Ion channels and the signal transduction pathways in the regulation of growth hormone secretion

The allostery and modification of hGHRH molecules and specific dimer produced significant fertility effect by proliferating and activating in-situ ovarian mesenchymal stem cells

The negative coordination of growth hormone secretagogue receptor (GHS-R) and growth hormone-releasing hormone receptor (GHRH-R) involves in the repair processes of cellular injury. The allosteric U- or H-like modified GHRH dimer Grinodin and 2Y were comparatively evaluated in normal Kunming mice and hamster infertility models induced by CPA treatment. 1-3-9 µg of Grinodin or 2Y per hamster stem-cell-exhaustion model was subcutaneously administered once a week, respectively inducing 75-69-46 or 45-13-50 % of birth rates. In comparison, the similar mole of human menopausal gonadotropin (hMG) or human growth hormone (hGH) was administered once a day but caused just 25 or 20 % of birth rates. Grinodin induced more big ovarian follicles and corpora lutea than 2Y, hMG, hGH. The hMG-treated group was observed many distorted interstitial cells and more connective tissues and the hGH-treated group had few ovarian follicles. 2Y had a plasma lifetime of 21 days and higher GH release in mice, inducing lower birth rate and stronger individual specificity in reproduction as well as only promoting the proliferation of mesenchymal-stem-cells (MSCs) in the models. In comparison, Grinodin had a plasma lifetime of 30 days and much lower GH release in mice. It significantly promoted the proliferation and activation of ovarian MSCs together with the development of follicles in the models by increasing Ki67 and GHS-R expressions, and decreasing GHRH-R expression in a dose-dependent manner. However, the high GH and excessive estrogen levels in the models showed a dose-dependent reduction in fertility. Therefore, unlike 2Y, the low dose of Grinodin specifically shows low GHS-R and high GHRH-R expressions thus evades GH and estrogen release and improves functions of organs, resulting in an increase of fertility.

Ion Channel Signature in Healthy Pancreas and Pancreatic Ductal Adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is the fourth most common cause of cancer-related deaths in United States and Europe. It is predicted that PDAC will become the second leading cause of cancer-related deaths during the next decades. The development of PDAC is not well understood, however, studies have shown that dysregulated exocrine pancreatic fluid secretion can contribute to pathologies of exocrine pancreas, including PDAC. The major roles of healthy exocrine pancreatic tissue are secretion of enzymes and bicarbonate rich fluid, where ion channels participate to fine-tune these biological processes. It is well known that ion channels located in the plasma membrane regulate multiple cellular functions and are involved in the communication between extracellular events and intracellular signaling pathways and can function as signal transducers themselves. Hereby, they contribute to maintain resting membrane potential, electrical signaling in excitable cells, and ion homeostasis. Despite their contribution to basic cellular processes, ion channels are also involved in the malignant transformation from a normal to a malignant phenotype. Aberrant expression and activity of ion channels have an impact on essentially all hallmarks of cancer defined as; uncontrolled proliferation, evasion of apoptosis, sustained angiogenesis and promotion of invasion and migration. Research indicates that certain ion channels are involved in the aberrant tumor growth and metastatic processes of PDAC. The purpose of this review is to summarize the important expression, localization, and function of ion channels in normal exocrine pancreatic tissue and how they are involved in PDAC progression and development. As ion channels are suggested to be potential targets of treatment they are furthermore suggested to be biomarkers of different cancers. Therefore, we describe the importance of ion channels in PDAC as markers of diagnosis and clinical factors.

The treatment effect of novel hGHRH homodimer to male infertility hamster

Extra-hypothalamic growth hormone-releasing hormone (GHRH) plays an important role in reproduction. To study the treatment effect of Grin (a novel hGHRH homodimer), the infertility models of 85 male Chinese hamsters were established by intraperitoneally injecting 20 mg/kg of cyclophosphamide once in a week for 5 weeks and the treatment with Grin or human menopausal gonadotropin (hMG) as positive control was evaluated by performing a 3-week mating experiment. 2-8 mg/kg of Grin and 200 U/kg of hMG showed similar effect and different pathological characteristics. Compared to the single cyclophosphamide group (0%), the pregnancy rates (H-, M-, L-Grin 26.7, 30.8, 31.3%, and hMG 31.3%) showed significant difference, but there was no difference between the hMG and Grin groups. The single cyclophosphamide group presented loose tubules with pathologic vacuoles and significant TUNEL positive cells. Grin induced less weight of body or testis, compactly aligned tubules with little intra-lumens, whereas hMG caused more weight of body or testis, enlarging tubules with annular clearance. Grin presented a dose-dependent manner or cell differentiation-dependentincrease in testicular GHRH receptor, and did not impact the levels of blood and testicular GH, testosterone. Grin promotes fertility by proliferating and differentiating primitive cells through up-regulating testicular GHRH receptor without triggering GH secretion, which might solve the etiology of oligoasthenozoospermia.

Novel hGHRH homodimer promotes fertility of female infertile hamster by up-regulating ovarian GHRH receptor without triggering GH secretion

Extra-hypothalamic growth hormone-releasing hormone (GHRH) plays an important role in infertility. The female infertility models were formed by intraperitoneally injecting cyclophosphamide in 5-week-old Chinese hamster once in a week for 5 weeks. All the models mated with healthy male hamster in the ratio of 1:1 in the experimental 6-8th week and the couples were separated to breed in the 9-10th week. 20 mg/kg of cyclophosphamide induced temporary interference of reproduction and did not cause significant difference in the weight of body, bilateral ovaries, or liver. By intramuscularly injecting twice in a week during the experimental 4-10th week, 2, 4, 8 mg/kg of Grin induced 30, 42.9, 60% of total pregnancy rates in a dose-dependent manner whereas 200 U/kg of hMG induced 50% of total pregnancy rates. The single cyclophosphamide dose caused strongly eosinophilic ovarian cells, scattered early follicles, many atretic follicles, and no corpora luteum was observed. The hMG group individually presents many follicles at all levels, especially secondary ones in the ovarian cortex and medulla. Much of loose connective tissue, vacuoles, and sparse interstitial cells distribute in the medulla. Grin induced many follicles at all dose levels and corpora lutea in the cortex, and the compactly aligned interstitial cells occurred in the whole ovarian tissue. The less TUNEL staining and higher expression of ki67 showed the proliferation and protection effect of Grin on ovarian cells. Grin obviously promotes fertility by up-regulating ovarian GHRH receptor and strengthening the development and maturation of follicles without triggering central and ovarian GH secretion.

In silico analysis of the transportome in human pancreatic ductal adenocarcinoma

The altered expression and/or activity of ion channels and transporters (transportome) have been associated with malignant behavior of cancer cells and were proposed to be a hallmark of cancer. However, the impact of altered transportome in epithelial cancers, such as pancreatic ductal adenocarcinoma (PDAC), as well as its pathophysiological consequences, still remains unclear. Here, we report the in silico analysis of 840 transportome genes in PDAC patients' tissues. Our study was focused on the transportome changes and their correlation with functional and behavioral responses in PDAC tumor and stromal compartments. The dysregulated gene expression datasets were filtered using a cut-off of fold-change values ≤-2 or ≥2 (adjusted p value ≤0.05). The dysregulated transportome genes were clearly associated with impaired physiological secretory mechanisms and/or pH regulation, control of cell volume, and cell polarity. Additionally, some down-regulated transportome genes were found to be closely linked to epithelial cell differentiation. Furthermore, the observed decrease in genes coding for calcium and chloride transport might be a mechanism for evasion of apoptosis. In conclusion, the current work provides a comprehensive overview of the altered transportome expression and its association with predicted PDAC malignancy with special focus on the epithelial compartment.

Ghrelin reduces voltage-gated calcium currents in GH₃ cells via cyclic GMP pathways

Ghrelin is an endogenous growth hormone secretagogue (GHS) causing release of GH from pituitary somatotropes through the GHS receptor. Secretion of GH is linked directly to intracellular free Ca2+ concentration ([Ca2+]i), which is determined by Ca2+ influx and release from intracellular Ca2+ storage sites. Ca2+ influx is via voltage-gated Ca2+ channels, which are activated by cell depolarization. The mechanism underlying the effect of ghrelin on voltage-gated Ca2+ channels is still not clear. In this report, using whole cell patch-clamp recordings, we assessed the acute action of ghrelin on voltage-activated Ca2+ currents in GH3 rat somatotrope cell line. Ca2+ currents were divided into three types (T, N, and L) through two different holding potentials (-80 and -40 mV) and specific L-type channel blocker (nifedipine, NFD). We demonstrated that ghrelin significantly and reversibly decreases all three types of Ca2+ currents in GH3 cells through GHS receptors on the cell membrane and down-stream signaling systems. With different signal pathway inhibitors, we observed that ghrelin-induced reduction in voltage-gated Ca2+ currents in GH3 cells was mediated by a protein kinase G-dependent pathways. As ghrelin also stimulates Ca2+ release and prolongs the membrane depolarization, this reduction in voltage-gated Ca2+ currents may not be translated into a reduction in [Ca2+]i, or a decrease in GH secretion.

