Prolactin cell subpopulations separated on discontinuous Percoll gradient: an immunocytochemical, biochemical, and physiological characterization

Investigating heterogeneity of intracellular calcium dynamics in anterior pituitary lactotrophs using a combined modelling/experimental approach

Cell responses are commonly heterogeneous, even within a subpopulation. In the present study, we investigate the source of heterogeneity in the Ca(2+) response of anterior pituitary lactotrophs to a Ca(2+) mobilisation agonist, thyrotrophin-releasing hormone. This response is characterised by a sharp increase of cytosolic Ca(2+) concentration as a result of mobilisation of Ca(2+) from intracellular stores, followed by a decrease to an elevated plateau level that results from Ca(2+) influx. We focus on heterogeneity of the evoked Ca(2+) spike under extracellular Ca(2+) free conditions. We introduce a method that uses the information provided by a mathematical model to characterise the source of heterogeneity. This method compares scatter plots of features of the Ca(2+) response obtained experimentally with those made from the mathematical model. The model scatter plots reflect random variation of parameters over different ranges, and matching the experimental and model scatter plots allows us to predict which parameters are most variable. We find that a large degree of variation in Ca(2+) efflux is a likely key contributor to the heterogeneity of Ca(2+) responses to thyrotrophin-releasing hormone in lactotrophs. This technique is applicable to any situation in which the heterogeneous biological response is described by a mathematical model.

Variations in the response of pituitary lactotrophs to oxytocin during the rat estrous cycle

Although removal of dopamine inhibition is established as a major factor in prolactin (PRL) release, a large body of evidence suggests that hypothalamic oxytocin (OT) may serve as a PRL-releasing hormone in the rat. PRL release is modulated by estradiol (E2), which rises between diestrus and proestrus of the estrous cycle, causing a PRL surge in the afternoon of proestrus. Given that E2 strongly modulates OT actions in both central and peripheral tissues, OT action on lactotrophs might also be modulated by the stage of the estrous cycle. To test this hypothesis, we have monitored PRL release and intracellular calcium levels ([Ca(2+)](i)) induced by OT in pituitary lactotrophs obtained from female rats in either diestrus 1 or proestrus. We found that both secretory and [Ca(2+)](i) responses to OT are significantly increased in lactotrophs obtained on proestrus. Moreover, we show that these differences are due to an increase in both the number of OT-responding lactotrophs and the magnitude of their individual [Ca(2+)](i) responses. Both secretory and [Ca(2+)](i) responses were abolished by a specific OT antagonist. Finally, dose-dependent studies show that the increased PRL-releasing effect of OT on proestrus is significant over a wide range of concentrations, particularly those observed in hypophyseal portal plasma. These results suggest that the rising E2 titers that culminate on proestrus facilitate the stimulatory action of OT on lactotrophs and support the notion that OT is a PRL-releasing hormone with an important role in the production of the proestrous surge of PRL.

Pituitary cell lines and their endocrine applications

The pituitary gland is an important component of the endocrine system, and together with the hypothalamus, exerts considerable influence over the functions of other endocrine glands. The hypothalamus either positively or negatively regulates hormonal productions in the pituitary through its release of various trophic hormones which act on specific cell types in the pituitary to secrete a variety of pituitary hormones that are important for growth and development, metabolism, reproductive and nervous system functions. The pituitary is divided into three sections-the anterior lobe which constitute the majority of the pituitary mass and is composed primarily of five hormone-producing cell types (thyrotropes, lactotropes, corticotropes, somatotropes and gonadotropes) each secreting thyrotropin, prolactin, ACTH, growth hormone and gonadotropins (FSH and LH) respectively. There is also a sixth cell type in the anterior lobe-the non-endocrine, agranular, folliculostellate cells. The intermediate lobe produces melanocyte-stimulating hormone and endorphins, whereas the posterior lobe secretes anti-diuretic hormone (vasopressin) and oxytocin. Representative cell lines of all the six cell types of the anterior pituitary have been established and have provided valuable information on genealogy of the various cell lineages, endocrine feedback control of hormone synthesis and secretions, intrapituitary interactions between the various cell types, as well as the role of specific transcription factors that determine each differentiated cell phenotype. In this review, we will discuss the morphology and function of the cell types that make up the anterior pituitary, and the characteristics of the various functional anterior pituitary cell systems that have been established to be representative of each anterior pituitary cell lineage.

Rhythmic secretion of prolactin in rats: action of oxytocin coordinated by vasoactive intestinal polypeptide of suprachiasmatic nucleus origin

Prolactin (PRL) is secreted from lactotrophs of the anterior pituitary gland of rats in a unique pattern in response to uterine cervical stimulation (CS) during mating. Surges of PRL secretion occur in response to relief from hypothalamic dopaminergic inhibition and stimulation by hypothalamic releasing neurohormones. In this study, we characterized the role of oxytocin (OT) in this system and the involvement of vasoactive intestinal polypeptide (VIP) from the suprachiasmatic nucleus (SCN) in controlling OT and PRL secretion of CS rats. The effect of OT on PRL secretion was demonstrated in cultured lactotrophs showing simultaneous enhanced secretion rate and increased intracellular Ca(2+). Neurosecretory OT cells of the hypothalamic paraventricular nucleus that express VIP receptors were identified by using immunocytochemical techniques in combination with the retrogradely transported neuronal tracer Fluoro-Gold (iv injected). OT measurements of serial blood samples obtained from ovariectomized (OVX) CS rats displayed a prominent increase at the time of the afternoon PRL peak. The injection of VIP antisense oligonucleotides into the SCN abolished the afternoon increase of OT and PRL in CS-OVX animals. These findings suggest that VIP from the SCN contributes to the regulation of OT and PRL secretion in CS rats. We propose that in CS rats the regulatory mechanism(s) for PRL secretion comprise coordinated action of neuroendocrine dopaminergic and OT cells, both governed by the daily rhythm of VIP-ergic output from the SCN. This hypothesis is illustrated with a mathematical model.

Steroid effects on secretion from subsets of lactotrophs: role of folliculo-stellate cells and annexin 1

Prolactin secretion is controlled by the hypothalamus, and by circulating steroids; oestrogens stimulate, but glucocorticoids inhibit prolactin release. Lactotrophs express intracellular receptors for oestrogens, but apparently not glucocorticoids. Therefore, a genomic effect of oestrogens could be direct, but that of glucocorticoids appears to be indirect. Lactotrophs are not a homogeneous cell population: some have large irregular dense-cored vesicles, others have small round vesicles, but the functional significance of this inhomogeneity is far from clear. Oestradiol and testosterone can stimulate rapid release of prolactin selectively from type II lactotrophs characterised by small round vesicles. Progesterone and other steroids do not exert this effect, which results from a non-genomic action of oestradiol and testosterone. Glucocorticoid inhibition of secretagogue-induced prolactin secretion is mimicked by annexin 1 (lipocortin 1), a protein induced by glucocorticoids in the pituitary and many other tissues, and can be blocked by annexin 1 immunoneutralisation and antisense. Glucocorticoid inhibition of ACTH and growth hormone secretion also involves annexin 1. Pituitary annexin 1 is located in folliculo-stellate cells; these express glucocorticoid receptors, and glucocorticoids induce annexin-1 synthesis. Annexin 1 is externalised from folliculo-stellate cells in response to glucocorticoids, despite the fact that it lacks a secretory signal sequence and is not packaged in vesicles. Inhibition of annexin 1 externalisation by glyburide suggests involvement of an ABC (ATP-binding cassette) transporter in externalisation. Both oestradiol and glucocorticoids therefore influence the secretion of prolactin by novel direct and indirect mechanisms, in addition to their much better understood effects on transcription via classical intracellular steroid receptors.

