Functional ATP receptors in rat anterior pituitary cells

Drosophila taste neurons as an agonist-screening platform for P2X receptors

The P2X receptor family of ATP-gated cation channels are attractive drug targets for pain and inflammatory disease, but no subtype-selective agonists, and few partially selective agonists have been described to date. As proof-of-concept for the discovery of novel P2X receptor agonists, here we demonstrate the use of Drosophila taste neurons heterologously expressing rat P2X2 receptors as a screening platform. We demonstrate that wild-type rat P2X2 expressed in Drosophila is fully functional (ATP EC₅₀ 8.7 µM), and that screening of small (2 µl) volumes of a library of 80 adenosine nucleotide analogues is rapid and straightforward. We have determined agonist potency and specificity profiles for rat P2X2 receptors; triphosphate-bearing analogues display broad activity, tolerating a number of substitutions, and diphosphate and monophosphate analogues display very little activity. While several ATP analogues gave responses of similar magnitude to ATP, including the previously identified agonists ATPγS and ATPαS, we were also able to identify a novel agonist, the synthetic analogue 2-fluoro-ATP, and to confirm its agonist activity on rat P2X2 receptors expressed in human cells. These data validate our Drosophila platform as a useful tool for the analysis of agonist structure-activity relationships, and for the screening and discovery of novel P2X receptor agonists.

Expression of P2X receptors in the rat anterior pituitary

In this study, the distribution patterns of P2X1 to P2X7 receptors in the anterior pituitary cells of rat were studied with single-, double-, and triple-labeling immunofluorescence, combined method of immunofluorescence and in situ hybridization, and Western blot. The results showed that the expression level of the P2X4 receptor protein was highest, followed by P2X5, P2X3, P2X2, P2X6, and P2X7 receptor proteins, but no P2X1 receptor protein was detected. Strong P2X4 receptor-immunoreactivity was detected in almost all the anterior pituitary cells. Different combinations of P2X receptors were detected in each individual cell type of the rat anterior pituitary. Gonadotrophs express P2X4, P2X5, and P2X6 receptors. Corticotrophs express P2X3 and P2X4 receptors. Folliculo-stellate cells express P2X2 and P2X4 receptors, and somatotrophs, lactotrophs, and thyrotrophs express only P2X4 receptors. The macrophages with Iba-1-ir expressed P2X7 receptors. The possible functions of these P2X receptors in each individual cell type of the rat anterior pituitary are discussed.

Neurotransmitter receptors as signaling platforms in anterior pituitary cells

The functions of anterior pituitary cells are controlled by two major groups of hypothalamic and intrapituitary ligands: one exclusively acts on G protein-coupled receptors and the other activates both G protein-coupled receptors and ligand-gated receptor channels. The second group of ligands operates as neurotransmitters in neuronal cells and their receptors are termed as neurotransmitter receptors. Most information about pituitary neurotransmitter receptors was obtained from secretory studies, RT-PCR analyses of mRNA expression and immunohistochemical and biochemical analyses, all of which were performed using a mixed population of pituitary cells. However, recent electrophysiological and imaging experiments have characterized γ-aminobutyric acid-, acetylcholine-, and ATP-activated receptors and channels in single pituitary cell types, expanding this picture and revealing surprising differences in their expression between subtypes of secretory cells and between native and immortalized pituitary cells. The main focus of this review is on the electrophysiological and pharmacological properties of these receptors and their roles in calcium signaling and calcium-controlled hormone secretion.

The Calcium-Sensing Receptor in Health and Disease

The extracellular calcium-sensing receptor (CaSR) is a unique G protein-coupled receptor (GPCR) activated by extracellular Ca²⁺ and by other physiological cations including Mg²⁺, amino acids, and polyamines. CaSR is the most important master controller of the extracellular Ca²⁺ homeostatic system being expressed at high levels in the parathyroid gland, kidney, gut and bone, where it regulates parathyroid hormone (PTH) secretion, vitamin D synthesis, and Ca²⁺ absorption and resorption, respectively. Gain and loss of function mutations in the CaSR are responsible for severe disturbances in extracellular Ca²⁺ metabolism. CaSR agonists (calcimimetics) and antagonists (calcilytics) are in use or under intense research for treatment of hyperparathyroidism secondary to kidney failure and hypocalcemia with hypercalciuria, respectively. Expression of the CaSR extends to other tissues and systems beyond the extracellular Ca²⁺ homeostatic system including the cardiovascular system, the airways, and the nervous system where it may play physiological functions yet to be fully understood. As a consequence, CaSR has been recently involved in different pathologies including uncontrolled blood pressure, vascular calcification, asthma, and Alzheimer's disease. Finally, the CaSR has been shown to play a critical role in cancer either contributing to bone metastasis and/or acting as a tumor suppressor in some forms of cancer (parathyroid cancer, colon cancer, and neuroblastoma) and as oncogene in others (breast and prostate cancers). Here we review the role of CaSR in health and disease in calciotropic tissues and others beyond the extracellular calcium homeostatic system.

