Effect of Topically Administered Epinephrine, Norepinephrine, and Acetylcholine on Cerebrocortical Circulation and the NAD/NADH Redox State

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Abstract
We investigated the effects of topically administered catecholamines and acetylcholine (ACh) on the cerebrocortical microcirculation and NAD/NADH redox state in chloralose-anesthetized cats. NADH fluorescence of the brain cortex and the volume of small intracortical vessels were measured by fluororeflectometry, and in most of the experiments the pial vessels were photographed simultaneously through a cranial window. Cerebrocortical vascular volume (CVV) and the diameter of the pial vessels were decreased, and NADH was oxidized by concentrations of epinephrine and norepinephrine as low as 3 × 10−8 M. Pial veins constricted approximately twice as much as pial arteries. ACh dilatated pial arteries, slightly constricted pial veins, and increased CVV, but had no effect on the NAD/NADH redox state. Since pial and intracortical vessels were constricted markedly by catecholamines, and since these vascular reactions appeared at a lower concentration than is presumed to occur in the synaptic cleft, our results support the regulating role of these substances in cerebral circulation. NADH oxidation, obtained with catecholamines, was interpreted to be due to enhanced tissue respiration. The finding that ACh dilatated pial arteries and increased CVV, but failed to influence the NAD/NADH redox state, might indicate that the brain cortices of normal animals are bioenergetically nonhypoxic. If cortical microregions where the oxygen tension is close to zero were biochemically hypoxic, NADH oxidation should have occurred during ACh administration.