Melanotrope cell plasticity: a key mechanism for the physiological adaptation to background color changes

The intermediate lobe of the pituitary secretes the melanotropic hormone alpha-MSH, which in amphibians plays a crucial role in skin color adaptation. It has been previously demonstrated that, in the frog Rana ridibunda, the intermediate lobe is composed of two distinct subpopulations of melanotrope cells that can be separated in vitro by using Percoll density gradients. These two melanotrope cell subsets, referred to as high-density (HD) and low-density (LD) cells, differ in their ultrastructural characteristics as well as in their biosynthetic and secretory activity. However, the specific, physiological role of the heterogeneity displayed by melanotrope cells remains elusive. In the present study, we investigated the effects of background color adaptation on melanotrope cell subpopulations. We found that adaptation of frogs to dark or white environment did not modify either the overall number of cells per intermediate lobe or the apoptotic and proliferation rates of melanotrope cells. On the other hand, adaptation of the animals to a white background significantly increased the proportion of hormone-storage HD cells and caused a concomitant decrease in that of LD cells (which exhibit higher levels of alpha-MSH release and POMC messenger RNA than HD cells). Conversely, after black-background adaptation the proportion of LD cells was markedly increased, suggesting that interconversion of HD cells to LD cells occurs during physiological activation of the intermediate lobe. In addition, black-background adaptation also enhanced alpha-MSH release by both cell subpopulations and increased inositol phosphate production in LD cells. These data indicate that, in frog, the proportions of the two melanotrope cell subsets undergo marked modifications during skin color adaptation, likely reflecting the occurrence of a secretory cell cycle whose dynamics are highly correlated to the hormonal demand imposed by the environment.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Routing, processing and export of rat pituitary prolactin: identification of a 36 kDa disulphide-bridged oligomeric preprolactin

To gain an insight in the routing, processing and export of rat prolactin, rat pituitary cells were cultured in serum-free medium in the presence of cycloheximide, carbonyl cyanide m-chlorophenylhydrazone, Brefeldin A and monensin. The potential influence of these perturbants, whose well documented effects are the altering of protein synthesis and transport, was studied on rat prolactin molecular size isoforms appearing in cellular extracts and in culture medium. The outcome of the culture experiments as recorded in vertical SDS-PAGE, thiol gradient electrophoresis and sequential SDS-PAGE followed by prolactin specific immunoblotting and densitometry, was as follows: (1) at the cellular level we were able to characterize a novel 36 kDa protein as a disulphide-bridged oligomeric precursor prolactin, which is presumably rapidly transformed in the cis/medial Golgi; to designate monomeric rat prolactin as an early Golgi protein and t o advance evidence that the main processing of the glycosylated rat prolactin is a cis/medial Golgi event; (2) in release none of the perturbants disturbed the relative distribution of monomeric and glycosylated rat prolactin, the main molecular size isoforms currently secreted by untreated pituitary cells, or induced the appearance of transformed molecular size isoforms; (3) the secretion mode indicates that rat prolactin is released via the regulated pathway in the presence of the perturbants used.

Mitogen-activated protein kinase-dependent stimulation of proliferation of rat lactotrophs in culture by 3',5'-cyclic adenosine monophosphate

Intracellular cAMP regulates cell proliferation as a second messenger of extracellular signals in a number of cell types. We investigated, by pharmacological means, whether an increase in intracellular cAMP levels changes proliferation rates of lactotrophs in primary culture, whether there are interactions between signal transduction pathways of cAMP and the growth factor insulin, and where the dopamine receptor agonist bromocriptine acts in the cAMP pathway to inhibit lactotroph proliferation. Rat anterior pituitary cells, cultured in serum-free medium, were treated with cAMP-increasing agents, followed by 5-bromo-2'-deoxyuridine (BrdU) to label proliferating pituitary cells. BrdU-labeling indices indicative of the proliferation rate of lactotrophs were determined by double immunofluorescence staining for PRL and BrdU. Treatment with forskolin (an adenylate cyclase activator) or (Bu)2cAMP (a membrane-permeable cAMP analog) increased BrdU-labeling indices of lactotrophs in a dose- and incubation time-dependent manner. The cAMP-increasing agents were also effective in increasing BrdU-labeling indices in populations enriched for lactotrophs by differential sedimentation. The stimulatory action of forskolin was observed, regardless of concentrations of insulin that were added in combination with forskolin. Inhibition of the action of endogenous cAMP by H89 or KT5720, a protein kinase A inhibitor, attenuated an increase in BrdU-labeling indices by insulin treatment. On the other hand, the specific mitogen-activated protein kinase inhibitor PD98059, which was effective in blocking the mitogenic action of insulin, markedly suppressed the forskolin-induced increase in BrdU-labeling indices. (Bu)2cAMP antagonized not only inhibition of BrdU labeling indices but also changes in cell shape induced by bromocriptine treatment, although forskolin did not have such an antagonizing effect. These results suggest that: 1) intracellular cAMP plays a stimulatory role in the regulation of lactotroph proliferation; 2) cAMP and insulin/mitogen-activated protein kinase signalings require each other for their mitogenic actions; and 3) the antimitogenic action of bromocriptine is, at least in part, caused by inhibition of cAMP production.

Percoll Density Gradient-Enriched Populations of Rat Pituitary Cells: Interleukin 6 Secretion, Proliferative Activity, and Nitric Oxide Synthase Expression

We used a discontinuous Percoll density gradient centrifugation to prepare enriched populations of prolactin (PRL), growth hormone (GH), and folliculo-stellate (FS) cells from rat anterior pituitaries in order to characterize these various cell populations. After cell dissociation and centrifugation, enriched PRL cells (55% of total cells as determined by immunocytochemistry [ICC] were present in Fraction 1 (Fr1) (density ([d]) = 1.059). Fr2 (d=1.071) had enriched S100-positive FS cells (31% of total cells), but enriched GH cell (60% of total cells) were present in Fr3 (d=1.094). Interleukin 6 (IL-6) was secreted mainly by enriched PRL cells in Fr1 (350 pg/mL/106 cells) and Fr2 (194 pg/mL/106 cells), and much less by the enriched GH cells inFr3 (16 pg/mL/106). Proliferation studies with combined 3H-thymidine and ICC for pituitary hormones showed that only the PRL cell had significant prolifereative activity. Immunostaining showed that immediately after separation, all three isoforms of nitric oxide synthase (NOS) were expressed anterior pituitary cells. After 3 d of culture, there was a marked increase in nuclear staining for neuronal NOS (nNOS) in all three fractions, whereas inducible NOS (iNOS) and endothelial NOS (eNOS) rexpression did not change significantly. These results indicate that: 1. Enriched populations of PRL, FS, and GH pituitary cells can be readily obtained with a rapid discontinuous percoll density separation procedure. 2. PRL cells from different fractions of the gradient show differenet proliferation rates and IL-6 secretion varied in different enriched cell populations. 3. Although all three isoforms of NOS were expressed in rat pituitary cells, nNOS is the prindipal isoform in anterior pituitary cells, and its expression was icreased after 3 d of culture of anterior pitutuitary cells.