Landscape of somatic mutations in sporadic GH-secreting pituitary adenomas

CONTEXT

Alterations in the cAMP signaling pathway are common in hormonally active endocrine tumors. Somatic mutations at GNAS are causative in 30-40% of GH-secreting adenomas. Recently, mutations affecting the USP8 and PRKACA gene have been reported in ACTH-secreting pituitary adenomas and cortisol-secreting adrenocortical adenomas respectively. However, the pathogenesis of many GH-secreting adenomas remains unclear.

AIM

Comprehensive genetic characterization of sporadic GH-secreting adenomas and identification of new driver mutations.

DESIGN

Screening for somatic mutations was performed in 67 GH-secreting adenomas by targeted sequencing for GNAS, PRKACA, and USP8 mutations (n=31) and next-generation exome sequencing (n=36).

RESULTS

By targeted sequencing, known activating mutations in GNAS were detected in five cases (16.1%), while no somatic mutations were observed in both PRKACA and USP8. Whole-exome sequencing identified 132 protein-altering somatic mutations in 31/36 tumors with a median of three mutations per sample (range: 1-13). The only recurrent mutations have been observed in GNAS (31.4% of cases). However, seven genes involved in cAMP signaling pathway were affected in 14 of 36 samples and eight samples harbored variants in genes involved in the calcium signaling or metabolism. At the enrichment analysis, several altered genes resulted to be associated with developmental processes. No significant correlation between genetic alterations and the clinical data was observed.

CONCLUSION

This study provides a comprehensive analysis of somatic mutations in a large series of GH-secreting adenomas. No novel recurrent genetic alterations have been observed, but the data suggest that beside cAMP pathway, calcium signaling might be involved in the pathogenesis of these tumors.

Purinergic signaling pathways in endocrine system

Adenosine-5'-triphosphate is released by neuroendocrine, endocrine, and other cell types and acts as an extracellular agonist for ligand-gated P2X cationic channels and G protein-coupled P2Y receptors in numerous organs and tissues, including the endocrine system. The breakdown of ATP by ectonucleotidases not only terminates its extracellular messenger functions, but also provides a pathway for the generation of two additional agonists: adenosine 5'-diphosphate, acting via some P2Y receptors, and adenosine, a native agonist for G protein-coupled adenosine receptors, also expressed in the endocrine system. This article provides a review of purinergic signaling pathways in the hypothalamic magnocellular neurosecretory cells and neurohypophysis, hypothalamic parvocellular neuroendocrine system, adenohypophysis, and effector glands organized in five axes: hypothalamic-pituitary-gonadal, hypothalamic-pituitary-thyroid, hypothalamic-pituitary-adrenal, hypothalamic-pituitary-growth hormone, and hypothalamic-pituitary-prolactin. We attempted to summarize current knowledge of purinergic receptor subtypes expressed in the endocrine system, including their roles in intracellular signaling, hormone secretion, and other cell functions. We also briefly review the release mechanism for adenosine-5'-triphosphate by neuroendocrine, endocrine and surrounding cells, the enzymes involved in adenosine-5'-triphosphate hydrolysis to adenosine-5'-diphosphate and adenosine, and the relevance of this pathway for sequential activation of receptors and termination of signaling.

