Carla Schramme

Beta-adrenergic stimulation of adenosine-3',5'-monophosphate (c-AMP) in primary cultures of rat anterior pituitary cell populations separated by unit gravity sedimentation. Relationship to growth hormone and prolactin release and to nonsecreting cells

The effect of the beta-adrenergic agonist isoproterenol (ISO) on c-AMP accumulation and on growth hormone (GH) and prolactin (PRL) release was studied in primary cultures of anterior pituitary cell populations with different proportional number of somatotrophs and lactotrophs, obtained by velocity sedimentation at unit gravity. ISO stimulated c-AMP levels in highly-enriched lactotroph (approximately 70%) as well as in highly-enriched somatotroph (approximately 65%) populations. ISO-stimulated c-AMP accumulation was attenuated by dopamine (DA) in highly enriched lactotroph but not somatotroph populations. In a small-cell population consisting of a much lower proportional number of lactotrophs and somatotrophs, the proportional increase of c-AMP accumulation was several times higher than in the other populations and the effect was not suppressible by DA. In this small-cell population, GH but not PRL release was more responsive to ISO than in the other populations. The present observations are consistent with the interpretation that the changes in c-AMP levels induced by ISO originate in part in somatotrophs and lactotrophs. However, the present data also demonstrate that the most responsive cell population in terms of c-AMP accumulation consists of small cells. The majority of these cells could not be identified. The possibility that these cells may be nonsecreting folliculostellate cells is discussed.

Stimulation of growth hormone release by vasoactive intestinal peptide and peptide PHI in rat anterior pituitary reaggregates. Permissive action of a glucocorticoid and inhibition by thyrotropin-releasing hormone

In superfused rat anterior pituitary cell reaggregates, cultured for 5 days in serum-free defined medium, vasoactive intestinal peptide (VIP) concentration-dependently stimulated prolactin (Prl) release but had only a marginal effect on growth hormone (GH) release. When reaggregates were cultured in the presence of 80 nM dexamethasone (Dex) VIP strongly stimulated GH release from a concentration as low as 0.1 nM. VIP did not stimulate LH release. Peptide PHI also stimulated GH release but thyrotropin-releasing hormone (TRH) or angiotensin II did not. In fact, TRH slightly but transiently inhibited basal GH release and strongly inhibited VIP-stimulated GH release. GH-releasing factor (GRF) stimulated GH more potently and with higher intrinsic activity than VIP but GRF did not increase Prl release. The present data indicate that under defined hormonal conditions VIP and PHI are capable of stimulating GH release and that TRH can antagonize this effect by a direct action on the pituitary.

Regulation of Prolactin Release by Angiotensin

In superfused anterior pituitary cell aggregates, prolactin release is stimulated by angiotensin II (AII) in a concentration-dependent fashion between 0.1 and 10 nM. When studied in aggregates prepared from pituitary cell populations separated according to size by unit gravity sedimentation, the PRL response to AII was weak in a population enriched in lactotrophs but deprived of gonadotrophs. In other separated populations, the response increased with the proportional number of gonadotrophs. The response also increased when lactotrophs were co-aggregated with an enriched population of gonadotrophs. It is proposed that the PRL response to AII is augmented by an intercellular messenger system presumably operating between gonadotrophs and lactotrophs.

Stimulation of spontaneous and dopamine-inhibited prolactin release from anterior pituitary reaggregate cell cultures by angiotensin peptides☆

In superfused anterior pituitary reaggregate cell cultures angiotensin II (AII) stimulated both spontaneous and dopamine-inhibited prolactin (PRL) release from subnanomolar concentrations. Angiotensin I (AI) and angiotensin III (AIII) also stimulated PRL release. The magnitude and rate of response to AI was equal to or only slightly lower than that to AII. However, the angiotensin converting enzyme (ACE) inhibitors captopril and teprotide (1 microM) completely abolished the PRL response to 0.1 nM AI and strongly reduced that to 1 nM AI. The intrinsic activity of AIII was lower than that of AII but could be enhanced by adding 2 microM of the aminopeptidase inhibitor amastatin to the superfusion medium. After withdrawal of AIII, PRL secretion rate rapidly returned to baseline levels, whereas after withdrawal of AI or AII, secretion fell to a level remaining significantly higher than basal release. The present findings indicate that stimulation of PRL release by AI is weak unless it is converted into AII by ACE and that aminopeptidase may be important in determining the magnitude and termination of the PRL response. Furthermore, the active peptides induce a different pattern of response.