Endothelin activates large-conductance K+ channels in rat lactotrophs: reversal by long-term exposure to dopamine agonist

Endothelin-1 (ET-1) inhibits PRL secretion from cultured rat lactotrophs. However, ET-1 stimulates PRL secretion after cultured lactotrophs have been exposed for 48 h to dopamine or D2 dopamine agonists. In the present study, we have used cell-attached and inside-out patch recordings to establish an ionic basis for these effects. Bath application of 20 nM ET-1 to untreated lactotrophs evoked a robust and persistent activation of large-conductance K+ channels in cell-attached patches. This effect of ET-1 had a long latency to onset, was maintained for as long as ET-1 was present, and required at least 10 min of washing in control saline before complete recovery was achieved. The stimulatory effect of 20 nM ET-1 on these channels was markedly attenuated in the presence of the selective ET(A) receptor antagonist BQ-610 (200 nM), or after pertussis toxin (200 ng/ml, 16 h) pretreatment. The unitary slope conductance of the ET-1 activated channels in cell attached patches was 165 and 95 pS when the recording electrodes contained 150 and 5.4 mM KCl, respectively. These channels were voltage-sensitive and their activity increased upon patch depolarization. Previously activated channels in cell-attached patches became quiescent immediately upon patch excision into Ca2+-free bath saline. Exposure of the intracellular surface to 0.1 microM Ca2+ restored the activity of these channels similar to the level seen before patch excision. In addition, preincubating the cells with the membrane-permeable Ca2+-chelator BAPTA-AM, or using Ca2+-free solution in the recording pipettes, prevented the activation of these channels by ET-1. The ET-1 activated large-conductance Ca2+-dependent K+ (BK(Ca)) channels were blocked by 20 mM tetraethylammonium but were insensitive to the K+ channel blockers apamin (1 microM), charybdotoxin (200 nM), or iberiotoxin (200 nM). Acute application of 10 microM dopamine and 20 nM ET-1 caused activation of BK(Ca) channels with indistinguishable kinetic properties, although the effect of dopamine occurred with shorter latency. After 48-h exposure to the specific D2 dopamine receptor agonist (+/-)-2-(N-phenyl-N-propyl) amino-5-hydroxytetralin hydrochloride (PPHT, 500 nM), bath application of 20 nM ET-1 resulted in inhibition of spontaneously active BK(Ca) channels. These data suggest that both the stimulatory and inhibitory effects of ET-1 on PRL secretion are mediated, at least in part, by actions on BK(Ca) channels, and that long term exposure to dopamine or D2 agonists alters the signaling pathways from the ET(A) receptor to BK(Ca) channels.

Heterogeneity of pituitary lactotrophs: immunocytochemical identification of functional subtypes

The existence of functional lactotroph subpopulations was confirmed in primary pituitary cell cultures of female rats submitted to estrogen treatment and stimulation with thyrotrophin releasing hormone (TRH) and angiotensin II (A-II). In cell cultures of pituitary tissue, prolactin (PRL) producing cells represent about 50% of the total cell count, most of which (90%) correspond to a typical lactotroph subpopulation characterized by large secretory granules, 500-900 nm in diameter, and well developed rough endoplasmic reticulum (RER) and Golgi complex. Few atypical lactotrophs were detected with a quiescent appearance and containing smaller secretory granules, often indistinguishable from granular content of other pituitary cells. Depletion of endogenous estrogen caused by ovariectomy (OVX) decreased the pituitary lactotroph population about 34%, with a relative increase of atypical forms (56%). Replacement therapy with benzoate estradiol (EB) to OVX rats did not reverse the proportion of typical and atypical lactotrophs gauged in control pituitary glands. The predominant lactotroph population of OVX rat was an atypical PRL producing cell which displayed a quiescent appearance compatible with a reduced secretory activity. By contrast, estrogen administration to OVX rats caused a striking development of the RER, a hypertrophy of the Golgi complex and an increased storage of mature and immature secretory granules in the majority of lactotrophs. These features are compatible with a reactivated protein synthesis. Estrogen also enhanced significantly (p < 0.05) the responsiveness of lactotrophs to A-II and the PRL secretion in both intact and OVX + EB treated rats increased by 40% and 30% respectively. By contrast, A-II did not produce any statistically significant response of lactotrophs from OVX female rats. At variance to this observation, in all models tested TRH increased significantly the PRL secretion (p < 0.05). The correlation of PRL secretion and morphology of different lactotroph subtypes authenticates the existence of a lactotroph subpopulation unresponsive to A-II in pituitary cell cultures from rats depleted of estrogen.

Different behavior of lactotroph cell subpopulations in response to angiotensin II and thyrotrophin-releasing hormone

1. In the present investigation we have extended the study of lactotroph subpopulations in primary pituitary cell cultures. Male rats with or without previous estrogenization followed by A-II or TRH treatments were selected as experimental models. 2. The TRH increased up to 50% the PRL released in both whole and OROX+EB rats (P < 0.05). In contrast, A-II treatment introduced no changes in PRL secretion from cell cultures derived from whole male rats but attained a significant augmentation (about 75%) of PRL secreted by ORQX+EB pituitary cells. 3. The addition of TRH and A-II to cultures of ORQX+EB-derived lactotrophs induced cytological changes compatible with a high secretory activity. In estrogen-treated rats the prevailing lactotroph subpopulation is type I. In cell cultures from control and A-II treated whole male pituitaries, the majority of lactotrophs consists of atypical subpopulations of II and III cells, with smaller secretory granules (between 150 and 300 nm in diameter). 4. Morphometry of immunostained lactotrophs performed on light microscopic preparations revealed that about 30-36% of the total cell count were lactotrophs. This percentage was fixed and did not change significantly after TRH and A-II treatments. 5. The present results confirm the presence of morphological and functional subtypes of lactotroph cells in rat pituitary. Typical PRL cell population shows the highest responsiveness to angiotensin II and TRH action. This functional heterogeneity of lactotroph subtypes may reflect an important and scarcely explored factor in the regulatory process of prolactin secretion.

Two frog melanotrope cell subpopulations exhibiting distinct biochemical and physiological patterns in basal conditions and under thyrotropin-releasing hormone stimulation

Cell heterogeneity designates the phenomenon by which a particular cell type is composed of morphologically and physiologically distinct cell subpopulations. We have previously isolated two subsets of melanotrope cells in the intermediate lobe of the frog pituitary by means of a separation procedure based on a Percoll density gradient High density (HD) melanotrope cells were found to exhibit a more granulated cytoplasm and a lower secretory rate than low density (LD) cells. In the present study, we have investigated the biochemical and functional characteristics of each melanotrope cell subpopulation by using various approaches, including chromatographic analysis for the measurement of the proportion of acetylated alpha MSH, microfluorimetric measurement of the cytosolic free calcium concentration ([Ca2+]i) and in situ hybridization for quantification of POMC messenger RNA (mRNA). Under basal conditions, LD melanotrope cells showed higher secretory activity, acetylation rate, [Ca2+]i, and POMC mRNA content compared to HD cells. Incubation of the cells with 100 nM TRH for 2 h induced a more pronounced activation of alpha MSH secretion, [Ca2+]i mobilization, and POMC mRNA accumulation in LD than in HD melanotrope cells. Conversely, TRH increased the rate of acetylation of alpha MSH in HD cells, but did not affect acetylation in LD cells. Taken together, these results demonstrate that the frog intermediate lobe is composed of two subsets of endocrine cells with distinct biochemical and functional characteristics. The coexistence of two cell subpopulations in the frog pars intermedia is consistent with the idea of a cell secretory cycle, in which each melanotrope subset represents a specific state of cellular activity.