Treatment of endocrine pancreatic tumors

Endocrine pancreatic tumors are rare with an incidence of 4 per million inhabitants. Most tumors are malignant except for insulinomas that usually are benign. They are slowly growing in the majority of cases but there are exceptions with rapidly progressing malignant carcinomas. Because of the rarity of these tumors large randomized trials are difficult to accomplish. However, most physicians treating these patients agree that surgery should be considered in all cases and that medical treatment with chemotherapy and biotherapy is well established for this group of patients.

Characterization of intracellular signaling mediated by human somatostatin receptor 5: role of the DRY motif and the third intracellular loop

Somatostatin (SST) exerts inhibitory effects on hormone secretion and cell proliferation by interacting with five different receptors (SST1-SST5) linked to multiple cellular effectors. The receptor structural domains involved in these effects have been only partially elucidated. The aim of the study was to investigate the molecular determinants mediating the interaction of the human SST5 with intracellular signaling in the pituitary cell line GH3, focusing on the BBXXB domain in the third intracellular loop and the DRY motif in the second intracellular loop. We analyzed the effects of the SST5 agonist BIM23206 on cAMP accumulation, intracellular calcium, GH secretion, cell proliferation, and ERK1/2 phosphorylation in cells expressing either wild-type SST5 or mutant receptors, in particular the naturally occurring mutant R240W in the BBXXB domain and the D136A and R137A mutants in the DRY motif. We found that residues D136 and R137 were critical for SST5 signaling because their substitutions abolished all the intracellular responses. Conversely, third intracellular loop mutations resulted in receptor that inhibited intracellular cAMP levels similar to the wild-type (50 +/- 9 vs. 53 +/- 12% inhibition) but failed to mediate the other responses elicited by wild-type SST5, i.e. reduction of intracellular calcium levels as well as inhibition of ERK1/2. These events resulted in an absent inhibition of GH release and an impaired reduction of cell proliferation (38 +/- 7 vs. 76 +/- 6% inhibition in wild type, P < 0.05). These data indicate that different regions of SST5 are required for the activation of different signaling pathways.

Upregulation of voltage-gated Na+ channels by long-term activation of the ghrelin-growth hormone secretagogue receptor in clonal GC somatotropes

A central question in adenohypophyseal cell physiology concerns the role of transmembrane ionic fluxes in the initiation of the hormone secretion process. In the current report, we investigated the effects of the growth hormone (GH) secretagogues ghrelin and GH-releasing peptide-6 (GHRP-6) on the regulation of the functional expression of voltage-gated Na(+) channels using the tumoral somatotrope GC cell line as a model. Cells were cultured under control conditions or in presence of the GH secretagogues (GHS) for 96 h, and Na(+) currents (I(Na)) were characterized in whole cell patch-clamp experiments. GHS treatment significantly increased I(Na) density in a dose-dependent manner. The effects of GHRP-6 were accompanied by an augment in conductance without changes in the kinetics and the voltage dependence of the currents, suggesting an increase in the number of channels in the cell membrane. Sustained inhibition of L-type Ca(2+) channel activity decreased I(Na) density and prevented the effects of the GHS, whereas long-term exposure to an L-channel agonist increased I(Na) density and enhanced the actions of GHRP-6, indicating that Ca(2+) entry through these channels plays a role in the regulation of Na(+) channel expression. Likewise, GHRP-6 failed to enhance Na(+) channel expression in the presence of membrane-permeable inhibitors of protein kinases A and C, as well as the Ca(2+)/calmodulin-dependent kinase II. Conversely, treatment with a cAMP analog or a protein kinase C activator enhanced both basal and GHS-induced secretion of GH measured by enzyme-linked immunoassay, suggesting that GHRP-6 acting through the ghrelin receptor and different signaling pathways enhances Na(+) channel membrane expression, which favors hormone release from GC somatotropes.

Involvement of tetrodotoxin-resistant Na+ current and protein kinase C in the action of growth hormone (GH)-releasing hormone on primary cultured somatotropes from GH-green fluorescent protein transgenic mice

GHRH depolarizes the membrane of somatotropes, leading to an increase in intracellular free Ca2+ concentration and GH secretion. Na+ channels mediate the rapid depolarization during the initial phase of the action potential, and this regulates Ca2+ influx and GH secretion. GHRH increases a tetrodotoxin-sensitive somatotrope Na+ current that is mediated by cAMP. TTX-resistant (TTX-R) Na+ channels are abundant in sensory neurons and cardiac myocytes, but their occurrence and/or function in somatotropes has not been investigated. Here we demonstrate expression of TTX-R Na+ channels and a TTX-R Na+ current, using patch-clamp method, in green fluorescent protein-GH transgenic mouse somatotropes. GHRH (100 nm) increased the TTX-R Na+ current in a reversible manner. The GHRH-induced increase in TTX-R Na+ current was not affected by the cAMP antagonist Rp-cAMP or protein kinase A inhibitors KT5720 or H89. The TTX-R current was increased by 8-bromoadenosine-cAMP (cAMP analog), forskolin (adenylyl-cyclase activator), and 3-isobutyl-1-methylxanthine (phosphodiesterase inhibitor), but the additional, GHRH-induced increase in TTX-R Na+ currents was not affected. U-73122 (phospholipase C inhibitor) and protein kinase C (PKC) inhibitors, Gö-6983 and chelerythrine, blocked the effect of GHRH. PKC activators, phorbol dibutyrate and phorbol myristate acetate, increased the TTX-R Na+ current, but GHRH had no further effect on the current. Na+-free extracellular medium significantly reduced GHRH-stimulated GH secretion. We conclude that GHRH-induced increase in the TTX-R Na+ current in mouse somatotropes is mediated by the PKC system. An increase in the TTX-R Na+ current may contribute to the GHRH-induced exocytosis of GH granules from mouse somatotropes.

Ghrelin induces growth hormone secretion via a nitric oxide/cGMP signalling pathway

The presence of ghrelin and its receptor, growth hormone (GH) secretagogue receptor, in the hypothalamus and pituitary, and its ability to stimulate GH release in vivo and in vitro, strongly support a significant role for this peptide in the control of somatotroph function. We previously demonstrated that ghrelin elicits GH secretion directly in somatotrophs by activating two major signalling cascades, which involve inositol phosphate and cAMP. In as much as nitric oxide (NO) and its mediator cGMP have been recently shown to contribute substantially to the response of somatotrophs to key regulatory hormones, including GH-releasing hormone, somatostatin and leptin, we investigated the possible role of this signalling pathway in ghrelin-induced GH release in vitro. Accordingly, cultures of pituitary cells from prepuberal female pigs were challenged with ghrelin (10(-8) m, 30 min) in the absence or presence of activators or blockers of key steps of the NO synthase (NOS)/NO/guanylate cyclase (GC)/cGMP route and GH secretion was measured. Two distinct activators of the NO route, S-nitroso-N-acetylpenicillamine (SNAP) (5 x 10(-4) m) and L-arginine methyl ester hydrochloride (L-AME) (10(-3) m), comparably stimulated GH secretion when applied alone. The presence of L-AME enhanced ghrelin-stimulated GH secretion, whereas SNAP did not alter its effect. Conversely, two different NOS/NO pathway inhibitors, N(w)-nitro-L-arginine methyl ester hydrochloride (10(-5) m) or haemoglobin (20 microg/ml), similarly blocked ghrelin-induced (but not basal) GH release, thus indicating that NO contributes critically to ghrelin action in somatotrophs. Moreover, incubation with a permeable cGMP analogue, 8-Br-cGMP (10(-8) m) stimulated GH secretion, but did not modify the stimulatory action of ghrelin, suggesting that cGMP could mediate the action of NO. Indeed, inhibition of GC by 10 microm LY-53,583 did not alter basal GH secretion but abolished the GH-releasing action of ghrelin. Taken together, our results provide novel evidence indicating that ghrelin requires activation of the NOS/NO route, and its subsequent GC/cGMP signal transduction pathway, as necessary steps to induce GH secretion from somatotrophs.