Influence of corticosteroids on prolactin release from anterior pituitary cell aggregates cultured in serum-free medium. differential effects on dopamine-induced inhibition, post-dopamine rebound and stimulation by TRH, Vasoactive Intestinal Peptide (VIP), angiotensinii and isoproterenol

Dispersed rat anterior pituitary cells were allowed to reassociate into spherical aggregates by gyrotory shaking in serum-free chemically defined culture medium. When aggregates were superfused after being cultured for 5 days in this medium, stimulation of PRL release by TRH, VIP, angiotensin II and the beta-adrenergic agonist isoproterenol was comparable to that of aggregates cultured in serum-supplemented culture medium. Addition to the serum-free medium of 80 nM dexamethasone (Dex) resulted in a significant enhancement of the stimulation of PRL release by TRH, VIP and angiotensin II but not of the stimulation of PRL release by isoproterenol. Dex also failed to influence the inhibition of PRL release by 10 min exposure to 10 nM dopamine (DA). However, Dex significantly enhanced the post-DA rebound secretion of PRL. After 3 weeks in culture Dex provoked a similar potentiation of the response to angiotensin as at 5 days in culture but it abolished almost completely the stimulatory effect of isoproterenol. It is concluded that pituitary cell aggregates cultured in defined serum-free medium are a reliable system to study the multifactorial control of PRL release. The data show that peptidergic, dopaminergic and beta-adrenergic control at the pituitary level is differentially modulated by corticosteroids.

The Role of Cell-Cell Communication in Neuropeptide-Stimulated and Dopamine-Inhibited Prolactin Release

The anterior pituitary is composed of different cell types producing different protein or peptide hormones. Although at first look these different cells are scattered throughout the gland, a more careful examination clearly shows that the topographical arrangement of the different cells is not random (1). Certain cell types are more abundant in certain areas than in others. Moreover, one cell type may display a selective topographical affinity for another. More than 10 years ago Nakane mentioned in his paper on immunocytochemical studies of the anterior pituitary cell types of the adult male rat that gonadotrophs and lactotrophs are frequently found in close association with each other (2). Many of these lactotrophsare cup-shaped, embracing and sometimes completely surrounding a gonadotroph. It has also been mentioned that gonadotrophs may have some affinity for somatotrophs (3). Nakane as well as others also found that corticotrophs (2,4,5) and thyrotrophs (6) are in close juxtaposition with somatotrophs. Horvath et c... (7) reported that gonadotrophs and lactotrophs form between each other specialized junctional complexes of the “macula adhaerens diminuta” type, described by Overton (8). The length of attachment of these adherence junctions varies between 50 and 300 nm, the intercellular gap measuring approximately 150 R. It has also been demonstrated that not all lactotrophs have affinity for a gonadotroph. Nogami and Yoshimura distinguished 4 morphologically distinct subtypes in adult male rat pituitary (9) : 1) oval or polygonal cells with only small spherical granules (130–200 nm diam.); 2) oval or polygonal cells with medium-sized spherical and polymorphic granules (250–300 nm); 3) polygonal cells with only large polymorphic granules (300–700 nm diam.) in the cytoplasm and small granules in the Golgi region; 4) cup-shaped cells with usually spherical (300 nm diam.) and a few polymorphic granules (300–700 nm diam.). Type 3 is the commonly accepted lactotroph in the female rat but it is not predominant in the male. The most frequently found lactotrophs in the male are the type 2 and the cup-shaped cells and these are the cells which have a special affinity for gonadotrophs (9,10). According to Sato (10) polygonal cells appear to gather around an enlarged gonadotroph and extend cytoplasmic processes to the gonadotroph to envelop it. The latter are the cup-shaped cells and some of them can completely surround a gonadotroph. cup-shaped cells, while intermingled with oval and polygonal cells, accumulate in the marginal layer of the gland, particularly in the vicinity of the sex zone (10), in which there is a high proportional number of gonadotrophs (2). However, these different cell types are also found to be scattered throughout the gland and sometimes in clusters (9,10).