Vascular resistance was measured after acute surgical denervation in the blood-perfused hind limbs of adult normal rats and rats which had been treated with 6-hydroxydopamine in the first 2 weeks of postnatal life. Vascular resistance was significantly higher in the limbs of 6-hydroxydopamine treated rats. 2. No excess of vasoconstrictor materials was detected when blood from 6-hydroxydopamine treated rats was used to perfuse the hind limbs of untreated rats or other 6-hydroxydopamine treated rats. 3. Hind limb blood vessels of 6-hydroxydopamine treated rats showed increased sensitivity to normal amounts of circulating vasoconstrictors of adrenal origin and to intra-arterial infusions of noradrenaline and angiotensin. 4. Infusion of desipramine into the hindlimb blood vessels of 6-hydroxydopamine treated rats did not increase vasoconstrictor responses to infused noradrenaline. Infusion of desipramine into the limbs of untreated rats increased responses to noradrenaline but these never attained the magnitude and duration of responses seen in limbs of 6-hydroxydopamine treated rats. 5. The increased sensitivity to noradrenaline of hindlimb vessels from 6-hydroxydopamine treated rats could partly be accounted for by an absence of neuronal uptake sites and partly by the development of non-specific post-junctional supersensitivity.