Involvement of somatostatin receptor subtypes in membrane ion channel modification by somatostatin in pituitary somatotropes

1. Growth hormone (GH) secretion from pituitary somatotropes is mainly regulated by two hypothalamic hormones, GH-releasing hormone (GHRH) and somatotrophin releasing inhibitory factor (SRIF). 2. Somatotrophin releasing inhibitory factor inhibits GH secretion via activation of specific membrane receptors, somatostatin receptors (SSTRs) and signalling transduction systems in somatotropes. 3. Five subtypes of SSTRs, namely SSTR1, 2, 3, 4 and 5, have been identified, with the SSTR2 subtype divided into SSTR2A and SSTR2B. All SSTRs are G-protein-coupled receptors. 4. Voltage-gated Ca(2+) and K(+) channels on the somatotrope membrane play an important role in regulating GH secretion and SRIF modifies both channels to reduce intracellular free Ca(2+) concentration and GH secretion. 5. Using specific SSTR subtype-specific agonists, it has been found that reduction in Ca(2+) currents by SRIF is mediated by SSTR2 and an increase in K(+) currents is mediated by both SSTR2 and SSTR4 in rat somatotropes.

Somatostatin decreases voltage-gated Ca2+ currents in GH3 cells through activation of somatostatin receptor 2

The secretion of growth hormone (GH) is inhibited by hypothalamic somatostatin (SRIF) in somatotropes through five subtypes of the somatostatin receptor (SSTR1-SSTR5). We aimed to characterize the subtype(s) of SSTRs involved in the Ca2+ current reduction in GH3 somatotrope cells using specific SSTR subtype agonists. We used nystatin-perforated patch clamp to record voltage-gated Ca2+ currents, using a holding potential of -80 mV in the presence of K+ and Na+ channel blockers. We first established the presence of T-, L-, N-, and P/Q-type Ca2+ currents in GH3 cells using a variety of channel blockers (Ni+, nifedipine, omega-conotoxin GVIA, and omega-agatoxin IVA). SRIF (200 nM) reduced L- and N-type but not T- or P/Q-type currents in GH3 cells. A range of concentrations of each specific SSTR agonist was tested on Ca2+ currents to find the maximal effective concentration. Activation of SSTR2 with 10(-7) and 10(-8) M L-797,976 decreased the voltage-gated Ca2+ current and abolished any further decrease by SRIF. SSTR1, SSTR3, SSTR4, and SSTR5 agonists at 10(-7) M did not modify the voltage-gated Ca2+ current and did not affect the Ca2+ current response to SRIF. These results indicate that SSTR2 is involved mainly in regulating voltage-gated Ca2+ currents by SRIF, which contributes to the decrease in intracellular Ca2+ concentration and GH secretion by SRIF.

Somatostatin increases voltage-gated K+ currents in GH3 cells through activation of multiple somatostatin receptors

The secretion of GH by somatotropes is inhibited by somatostatin (SRIF) through five specific membrane receptors (SSTRs). SRIF increases both transient outward (IA) and delayed rectifying (IK) K+ currents. We aim to clarify the subtype(s) of SSTRs involved in K+ current enhancement in GH3 somatotrope cells using specific SSTR subtype agonists. Expression of all five SSTRs was confirmed in GH3 cells by RT-PCR. Nystatin-perforated patch clamp was used to record voltage-gated K+ currents. We first established the presence of IA and IK type K+ currents in GH3 cells using different holding potentials (-40 or -70 mV) and specific blockers (4-aminopirimidine and tetraethylammonium chloride). SRIF (200 nM) increased the amplitude of both IA and IK in a fully reversible manner. Various concentrations of each specific SRTR agonist were tested on K+ currents to find the maximal effective concentration. Activation of SSTR2 and SSTR4 by their respective agonists, L-779,976 and L-803,087 (10 nM), increased K+ current amplitude without preference to IA or IK, and abolished any further increase by SRIF. Activation of SSTR1 and SSTR5 by their respective agonists, L-797,591 or L-817,818 (10 nM), increased K+ current amplitude, but SRIF evoked a further increase. The SSTR3 agonist L-797,778 (10 nM) did not affect the K+ currents or the response to SRIF. These results indicate that SSTR1, -2, -4, and -5 may all be involved in the enhancement of K+ currents by SRIF but that only the activation of SSTR2 or -4 results in the full activation of K+ current caused by SRIF.

Rapid suppressing action of insulin-like growth factor-I (IGF-I) on GH release from anterior pituitary cells of goats

Goat anterior pituitary cells were cultured to investigate the effects of insulin-like growth factor-I (IGF-I), insulin, and growth hormone (GH) on basal and GH-releasing hormone (GHRH)-induced GH release. Changes in cellular Ca2+ concentrations were also assessed to enable discussion of the cellular mechanisms of IGF-I. The cells were cultured for 48 h, and then stimulated with GHRH (10 nmol/l) for 30 min, with or without each test substance. In the control cells, IGF-I (10 and 100 ng/ml) significantly raised the basal, but did not change GHRH-induced GH release, resulting in the abolishment of GH release induced by GHRH in the presence of 100 ng/ml IGF-I. However, there was no significant effect of insulin (10, 100, and 1000 microU/ml) on basal and GHRH-induced GH release. In the cells cultured for 48 h with each test substance but stimulated for 30 min without the test substance, no significant change in the basal and GHRH-stimulated GH release was observed. Regardless of treatment, there was no significant effect on intra-cellular GH content. Analysis with a confocal laser microscope revealed that IGF-I (100 ng/ml) significantly increased the basal, but significantly reduced GHRH (10 nmol/l)-induced increase in cellular Ca2+ concentrations. From these findings we conclude that IGF-I exerts an acute suppressing action on the GHRH-induced GH release, which partly involves changes in cellular Ca2+ metabolism in goat somatotrophs.

Expression and distribution of somatostatin receptor subtypes in the pancreatic islets of mice and rats

Somatostatin acts on specific membrane receptors (sst(1-5)) to inhibit exocrine and endocrine functions. The aim was to investigate the distribution of sst(1-5) in pancreatic islet cells in normal mice and rats. Pancreatic samples from five adult C57BL/6 mice and Sprague-Dawley rats were stained with antibodies against sst(1-5) and insulin, glucagon, somatostatin, or pancreatic polypeptide (PP). A quantitative analysis of the co-localization was performed. All ssts were expressed in the pancreatic islets and co-localized on islet cells to various extents. A majority of the beta-cells expressed sst(1-2) and sst(5) in mouse islets, while < or =50% in the rat expressed sst(1-5). The expression of sst(1-5) on alpha-cells did not differ much among species, with sst(2) and sst(5) being highly expressed. About 70% of the delta-cells expressed sst(1-4) in the rat pancreas, whereas 50% of the islet cells expressed sst(1-5) in the mouse. Furthermore, 60% of the PP-cells expressed sst(1-5) in the mouse, while the rat islets had lower values. Co-expression with the four major islet hormones varies among species and sst subtypes. These similarities and differences are interesting and need further evaluation to elucidate their physiological role in islets.

Intracellular signaling mechanisms mediating ghrelin-stimulated growth hormone release in somatotropes

Ghrelin is a newly discovered peptide that binds the receptor for GH secretagogues (GHS-R). The presence of both ghrelin and GHS-Rs in the hypothalamic-pituitary system, together with the ability of ghrelin to increase GH release, suggests a hypophysiotropic role for this peptide. To ascertain the intracellular mechanisms mediating the action of ghrelin in somatotropes, we evaluated ghrelin-induced GH release from pig pituitary cells both under basal conditions and after specific blockade of key steps of cAMP-, inositol phosphate-, and Ca2+-dependent signaling routes. Ghrelin stimulated GH release at concentrations ranging from 10-10 to 10-6 m. Its effects were comparable with those exerted by GHRH or the GHS L-163,255. Combined treatment with ghrelin and GHRH or L-163,255 did not cause further increases in GH release, whereas somatostatin abolished the effect of ghrelin. Blockade of phospholipase C or protein kinase C inhibited ghrelin-induced GH secretion, suggesting a requisite role for this route in ghrelin action. Unexpectedly, inhibition of either adenylate cyclase or protein kinase A also suppressed ghrelin-induced GH release. In addition, ghrelin stimulated cAMP production and also had an additive effect with GHRH on cAMP accumulation. Ghrelin also increased free intracellular Ca2+ levels in somatotropes. Moreover, ghrelin-induced GH release was entirely dependent on extracellular Ca2+ influx through L-type voltage-sensitive channels. These results indicate that ghrelin exerts a direct stimulatory action on porcine GH release that is not additive with that of GHRH and requires the contribution of a multiple, complex set of interdependent intracellular signaling pathways.