Substance P increases intracellular Ca2+ in individual rat pituitary lactotrophs, somatotrophs, and gonadotrophs

The present study has investigated transients in the intracellular calcium concentration [Ca2+]i in response to substance P (SP) in single pituitary cells. SP raised [Ca2+]i in three subtypes of pituitary cells: lactotrophs, somatotrophs, and gonadotrophs. In all three cell subtypes the [Ca2+]i response to SP was amplitude-modulated and a concentration of 100 nM was necessary to elicit well pronounced two phased [Ca2+]i transients. The first phase was associated with increased generation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) in all three cell types. In lactotrophs, the second phase, but not the first, was blunted by depletion of extracellular Ca2+ (Ca2+ free EGTA incubation buffer) and by addition of dopamine (1 microM). In somatotrophs, the second phase of the SP-induced [Ca2+]i response was inhibited by depletion of extracellular Ca2+ and by addition of somatostatin (100 nM), while the first phase was unaffected by this treatment. In gonadotrophs, the second phase, but not the first, was inhibited by the Ca2+ channel blocker methoxyverapamil and depletion of extracellular Ca2+. SP was compared with other agonists having an action on lactotrophs, somatotrophs or gonadotrophs. These experiments demonstrated that SP was a weaker agonist in terms of maximal [Ca2+]i response than thyrotropin-releasing hormone (TRH) (in lactotrophs), growth hormone-releasing hexapeptide (in somatotrophs) and GnRH (in gonadotrophs). On the basis of these results it is concluded that SP exerts direct Ca2+ mobilizing effects in single lactotrophs, somatotrophs, and gonadotrophs derived from male peripubertal rats. The first phase in SP-induced [Ca2+]i transients is likely to be brought about by inositol 1,4,5-trisphosphate-mediated Ca2+ release from internal stores while the second phase reflects an influx of calcium through voltage-gated calcium channels.

Heterogeneity of growth hormone (GH)-producing cells in aging male rats: in vitro GH releasing activity of somatotrope subpopulations

Studies on the age-related decline of growth hormone (GH) release have ignored that the population of GH-producing cells (somatotropes) is heterogeneous. In aging male rats, centrifugation of dispersed pituitary cells in a density gradient yields two somatotrope subpopulations, i.e. low- (LD) and high-density (HD) cells. A previous analysis of ultrastructure and GH mRNA levels has shown that storage and biosynthetic features were inversely related in both subsets. Furthermore, ultrastructural and molecular differences between LD- and HD-cells were retained throughout the rat lifespan, suggesting that the heterogeneity of somatotropes may have a biological meaning. Accordingly, the main objective of the present study was to analyze the functional heterogeneity of the somatotrope population during the aging process in male rats. For this purpose, the response of LD- and HD-somatotropes from 5-, 19-, and 26-month-old male rats was analyzed with an optimized cell immunoblot assay both under basal conditions, and after GH-releasing factor (GRF) and/or somatostatin (SS) treatments. Simultaneous measurements of hormonal release, intracellular GH content, and cell size were performed at the single-somatotrope level. Average values for those parameters were significantly higher in HD- than in corresponding LD-cells, such differences being irrespective of age or treatment. Releasing activity and GH content were significantly reduced with age in both subpopulations. GRF stimulated GH release from LD- and HD-somatotropes, and the GRF responsiveness was similar in both subpopulations and in all ages. On the other hand, SS prevented GRF-stimulated GH release in most cases. At the level of single cells, both releasing activity and cell size showed a significant, linear dependence on intracellular GH content, correlations being irrespective of age, subpopulation, or treatment. Taken together, our results demonstrate that LD- and HD-somatotrope subpopulations display quantitative differences in releasing activity that are essentially retained through aging. This functional heterogeneity is more dependent on the basal GH release of these somatotrope subsets than in their responsiveness to GRF and SS. The present findings suggest that the reduction in secretory activity at the single somatotrope level observed in both subpopulations underlies the age-related decline of pituitary GH release. Finally, a theoretical model of secretory cycle is proposed which might contribute to the understanding of the biological meaning of the somatotrope subpopulations in aging male rats.

Heterogeneity of growth hormone (GH)-producing cells in aging male rats: ultrastructure and GH gene expression in somatotrope subpopulations

Mammalian aging is characterized by a decline in the content and release of pituitary growth hormone (GH). However, few studies on the age-related changes in the population of GH-producing cells (somatotropes) have been carried out. We have investigated whether changes in number, ultrastructure and GH gene expression in subpopulations of somatotropes could explain the reduced GH release in aged rats. Three representative ages were studied: adult (5-month-old), old (19-month-old), and senescent (26-month-old) male rats. The total number of immunoreactive-GH cells per pituitary gland remained invariable to age. The separation of dispersed pituitary cells on a density gradient yielded two somatotrope subpopulations, of low density (LD) and high density (HD). Both subpopulations were equally represented in adults, whereas in old and senescent rats a predominance of LD-somatotropes was observed. Morphometric analysis showed that subpopulations exhibited storage and biosynthetic features inversely related. In LD-somatotropes, rough endoplasmic reticulum (RER) was more prominent but secretory granules (SG) were less abundant than in HD somatotropes. Concurrently, in situ hybridization for GH mRNA showed that GH gene expression was higher in LD-cells. Differences between subpopulations were essentially retained through the animals' lifespan, but small-sized SG, reduced RER, and low GH mRNA levels were inherent to aging both in LD- and in HD-somatotropes. The present findings demonstrate that the reduced content of pituitary GH in aged male rats is not due to a diminished number of GH-producing cells, but to the numerical predominance of scarcely granulated LD-somatotropes, combined with the decline in GH biosynthetic capacity observed in both subpopulations. In addition, age-related changes in ultrastructure and GH gene expression suggest a chronic inhibition of GH release and/or a weak stimulation of GH biosynthesis affecting both subpopulations.

Differential alternative splicing of PACAP receptor in pituitary cell subpopulations

The capability of rat pituitary cells to express receptors for pituitary adenylate cyclase activating polypeptide (PACAP) and VIP was evaluated by binding studies and measurement of adenylate cyclase activity on whole gland preparations and by reverse transcriptase-polymerase chain reaction (TR-PCR) using specific primers on preparations from isolated cell populations enriched in PRL- and GH-producing cells. Data obtained on whole gland preparations indicated that selective PACAP receptors (PACAP Type I) predominated. The mRNA coding for PACAP Type I and for the non-selective PACAP receptors Type II VIP2 (but not VIP1) were identified. The mRNA coding for four different spliced variants of the PACAP Type I receptor were detected. In PRL producing cells, three variants and the VIP2 mRNA were detected, whereas in GH-producing cells the mRNA coding for the variant having a 28-amino acid insert (termed HOP) in the third intracellular loop was the only present.

