Dopamine in Plasma of Lateral and Medial Hypophysial Portal Vessels: Evidence for Regional Variation in the Release of Hypothalamic Dopamine into Hypophysial Portal Blood*

Abstract

The plasma concentrations of dopamine in blood from hypophysial portal vessels in various locations on the pituitary stalk were evaluated in diestrous rats. It was found that the mean concentration of dopamine in blood from lateral hypophysial portal vessels, which contain the venous effluent of the lateral median eminence, was significantly less (P less than 0.005) than that in blood from medial portal vessels, which contain the venous effluent of the medial median eminence [1.59 +/- (SE) 0.23 ng/ml vs. 3.12 +/- 0.48 ng/ml]. The mean plasma concentration of dopamine in blood of lateral portal vessels and of medial portal vessels was at least 20-40 times greater than that in arterial blood of these animals. It was calculated that the rate of release of hypothalamic dopamine was 174 +/- 38 pg/h into a medial portal vessel and 73 +/- 15 pg/h into a lateral portal vessel. The mean plasma concentration of norepinephrine or epinephrine in blood from a medial portal vessel was not different from that from a lateral portal vessel. To address the issue of whether the rate of release of dopamine into a medial portal vessel and into a lateral portal vessel was correlated with the rate of synthesis of dopamine in discrete regions of the median eminence, the concentration of L-dihydroxyphenylalanine (DOPA), the precursor of dopamine, was evaluated in lateral and medial segments of the median eminence of diestrous rats treated with 3-hydroxybenzylhydrazine, an inhibitor of DOPA decarboxylase activity. The concentration of DOPA was similar in the medial and lateral segments of the median eminence, suggesting that the rate of synthesis of dopamine did not account for the difference in the rate of release of dopamine into portal blood. The finding of different concentrations of dopamine in blood from various hypophysial portal vessels may be important in view of the heterogenous perfusion of the pars distalis with hypophysial portal blood. We suggest that topographic differences may exist in the release of PRL by cells of the pituitary gland as a consequence of uneven concentrations of dopamine in portal blood perfusing the lactotropes.

Mass and in situ Molar Activity of Tyrosine Hydroxylase in the Median Eminence

“…The increase in the amount of TH may represent a form of TH that is more active than that of the euthyroid state. It is known that an increase in phosphorylated TH in the ME and SCG is associated with an increase in the activity of the enzyme [15,25]. It is possible that the amount of the substrate, tyrosine, in the TID neurites of the ME is increased by thyroidectomy.…”

“…), and hypophysial portal blood was collected for 2 h from a single portal vessel, using the vessel drai ning the medial region of the ME as described previously [14,15]. At the end o f the collection, the plasma was mixed with an equal volume of 0.6 AT PCA containing 2.7 mAT EGTA and centrifuged at 10,000 g for I min.…”

Relationship between Dopamine Release into Hypophysial Portal Blood and Prolactin Release after Morphine Treatment in Rats

“…The concentration of DA was adjusted for each animal so that the infusion rate of the solution of DA or the solvent vehicle was 5.6 pl/min. Within 20 min after beginning the infusion, a single hypophysial portal vessel in the medial zone of the stalk was cannulated for collection of hypophysial portal blood, as described elsewhere [22,25]. Portal blood was collected by gravity flow (approximately 1.6-2.8 pl/m in) for 2 h and was kept in an ice bath during the collection period.…”

High concentrations of catecholamines in human hypothalamic-hypophysial blood.

“…*P < 0.04, **P < 0.007, ***P < 0.003, ****P < 0.002 vs. suprapituitry blood. (27) in hypophysial portal than in peripheral blood, whereas NE concentrations are similar (28).…”