The in vitro regulation of growth hormone secretion by orexins

Orexins, orexigenic neuropeptides, have recently been discovered in lateral hypothalamus and play an important role in the regulation of pituitary hormone secretion. Two subtypes of orexin receptors (orexin-1 and orexin-2) have been demonstrated in pituitaries. In this experiment, the effects of orexins on voltage-gated Ca2+ currents and the GH release in primary cultured ovine somatotropes were examined. Voltage-gated Ca2+ currents were isolated in ovine somatotropes as L, T, and N currents using whole-cell patch-clamp techniques and specific Ca2+ channel blocker and toxin. Application of orexin-A or orexin-B (100 nM) significantly, dose-dependently, and reversibly increased only nifedipine-sensitive L-type Ca2+ current. Inhibitors of PKC (calphostin C, PKC inhibitory peptide) but not inhibitors of PKA (H89, PKA inhibitory peptide) cancelled the increase in the L current by orexins. Co-administration of orexin-A and GHRH (10 nM) showed an additive effect on the L current. Specific intracellular Ca2+-store-depleting reagent, thapsigargin (1 microM), did not affect the orexin-induced increase in the L current. Orexin-B alone slightly increased GH release and co-administration of orexin-A and GHRH synergistically stimulated GH secretion in vitro. It is therefore suggested that orexins may play an important role in regulating GHRH-stimulated GH secretion through an increase in the L-type Ca2+ current and the PKC-mediated signaling pathways in ovine somatotropes.

International Union of Pharmacology. XXXV. The glucagon receptor family

Peptide hormones within the secretin-glucagon family are expressed in endocrine cells of the pancreas and gastrointestinal epithelium and in specialized neurons in the brain, and subserve multiple biological functions, including regulation of growth, nutrient intake, and transit within the gut, and digestion, energy absorption, and energy assimilation. Glucagon, glucagon-like peptide-1, glucagon-like peptide-2, glucose-dependent insulinotropic peptide, growth hormone-releasing hormone and secretin are structurally related peptides that exert their actions through unique members of a structurally related G protein-coupled receptor class 2 family. This review discusses advances in our understanding of how these peptides exert their biological activities, with a focus on the biological actions and structural features of the cognate receptors. The receptors have been named after their parent and only physiologically relevant ligand, in line with the recommendations of the International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC-IUPHAR).

Orexin-A augments voltage-gated Ca2+ currents and synergistically increases growth hormone (GH) secretion with GH-releasing hormone in primary cultured ovine somatotropes

Orexins are recently discovered neuropeptides that play an important role in the regulation of hormone secretion, and their receptors have been recently demonstrated in the pituitary. The effects of orexin-A on voltage-gated Ca2+ currents and GH release in primary cultured ovine somatotropes were examined. The expression of orexin-1 receptor was demonstrated by RT-PCR in ovine somatotropes, from which Ca2+ currents were also isolated as L, T, and N currents. Application of orexin-A (100 nM) significantly and reversibly increased only the L current, and coadministration of orexin-A and GHRH (10 nM) showed an additive effect on this current, but no effect of orexin-A was observed on either T or N current. Furthermore, the orexin-A-induced increase in the L current was completely abolished by the inhibition of protein kinase C (PKC) activity using calphostin C (100 nM), phorbal 12,13-dibutyrate pretreatment (0.5 micro M) for 16 h or specific PKC inhibitory peptide PKC(19-36) (1 mM). However, the increase in L current by orexin-A was sustained when cells were preincubated with a specific protein kinase A blocker H89 (1 micro M) or a specific intracellular Ca2+ store depleting reagent thapsigargin (1 micro M). Finally, orexin-A alone did not significantly increase GH release, but coadministration of orexin-A and GHRH showed a synergistic effect on GH secretion in vitro. Our results therefore suggest that orexin-A may play an important role in regulating GHRH-stimulated GH secretion through the enhancement of the L-type Ca2+ current and the PKC-mediated signaling pathway in ovine somatotropes.

Growth hormone-releasing peptide-2 reduces inward rectifying K+ currents via a PKA-cAMP-mediated signalling pathway in ovine somatotropes

Inward-rectifying potassium (Kir) channels are essential for maintaining the resting membrane potential near the K(+) equilibrium and they are responsible for hyperpolarisation-induced K(+) influx. We characterised the Kir current in primary cultured ovine somatotropes and examined the effect of growth hormone-releasing peptide-2 (GHRP-2) on this current and its related intracellular signalling pathways. The Kir current was, in most cases, isolated using nystatin-perforated patch-clamp techniques. In bath solution containing 5 mM K(+), the Kir current was composed of both transient (fast activated) and delayed (slowly activated) components. An increase in the external K(+) concentration from 5 to 25 mM induced an augmentation of approximately 4-fold in the delayed part of the Kir current and both BaCl(2) and CsCl dose-dependently inhibited this current, confirming the presence of the Kir current in ovine somatotropes. Moreover, this specific effect of high K(+) on the Kir current was only observed in the cells that showed positive staining with anti-growth hormone (GH) antibodies, or in GC cells that belong to a rat somatotrope cell line. Application of GHRP-2 (100 nM) reversibly and significantly reduced the Kir current in bath solutions with 5 or 25 mM K(+) in ovine somatotropes. In addition, we found that the reduction in the Kir current mediated by GHRP-2 was totally abolished by the pretreatments with H89 (1 microM) or Rp-cAMP (100 microM) or by intracellular dialysis of a specific protein kinase A (PKA) inhibitory peptide PKI (10 microM). The specific PKC blocker chelerythrine (1 microM) or inhibitory peptide PKC(19-36) (10 microM) did not show any effects on the GHRP-2-induced decrease in the Kir current. These results suggest that the inhibition of Kir current through PKA-cAMP pathways may play an integral role in GHRP-2-induced depolarisation and GH release in ovine somatotropes.

The effect of two-day treatment of primary cultured ovine somatotropes with GHRP-2 on membrane voltage-gated K+ currents

Long-term in vivo treatment with synthetic GH-releasing peptides (GHRPs) enhances the release of GH induced by endogenous GHRH. The mechanism for such an enhancement on GH release is unknown. In this experiment, somatotropes were obtained from ovine pituitaries by enzyme dissociation and enriched by density centrifugation. Membrane voltage and currents were recorded with whole-cell patch-clamp configuration. After 48-h treatment with GHRP-2 (10(-8) M), the percentage of cells with spontaneous action potential was increased (51 vs. 27%) without change of resting potential. This GHRP-2 treatment also increased the amplitude of voltage-gated K+ currents (predominantly transient A-type-like current but also delayed rectifier or K-type-like current) without modification of biophysical kinetics. Down-regulation of protein kinase C (PKC) with phorbol 12-myristate 13-acetate at the time of adding GHRP-2 blocked the increase in K+ currents. Inclusion of calphostin C (PKC inhibitor) but not H(89) (protein kinase A inhibitor) significantly reduced the increase in K+ currents by GHRP-2. Inclusion of actinomycin D (transcription inhibitor) or cycloheximide (protein synthesis inhibitor) abolished the increase in K+ currents. These data indicate that 48-h GHRP-2 treatment increases the density of K+ channels via PKC and channel protein synthesis. Such a modification on K+ channels by GHRP-2 may be partially responsible for the change of somatotrope electrophysiological properties and sensitivity to GHRH stimulation.

Research progress in the stimulatory inputs regulating growth hormone (GH) secretion

A review is presented on progress in the research of stimulatory inputs that regulate growth hormone secretion, including recent results on the action of the hypothalamic peptides growth-hormone releasing factor (GHRH) and pituitary adenylate cyclase-activating polypeptide (PACAP), as well as that of both peptidic (growth hormone-releasing hexapeptide; GHRP-6) and non-peptidyl (L-163,255) synthetic GHSs on somatotrope cell function.

