The capacity of the hypothalamus to transport dopamine from brain tissue into hypophysial portal blood was investigated. One hour after injecting 10 μCi of 3H-dopamine into a lateral ventricle of male rats, radioactivity was present in the brain, in the anterior pituitary gland, and in systemic plasma. The highest concentration of radioactivity was found in the hypothalamus, whereas the lowest was found in systemic plasma. The cerebrum and systemic plasma contained over 50% of the dose injected, whereas the anterior pituitary gland contained no more than 0.2%. Three doses of 3H-dopamine (0.1, 1, and 10 μCi) were injected into a lateral ventricle, and hypophysial portal and arterial blood were collected at 7 μl/min for two hours. Ten min after injection, radioactivity was detected in hypophysial portal blood, reached a peak within 15–20 min, and then declined gradually to attain a concentration similar to that in arterial blood. Portal and arterial plasma and tissue extracts from rats injected intraventricularly with 3H-dopamine were subjectedto gel filtration on a Sephadex G-10 column and to paper electrophoresis. The hypothalamus as well as the cerebrum contained several radiolabeled substances one of which appeared to behave like 3H-dopamine. However, no free 3H-dopamine was evident at this time either in blood or in the ante.rior pituitary gland. The results of chromatography and electrophoresis of dialyzed portal plasma indicate that much of the radioactivity was bound to macromolecules (probably plasma proteins) larger than 10, 000–12, 000 molecular weight. When the radioactive compound was dissociated from the macromolecule with perchloric acid, the dissociated compound did not behave as free 3H-dopamine as judged by gel filtration and electrophoresis.It is concluded that a transformation of 3H-dopamine occurred during passage from theCSF to blood and that no 3H-dopamine was present in portal blood. A large portion of the radioactivity was bound to plasma macromolecules.
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