Chronic hypoxia alters prejunctional alpha(2)-receptor function in vascular adrenergic nerves of adult and fetal sheep

The impact of development and chronic high-altitude hypoxia on the function of prejunctional alpha (2)-adrenoceptors was studied by measuring norepine phrine release in vitro from fetal and adult sheep middle cerebral and faci al arteries. Blockade of prejunctional alpha (2)-adrenoceptors with idazoxa n significantly increased stimulation-evoked norepinephrine release in norm oxic arteries. This effect was eliminated after chronic hypoxia in cerebral arteries, with a tendency to decline in fetal facial arteries. After chron ic hypoxia, the capacity to release norepinephrine declined in fetal middle cerebral arteries with a similar trend in facial arteries. Norepinephrine release was maintained in adult arteries. During development, stimulation-e voked norepinephrine release from middle cerebral and facial arteries was h igher compared with adult arteries. In fetal arteries, adrenergic nerve fun ction declined after chronic hypoxia. However, in adult arteries, adrenergi c nerves adapted to chronic hypoxia by maintaining overall function. This d ifferential adaptation of adrenergic nerves mi fetal arteries may reflect d ifferences in fetal distribution of blood flow in response to chronic hypox ic stress.