Somatostatin stimulates GH secretion in two porcine somatotrope subpopulations through a cAMP-dependent pathway

Somatostatin (SRIF) inhibits GH release from rat somatotropes by reducing adenylate cyclase (AC) activity and the free cytosolic calcium concentration ([Ca(2+)](i)). In contrast, we have reported that SRIF can stimulate GH release in vitro from pig somatotropes. Specifically, 10(-7) and 10(-15) M SRIF stimulate GH release from a subpopulation of high density (HD) somatotropes isolated by Percoll gradient centrifugation, whereas in low density (LD) somatotropes only 10(-15) M SRIF induces such an effect. To ascertain the signaling pathways underlying this phenomenon, we assessed SRIF effects on second messengers in cultured LD and HD cells by measuring cAMP, IP turnover, and [Ca(2+)](i). Likewise, contribution of the corresponding signaling pathways to SRIF-induced GH release was evaluated by blocking AC, PLC, extracellular Ca(2+) influx, or intracellular Ca(2+) mobilization. Both 10(-7) and 10(-15) M SRIF increased cAMP, IP turnover, and [Ca(2+)](i) in HD cells. Conversely, in LD cells 10(-7) M SRIF reduced [Ca(2+)](i), but did not alter cAMP or IP, and 10(-15) M SRIF was without effect. Interestingly, SRIF-stimulated GH release was abolished in both subpopulations by AC blockade, but not by PLC inhibition. Furthermore, SRIF-induced GH release was not reduced by blockade of extracellular Ca(2+) influx through voltage-sensitive channels or by depletion of thapsigargin-sensitive intracellular Ca(2+) stores. Therefore, SRIF stimulates GH secretion from cultured porcine somatotrope subpopulations through an AC/cAMP pathway-dependent mechanism that is seemingly independent of net increases in IP turnover or [Ca(2+)](i). These novel actions challenge classic views of SRIF as a mere inhibitor for somatotropes and suggest that it may exert a more complex, dual function in the control of porcine GH release, wherein molecular heterogeneity of somatotropes would play a critical role.

Diverse intracellular signalling systems used by growth hormone-releasing hormone in regulating voltage-gated Ca2+ or K channels in pituitary somatotropes

Influx of Ca2+ via Ca2+ channels is the major step triggering exocytosis of pituitary somatotropes to release growth hormone (GH). Voltage-gated Ca2+ and K+ channels, the primary determinants of the influx of Ca2+, are regulated by GH-releasing hormone (GHRH) through G-protein-coupled intracellular signalling systems. Using whole-cell patch-clamp techniques, the changes of the Ca2+ and K+ currents in primary cultured ovine and human somatotropes were recorded. Growth hormone-releasing hormone (10 nmol/L) increased both L- and T-type voltage-gated Ca2+ currents. Inhibition of the cAMP/protein kinase A (PKA) pathway by either Rp-cAMP or H89 blocked this increase in both L- and T-type Ca2+ currents. Growth hormone-releasing hormone also decreased voltage-gated transient (IA) and delayed rectified (IK) K+ currents. Protein kinase C (PKC) inhibitors, such as calphostin C, chelerythrine or downregulation of PKC, blocked the effect of GHRH on K+ currents, whereas an acute activation of PKC by phorbol 12, 13-dibutyrate (1 micromol/L) mimicked the effect of GHRH. Intracellular dialysis of a specific PKC inhibitor (PKC19-36) also prevented the reduction in K+ currents by GHRH. It is therefore concluded that GHRH increases voltage-gated Ca2+ currents via cAMP/PKA, but decreases voltage-gated K+ currents via the PKC signalling system. The GHRH-induced alteration of Ca2+ and K+ currents augments the influx of Ca2+, leading to an increase in [Ca2+]i and the GH secretion.

Expression of the five different somatostatin receptor subtypes in endocrine cells of the pancreas

Knowledge concerning tissue-specific expression of the five somatostatin receptor subtypes is of great importance in understanding their physiological function. We developed rabbit polyclonal antibodies specific for each human somatostatin receptor subtype and report our results concerning the expression in normal endocrine pancreatic cells. The antibodies were produced by immunizing rabbits with fragments specific for the five cloned somatostatin receptor subtypes. Colocalization of these somatostatin receptors with the four major islet hormones--insulin, glucagon, somatostatin, and pancreatic polypeptide--was studied in normal human endocrine pancreatic cells, using double-immunofluorescence staining. High expression of somatostatin receptor subtypes 1, 3, and 4 was found in all endocrine pancreatic cells. Somatostatin receptor subtype 2 was frequently expressed in alpha and beta cells, whereas expression was low in pancreatic polypeptide cells and intermediate in delta cells. Somatostatin receptor subtype 5 was expressed in most beta and delta cells but almost absent in alpha and pancreatic polypeptide cells. There is a variability in the normal expression of somatostatin receptor subtypes among the different human endocrine pancreatic cells. Knowledge of this expression and the physiological function mediated by these receptors will be valuable in the future when considering treatment of endocrine disorders.

Growth hormone secretagogue actions on the pituitary gland: multiple receptors for multiple ligands?

1. Growth hormone (GH) secretion is thought to occur under the reciprocal regulation of two hypothalamic hormones, namely GH-releasing hormone (GHRH) and somatostatin (SRIF), through their engagement with specific cell-surface receptors on the anterior pituitary somatotropes. 2. In addition to GHRH and SRIF, synthetic GH-releasing peptides (GHRP) or GH secretagogue(s) (GHS) regulate GH release through the activation of a novel receptor, the GHS receptor (GHS-R). 3. The cloning of the GHS-R from human, swine and rat identifies a novel G-protein-coupled receptor involved in the control of GH secretion and supports the existence of an undiscovered hormone that may activate this receptor. 4. Varieties of intracellular signalling systems are suggested to mediate the action of GHS, which include changes in intracellular free Ca2+ ([Ca2+]i), cAMP, protein kinases A and C, phospholipase C etc. 5. With regard to the use of signalling systems by GHS, especially a new form of GHRP or GHRP-2, a clear species difference has been demonstrated, supporting the possibility of more than one type of GHS-R.

Growth hormone-releasing hormone decreases voltage-gated potassium currents in GH4C1 cells

The electrophysiological properties of anterior pituitary somatotropes integrally involve the function of voltage-gated K+ currents. In this study, we have used GH4C1 cell lines to investigate the effect of human GHRH on voltage-gated K+ currents. Because of a clear 'rundown' of the K+ current with classic whole cell recording (WCR) without ATP in pipette solution, nystatin-perforated WCR was the major recording configuration used. Using a low Ca2+ (0.5 mM) bath solution containing Co2+ (1 mM) and TTX (1 microM), GH4 cells predominantly exhibited an outward delayed rectifier K+ current (IK). Local application of growth hormone releasing hormone (GHRH) (100 nM) reversibly reduced the amplitude of the K+ currents (to 83% of control). There was no effect of GHRH on the activation curve of the K+ current and no difference observed using 2.5 mM Ca2+ or low Ca2+ (0.5 mM Ca2++1 mM Co2+) bath solutions. Under the condition of low Ca2+ bath solution, the application of apamin (1 microM) or charybdotoxin (1 microM), two specific blockers of the Ca2+-activated K+ current, did not alter the K+ current or the response to GHRH. This reduction in the K+ current by GHRH was also observed with classic WCR with a pipette solution containing ATP (2 mM). The GHRH-induced reduction in the K+ current was completely abolished by the presence of GDP-beta-s (500 microM) in the pipette solution or by addition of PKC inhibitors, calphostin C (100 nM) and chelerythrine (1 microM), in bath solution. Inhibitor for cAMP-PKA system (Rp-cAMP and H89) did not affect the K+ current response to GHRH. These results suggest that GHRH reduces the voltage-gated K+ current in GH4C1 cells, a response that is mediated by G-proteins and PKC system but not by cAMP-PKA system. The reduction in the K+ current may partially contribute to the GHRH-stimulated growth hormone (GH) secretion.

Somatostatin receptor scintigraphy during treatment with lanreotide in patients with neuroendocrine tumors

To investigate possible changes in somatostatin receptor expression during treatment with high dose lanreotide, eight patients with neuroendocrine tumors were investigated by [(111)In-DTPA-D-Phe1]-octreotide scintigraphy before and during treatment. The spleen-to-background ratio decreased in all patients, whereas tumor-to-background ratio revealed a heterogeneous pattern with an average increase of 50% (-79% to +1,087%). This finding indicates that lanreotide treatment may influence the binding of radioactively labeled somatostatin to the spleen, while changes in the binding to functioning somatostatin receptors in tumor cells are more complex and not clearly related to treatment.

