The effects of adrenergic blockade on fetal response to hypoxia.

The actions of the adrenergic blocking agents propranolol and phentolamine upon the responses of 124-135 days fetal sheep to hypoxia induced by causing pregnant ewes to breathe 9% O2 and 3% CO2 in N2 have been studied. During hypoxia fetal heart rate fell and any tendency for this to return was prevented by propranolol and stimulated by phentolamine. The ability of the fetal heart rate to return during hypoxia appears to be related to the rise in plasma catecholamines. Hypoxia induced increases in plasma ACTH and cortisol and in plasma metabolites appear to have the same characteristics as those changes induced by catecholamine infusion; the former being largely an alpha-receptor effect and the latter being beta-receptor mediated. The results indicate but do not prove that many of the fetal responses to hypoxia could be caused by the rise in plasma catecholamines.