Concentrations of dopamine and noradrenaline in hypophysial portal blood in the sheep and the rat

Abstract

The concentrations of dopamine, noradrenaline and their respective primary neuronal metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and 3,4-dihydroxyphenylethyleneglycol (DHPG) were measured in the hypophysial portal and peripheral plasma of sheep and rats by combined gas chromatography-mass spectrometry. Hypophysial portal and jugular blood samples were taken at 5- to 10-min intervals for 3-7 h from six conscious ovariectomized ewes. Blood was also collected for 30 min under urethane anaesthesia from the cut pituitary stalk from 16 pro-oestrous female and five intact male rats. In ovariectomized ewes, noradrenaline concentrations were higher in hypophysial portal plasma than in peripheral plasma (6.6 +/- 0.8 vs 2.2 +/- 0.4 nmol/l). In contrast, dopamine was undetectable (less than 1 nmol/l) in the portal and peripheral plasma of all ewes. Plasma levels of DOPAC and DHPG in portal and jugular samples were similar. In all pro-oestrous female rats, plasma concentrations of dopamine were higher in portal blood than in jugular blood (8.0 +/- 1.4 vs 4.8 +/- 0.6 nmol/l). Detectable concentrations of dopamine were measured in the portal plasma of two out of five male rats. Noradrenaline concentrations were higher in portal plasma than in peripheral plasma of both female (8.3 +/- 1.7 vs 3.7 +/- 0.6 nmol/l) and male (14.8 +/- 2.7 vs 6.1 +/- 1.2 nmol/l) rats. These data show that noradrenaline, but not dopamine, is secreted into the long portal vessels in sheep. The results suggest that there are species differences in the secretion of hypothalamic dopamine into hypophysial portal blood.

Monoamine Content of the Stalk‐Median Eminence and Hypothalamus in Adult Female Sheep as Affected by Daylength

“…In male lambs, castration or immunoneutralization of oestradiol are able to induce a modification of both LH secretion and DA content in the SME without altering PRL secretion (11). In adult ewes, stalk section only induced a transient increase in PRL release (19,20) and D A was absent from the capillaries of long portal vessels (21). In sheep, this amine could reach the pituitary through a more dorsal or posterior route as suggested by the increase in PRL secretion after premammillary deafferentations (6) or lesion of the hypothalamo-hypophysial tract posterior to the SME (22) in this species.…”

Prolactin secretion in sheep after dehydration followed by restraint or administration of ovine corticotrophin‐releasing factor

“…Tonic inhibition of prolactin secretion in the non-stressed sheep appears to involve the transfusion of dopamine from the posterior to the anterior pituitary by the secondary portal vessels (Thomas, Cummins, Canny, Rundle, Griffin, Katsahambas & Clarke, 1989), since dopamine is not present in long portal vessel plasma (Thomas, Cummins, Smythe, Gleeson, Dow, Fink & Clarke, 1989). Therefore, the effect of dehydration on basal prolactin secretion may involve some enhancement of dopaminergic inhibition and this too could be due to a central action of vasopressin, as suggested in the rat (DePaolo, Berardo & Carrillo, 1986).…”

Epinephrine in Rat Hypophysial Portal Blood Is Derived Mainly from the Adrenal Medulla

“…However, NE levels are 2 times higher in portal blood of intact rats than in peripheral blood in another study where this amine has been measured with the HPLC-ED method [19], Our studies as well as other investigations in the rat have been performed in anesthetized and surgically stressed animals. It is of interest to notice that NE concentration in hypophysial portal blood of unanesthetized and unstressed sheep in 2-fold higher than in jugular blood [32], meanwhile DA is undetec table in both sources of blood of the same animals. Then, one must keep in mind that the regulation of catecholamines secre tion into portal and peripheral blood may be different accord ing to the species and the state of the animals (anesthesia, stress).…”