Dopamine in Hypophysial Portal Blood: Relationship to Circulating Prolactin in Pregnant and Lactating Rats*

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Abstract
The relationship between dopamine (DA) concentration in hypophysial portal blood and that of PRL in systemic blood during gestation and lactation was investigated. PRL level in plasma collected by decapitation was low during the 18–21 days of gestation, increased on day 22, and remained elevated throughout the first 10 days of lactation. Separation of mothers from pups for 24 h resulted in a marked reduction in plasma PRL concentration. DA, but not norepinephrine or epinephrine, was higher in portal than in arterial plasma in all rats. DA in portal plasma of 18- to 21-day pregnant rats [4.9 ± 0.7 ng/ ml, (mean ± SEM)] was significantly higher than that in portal plasma from 1- to 5- and 6- to 10-day lactating rats (2.9 ± 0.3 and 2.7 ± 0.4 ng/ml, respectively). Separation of mothers from pups for 24 h resulted in a significant elevation of portal plasma dopamine compared with separation for 30 min. No differences were found in hypothalamic, anterior, or posterior pituitary catecholamine concentrations as a result of pup separation. In contrast to plasma collected by decapitation, the PRL level in plasma collected from femoral cannulae of anesthetized rats before removal of the pituitary was low, without apparent differences among the various groups of rats. Therefore, the effects of anesthesia and surgical trauma on catecholamine and PRL secretion were examined. Pentobarbital anesthesia and surgical trauma caused marked changes in circulating norepinephrine and epinephrine but not DA concentrations. In vitro incubation of dispersed rat pituitary cells with 100 μg/trA sodium pentobarbital resulted in a 50% inhibition of PRL secretion;, incubation with DA at a range of 1.5–15 ng/ml resulted in 30–75% inhibition, respectively. The following conclusions were reached. 1) Under physiological conditions of low PRL release, hypothalamic DA secretion is elevated, and when release is elevated, DA secretion is reduced. 2) DA levels in hypophysial portal blood are well within the range of concentrations which are very effective in inhibiting PRL secretion in vitro. 3) The inability to show reciprocal relations between DA in portal blood and PRL in systemic blood of the same rats might be due to the direct inhibition of PRL secretion from pituitary lactotrophs by pentobarbital rather than action via the hypothalamic dopaminergic system. (Endocrinology106: 690, 1980)