Differential Effects of Thyrotropin-Releasing Hormone on in vitro Release of in vivo or in vitro Newly Synthesized and Mature Prolactin by Lactating Rat Adenohypophyses

Abstract

We studied the basal and thyrotropin-releasing hormone (TRH)-stimulated in vitro release of newly synthesized and mature prolactin (PRL) by lactating rat pituitary fragments. Newly synthesized PRL was defined as that resulting from either in vivo pulse labelling with [³H]-leucine (injected i.v. 10 min before removing the pituitary glands for in vitro incubation); or in vitro labelled by a 5-min preincubation period of adenohypophyseal fragments. Mature PRL was defined as that labelled in vivo 8 h before incubation. Medium PRL was quantified by polyacrylamide-gel-electrophoresis densitometric and liquid scintillation techniques. Under basal conditions, both total (unlabelled) and 8-hour labelled (mature) PRLs showed an in vitro release pattern characterized by an initial period of faster and higher secretion that lasted for 30–60 min followed by a lower secretion rate that extended from 60 to 240 min of incubation. Secretion of newly synthesized PRLs was detected since the first 30 min, but did not reach maximum levels until 90–120 min of incubation. Accordingly, PRL release during the first 30–60 min of incubation consisted primarily of mature PRL, whereas secretion during the rest of incubation included the release of newly synthesized hormone. These results confirm previous data on the sequential, rather than preferential, release of different age PRLs. TRH (0.35 µM) stimulated (about 30% above control) the release of both total and mature PRLs. No effect could be detected upon the release of in vivo labelled new PRL, while the same dose of TRH strongly stimulated (about 100% above control) the release of in vitro labelled new PRL. These data show that TRH exerted different effects upon PRL secretion according to the age of the hormone since biosynthesis and whether new PRL was synthesized in vivo or in vitro. These findings suggest that qualitative differences between in vivo and in vitro synthesized PRL may exist, which may affect hormonal releasability to hypothalamic regulation.

Vesicular Release of Prolactin from Preformed Prolactin Granules Is Stimulated by Soluble Factor(s) from the Anterior Pituitary of Lactating Rats

“…It shows that stimulation of PRL secretion does not involve mobilization of newly synthesized PRL. The rationale for analyzing the release of [ 3 H]-prelabeled PRL is that in estrogen-treated females [10, 33] or in lactating rats [11, 12], newly synthesized and mature [ 3 H] PRL show different patterns of secretion, mainly through the regulated secretory pathway [10], which may be influenced by hypothalamic secretagogues [13, 14]. Following PRL labeling with [ 3 H] leucine, we found that upon stimulation with CM, PRL release displays essentially the same pattern, regardless of the length of time (between 30 min and 24 h) that elapsed between [ 3 H] PRL labeling and incubation with CM.…”

“…Animals were euthanized by decapitation after light ether anesthesia. It has been shown that this procedure does not alter pituitary PRL content [11,12,13,14]. …”

Evidence for Independently Regulated Secretory Pathways of Proopiomelano‐cortin‐Related Peptides in the Mouse Pars Intermedia

“…The existence of multiple secretory pathways within endocrine cells has been receiving increasing attention (1)(2)(3)(4)(5). In some cases there is evidence for independently regulated pathways, for example in prolactin-producing cells (6) and parathyroid hormone-producing cells (7,8).…”