High concentrations of epinephrine derived from a central source and of 5-hydroxyindole-3-acetic acid in hypophysial portal plasma

This study was designed to determine if active secretion of epinephrine (EPI) and/or 5-hydroxytryptamine (5-HT) from the hypothalamus into the hypophysial portal vasculature takes place, in addition to the well-known secretion of dopamine (DA). Hypophysial portal plasma was collected from urethane-anesthetized male rats by stalk cannulation (Porter method) or by periodic aspiration of portal blood (Worthington and Fink method). Portal and peripheral plasma concentrations of EPI, 5-HT, DA and 5-hydroxyindole-3-acetic acid (5-HIAA) were concurrently measured by high performance liquid chromatography with electrochemical detection. Significantly higher concentrations of EPI were found in hypophysial portal than in peripheral plasma. After adrenalectomy (ADX), peripheral plasma levels of EPI were undetectable, whereas portal plasma EPI levels were only slightly attenuated. Although 5-HT levels in portal and peripheral plasma were not different, 5-HIAA levels were 3-fold higher in portal plasma. DA was 10-15fold higher in portal plasma. All the above differences were found independent of the collection method. The high level of 5-HIAA in portal plasma was not due to conversion of 5-HT to 5-HIAA by monoamine oxidase in plasma. The results indicate that in addition to DA, another amine (EPI) and an amine metabolite (5-HIAA) have a concentration gradient in portal vs peripheral plasma. Moreover, the presence of EPI in portal plasma after ADX is a strong indication that EPI is primarily derived from a central source, suggesting that the amine may have a direct physiological role in the regulation of anterior pituitary function.