Hemodynamic responses to alpha-adrenergic blockade during hypoxemia in the fetal lamb.

The mechanisms responsible for the increase in umbilical venous and hepatic vascular resistance during hypoxemia are poorly understood. To assess the relative importance of alpha-adrenergic receptors, we produced an acute, severe hypoxemia in chronically instrumented fetal sheep. While fetal arterial oxygen saturation was maintained at the same level, we then injected phentolamine, a selective alpha-adrenoreceptor blocker. We found that the hypoxemia-induced vasoconstriction of the umbilical veins and hepatic vasculature was reversed by alpha blockade. Thus, alpha-adrenergic stimulation is necessary to maintain vasoconstriction of the umbilical veins and hepatic vasculature during acute fetal hypoxemia. Furthermore, alpha-adrenergic stimulation is responsible for the hypoxemia-induced vasoconstriction of the gut, spleen, and lower carcass. Thus, the alpha-adrenergic system mediates important fetal hemodynamic adaptations to acute hypoxemia. However, the alpha-adrenergic system is not responsible for the hypoxemia-induced constriction of the renal vasculature.