Perifused pituitary cells as a model for studies of gonadotropin biosynthesis and secretion

Evidence for a novel intrapituitary autocrine/paracrine feedback loop regulating growth hormone synthesis and secretion in grass carp pituitary cells by functional interactions between gonadotrophs and somatotrophs

Gonadotropin (GTH) and GH released from the pituitary are known to interact at multiple levels to modulate the functions of the gonadotrophic and somatotrophic axes. However, their interactions at the pituitary level have not been fully characterized. In this study, autocrine/paracrine regulation of GH synthesis and secretion by local interactions between gonadotrophs and somatotrophs was examined using grass carp pituitary cells as a cell model. Exogenous GTH and GH induced GH release and GH mRNA expression in carp pituitary cells. Removal of endogenous GTH and GH by immunoneutralization with GTH and GH antisera, respectively, suppressed GH release, GH production, and GH mRNA levels. GH antiserum also blocked the stimulatory effects of exogenous GTH on GH release and GH mRNA levels. In reciprocal experiments, GH release and GH mRNA expression induced by exogenous GH was significantly reduced by GTH antiserum. In addition, exogenous GH was found to be inhibitory to basal GTH release and treatment with GH antiserum elevated GTH secretion at low doses but suppressed GTH production at high doses. These results suggest that local interactions between gonadotrophs and somatotrophs may form an intrapituitary feedback loop to regulate GH release and synthesis. In this model, GTH released from gonadotrophs induces GH release and GH production in neighboring somatotrophs. GH secreted maintains somatotroph sensitivity to GTH stimulation, and at the same time, inhibits basal GTH release in gonadotrophs. This feedback loop may represent a novel mechanism regulating GH release and synthesis in lower vertebrates.

Studies on the prolactin-releasing mechanism of histones H2A and H2B

In previous studies we demonstrated that histone preparations possess multiple effects in vivo on pituitary hormone secretion. We have now studied the specificity and signal transduction pathways involved in the prolactin (PRL)-releasing activity of histones H2A and H2B on perifused and incubated rat pituitary cells. In the perifusion experiments, freshly dispersed pituitary cells were packed into short columns and were continuously perifused with serum-free medium. The substances to be tested (stimuli) were pumped through the perifusion circuit, at the end of which perifusate fractions were collected and PRL measured by specific RIA. In the incubation studies, freshly dispersed pituitary cells were incubated in a metabolic incubator with different stimuli at different doses and for varying times. Perifusion of cells with median eminence extract (1/30), histone H2A (30 microM) or histone H2B (30 microM), generated clear PRL release responses. Cells incubated with histone H2A and H2B showed a dose- and time-dependent stimulatory effect on PRL release which, for H2A, was blocked by peptide MB-35, an 86-120 amino acid synthetic fragment of histone H2A. The polycation, poly-lys was unable to mimic the action of histones. To detect the possible signal transduction pathways involved in the response of lactotrophs to histones, cells were incubated with the calcium ionophore A23187, the calcium chelator EGTA, the intracellular phosphoinositide enhancer LiCl, the intracellular cAMP enhancers caffeine, NaF and forskolin, and the protein kinase C inhibitor, trifluoperazine (TFP). Both EGTA (or EGTA plus A23187 ionophore) and TFP were able to reduce significantly the response of lactotrophs to histones. Our results confirm previous evidence that histones may act as hypophysotropic signals. The data also suggest that calcium- and diacylglycerol-associated pathways participate in these effects.