Human GHRH reduces voltage-gated K+ currents via a non-cAMP-dependent but PKC-mediated pathway in human GH adenoma cells

1. Whole-cell voltage-gated K+ currents and the K+ current response to growth hormone-releasing hormone (GHRH) were characterised in primary cultures of human acromegalic somatotropes. 2. Both delayed rectifier (IK) and transient (IA) K+ currents were recorded from human somatotropes held at -80 mV and bathed in a solution containing Cd2+ (1 mM), TTX (1 microM) and a low concentration of Ca2+ (0.5 mM). Only IK was recorded, however, when a holding potential of -40 mV was used. 3. GHRH (10 nM) immediately and significantly reduced the amplitude of both IA and IK. While the reduction in the amplitude of IA was fully reversed following the removal of GHRH, the amplitude of IK had only partially recovered 10 min after GHRH removal. In addition, GHRH shifted the voltage-dependent inactivation curve of IA by 13.5 mV in the negative direction. 4. In a low Ca2+ and Cd2+-containing solution, the Ca2+-activated K+ channel blockers apamin (100 nM and 1 microM) and charybdotoxin (1 microM) did not alter K+ currents or the effect of GHRH on the recorded K+ currents. 5. The whole-cell K+ currents and their responses to GHRH were unaffected by the application of 8-bromo-cAMP (100 microM), Rp-cAMP (100 microM) or the protein kinase A (PKA) inhibitor H89 (1 microM). In addition, intracellular dialysis of the PKA inhibitory peptide PKI (10 microM) had no effect on the K+ current response to GHRH. 6. While the application of protein kinase C (PKC) inhibitors calphostin C (100 nM) or chelerythrine (1 microM) did not affect the amplitude of the K+ currents, the K+ current response to GHRH was significantly attenuated. Downregulation of PKC with phorbol 12,13-dibutyrate (PDBu, 0.5 microM for 16 h) also abolished the K+ current response to GHRH. In addition, intracellular dialysis of somatotropes with the PKC inhibitory peptide PKC19-36 (1 microM) prevented the GHRH-induced decrease in the amplitude of the voltage-gated K+ currents. Local application of PDBu (1 microM) significantly reduced the amplitude of the voltage-gated K+ currents in a similar manner to that induced by GHRH, but without clear recovery upon removal. 7. This study demonstrates that GHRH decreases voltage-gated K+ currents via a PKC-mediated pathway in human adenoma somatotropes, rather than by the cAMP-PKA pathway that is usually implicated in the actions of GHRH.

Suppressing effects of short-chain fatty acids on growth hormone (GH)-releasing hormone-induced GH release in isolated anterior pituitary cells of goats

The involvement of tetrodotoxin-sensitive Na+ channels and receptor-operated nonspecific Ca2+ channels, and the effects of short-chain fatty acids, on growth hormone (GH) release induced by GH-releasing hormone (GHRH) were investigated in cultured and freshly isolated caprine anterior pituitary cells. In 3-d cultured cells in Dulbecco's modified Eagle's medium, an increase in GH release induced by GHRH (10 nmol/l) was moderately, but significantly, reduced by a voltage-sensitive Na+ channel antagonist tetrodotoxin (1 micromol). The GHRH-induced GH increase, which was not affected by a simultaneous addition of a receptor-operated nonspecific Ca2+ channel antagonist tetramethrine (0.1 mmol/l), was significantly reduced by a voltage-sensitive L-type Ca2+ channel antagonist nifedipine (1 micromol/l). Propionate and butyrate at 10 mmol/l, however, not only suppressed basal GH release but also significantly reduced the GH increase induced by 10 nmol/l of GHRH. The inhibitory action of these acids was also reproduced by an addition of beta-hydroxy butyrate (10 mmol/l) and octanoate (10 mmol/l). In freshly isolated and perifused cells, butyrate (10 mmol/l) as well as somatostatin (100 nmol/l) significantly reduced the GH increase induced by GHRH. From these findings we conclude that tetrodotoxin-sensitive Na+ channels and voltage-dependent L-type Ca2+ channels are involved in the cellular mechanism for GHRH-induced GH release, and that short-chain fatty acids such as propionate and butyrate have a direct action on somatotrophs to reduce basal and GHRH-induced GH release, in caprine somatotrophs.

Growth hormone (GH)-releasing factor differentially activates cyclic adenosine 3',5'-monophosphate- and inositol phosphate-dependent pathways to stimulate GH release in two porcine somatotrope subpopulations

Somatotropes comprise two morphologically and functionally distinct subpopulations of low (LD) and high (HD) density cells. We recently reported that GRF induces different patterns of increase in the cytosolic free Ca2+ concentration in single porcine LD and HD somatotropes, which for LD cells required not only Ca2+ influx but also intracellular Ca2+ mobilization. This suggested that GRF may activate multiple signaling pathways in pig LD and HD somatotropes to stimulate GH secretion. To address this question, we first assessed the direct GRF effect on second messenger activation in cultures of LD and HD cells by measuring cAMP levels and [3H]myo-inositol incorporation. Secondly, to determine the relative importance of cAMP- and inositol phosphate (IP)-dependent pathways, and of intra- and extracellular Ca2+, GRF-induced GH release from cultured LD and HD somatotropes was measured in the presence of specific blockers. GRF increased cAMP levels in both subpopulations, whereas it only augmented IP turnover in LD cells. Accordingly, adenylate cyclase inhibition by MDL-12,330A abolished GRF-stimulated GH release in both subpopulations, whereas phospholipase C inhibition by U-73122 only reduced this effect partially in LD cells. Likewise, blockade of Ca2+ influx with Cl2Co reduced GRF-stimulated GH secretion in both LD and HD somatotropes, whereas depletion of thapsigargin-sensitive intracellular Ca2+ stores only decreased the secretory response to GRF in LD cells. These results demonstrate that GRF specifically and differentially activates multiple signaling pathways in two somatotrope subpopulations to stimulate GH release. Thus, although the prevailing signaling cascade employed by GRF in both subpopulations is adenylate cyclase/cAMP/extracellular Ca2+, the peptide also requires activation of the phospholipase C/IP/intracellular Ca2+ pathway to exert its full effect in porcine LD somatotropes.

Muscarinic M1 and M3 receptors are present and increase intracellular calcium in adult rat anterior pituitary gland

Physiological and biochemical evidence indicates the existence of functional muscarinic cholinergic receptors in the anterior pituitary. The selectivity of these receptors has been characterised by studying the binding of [3H]quinuclidinyl benzilate ([3H]QNB) and [3H]diphenyl-acetoxy-N-methyl-piperidine ([3H]4-DAMP) in membrane preparation of male rat anterior pituitary at 25 degrees C. Competition experiments with receptor selective muscarinic antagonists were used to characterise specific selective muscarinic receptor binding. Both [3H]QNB and [3H]4-DAMP bound to anterior pituitary membranes at low concentrations, binding was saturable and was potently displaced by 4-DAMP (M1, M3 subtypes selective antagonist) > atropine (general) > pirenzepine (M1). Methoctramine (M2) didn't antagonise the [3H]QNB binding efficiently. Acetylcholine and carbachol increased the intracellular Ca2+ level in 62% and 65% of cultured rat anterior pituitary cells in a dose-dependent manner, and this effect was prevented by pirenzepine. Based on these results we suggest that both M1 and M3 muscarinic receptors are present and active in the majority of cells in the rat anterior pituitary gland, but their physiological role in the adult rat remains to be examined.

Gi-3 protein mediates the increase in voltage-gated K+ currents by somatostatin on cultured ovine somatotrophs

Voltage-gated K+ currents in rat somatotrophs are increased by somatostatin (SRIF) through unidentified G protein. In this experiment, somatotroph-enriched cells (up to 85%) were obtained from ovine pituitary glands and further identified by the increase in K+ currents by SRIF. The whole cell recording was employed to study the voltage-gated K+ currents. A reversible increase in K+ currents (up to 150% of control) was obtained in response to local application of SRIF (10 nM) but not vehicle. When the guanosine 5'-O-(3-thiotriphosphate) was included in the pipette solution (200 microM), the recovery phase of K+ current response to SRIF was abolished. Inclusion of guanosine 5'-O-(2-thiodiphosphate) (200 microM) in pipette solution blocked the K+ current response to SRIF. Intracellular dialysis of antibodies against alphao-, alphai-, alphai-1-2-, or alphai-3-subunits of G proteins via patch pipettes was confirmed by immunofluorescent staining of the antibodies. Antibody dialysis alone did not modify voltage-gated K+ currents. Dialysis of anti-alphai or anti-alphai-3 antibodies significantly attenuated the increase in K+ currents that was obtained after application of 10 or 100 nM SRIF. Dialysis with anti-alphao, anti-alphai-1-2, or heat-inactivated (60 degreesC for 10 min) anti-alphai antibodies did not diminish the effect of SRIF on K+ currents. We conclude that the Gi-3 protein mediates the effect of SRIF on voltage-gated K+ currents in ovine somatotrophs.