Heterogeneous response of porcine gonadotrope subpopulations to gonadotropin releasing hormone (GnRH) during postnatal development

In previous papers, we showed the porcine gonadotrope population to be composed of three GtH subpopulations that can be separated by density using a continuous Percoll density gradient. We also demonstrated that these subpopulations exhibited different hormonal storage patterns and morphological features during porcine postnatal development at three representative ages: neonates (30 days), prepubers (5-6 months) and matures (16-18 months). In this work, we investigated whether these morphologically heterogeneous subpopulations are also functionally different. Thus, the effect of the hypothalamic gonadotropic hormone-releasing factor (GnRH) on these subpopulations was assessed in order to ascertain whether a mutual relationship between the reported morphological features, hormonal storage patterns and physiological response to the stimulation can be established. For this purpose, gonadotropin secretion was measured by cell immunoblot assay and hormonal content by scanning cytophotometry. Low-density gonadotropes (1.049 g/cm3), present in the three age groups studied, were mainly composed of bihormonal LH/FSH cells in neonates and monohormonal LH cells in prepubers and matures. GnRH stimulation was found to increase both LH and FSH secretion, as well as the intracellular content. These results indicate that GnRH can stimulate both the synthesis and release of both gonadotropins in this subpopulation. Middle-density gonadotropes (1.062 g/cm3), present in prepubers and matures only, were composed of bihormonal cells. GnRH stimulated the secretion of LH and FSH in prepubers and matures, but decreased hormonal contents except that of LH in prepubers. However, GnRH stimulation increased the proportion of immunoreactive gonadotropes (particularly monohormonal cells). Finally, high-density cells (1.087 g/cm3), present in neonates and prepubers only, were mostly composed of bihormonal LH/FSH gonadotropes, and exhibited low (neonates) or no response (prepubers) in terms of LH release and content when treated with GnRH. In conclusion, these results indicate that porcine gonadotrope subpopulations are morphologically and physiologically heterogeneous. The heterogeneity remained through porcine postnatal development, thus suggesting that all the subpopulations are physiologically relevant. However, the different hormonal storage patterns between subsets of the same density suggest age-related differences within each subpopulation due, at least in part, to the different physiological condition of the animals during development.

Cyclosporin A, rapamycin and FK506 decrease prolactin release from rat pituitary cells in primary culture

It is at present well established that prolactin exerts a non-specific immunoactivating function. In this work we tested whether the immunosuppressant drugs cyclosporin A, FK506 and rapamycin influence prolactin release from rat pituitary cells in primary culture. The tested drugs had no effect on the prolactin release measured during a 2h incubation period, indicating that they do not influence the secretion of prolactin from intracellular stores into the culture medium. During longer incubation times (48h), however, prolactin release was diminished to 56% +/- 18 (10 microM cyclosporin A), 64% +/- 14 (1 microM rapamycin) or 64% +/- 7 (1 microM FK506), suggesting an effect on prolactin production. At these drug concentrations no toxic effects were observed. The data indicate that inhibition of pituitary prolactin synthesis might contribute to the immunosuppressant action of cyclosporin A, rapamycin and FK506.

Calcitonin inhibits prolactin gene transcription in rat pituitary cells

Our recent studies have shown that calcitonin (CT)-like immunoreactive peptide is synthesized and released from cultured rat anterior pituitary (AP) cells, and may serve as a paracrine inhibitor of PRL release. The present studies investigated effects of CT on basal and TRH-induced PRL mRNA levels in rat AP and rat pituitary tumor GH(3) cells. CT attenuated steady-state PRL mRNA levels in a dose-dependent fashion in primary rat AP and GH(3) cells. The kinetics of CT action suggests that 100NM: CT caused a significant decline after 3 h, and the inhibition was sustained at least until the longest tested incubation period of 30 h. Results from nuclear run-on assays suggest that 100 nM CT decreased the rate of PRL gene transcription by 80% after 30 min of incubation. CT did not affect PRL mRNA levels in Ca(2+)-depleted GH(3) cells but dramatically decreased them in Ca(2+)-repleted cells. Bay K 8644 induced increase in PRL mRNA levels of Ca(2+)-repleted GH(3) cells and CT did not affect this increase. These results suggest that CT rapidly and selectively inhibits PRL gene transcription in primary AP and GH(3) cells, and support a possibility that CT-induced attenuation of PRL mRNA may involve cytoplasmic Ca(2+).

Effect of angiotensin II on the proliferation of mammotrophs from the adult rat anterior pituitary in culture

We measured the influence of prolactin-releasing neuropeptides on mammotroph proliferation in cultures of rat adenohypophysis cells using flow cytometry. Angiotensin II (AII) increased mammotroph proliferation. Other peptides with hormone-releasing activities did not promote growth. Tamoxifen inhibited mammotroph proliferation in control and AII-containing cultures and the inhibition was reversed with beta-estradiol. Saralasin, an AII receptor antagonist, suppressed not only AII-induced mammotroph proliferation but also luteinizing hormone-releasing hormone (LHRH)-induced proliferation. These results suggest that hypothalamic LHRH stimulates AII release from gonadotrophs and that AII, with estrogen, controls mammotroph proliferation in rat pituitary.

Development and heterogeneity of prolactin cells

Prolactin (PRL) is synthesized in pituitary cells called mammotrophs (PRL cells). Ample evidence demonstrates that the PRL cell population consists of structurally and functionally heterogeneous PRL cells. Multiple variants of PRL molecules are found in various species. Prolactin cells may be divided into various subtypes in the rat and mouse. Secretory activities differ among the PRL cell population. These heterogeneities may reflect various phases of the maturation process of PRL cells, or the integrated outcome of various functional differences in PRL cells. To clarify the significance of heterogeneities among PRL cells, we present updated reports on the differentiation, proliferation, and development of PRL cells, and discuss factors responsible for the functional differences in PRL cell population. The age-related alteration in PRL secretion in the rat is summarized, because it is one of the most important aspects of the developmental changes in PRL cells. A mammosomatotroph, which secretes growth hormone and PRL, is found in various species. Prolactin cells and somatotrophs are derived from the same lineage. The possible relationship among PRL cells, somatotrophs, and mammosomatotrophs is discussed.

Distribution of lactotroph subpopulations in different regions of the rat pituitary

In this study, we examined the distribution of lactotrophs in different regions of pituitaries of ovariectomized (OVX) rats either untreated or treated with estradiol, progesterone or a combination of estradiol and progesterone. Anterior pituitaries were cut into inner and outer zones. Each of these regions was enzymatically dispersed and the resulting cells subjected to density sedimentation through a discontinuous Percoll gradient. The light and heavy cell subpopulations obtained were compared with cells not subjected to Percoll (unseparated cells). Cell numbers were determined and prolactin positive cells were assessed by immunocytochemistry. In OVX rats, the percentage of lactotrophs in the unseparated cell fraction of the outer zone was significantly (P < 0.05) lower than that in the inner zone. Estradiol increased the percentage of PRL cells in all cell populations, but the effect on inner zone light cells was significantly less than on any other type of cells. Progesterone given alone did not affect the percentage of lactotrophs in pituitary compared to untreated OVX rats nor did it alter the effects of estradiol. When the number of lactotrophs was calculated, the unseparated cells of inner and outer zone were equivalent except in the untreated OVX group where there was a significantly greater number of lactotrophs in the outer zone. In untreated OVX rats there was an equal distribution of lactotrophs between light and heavy cells in both inner and outer zones. Estradiol significantly increased the number of PRL cells in both zones and the effect was greater in the heavy cell fractions especially in the outer zone. In contrast, progesterone decreased the number of PRL positive cells in both zones and across cell types, but this effect was most pronounced in the outer zone. Progesterone also decreased the total pituitary cell number and this effect was greater than could be accounted for by the decrease in lactotroph numbers. We conclude that, except in untreated OVX rats, the numbers of lactotrophs in inner and outer zones are similar. The results also indicate that estradiol and progesterone can significantly alter the proportion and/or numbers of PRL positive cells within pituitary zones and may also affect cells that are not identified as PRL positive.