Purinergic signalling in endocrine organs

There is widespread involvement of purinergic signalling in endocrine biology. Pituitary cells express P1, P2X and P2Y receptor subtypes to mediate hormone release. Adenosine 5'-triphosphate (ATP) regulates insulin release in the pancreas and is involved in the secretion of thyroid hormones. ATP plays a major role in the synthesis, storage and release of catecholamines from the adrenal gland. In the ovary purinoceptors mediate gonadotrophin-induced progesterone secretion, while in the testes, both Sertoli and Leydig cells express purinoceptors that mediate secretion of oestradiol and testosterone, respectively. ATP released as a cotransmitter with noradrenaline is involved in activities of the pineal gland and in the neuroendocrine control of the thymus. In the hypothalamus, ATP and adenosine stimulate or modulate the release of luteinising hormone-releasing hormone, as well as arginine-vasopressin and oxytocin. Functionally active P2X and P2Y receptors have been identified on human placental syncytiotrophoblast cells and on neuroendocrine cells in the lung, skin, prostate and intestine. Adipocytes have been recognised recently to have endocrine function involving purinoceptors.

Expression and roles of pannexins in ATP release in the pituitary gland

Pannexins are a newly discovered three-member family of proteins expressed in the brain and peripheral tissues that belong to the superfamily of gap junction proteins. However, in mammals pannexins do not form gap junctions, and their expression and function in the pituitary gland have not been studied. Here we show that the rat pituitary gland expresses mRNA and protein transcripts of pannexins 1 and 2 but not pannexin 3. Pannexin 1 was more abundantly expressed in the anterior lobe, whereas pannexin 2 was more abundantly expressed in the intermediate and posterior pituitary. Pannexin 1 was identified in corticotrophs and a fraction of somatotrophs, the S100-positive pituicytes of the posterior pituitary and AtT-20 (mouse pituitary adrenocorticotropin-secreting cells) and rat immortalized pituitary cells secreting prolactin, whereas pannexin 2 was detected in the S100-positive folliculostellate cells of the anterior pituitary, melanotrophs of the intermediate lobe, and vasopressin-containing axons and nerve endings in the posterior lobe. Overexpression of pannexins 1 and 2 in AtT-20 pituitary cells enhanced the release of ATP in the extracellular medium, which was blocked by the gap junction inhibitor carbenoxolone. Basal ATP release in At-T20 cells was also suppressed by down-regulating the expression of endogenous pannexin 1 but not pannexin 2 with their short interfering RNAs. These results indicate that pannexins may provide a pathway for delivery of ATP, which is a native agonist for numerous P2X cationic channels and G protein-coupled P2Y receptors endogenously expressed in the pituitary gland.

Expression of P2Y receptors in the rat anterior pituitary

In this study, the distribution patterns of P2Y(1), P2Y(2) P2Y(4), P2Y(6), P2Y(12), and P2Y(13) receptors in the anterior pituitary cells of rat were studied with double-labeling immunofluorescence and Western blot. The results showed that P2Y receptors were widely expressed in the anterior pituitary. P2Y(1) and P2Y(4) receptors were found to be expressed in the majority of gonadotrophs and thyrotrophs, P2Y(2) receptors were expressed in a small subpopulation of lactotrophs and almost all the folliculo-stellate cells, that were also stained with S100 protein immunoreactivity. P2Y(6) receptors were expressed in macrophages. P2Y(13) receptors were expressed in a small subpopulation of cells in the rat anterior pituitary, the identity of which needs to be clarified. P2Y(1) and P2Y(4) receptors are co-expressed in some gonadotrophs and thyrotrophs. Corticotrophs and somatotrophs were found not to express P2Y receptors in this study. FSH and TSH were shown to coexist in the same endocrine cells in rat anterior pituitary. The present data suggests that purines and/or pyrimidines could be involved in regulating the functions of gonadotrophs and thyrotrophs via P2Y(1) and P2Y(4) receptors, some lactotrophs via P2Y(2) receptors, and folliculo-stellate cells via P2Y(2) receptors in the rat anterior pituitary.