Studies on the gonadotropin-releasing activity of thymulin: changes with age

We assessed the ability of thymulin, a zinc-dependent nonapeptide produced by the thymic epithelial cells, to influence the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from dispersed anterior pituitary (AP) cells from young, adult, and senescent female rats. Perifusion of young and senescent AP cells with thymulin doses of 10(-6) to 10(-5) M gave a significant stimulatory response for LH but not FSH. Gonadotropin release was always lower in the senescent cells. AP cells from both age groups incubated with 10(-8) to 10(-3) M thymulin showed a time- and dose-dependent response for both gonadotropins, with a maximal stimulation at 10(-7) M. Preincubation of thymulin with an antithymulin serum completely quenched the secretagogue activity of the hormone. Coincubation of thymulin with the secretagogue gonadotropin-releasing hormone (GnRH) revealed a synergistic effect on LH release and an additive effect on the release of FSH. The calcium chelator EGTA blocked the gonadotropin-releasing activity of thymulin in AP cells. The cAMP enhancers, caffeine, NaF, and forskolin significantly increased the thymulin-stimulated release of gonadotropins. The inositol phosphate enhancer LiCl potentiated the action of thymulin on gonadotropins. It is concluded that the gonadotropin-releasing activity documented here for thymulin is an age- and receptor-dependent effect mediated in part by calcium, cAMP, and inositol phosphates.

A mathematical model of luteinizing hormone release from ovine pituitary cells in perifusion

We model the effect of gonadotropin-releasing hormone (GnRH) on the production of luteinizing hormone (LH) by the ovine pituitary. GnRH, released by the hypothalamus, stimulates the secretion of LH from the pituitary. If stimulus pulses are regular, LH response will follow a similar pattern. However, during application of GnRH at high frequencies or concentrations or with continuous application, the pituitary delivers a decreased release of LH (termed desensitization). The proposed mathematical model consists of a system of nonlinear differential equations and incorporates two possible mechanisms to account for this observed behavior: desensitized receptor and limited, available LH. Desensitization was provoked experimentally in vitro by using ovine pituitary cells in a perifusion system. The model was fit to resulting experimental data by using maximum-likelihood estimation. Consideration of smaller models revealed that the desensitized receptor is significant. Limited, available LH was significant in three of four chambers. Throughout, the proposed model was in excellent agreement with experimental data.

Thymulin stimulates prolactin and thyrotropin release in an age-related manner

Thymulin is a Zn-bound nonapeptide produced by the thymic epithelial cells (TEC) whose secretion is modulated by prolactin (PRL) and thyroid hormones, among other hormones. We assessed the ability of thymulin to influence the release of PRL and thyroid stimulating hormone (TSH) from dispersed anterior pituitary (AP) cells from young, middle-aged and senescent Sprague-Dawley female rats. Perifused and incubated AP cells were used in different sets of experiments and PRL and TSH release was measured by radioimmunoassay. Perifusion of young and senescent AP cells with thymulin doses ranging from 10(-8) to 10(-5) M gave a logarithmic dose response pattern for both hormones. Supernatants from TEC lines also showed PRL and TSH secretagogue activity. Hormone release was always lower in the senescent cells. AP cells incubated with 10(-8) to 10(-3) M thymulin showed a time- and dose-dependent response for both hormones, the latter being bell-shaped with a maximum at 10(-7) M thymulin. Preincubation of thymulin with an anti-thymulin serum completely quenched the secretagogue activity of the thymic hormone. Coincubation of thymulin with TRH revealed a synergistic release of PRL and TSH in AP cells from all age groups. The calcium chelator EGTA but not the calcium ionophore A23187, blocked the hormone-releasing activity of thymulin in AP cells. The cAMP enhancers, caffeine, NaF and forskolin, significantly increased the thymulin-stimulated release of PRL and TSH, while trifluoperazine, a protein kinase C inhibitor, had no effect. The inositol phosphate enhancer LiCl, potentiated the action of thymulin on PRL and TSH. The present results suggest that the TRH-like activity documented here for thymulin is a receptor-mediated effect which appears to involve calcium, cAMP and inositol phosphates. Senescence but not middle age, appears to impair AP cell responsiveness to thymulin.