Hormonal storage patterns and morphological heterogeneity of porcine gonadotrope cells during postnatal development

Previous reports indicate that gonadotrope cells of the porcine pituitary gland can be separated into three subpopulations of low- (1.049 g/cm3), middle- (1.062 g/cm3) and high- (1.087 g/cm3) density in a continuous Percoll density gradient. The aim of this work was to study the hormonal storage patterns and morphological features of these subpopulations at three representative ages of the postnatal development: neonatals (30-day-old animals), prepubertals (5-6-month-old animals) and matures (16-18-month-old animals). The low-density subpopulation, present at the three ages studied, was mainly composed of bihormonal LH/FSH cells in neonatal and monohormonal LH cells in prepubertal and mature animals. On the other hand, middle- (only present in prepubertal and mature animals) and high-density subpopulations (only present in neonatal and prepubertal animals) were mainly composed of bihormonal LH/FSH gonadotropes. In ultrastructural terms, these subpopulations exhibit a correlation between density and morphology irrespective of the animal's age. The low-density subpopulation was composed of poorly granulated cells with highly developed biosynthetic machinery (rough endoplasmic reticulum and Golgi complex), while high-density cells were of opposite morphology, with a highly granulated cytoplasm and poorly developed rough endoplasmic reticulum and Golgi complex. The middle-density subpopulation was composed of poorly granulated cells with scarcely developed biosynthetic machinery. In conclusion, these results indicate that porcine gonadotrope cells during postnatal development are composed of three subpopulations of different hormonal storage patterns and morphology. The presence of these subpopulations at the different stages of postnatal development strongly suggests that their proportions may play a major role in the endocrine control process.

Application of a Percoll density gradient to separate and enrich porcine pituitary cell types

The application of a Percoll density gradient cell separation procedure to the pituitary gland of neonatal, prepubertal and mature pigs is described. After enzymatic dispersion, cell viability was 90% to 98% as determined by uptake of Trypan Blue. Recovery of dissociated cells after application of the gradient ranged from 80% to 95%. The dissociated cells were separated in several fractions that were characterized immunocytochemically using different antisera. We obtained highly enriched fractions of gonadotrope (gonadotropins), somatotrope (growth hormone) and lactotrope (prolactin) cells for each age. Concentration of the cell types (purity) was higher than 60% in the following fractions: 1) gonadotropin cells, fraction 15 (1.033 g/cc of density) from mature animals; 2) growth hormone cells, fraction 3 (1.121 g/cc of density) from neonatal animals and fraction 9 (1.087 g/cc) from prepubertal animals; and 3) prolactin cells, fraction 7 (1.094 g/cc) and 10 (1.082 g/cc) from neonatal animals and fraction 14 (1.051 g/cc) from mature pigs. In thyrotrope (thyroid-stimulating hormone) and corticotrope (adrenocorticotropin) cells, enriched fractions were also obtained, although the values of purity were lower (20% to 58%). In conclusion, the proposed cell separation and enrichment technique is suitable for the isolation, purification and examination of porcine pituitary cell types and subpopulations, and offers major advantages such as simplicity, rapidity, efficiency and reproducible results.

Reduction of plasma prolactin by acute administration of CB-154 in ovariectomized F344 and Holtzman rats treated with diethylstilbestrol: a comparison of RIA and Nb2 lymphoma bioassay

Fischer 344 (F344) and Holtzman Sprague-Dawley female rats were ovariectomized and implanted with a s.c. Silastic capsule of diethylstilbestrol (DES). At 1, 4 and 8 weeks of DES exposure, blood samples were obtained by infraorbital sinus puncture under light ether anesthesia before and 2 hours after s.c. administration of CB-154 (2.5 mg/rat). The plasma obtained was assayed for prolactin by radioimmunoassay (RIA) and Nb2 lymphoma cell bioassay (BA). At 1 week of DES treatment plasma prolactin in F344 rats measured by RIA was decreased by CB-154 (approximately 60%) whereas the level measured by BA was not changed and the BA:RIA ratio was increased from 2.4 +/- 0.2 to 4.8 +/- 0.9. CB-154 decreased plasma prolactin levels at 4 weeks but the effect seen by RIA (approximately 80%) was greater than that seen by BA (approximately 60%) and the BA:RIA ratio was increased (2.2 +/- 0.2 vs 3.4 +/- 0.5). By 8 weeks of DES exposure, CB-154 was as effective in reducing the levels measured by BA (approximately 89%) as those measured by RIA (approximately 85%) and the BA: RIA was not affected by the dopamine agonist. In Holtzman rats CB-154 decreased prolactin levels measured by RIA and BA to the same extent at both 1 and 4 weeks resulting in no change in the BA:RIA ratio, but at 8 weeks the BA:RIA was decreased by CB-154 (2.1 +/- 0.1 to 1.7 +/- 0.1). As was observed in F344 rats, the reduction in plasma prolactin induced by CB-154 increased as the duration of DES treatment increased (1 week approximately 45%; 4 weeks approximately 55-60%; 8 weeks approximately 80-85% inhibition) regardless of assay method. It is concluded that DES increases the BA:RIA ratio of plasma prolactin and that acute CB-154 treatment increases, decreases or does not change the BA:RIA ratio depending on the strain of rat used and the duration of DES treatment.

Long-term in-vitro treatment of human growth hormone (GH)-secreting pituitary adenoma cells with octreotide causes accumulation of intracellular GH and GH mRNA levels

OBJECTIVE

We studied the effects of long-term in-vitro exposure of human GH secreting pituitary adenoma cells to octreotide on GH release, intracellular GH concentrations and GH messenger ribonucleic acid (mRNA) levels.

DESIGN

Human GH-secreting pituitary adenoma cells were cultured for periods from 4 days up to 3 weeks without or with octreotide (10 nM) and/or bromocriptine (10 nM). The effects of these drugs were measured on GH release, intracellular GH concentrations and intracellular GH mRNA levels.

PATIENTS

Thirteen patients with GH-secreting pituitary adenomas were studied. Twelve patients were untreated, one had been pretreated with octreotide (12 weeks, 3 x 100 micrograms daily).

MEASUREMENTS

GH, PRL, alpha-subunit and IGF-I concentrations in plasma, media and cell extracts were determined by immunoradiometric or radioimmuno-assays. GH mRNA levels were determined by automatic quantification of grain numbers in individual adenoma cells.

RESULTS

Incubation of the adenoma cells for 4 days with 10 nM octreotide induced a dose-dependent inhibition of GH release and a parallel increase (increase varying between 124 and 617% of control) in the intracellular GH levels was observed in six of seven adenomas. In addition, bromocriptine, when effective in inhibiting GH release by the adenomas, also induced an increase in intracellular GH levels. Even after 3 weeks of exposure to 10 nM octreotide in vitro there was a statistically significant increase in intracellular GH levels (between 191 and 923% of control). Withdrawal of octreotide after 6 days of incubation resulted in a lowering of intracellular GH levels to control values, showing that the octreotide-induced increase in intracellular GH is reversible. In a 96-hour incubation with 10 nM octreotide, GH mRNA levels were increased in two, and slightly decreased in one of the three adenomas tested. This effect was time dependent in that there was no significant effect of 10 nM octreotide on GH mRNA levels in a 24-hour incubation.