G protein abnormalities in pituitary adenomas

It has been demonstrated that the majority of secreting and nonsecreting adenomas is monoclonal in origin suggesting that these neoplasia arise from the replication of a single mutated cell, in which growth advantage results from either activation of protooncogenes or inactivation of antioncogenes. Although a large number of genes has been screened for mutations, only few genetic abnormalities have been found in pituitary tumors such as allelic deletion of chromosome 11q13 where the MEN-1 gene has been localised, and mutations in the gene encoding the alpha subunit of the stimulatory Gs and Gi2 protein. These mutations constitutively activate the alpha subunit of the Gs and Gi2 protein by inhibiting their intrinsic GTPase activity. Both Gs alpha and Gi2alpha can be considered products of protooncogenes (gsp and gip2, respectively) since gain of function mutations that activate mitogenic signals have been recognized in human tumors. Gsp oncogene is found in 30-40% of GH-secreting adenomas, in a low percentage of nonfunctioning and ACTH-secreting pituitary adenomas, in toxic thyroid adenomas and differentiated thyroid carcinomas. The same mutations, occurred early in embriogenesis, have been also identified in tissues from patients affected with the McCune Albright syndrome. These mutations result in an increased cAMP production and in the subsequent overactivation of specific pathways involved in both cell growth and specific programmes of cell differentiation. By consequence, the endocrine tumors expressing gsp oncogene retain differentiated functions. The gip2 oncogene has been identified in about 10% of nonfunctioning pituitary adenomas, in tumors of the ovary and the adrenal cortex. However, it remains to be established whether Gi proteins activate mitogenic signals in pituitary cells. Since Gi proteins are involved in mediating the effect of inhibitory neurohormones on intracellular effectors, it has been proposed that in pituitary tumors the low expression of these proteins, particularly Gi1-3alpha, may contribute to uncontrolled pituitary cells growth by preventing the transduction of inhibitory signals. While by in vitro mutagenesis it has been demonstrated that activated mutant of Gq alpha, G12alpha, G13alpha and Gz alpha are fully oncogenic, it remains to be proved whether or not these abnormalities might naturally occur in human tumors and, in particular, in pituitary adenomas.

Effect of growth hormone-releasing peptide-2 (GHRP-2) and GH-releasing hormone (GHRH) on the the cAMP levels and GH release from cultured acromegalic tumours

There is a difference between the sheep and rat somatotrophs in the response to growth hormone-releasing peptide-2 (GHRP-2), which raises the question of what the response may be in human somatotrophs. In the present study, cells were obtained from seven human acromegalic tumours and the effects of GHRP-2 were studied. Cells were dissociated and kept in primary culture for 1-3 weeks before experimentation. Application of GHRP-2 for 30 min induced a significant increase in GH secretion from the cultured cells from all seven tumours whereas human GH-releasing hormone (hGHRH) at a dose of 10 nM induced a significant GH release in only four of seven tumours. The intracellular levels of cAMP in all seven tumours were significantly increased by both 10 nM GHRP-2 and GHRH, but the response to GHRH was significantly higher than the response to GHRP-2. The adenylyl cyclase inhibitor, MDL 12330A, blocked the effect of GHRH and GHRP-2 on intracellular cAMP levels, whereas the Ca2+ channel blocker Co2+ (0.5 mM) did not attenuate the cAMP response. For the tumours in which GH secretion was increased by GHRH and GHRP-2, the cAMP antagonist Rp-cAMP blocked the GH response to GHRH but not to GHRP-2. When a protein kinase A (PKA) inhibitor (H89) was applied, GHRH stimulated GH release was blocked, but cAMP accumulation was not affected. The response to GHRP-2 was not altered by H89. Calphostin C [a protein kinase C (PKC) inhibitor] reduced the effect of GHRP-2 on the secretion of GH but did not affect the response to GHRH. Both GHRH and GHRP-2 increased the intracellular Ca2+ concentration in a concentration-dependent manner. We conclude that (1) GHRH increases GH secretion from human GH tumours via the cAMP pathway whereas GHRP-2 increases GH secretion mainly via the PKC pathway; (2) GHRH increases cAMP (without GH release) in a subset of tumours whereas GHRP-2 increases cAMP levels (slightly) and GH secretion in all tumours; and (3) GHRP-2 and GHRH do not act on the same receptor on human somatotrophs derived from acromegalic tumours.

Pituitary adenylate cyclase-activating polypeptides 38 and 27 increase cytosolic free Ca2+ concentration in porcine somatotropes through common and distinct mechanisms

Ca2+ plays an essential role in pituitary adenylate cyclase-activating polypeptide (PACAP)-stimulated growth hormone (GH) secretion from porcine somatotropes. Here, Indo-1 microfluorimetry was used to investigate the dynamics of free cytosolic Ca2+ concentration ([Ca2+]i) in single porcine somatotropes in response to PACAP38 and PACAP27. We also evaluated the relative contributions of extra- and intracellular Ca2+ sources and of cAMP-dependent protein kinase (PKA) and phospholipase C (PLC). A high proportion of somatotropes responded to PACAP38 (79.4%) and PACAP27 (68.4%) with [Ca2+]i rises that could be followed by a refractory plateau (type 1 response), or by a decrease in [Ca2+]i during which somatotropes were responsive to a subsequent PACAP pulse (type II response). Although Ca2+ profiles in response to both peptides were similar, PACAP38-induced [Ca2+]i rises were higher. Somatotrope response to PACAP38 or PACAP27 was markedly reduced by removing extracellular Ca2+, blocking Ca2+ entry through L-type voltage sensitive Ca2+ channels (VSCC), or inhibiting PKA. Conversely, Ca2+ depletion from intracellular stores or PLC inactivation did not affect the response to PACAP27 but considerably reduced maximal [Ca2+]i induced by PACAP38. We conclude that both peptides stimulate extracellular Ca2+ influx through L-type VSCC by a PKA-dependent mechanism. However, PACAP38 also triggers a PLC-mediated Ca2+ mobilization from intracellular stores, thereby indicating that the two molecular forms of PACAP activate common and distinct second messenger pathways within porcine somatotropes.

Growth hormone-releasing factor mobilizes cytosolic free calcium through different mechanisms in two somatotrope subpopulations from porcine pituitary

Porcine somatotropes can be separated by Percoll density gradient centrifugation into low (LD) and high density (HD) subpopulations that differ ultrastructurally and functionally. Here, we report the effects of growth hormone-releasing factor (GRF) on the cytosolic free calcium concentration ([Ca2+]i) of single LD and HD somatotropes. Resting [Ca2+]i in LD somatotropes was 2-fold higher than in HD cells. GRF induced [Ca2+]i increases in a similar percentage of somatotropes from both subsets. However, amplitude and kinetics of the responses were markedly different. In all responsive LD somatotropes, GRF evoked a rapid initial peak followed by a sustained plateau (plateau-type response). Blockade of extracellular Ca2+ entry by 3 mM EDTA, 2 mM CoCl2, or 100 microM verapamil completely abolished the plateau phase without affecting the initial Ca2+ spike. Conversely, only the plateau phase was preserved in thapsigargin (TG)-treated LD cells. The vast majority of GRF-responsive HD somatotropes exhibited a transient [Ca2+]i peak that returned gradually to baseline (transient-type response). This response was completely blocked by removal of extracellular Ca2+, whereas TG treatment had no effect. Taken together, our results indicate that the response of LD somatotropes to GRF depends on mobilization of Ca2+ of both extra- and intracellular origin, whereas that of HD somatotropes seems to be exclusively dependent on extracellular Ca2+ entry through L-type voltage sensitive Ca2+ channels (VSCC). These findings are the first to demonstrate a differential effect of GRF on Ca2+ mobilization in two somatotrope subpopulations, and suggest the existence of differences in the GRF receptor(s) expressed in each subpopulation and/or in the intracellular signalling pathways activated upon GRF binding.