Somatostatin inhibits (d-Arg6, Pro9-NEt) salmon gonadotropin-releasing hormone- and dopamine D1-stimulated growth hormone release from perifused pituitary cells of chinese grass carp, ctenopharyngodon idellus

In this study, a heterologous radioimmunoassay (RIA) for grass carp GH has been validated and used to monitor the kinetics of GH release from perifused grass carp pituitary cells. To establish the anatomical specificity of GH antiserum used in this RIA, immunohistochemical staining was performed in grass carp pituitary sections. Somatotrophs recognized by this GH antiserum were located mainly in the proximal pars distalis without overlapping with gonadotrophs located in the same area or with lactotrophs located in the rostral pars distalis. The immunoreactivity of somatotrophs was abolished by preabsorbing GH antiserum with purified grass carp GH, suggesting that the possibility of a cross-reactivity of antiserum with other grass carp pituitary hormones is unlikely. Using 125I-labeled carp GH as the RIA tracer, parallelism was observed among the displacement curves of grass carp GH standard, grass carp serum, and culture medium conditioned by grass carp pituitary cells, suggesting that this RIA can be used to quantitate grass carp GH levels in biological samples. Using an in vitro column perifusion system, a superactive gonadotropin-releasing hormone (GnRH) analog (d-Arg6, Pro9-NEt)-sGnRH(sGnRHa, 0.3-30 nM), dopamine (DA, 0.1-10 muM), and the nonselective DA agonist apomorphine (0.1-10 muM) stimulated GH release from grass carp pituitary cells in a dose-dependent manner. The GH-releasing effect of DA was mimicked by the D1 agonists SKF38393 (0.1-10 muM) and SKF77434 (0.1-10 muM), but not by the D2 agonist LY171555 (3 muM). In addition, the GH response to DA (1 muM) was blocked by the D1 antagonist SCH23390 (5 muM) but not by the D2 antagonist (+/-) sulpiride (5 muM), suggesting that the GH-releasing action of DA is mediated through receptors resembling mammalian D1 receptors. Somatostatin-14 (SRIF14, 0.01-100 nM), unlike sGnRHa and DA, induced a dose-dependent suppression on basal GH release. At a high dose (100 nM), SRIF14 also abolished the GH responses to sGnRHa (100 nM), DA (10 muM), and the D1 agonist SKF38393 (3 muM). These results, as a whole, provide evidence that GH release in the grass carp is under the direct regulation of GnRH, DA, and SRIF at the pituitary cell level. The present study also suggests that DA D1 receptors are present in grass carp pituitary cells mediating the GH-releasing action of DA.

Identification of an inhibin receptor in gonadal tumors from inhibin alpha-subunit knockout mice

Inhibins and activins are dimeric proteins that are functional antagonists and are structurally related to the transforming growth factor-beta (TGFbeta) family of growth and differentiation factors. Receptors for activin and TGFbeta have been identified as dimers of serine-threonine kinase subunits that regulate cytoplasmic proteins known as Smads. Despite major advances in our understanding of activin and TGFbeta receptors and signaling pathways, little is known about inhibin receptors or the mechanism by which this molecule provides a functionally antagonistic signal to activin. Studies described in this paper indicate that an independent inhibin receptor exists. Numerous tissues were examined for inhibin-specific binding sites, including the developing embryo, in which the spinal ganglion and trigeminal ganglion-bound iodinated inhibin A. Sex cord stromal tumors, derived from male and female inhibin alpha-subunit-deficient mice, were also identified as a source of inhibin receptor. Abundant inhibin and few activin binding sites were identified in tumor tissue sections by in situ ligand binding using iodinated recombinant human inhibin A and 125I-labeled recombinant human inhibin A. Tumor cell binding was specific for each ligand (competed by excess unlabeled homologous ligand and not competed by heterologous ligand). Based on these results and the relative abundance and homogeneity of tumor tissues versus the embryonic ganglion, tumor tissues were homogenized, membrane proteins were purified, and putative inhibin receptors were isolated using an inhibin affinity column. Four proteins were eluted from the column that bind iodinated inhibin but not iodinated activin. These data suggest that inhibin-specific membrane-associated proteins (receptors) exist.