Characterization of purinergic P2X4 receptor channels expressed in anterior pituitary cells

Anterior pituitary cells express cation-conducting P2X receptor channels (P2XRs), but their molecular identity, electrophysiological properties, cell-specific expression pattern, and physiological roles have been only partially characterized. In this study, we show by quantitative RT-PCR that mRNA transcripts for the P2X(4) subunit are the most abundant in rat anterior pituitary tissue and confirm the P2X(4)R protein expression by Western blot analysis. Single-cell patch-clamp recordings show that extracellular ATP induced an inward depolarizing current in a majority of thyrotropin-releasing hormone-responsive pituitary cells, which resembled the current profile generated by recombinant P2X(4)R. The channels were activated and desensitized in a dose-dependent manner and deactivated rapidly. Activation of these channels led to stimulation of electrical activity and promotion of voltage-gated and voltage-insensitive Ca(2+) influx. In the presence of ivermectin, a specific allosteric modulator of P2X(4)Rs, there was an approximately fourfold increase in the maximum amplitude of the ATP-induced inward current, accompanied by an increase in the sensitivity of receptors for ATP, slowed deactivation of receptors, and enhanced ATP-induced prolactin release. These results indicate that thyrotropin-releasing hormone-responsive cells, including lactotrophs, express homomeric and/or heteromeric P2X(4)Rs, which facilitate Ca(2+) influx and hormone secretion.

Signaling by purinergic receptors and channels in the pituitary gland

Adenosine 5'-triphosphate is frequently released by cells and acts as an agonist for G protein-coupled P2Y receptors and ligand-gated P2X cationic channels in numerous tissues. The breakdown of ATP by ectonucleotidases not only terminates its extracellular messenger functions, but also provides a pathway for the generation of two additional agonists: adenosine 5'-diphosphate, acting via some P2Y receptors, and adenosine, a native agonist for G protein-coupled adenosine receptors. In the pituitary gland, adenosine 5'-triphosphate is released from the endings of magnocellular hypothalamic neurons and by anterior pituitary cells through pathway(s) that are still not well characterized. This gland also expresses several members of each family of purinergic receptors. P2X and adenosine receptors are co-expressed in the somata and nerve terminals of vasopressin-releasing neurons as well as in some secretory pituitary cells. P2X receptors stimulate electrical activity and modulate InsP(3)-dependent calcium release from intracellular stores, whereas adenosine receptors terminate electrical activity. Calcium-mobilizing P2Y receptors are expressed in pituicytes, folliculo-stellate cells and some secretory cells of the anterior pituitary.

Accounting for nonspecific enhancement in neuronal tract tracing using manganese enhanced magnetic resonance imaging

Manganese enhanced MRI (MEMRI) is an emerging technique for tracing neuronal pathways in vivo. However, manganese may leak into blood vessels or cerebrospinal fluid (CSF) after local injection and can be circulated to and taken up by brain regions that may not have connections to the targeted pathways. Comparing enhancement time courses after intranasal injection with intravenous infusion of MnCl(2) in rats, the early enhancements in the pituitary gland (Pit) and hippocampus indicate the contrasts in those regions in the olfactory tract-tracing experiment were caused by such systemic effects. Since the Pit has easy access to manganese from the blood and its signal is proportional to other brain regions after intravenous infusion, it was used as an internal reference for the systemic effects. Applying intensity normalization by the Pit signal to tract-tracing data from the olfactory bulb led to reduced contrast in the hippocampus. These results demonstrate that nonspecific enhancements in MEMRI tract-tracing studies may have to be taken into account and that normalization by the Pit signal can compensate these effects.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

P2Y receptor-mediated excitation in the posterior hypothalamus

Histaminergic neurons located in the posterior hypothalamus (tuberomamillary nucleus, TMN) project widely through the whole brain controlling arousal and attention. They are tonically active during wakefulness but cease firing during sleep. As a homeostatic theory of sleep involves ATP depletion and adenosine accumulation in the brain, we investigated the role of ATP and its analogues as well as adenosine on neuronal activity in the TMN. We show increased firing of rat TMN neurons by ATP, ADP, UTP and 2meSATP, indicating activation of receptors belonging to the P2Y family. Adenosine affected neither membrane potential nor firing of these cells. Single-cell reverse transcriptase-polymerase chain reaction revealed that P2Y1 and P2Y4 are prevailing receptors in TMN neurons. P2Y1 receptor mRNA was detected with a higher frequency in 2-week-old than in 4-week-old rats; in accordance, 2meSATP was more potent than ATP. Semi-quantitative real-time polymerase chain reaction revealed a developmental downregulation of mRNA levels for P2Y1 and P2Y4 receptors. Immunocytochemistry demonstrated neuronal and glial localization of the P2Y1 receptor protein. Network activity measured with multielectrode arrays in primary cultures made from the posterior hypothalamus was enhanced by UTP and 2meSATP (P2Y4 and P2Y1 agonists, respectively). ATP caused an inhibition of firing, which was reversed in the presence of suramin or gabazine [gamma-aminobutyric acid (GABA)A receptor antagonist], indicating that GABAergic neurons are preferentially activated by ATP in this network. Excitation of the wake-active TMN neurons by nucleotides and the lack of adenosine action may be important factors in sleep-wake regulation.

