied by increased CSF cGMP, which also was unchanged in the presence of l-NIO (394 ± 19 and 776 ± 63 versus 323 ± 13 and 739 ± 25 fmol/mL for control and hypoxia in the absence and presence of l-NIO, respectively, n = 6). Additionally, hypoxic pial dilation was associated with increased CSF methionine enkephalin, which also was unchanged in the presence of l-NIO (992 ± 73 and 2469 ± 197 versus 984 ± 18 and 2275 ± 185 pg/mL, respectively, n = 6). In contrast, methionine enkephalin–induced dilation was blocked by l-NIO (6 ± 1, 10 ± 1, and 16 ± 1 versus 1 ± 1, 1 ± 1, and 2 ± 1% for 10⁻¹⁰, 10⁻⁸, 10⁻⁶ mol/L methionine enkephalin, respectively, before and after l-NIO, n = 8). Substance P–induced pial dilation was blunted by l-NIO, whereas responses to sodium nitroprusside and N-methyl-d-aspartate were unchanged. These data indicate that endothelial NO synthase contributes to opioid-induced pial artery dilation but not hypoxia-induced dilation. Additionally, these data suggest that neuronally derived NO contributes to hypoxic pial dilation.