Stimulation of Prolactin Secretion from Rat Pituitary by Luteinizing Hormone-Releasing Hormone: Evidence against Mediation by Angiotensin II Acting through a (Sar¹-Ala⁸)-Angiotensin II-Sensitive Receptor

Abstract

In aggregate cell cultures of 15- to 20-day-old rat pituitary maintained in serum-free medium, luteinizing hormone-releasing hormone (LHRH) (10 nM) stimulated prolactin (PRL) release, confirming our previous results and those of others with serum-supplemented medium. Since angiotensin II (AII) stimulates PRL release and a renin-angiotensin system is expressed in gonadotrophs, LHRH stimulation of PRL release might be mediated by AIL To evaluate this hypothesis, the influence of (Sar¹, Ala⁸)AII and (Sar¹, Ile⁸)AII two peptide AII receptor antagonists, of DUP753, a nonpeptide and stable AII receptor antagonist, of a converting enzyme inhibitor, and of angiotensinogen on LHRH-induced PRL release was tested in various in vitro conditions of 15- to 20-day-old female rat pituitary. In aggregates maintained in serum-free medium with or without dexamethasone (DEX) and triiodothyronine (T₃), or maintained in serum-supplemented medium, the effect of LHRH on PRL release was not affected by (Sar¹ Ala⁸)AII (0.1 µM), (Sar¹, Ile⁸)AII (10 µM) or DUP753 (10 µM). Only a high dose (10 µM) of (Sar¹, Ala⁸)AII attenuated the LHRH-induced PRL release. The latter attenuation was seen only with aggregates cultured in the DEX/T₃ medium and not with aggregates cultured in the presence of serum. A dose of 1 or 10 nM (Sar¹, Ala⁸) AII also failed to block the effect of LHRH used at 1 nM. In contrast, (Sar¹, Ala⁸) AII dose dependently as well as DUP753 (10 µM) abolished the AII-induced PRL release. (Sar¹, Ala⁸)AII also failed to affect the LHRH-induced PRL release in pituitary cell aggregates from 6-week-old male rats. However, in aggregates from both immature and 6-week-old rats, (Sar¹, Ala⁸) AII provoked a small and statistically significant attenuation of the LHRH-induced PRL release when a 100 nMdose of LHRH was used. In freshly isolated hemipituitaries from 5-day-old rats, (Sar¹, Ala⁸) AII (1 or 10 µM) did not affect the LHRH- (10 nM) induced PRL release. In single cells obtained by redispersion of aggregates and mounted in a Biogel P2 column, LHRH still stimulated PRL release. Again this effect could not be blocked by DUP753. Treatment of aggregate cell cultures with the angiotensin-converting enzyme inhibitor captopril or with angiotensinogen did not alter the LHRH-induced PRL release. It is concluded that AII is not the paracrine factor mediating the effect of LHRH at low nanomolar doses on PRL release, at least not through the classical AII receptor. The involvement of AII acting on a non-(Sar¹Ala⁸) AII-sensitive receptor cannot be excluded and warrants further investigation.

Multi-responsiveness of single anterior pituitary cells to hypothalamic-releasing hormones: A cellular basis for paradoxical secretion

“…In addition, TRH has been proposed to be a GH-releasing factor (2) on the basis of the ability to induce GH release by pituitary cells from rats and other mammals (20). There also have been reports that LHRH can stimulate secretion of GH and TSH from AP cell cultures (21) as well as PRL release from aggregate AP cells (22). Reports about so-called ''paradoxical'' responses to HRHs are likewise quite common for healthy and infirm human subjects (2), but such observations are not fully interpretable, owing to the potential for indirect effects of HRHs administered in vivo.…”

Secretion of Renin–Angiotensin System (RAS) Components by Normal and Tumoral Lactotropes: A Comparative Study Using Reverse Hemolytic Plaque Assay (RHPA) and Immunoelectron Microscopy

“…The most likely function is autocrine regulation of PRL-secreting cells because of the important role played by the final product of the RAS cascade, A II, in the release of prolactin (Steele et al 1981; see reviews in Ganong 1993Ganong ,1994Ganong ,1995. Paracrine interactions with other cell types in the intact pituitary and in primary cultures (Steele and Myers 1990;Vankelecom and Denef 1997), as well as the actual role of AII in these interactions, have been questioned (Houben and Denef 1991;Robberecht et al 1992). Nevertheless, the presence of A II receptors in lactotropes, demonstrated by A II binding (Aguilera et al 1982) and by RT-PCR (Moreau et al 1997) on partially purified cells, is a potent argument for an autocrine action.…”

Interaction off αT3-1 Cells with Lactotropes and Somatotropes of Normal Pituitary in vitro

“…Final interpretation of these findings cannot be giv en unless the factors involved are identified. Another point of concern is that GnRH is capable of stimulating PRL release from normal immature pituitary cells at least with a factor 2 (area under the curve) [8,9], while in the coaggregates of lactotropes with aT3-l cells this was less than 25%. However, this effect was consistently found and statistically significant.…”

“…We have presented experimental evidence using pituitary reaggregate cell cultures of 14-day-old female rats that the action of GnRH on PRL secretion as well as on the in vitro differentiation and proliferation of lacto tropes and somatotropes is mediated by paracrine factors released from gonadotropes [8,21,22,28]. As far as secre tion is concerned, the effects were shown not to be due to an artefact of culture [9,18]. Effects of GnRH were absent in lactotrope/somatotrope populations deprived of go nadotropes.…”