Purinergic receptor ligands stimulate pro-opiomelanocortin gene expression in AtT-20 pituitary corticotroph cells

Although recent studies have suggested that purinergic receptors are expressed in the anterior pituitary gland, their involvement in the regulation of pituitary hormone gene expression is not completely understood. In the present study, we examined the expression of purinergic receptors and the effects of purinergic receptor ligands on pro-opiomelanocortin (POMC) gene expression, in AtT20 mouse corticotroph cells. We identified the expression of most of the purinergic receptor subtypes (A1, A2, P2X1, 3-7, P2Y1, 2, 4) mRNAs, analysed by the reverse transcriptase-polymerase chain reaction. We also found that adenosine and ATP, two representative and endogenous agonists of A1-3 and P2X/P2Y receptors, respectively, stimulated the 5'-promoter activity of the POMC gene in a dose- and time-related manner. When these ligands were simultaneously used with corticotrophin-releasing hormone (CRH), effects that were more than additive were observed, suggesting an enhancing role of these compounds in CRH-mediated adrenocorticotrophic hormone (ACTH) synthesis. These ligands also stimulated the expression of transcription factors involved in the regulation of the POMC gene, but did not enhance ACTH secretion. Finally, the positive effect of adenosine as well as CRH was completely inhibited by the protein kinase A inhibitor H89, whereas that of ATP was not influenced, indicating that different intracellular signalling pathways mediate these effects. Altogether, our results suggest a stimulatory role for these purinergic receptor ligands in the regulation of POMC gene expression in corticotroph cells. Because adenosine and ATP are known to be produced within the pituitary gland, it is possible they may be acting in an autocrine/paracrine fashion.

Roles of purinergic P2X receptors as pacemaking channels and modulators of calcium-mobilizing pathway in pituitary gonadotrophs

Anterior pituitary cells release ATP and express several subtypes of purinergic P2 receptors, but their biophysical properties and roles in spontaneous and receptor-controlled electrical activity have not been characterized. Here we focused on extracellular ATP actions in gonadotrophs from embryonic, neonatal, and adult rats. In cells from all three age groups, the Ca2+-mobilizing agonist GnRH induced oscillatory, hyperpolarizing, nondesensitizing, and slow deactivating currents. In contrast, ATP induced nonoscillatory, depolarizing, slowly desensitizing, and rapidly deactivating current, indicating that these cells express cation-conducting P2X channels but not Ca2+-mobilizing P2Y receptors. The amplitudes of P2X current response and the rates of receptor desensitization were dependent on ATP concentration. The biophysical and pharmacological properties of P2X currents were consistent with the expression of P2X2 subtype of channels in these cells. ATP-induced rapid depolarization of gonadotrophs lead to initiation of firing in quiescent cells, an increase in the frequency of action potentials in spontaneously active cells, and a transient stimulation of LH release. ATP also influenced GnRH-induced current and membrane potential oscillations and LH release in an extracellular Ca2+-dependent manner. These inositol 1,4,5-triphosphate-dependent oscillations were facilitated, slowed, or stopped, depending of ATP concentration, the time of its application, and the level of Ca2+ content in intracellular stores. These results indicate that, in gonadotrophs, P2X receptors could operate as pacemaking channels and modulators of GnRH-controlled electrical activity and secretion.

