Dopamine in Hypophysial Portal Plasma of the Rat During the Estrous Cycle and Throughout Pregnancy1

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Abstract
Catecholamine levels in hypophysial portal plasma were determined in pregnant and non-pregnant female rats and in intact and castrated male rats, using a radioenzymatic assay for the simultaneous determination of dopamine, norepinephrine [NE], and epinephrine [E] in 50 .mu.l of plasma. Portal and arterial blood were collected from anesthetized rats at 7 .mu.l/min for 60 min. During the collection, blood was kept at 0.degree. C, a temperature at which endogenous catecholamines were relatively stable. Dopamine was present in high concentrations in hypophysial portal plasma throughout pregnancy, attaining a level near 20 ng/ml on the 20th day of gestation. Dopamine levels in arterial plasma from the same rats were low or undetectable (0.4-0.8 ng/ml). NE and E were undetectable (< 0.6 ng/ml) in portal and arterial plasma from these rats. The major catecholamine in extracts of the hypothalamus from pregnant rats was NE, whereas that in the posterior pituitary was dopamine. Dopamine levels in portal plasma collected during proestrus, estrus, diestrus 1 and diestrus 2 were 1.32 .+-. 0.21 (mean .+-. SE), 3.87 .+-. 0.96, 3.11 .+-. 0.73, and 2.3 .+-. 0.45, respectively. Dopamine in portal plasma from intact and from castrated male rats was approximately 0.6 ng/ml. NE and E were not detectable in either portal or arterial plasma from these animals. Dopamine is secreted into hypophysial portal blood in significant quantities during pregnancy. Hypothalamic secretion of dopamine in cyclic rats is greatest during the day of estrus and early diestrus and least on the day of proestrus, and these findings support the view that dopamine of hypothalamic origin may have an important role in the regulation of anterior pituitary function.