CONCLUSIONS

(1) Long-term in-vitro exposure of GH-adenoma cells to octreotide causes an increase in intracellular GH levels in the majority of the adenomas, probably because of an increase in GH mRNA levels in the adenoma cells; and (2) this considerable increase in intracellular GH levels may be one of the explanations for the relatively poor effect of octreotide on tumour shrinkage in patients with GH-secreting pituitary adenomas.

Characterization of bone cells isolated on discontinuous Percoll gradients: distribution in sequentially derived populations

Bone cell populations obtained by sequential digestion of newborn mouse calvariae remain morphologically heterogeneous despite well-documented biochemical differences. Fractionation of these populations on Percoll gradient reveal three major cell groups of low, intermediate, and high buoyant density (1.056, 1.070, and 1.095 g/ml) that are present in different ratios in early and late released populations. Cells of low and intermediate density dominate in early released populations. In contrast, late released populations contain mostly high-density cells. Basal levels of alkaline phosphatase are highest in cells of intermediate buoyant density. All cells respond to PTH with cAMP production and morphologic transformation, but biochemical responses to PTH, such as secretion of insulin-like growth factor I (IGF-I) and stimulation of alkaline phosphatase activity, occur mostly in cells of intermediate density. These data suggest that (1) subclasses of osteoblasts can be further separated by density and (2) PTH effects on alkaline phosphatase activity and IGF-I secretion are probably expressed by osteoblasts of a certain subclass and/or stage of development.

Further characterization of rat 26,000 prolactin as a glycoprotein with essentially o-linked carbohydrate chains

Abstract In the rat two major molecular variants of prolactin are recorded i.e. 23,000 M(r) and glycosylated 26,000 M(r). In order to further characterize the glycosylated 26,000 rat prolactin molecular variant, rat pituitary cell lysates were digested with several glycoen-zymes and the digestion products submitted to sodium dodecyl sulphate polyacrylamide gel electrophoresis and subsequent immunoblotting. The results were as follows: treatment with 1) neuraminidase, specific for sialic acid, yielded an M(r) decrease of the glycosidic variant from 26,000 to 24,500, 23,800, 23,000 and 22,000; 2) endo-alpha-N-acetylgalactosaminidase, which releases the disaccharide Gal (beta 1-3) GalNac from O-glycans, split 26,000 rat prolactin into a doublet of M(r) 26,000 to 25,500; and 3) mixed exoglycosidases from Turbo cornutus caused a gradual M(r) shift from 26,000 to 23,000. Affinity chromatography on wheat germ agglutinin Sepharose 6MB and soybean agglutinin agarose of rat pituitary homogenates and competitive inhibition tests showed that glycosylated rat prolactin has distinct affinity for these lectins. From the experimental data it is proposed that glycosylated rat prolactin is O-linked through threonine by the disaccharide Gal (beta 1-3) GalNac and possesses at least GalNac, and/or Gal and sialyl residues.

Identification of a D1 dopamine receptor, not linked to adenylate cyclase, on lactotroph cells

1. We studied the lactotroph cells of the rat by both in vivo and in vitro pharmacological techniques for the presence of D1-receptors. Both approaches revealed the presence of D2-receptor, stimulated by quinpirole (resulting in an inhibition of prolactin secretion) and blocked by domperidone. 2. Administration of fenoldopam, the most selective D1-receptor agonist currently available, resulted in a dose-dependent decrease of prolactin secretion in vivo (after pretreatment with alpha-methyl-p-tyrosine) and in vitro (cultured pituitary cells). This increase was dose-dependently blocked by the selective D1-receptor antagonist, SCH 23390, and although the effect of fenoldopam was less than that obtained by D2-receptor stimulation, these data suggest that a D1-receptor also controls prolactin secretion. 3. In order to detect the location of these dopamine receptors, autoradiographic studies were performed by use of [3H]-SCH 23390 and [3H]-spiperone as markers for D1- and D2-receptors, respectively. Specific binding sites for [3H]-SCH 23390 were demonstrated. Fenoldopam dose-dependently reduced [3H]-SCH 23390 binding, but had no effect on [3H]-spiperone binding. Immunocytochemical labelling of prolactin cells after incubation with [3H]-SCH 23390 revealed that the granulae and hence, D1 binding sites were present on the lactotroph cells. 4. Radioligand binding studies performed on membranes from anterior pituitary cells revealed the presence of the D2-receptor (54 fmol mg-1 protein) with a Kd of 0.58 nM for [3H]-spiperone, but failed to detect D1-receptors. 5. Finally, we studied the effect of dopamine and of fenoldopam on the adenosine 3':5'-cyclic monophosphate (cyclic AMP) content of anterior pituitary cells. Although cyclic AMP increased upon prostacyclin administration, indicating an intact adenylate cyclase system, fenoldopam failed to increase the cyclic AMP production. 6. It is tempting to speculate that fenoldopam reduces prolactin secretion through interaction with a non-cyclase-linked D1-receptor on the lactotroph cells.

Physiological characterization of two functional states in subpopulations of prolactin cells from lactating rats

1. Lactotroph cells from lactating female rat pituitary glands were dissociated, separated and enriched on a continuous gradient of bovine serum albumin at unit gravity. Two lactotroph subpopulations were observed in the light (F(3-5)) and the heavy (F(7-9)) fractions of the gradient. Both populations were maintained for at least 6 days in culture before experiments were performed. 2. Patch-clamp recordings, in the whole-cell mode, were performed on both lactotroph subpopulations in order to measure passive membrane properties and Ca2+ currents. Resting membrane potential as well as membrane capacitance values were found to be lower in light fraction cells. The two components of Ca2+ currents, called fast and slow deactivating (FD and SD) currents, were present with different proportions in each subpopulation; the ratio of current amplitudes, SD/FD, was 2.42 +/- 0.41 (n = 18) in light fraction cells and 1.17 +/- 0.27 (n = 17) in heavy fraction cells (P less than 0.02). 3. Reverse haemolytic plaque assay showed that in the light and heavy fractions, 68 and 47% of the lactotroph cells, respectively, were secreting. Population analysis of the plaque areas revealed a bimodal frequency distribution of plaque sizes consisting of small (1500 microns 2) and large plaques (3995 microns 2). A majority of light fraction cells produced large plaques whereas most of the heavy fraction cells produced small plaques. 4. Perifusion experiments performed on enriched prolactin cells showed that (1) basal prolactin (PRL) release was higher in light fraction than in heavy fraction cells, (2) the dopamine (10(-8)M)-induced inhibition of PRL release was greater in light fraction cells (86 +/- 15%) than in heavy fraction cells (41 +/- 21%), and (3) the thyrotrophin-releasing hormone (TRH, 10(-8)M)-induced increase of PRL release was 150 +/- 60% in light fraction versus 330 +/- 82% in heavy fraction cells. 5. Current-clamp recordings were performed using the intracellular technique. Lactotrophs were categorized according to their electrophysiological response following application of dopamine or TRH (both 10(-8)M). In the light fractions, the majority of the cells tested were hyperpolarized by dopamine (68%), whereas only 7% were depolarized by TRH application. In the heavy fractions, most of the cells (63%) responded to TRH application, while only 13% were dopamine sensitive. 6. Cytosolic free Ca2+ concentration ([Ca2+]i) measurements with the fluorescent probe Indo-1 revealed two lactotroph subtypes. Most cells in the light fractions (sixteen of twenty-two tested cells) exhibited an unstable level of [Ca2+]i with values fluctuating between 114.1 +/- 34.3 and 221 +/- 50 nM (mean +/- S.D.). Application of dopamine or of the D2 receptor agonist RU 24213 (10(-8)M) resulted in the disappearance of these fluctuations and in an accompanying decrease in basal [Ca2+]i level.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantitative in situ hybridization in neuro-endocrinology

