Resistance of noradrenaline in blood vessels to depletion by 6-hydroxydopamine or immunosympathectomy.

1. The degree of the decrease in the noradrenaline concentrations caused by 6-hydroxydopamine or immunosympathectomy was different in different areas of the cardiovascular system.2. In rats or guinea-pigs 6-hydroxydopamine depleted the noradrenaline content of the heart by 90%, of the mesenteric vein by 80% and of the mesenteric artery and aorta by 30-60%. Immunosympathectomy elicited a 70% reduction in the cardiac noradrenaline but only a 50% reduction in the noradrenaline of the blood vessels of the rat.3. The tyrosine hydroxylase activity of the heart, blood vessels, or adrenal glands was not significantly altered 2 weeks after 6-hydroxydopamine. Nor was the monoamine oxidase activity in heart or blood vessels changed.4. The inconsistent ability of both 6-hydroxydopamine and immunosympathectomy to abolish experimental hypertension may be due to the partial persistence of noradrenaline and functional sympathetic nervous system activity in the blood vessels.