Abstract

Appreciable concentrations of norepinephrine were found in the blood vessels of the rat, rabbit and guinea pig. Rat aorta had the lowest concentration of norepinephrine (0.4 µg/g), the guinea-pig and rabbit aorta had higher levels, 1 to 2 µg/g. The content in the mesenteric and renal arteries, the vena cava and mesenteric vein exceeded 1.5 µg/g. An uneven regional distribution was noted; the distal portions of the aorta and mesenteric artery had twice the content of the proximal tissue. After inhibition of the enzyme monoamine oxidase, the rat mesenteric artery and vein retained large amounts of norepinephrine whereas the aorta and heart did not, suggesting a relative absence of binding sites in the rat heart and aorta as compared to the mesenteric bed. Since blood vessels extracted less than one-fourth as much norepinephrine-³H from the circulation as the heart, it is possible that in the vasculature the reuptake mechanism may play an auxiliary role in the disposition of norepinephrine. Reserpine (1.5 mg/kg) decreased the content of norepinephrine throughout the cardiovascular system, depleting the heart to the greatest extent.