Impact of development and chronic hypoxia on NE release from adrenergic nerves in sheep arteries

To examine effects of development and chronic high-altitude hypoxia on symp athetic nerve function in sheep, norepinephrine release was measured in vit ro from middle cerebral and facial arteries. Capsaicin was used to test the role of capsaicin-sensitive sensory nerves; norepinephrine release was not altered by capsaicin treatment. N-omega-nitro-L-arginine methyl ester (L-N AME), an inhibitor of NO synthase, decreased stimulation-evoked norepinephr ine release in middle cerebral arteries from normoxic sheep with no effect in hypoxic arteries or facial arteries. Thus NO-releasing nerves augmented norepinephrine release. Furthermore, the function of NO-releasing nerves de clined after chronic hypoxia. Despite loss of the augmenting effects of NO, stimulation-evoked fractional norepinephrine release was unchanged after c hronic hypoxia, suggesting that middle cerebral arteries adapt to hypoxia b y increasing stimulation-evoked norepinephrine release. In fetal facial art eries, chronic hypoxia resulted in a decline in stimulation-evoked norepine phrine release, but there was an increase in the adult facial artery. In th e adult, adaptation to chronic hypoxia is similar in both cerebral and faci al arteries. However, differential adaptation in fetal adrenergic nerves ma y reflect differences in fetal redistribution of blood flow in the face of chronic hypoxia but could also possibly contribute to increased incidence o f fetal morbidity.