Hypothalamic and hypophyseal regulation of growth hormone secretion

1. Regulation of pulsatile secretion of growth hormone (GH) relies on hypothalamic neuronal loops, major transmitters involved in their operation are growth hormone releasing hormone (GHRH) synthetized mostly in arcuate nucleus (ARC) neurons, and somatostatin (SRIH), synthetized both in hypothalamus periventricular (PVe) and ARC neurons. 2. Neurons synthetizing both peptides can inhibit each other in a reciprocal manner. Other neuropeptides synthetized in ARC neurons, such as galanin, or in ARC interneurons, such as neuropeptide Y (NPY), are able to modulate synthesis and release of GHRH and SRIH into the hypothalamohypophyseal portal system. 3. In addition, the hitherto uncharacterized endogenous ligand of the recently cloned growth hormone releasing peptide receptor, expressed mostly in the ARC, triggers GH release, presumably by actions on ARC interneurons. 4. Thyroid, gonadal, and adrenal steroid hormones also affect the GHRH-SRIH balance; a differential distribution of sex steroid receptors in the ARC and the PVe is likely to account for the different pattern of GH secretion in male and female animals. 5. Growth hormone itself is able to inhibit the amplitude of GH secretory episodes and to increase their frequency, by entering the brain (presumably by receptor-mediated internalization at the level of the choroid plexus) and acting subsequently on ARC neurons. 6. At the pituitary level, major neurotransmitters regulating GH cells act on receptors of the VIP/PACAP/GHRH family and of the somatostatin family, in particular, sst2 and sst3. Those are coupled to accumulation of cAMP as a second messenger. 7. In addition, patch-clamp experiments and measurement of intracellular Ca2+ indicate that GH cells present characteristic, GHRH-dependent, but self-maintained Ca2+ spikes and [Ca2+]i transients, which reflect adaptive mechanisms to constraints of episodic release. 8. Recent data on transcription factors affecting GH gene expression and somatotrope differentiation are also summarized. 9. Regulation and differentiation of somatotropes also depend upon paracrine processes within the pituitary itself and involve growth factors and several neuropeptides, for instance, vasoactive intestinal peptide, angiotensin 2, endothelin, and activin. 10. Finally, characteristic changes occur in the GH secretory pattern under discrete, pathological conditions, such as abnormal growth and dwarfism, diabetes, and acromegaly, as well as during inflammatory processes.

G(o)2 and Gi3 proteins mediate the action of somatostatin on membrane Ca2+ and K+ currents in ovine pituitary somatotrophs

1. Growth hormone (GH) secretion from the anterior pituitary gland is mainly regulated by hypothalamic GH-releasing hormone (GHRH) and somatostatin (SRIF). Somatostatin reduces both spontaneous and GHRH-stimulated GH secretion. 2. Exocytosis of GH is mainly determined by the intracellular free Ca2+ concentration ([Ca2+]i), which is regulated by the influx of Ca2+ via membrane Ca2+ channels. Somatostatin reduces the influx of Ca2+ through two separate mechanisms, namely a direct action on Ca2+ channels and an indirect action on membrane potentials through the activation of K+ channels. 3. In the present experiments, somatotroph-enriched cells were obtained from the ovine pituitary gland by means of collagenase dissociation and Percoll-gradient centrifugation. Further identification was based on the effect of SRIF (10 nmol/L) on Ca2+ or K+ currents. 4. A significant reduction in Ca2+ currents and an increase in K+ currents was obtained in response to local application of SRIF (10 nmol/L), but vehicle application had no effect. The responses of Ca2+ and K+ currents to SRIF were reversible after removal of SRIF. 5. Dialysis of GTP-gamma-s (200 mumol/L) abolished the recovery phase of K+ current response to SRIF after its removal, whereas GDP-beta-s (200 mumol/L) totally blocked the response. Pretreatment of the cells with pertussis toxin (100 nmol/L) overnight abolished the Ca2+ current response to SRIF. 6. Intracellular dialysis of antibodies to alpha o, alpha i1-3, alpha i1-2 and alpha i3 subunits of the G-proteins into cells via whole-cell patch-clamp pipettes was confirmed by immunofluorescent staining of the antibodies. 7. Dialysis of anti-alpha i1-3 or anti-alpha i3 antibodies significantly attenuated the increase in the K+ current in response to 10 nmol/L SRIF, whereas neither anti-alpha o nor anti-alpha i1-2 antibodies diminished the effect of SRIF on the K+ current. 8. Dialysis of anti-alpha o antibodies significantly attenuated the reduction in the Ca2+ current that was obtained upon application of 10 nmol/L SRIF. Neither anti-alpha i1-2 nor anti-alpha i3 antibody dialysis diminished the effect of SRIF on the Ca2+ current. 9. Dialysis of the alpha o common antisense oligonucleotides (ASm) but not the alpha i3 AS significantly diminished the inhibitory effect of SRIF on the Ca2+ current. This effect of alpha o ASm dialysis occurred at 12 h incubation after dialysis, reaching a maximal level at 48 h and partially recovering at 72 h incubation. Antisense oligonucleotides specific for alpha o1 (alpha o1 AS) or alpha o2 (alpha o2 AS) were dialysed into somatotrophs and only alpha o2 AS significantly attenuated the inhibition of SRIF on the Ca2+ current. 10. It is concluded that the Gi3 protein mediates the effect of SRIF on the K+ current and that the G(o)2 protein mediates the effect of SRIF on the Ca2+ current in primary cultured ovine somatotrophs.

Growth hormone release induced by an amino acid mixture from primary cultured anterior pituitary cells of goats

The effects of amino acids on growth hormone (GH) release and cytosolic calcium concentration ([Ca2+]i) were investigated in caprine anterior pituitary cells cultured for 3 d in Dulbecco modified Eagle medium. The addition of an amino acid mixture consisting of seven nonessential amino acids (NEAA: L-Asp, Gly, L-Ala, L-Ser, L-Pro, L-Asn, and L-Glu; concentration of each 12.5-200 mumol/l) in the medium significantly raised GH release from the cultured cells in a concentration-dependent manner with the maximum release at 200 mumol/l NEAA. Although an addition of L-Asp (0.1-100 mumol/l) caused a significant rise in GH release in a concentration-dependent manner, neither the individual amino acids contained in NEAA except L-Asp nor others (L-Leu, L-Phe, L-Gln, L-Met, and L-Arg) caused a rise in GH release when added alone to the medium. The rise in GH release induced by NEAA (200 mumol/l) and GH-releasing hormone (GHRH, 10 nmol/l) was significantly reduced by the addition of EGTA (1.8 mmol/l) and nifedipine (1 mumol/l) to the medium, respectively. The addition of NEAA (200 mumol/l) caused a rapid and transient [Ca2+]i increase, followed thereafter by a steady increase. The prior addition of nifedipine (1 mumol/l), which itself significantly reduced the basal [Ca2+]i, completely abolished the response induced by NEAA or GHRH. From these findings, we conclude that: 1) NEAA raises GH release and [Ca2+]i in cultured caprine anterior pituitary cells, and 2) Ca2+ influx from the medium may be responsible for the cellular action of NEAA.

Activation of Na+ channels in GH3 cells and human pituitary adenoma cells by PACAP

The effects of pituitary adenylate cyclase activating polypeptide (PACAP) on ion channels were examined in GH3 cells human pituitary adenoma cells. In GH3 cells, PACAP-38 (10-9 M) reversibly activated tetrodotoxin-sensitive NA+ channels but had little effect on nicardipine-sensitive Ca2+ channels. PACAP-induced increase in Na+ currents was inhibited by PACAP (6-38), a specific PACAP receptor antagonist, and Rp-cAMPs, an inhibitor for protein kinase A, and mimicked by 8-bromo-cAMP. In human pituitary adenoma cells, PACAP also activated tetrodotoxin-sensitive Na+ channels and growth hormone secretion. These results suggest the possibility that PACAP can activate voltage-gated Na+ channels via adenylate cyclase-protein kinase A pathway in the pituitary.

Pituitary adenylate cyclase-activating polypeptide regulates [Ca(2+)](i) and electrical activity in pituitary cells through cell type-specific mechanisms

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a recently identified hypothalamic factor that acts on a variety of anterior pituitary cell types. It is clear, however, that its actions are not mediated by the same intracellular signaling mechanisms in each cell type. The signaling pathways by which PACAP regulates changes in [Ca(2+)], and electrical activity in rat somatotrophs and gonadotrophs is described in the present article. Finally, the possibility that the differences in PACAP-regulated signaling in anterior pituitary cells is due to the differential expression and coupling of PACAP receptor subtypes is discussed.

Neurotransmitter-induced inhibition of exocytosis in insulin-secreting beta cells by activation of calcineurin

Neurotransmitters and hormones such as somatostatin, galanin, and adrenalin reduce insulin secretion. Their inhibitory action involves direct interference with the exocytotic machinery. We have examined the molecular processes underlying this effect using high resolution measurements of cell capacitance. Suppression of exocytosis was maximal at concentrations that did not cause complete inhibition of glucose-stimulated electrical activity. This action was dependent on activation of G proteins but was not associated with inhibition of the voltage-dependent Ca2+ currents or adenylate cyclase activity. The molecular processes initiated by the agonists culminate in the activation of the Ca(2+)-dependent protein phosphatase calcineurin, and suppression of the activity of this enzyme abolishes their action on exocytosis. We propose that mechanisms similar to those we report here may contribute to adrenergic and peptidergic inhibition of secretion in other neuroendocrine cells and in nerve terminals.