In situ hybridization has become one of the most powerful tools for localizing specific gene products in a tissue. Labelled molecular probes are used on tissue sections or on cells for localization of their hybridization sites. In situ hybridization makes it possible to define the spatial and temporal distribution of specific genes. The use of probes dually labelled, or the combined use of a molecular probe and labelled antibodies provides unprecedented opportunities to examine gene expression in individual cells. In addition, the quantification of gene products can shed light on regulation of gene expression at the cellular level.

Growth hormone-releasing factor (GRF) suppresses the in vitro proliferation of mammotrophs from the adult rat

In order to examine the hypothalamic control of acidophilic proliferation, anterior pituitary cells from adult female rats were cultured with or without rat growth hormone-releasing factor fragment 1-29 (GRF-29). Changes in the numbers of mammotrophs and somatotrophs during culture were measured by immunocytochemical staining. The addition of GRF suppressed the increase in the number of mammotrophs even at the very low concentration of 10(-12) M. The number of somatotrophs increased in the medium containing GRF. The increase in mammotroph number was blocked by cytosine arabinoside, a mitotic inhibitor. GRF had no effect on the in vitro proliferation of fibroblasts. These results indicate the important role of hypothalamic GRF in the differential growth and secretion of the acidophils in vivo.

The relationship between growth hormone (GH) messenger ribonucleic acid levels and hormone release from individual cells derived from human GH-secreting pituitary adenomas

GH mRNA expression and GH release by individual cells derived from four GH-secreting pituitary adenomas were studied by in-situ hybridization and the reverse haemolytic plaque assay, respectively. In addition the percentage of PRL mRNA-containing cells was determined in these cell suspensions. The percentages of GH mRNA-containing cells varied between 52 and 89 while the percentages of GH plaque forming cells varied between 25 and 77. Frequency distributions of GH mRNA levels in individual cells and of individual GH plaque areas showed a majority of the cells having low GH mRNA levels and secreting low amounts of GH respectively, while there is a low proportion of cells expressing high GH mRNA levels and forming large GH plaques. There was a significant correlation between the GH mRNA levels and the GH plaque areas of individual cells from the four adenomas (P less than 0.001). The percentages of PRL mRNA-containing cells in the four different adenomas amounted to less than 1, 5, 2 and 18. Cultured cells from the adenomas consisting of 5 and 18% PRL mRNA-containing cells also contained and released measurable amounts of PRL. Our data show that individual cells from GH-secreting pituitary adenomas are heterogeneous with respect to GH mRNA expression, a small proportion of the cells expressing a high amount of GH mRNA. The heterogeneity in GH mRNA expression is correlated with the heterogeneity in GH release. These observations suggest that a considerable part of GH secreted from a GH-secreting pituitary adenoma is produced by a minority of the GH-secreting tumour cell population.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of dopaminergic antagonism by sulpiride on TRH and VIP-induced prolactin release in nonsuckled lactating rats

Prolactin (PRL) release was studied in female rats during midlactation using pharmacologic manipulations designed to mimic the hypothalamic effects of suckling. In the first experiment pituitary dopamine (DA) receptors were blocked by sulpiride (10 micrograms/rat i.v.). One hour later, thyrotropin-releasing hormone (TRH, 1.0 micrograms/rat i.v.) was given to induce PRL release. TRH released significantly more PRL following DA antagonism than when no DA antagonism was produced, suggesting that DA receptor blockade increased the sensitivity of the AP to TRH. In a second experiment, VIP (25 micrograms/rat) increased plasma prolactin 3-4 fold but this effect was not enhanced significantly by prior dopamine antagonism with sulpiride. We conclude that dopamine antagonism enhances the PRL releasing effect of TRH but not VIP in lactating rats.

Effect of tunicamycin, swainsonine, castanospermine, Beta-hydroxynorvaline and monensin on the post-translational processing of rat prolactin molecular forms

Abstract Prolactin cells derived from the anterior pituitaries of female rats were cultured in the presence of tunicamycin, swainsonine, castanospermine, beta-hydroxynorvaline and monensin in order to study their effect on the post-translational processing of the M(r) 17,000, 23,000 and 26,000 prolactin molecular forms. Sodium-dodecyl-sulphate polyacrylamide electrophoresis and subsequent immunoblotting revealed that: 1) tunicamycin, swainsonine and castanospermine, compounds that are essentially known as inhibitors of the N-glycosylation processus, had no effect on M(r) 17,000, 23,000 and 26,000 rat prolactin; 2) betahydroxynorvaline, which has been assumed to inhibit processing of pre-prolactin to mature 23,000 prolactin, did not increase the synthesis of 26,000 rat prolactin. In case of inhibition of the processing of a pre-prolactin to mature prolactin, one would expect an increase of the pre-prolactin; consequently, we could not establish the 26,000 rat prolactin, we revealed in immunoblotting, as a pre-prolactin; 3) monensin affected the post-translational processing of 17,000 and 26,000 rat prolactin, but left the 23,000 mature form intact. This is an important finding for the following reasons: monensin blocks the transport of secretory and membrane proteins, and this blockade prevents the cleavage of these molecules; indeed, production of 17,000 rat prolactin, a form of cleaved prolactin, was inhibited. Monensin also affects glycosylation and 26,000 rat prolactin has been identified as a presumably O-iinked glycosylated variant. The fact that its synthesis is inhibited by monensin treatment, but not by inhibitors of the N-linked process, particularly tunicamycin, and that 26,000 rat prolactin is susceptible to mild alkali and decomposition via beta-elimination are decisive arguments in favour of the O-linked glycosidic linkage.

Ontogeny of hormone-secreting cells of the rat pituitary gland: an immunocytochemical study on dissociated cells

An immunocytochemical study was undertaken in foetal, prepubertal and mature rats to determine the time of differentiation of various types of adenohypophyseal cells during development. Freshly dissociated pituitary cells from foetal (18-21 days postconception), neonatal (from birth up to 30 days) and adult rats (more than 8 weeks) were characterized using immunocytochemical methods. All types of hormone-producing cells were present at day 18 postconception, although only 20% of the cells were immunolabelled. Adrenocorticotropin (ACTH)-secreting cells accounted for the highest number of hormone-positive cells. Growth hormone-secreting cells increased remarkably from day 18 postconception onwards. Prolactin-secreting cells were not seen in the foetal adenohypophysis and did not start to increase until 10 days after birth, whereas by that time the number of ACTH, thyrotropin, follicle-stimulating and luteinizing hormone-secreting cells had stopped increasing. By day 30 after birth, 80-95% of the cells were immunoreactive.