Prolactin-releasing activity in methanol extracts of human plasma: Problems with the bioassay of this activity

Abstract

PRL-releasing activity was found in methanol extracts of plasma acquired from breast feeding women postsuckling and from females with normal menses. PRL-releasing activity was bioassayed by injecting the extracts into estrogenpretreated male rats. PRL release was not related to the high sodium, potassium, and osmolar content of the extracts.In cubation of the plasma before separation decreased the content of peptides with PRL-releasing activity, such as TRH and arginine vasopressin. In addition, no TSH release was observed after infusion of the extracts. The PRL-releasing activity, however, was only noted after a rapid 2-min infusion of the extracts via the carotid artery, and a 4-min carotid artery infusion or a ugular vein infusion did not produce significant PRL release. The administration of various amounts of extracts to either GH3 or normal rat pituitary monolayer culture produced either no PRL release or dissociation of the cells from attachment to the culture plates. Incubation of the extracts with hemipituitary organ cultures produced significant PRL release into the media. The pituitaries exposed to the extracts, however, showed a significant weight gain during the incubation, suggesting a toxic effect from the extracts. In conclusion, 1) methanol extracts of plasma from normal women produce significant PRL release in estrogenized male rats and hemipituitary organ cultures; 2) rapid carotid artery infusion of the extracts is required to demonstrate significant PRL release in the in vivo bioassay system, and swelling of the hemipituitaries in organ culture occurs with the addition of the extracts; and 3) these observations challenge the validity of attributing the PRL release in these systems to specific circulating PRL-releasing agents. In addition, these findings raise questions concerning the value of using methanol extraction for the concentration of small molecular weight peptides that will subsequently be tested for their ability to promote hormonal release in other bioassay systems. © 1981 by The Endocrine Society.