Release and extracellular metabolism of ATP by ecto-nucleotidase eNTPDase 1-3 in hypothalamic and pituitary cells

Hypothalamic and pituitary cells express G protein-coupled adenosine and P2Y receptors and cation-conducting P2X receptor-channels, suggesting that extracellular ATP and other nucleotides may function as autocrine and/or paracrine signaling factors in these cells. Consistent with this hypothesis, we show that cultured normal and immortalized pituitary and hypothalamic cells release ATP under resting conditions. RT-PCR analysis also revealed the presence of transcripts for ecto-nucleotidase eNTPDase 1–2 in these cells. These enzymes were functional as documented by degradation of endogenously released and exogenously added ATP. Blocking the activity of eNTPDases by ARL67156 led to an increase in ATP release in perifused pituitary cells and inhibition of degradation of extracellularly added ATP. Furthermore, the addition of apyrase, a soluble ecto-nucleotidase, and the expression of recombinant mouse eNTPDase-2, enhanced degradation of both endogenously released and exogenously added ATP. The released ATP by resting hypothalamic cells was sufficient to activate and desensitize high-affinity recombinant P2X receptors, whereas facilitation of ATP metabolism by the addition of apyrase protected their desensitization. These results indicate that colocalization of ATP release sites and ecto-nucleotidase activity at the plasma membrane of hypothalamic and pituitary cells provides an effective mechanism for the operation of nucleotides as extracellular signaling molecules.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Role of nucleotide P2 receptors in calcium signaling and prolactin release in pituitary lactotrophs

Anterior pituitary cells express nucleotide-gated G protein-coupled P2 receptors (P2YRs) and cation-conducting channels (P2XRs). However, the identification of P2 receptors subtypes and their native ligands, and the distribution and function of these receptors within the secretory and non-secretory pituitary cells has been incompletely characterized. The focus in this study was on lactotroph subpopulation of cells. ATP and ADP, but not UTP and UDP, triggered calcium signaling in a majority (85%) of lactotrophs and prolactin release in mixed pituitary cells. Consistent with the role of P2 receptors in signaling and secretion, the actions of ATP and ADP were abolished in the presence of apyrase, an ectonucleotidase. Transcripts for Gq-coupled calcium-mobilizing P2Y1R, P2Y2R, P2Y4R, and P2Y6R, as well as Gi-coupled P2Y12R, were identified in mixed anterior pituitary cells. The ligand-selectivity profile of calcium mobilization-dependent signaling and prolactin secretion and the blockade of these responses by pyridoxal 5-phosphate 6-azophenyl-2',4'-disulphonic acid indicated that P2Y1R mediates the stimulatory action of ATP and ADP. Within the channels expressed in anterior pituitary (P2X2R, P2X3R, P2X4R, and P2X7R), the P2X4R subtype provides a major pathway for calcium influx-dependent signaling and prolactin secretion. This conclusion was based on comparison of native to recombinant channels with respect to their ligand preference, sensitivity to pyridoxal 5-phosphate 6-azophenyl-2',4'-disulphonic acid, and the rates of calcium signal desensitization.

Purinergic regulation of intracellular Ca2+ concentration of rat pituitary folliculo-stellate cells in primary culture

Pituitary folliculo-stellate cells (FSCs) are glia-like cells in the anterior pituitary and are believed to modulate the activity of the pituitary endocrine cells. However, little is known what regulates the activity of FSCs. We hypothesized that ATP could act on FSCs, because ATP is coreleased with pituitary hormones from endocrine cells. To test this possibility, we examined the effect of ATP by measuring intracellular Ca2+ concentration [Ca2+]i of FSCs in primary culture. Both ATP and UTP increased the [Ca2+]i in a concentration-dependent manner in a range between 0.1 microM and 10 microM. The response was completely suppressed by thapsigargin, an inhibitior of endoplasmic reticulum Ca2+-ATPase, and was significantly suppressed by U-73122, an inhibitor of phospholipase C. The response was also suppressed by caffeine, a blocker of IP3 receptor, whereas that was not suppressed by ryanodine, an antagonist of ryanodine receptor. These results indicate that ATP increases [Ca2+]i of FSCs by activating phospholipase C via P2Y purinergic receptor and suggest that ATP would be one of paracrine factors to FSCs in the anterior pituitary.

Activation of purinergic receptors by ATP inhibits secretion in bovine adrenal chromaffin cells

Autoinhibition is a common mechanism observed in neurons to regulate neurotransmission. Released neurotransmitter interacts with presynaptic autoreceptors to inhibit subsequent release. The requisite elements for autoinhibition are present in chromaffin cells: secretory granules contain millimolar levels of ATP which is coreleased with catecholamines upon stimulation and the cells express purinergic receptors. We were interested to determine whether autoinhibition produced by ATP binding to purinergic receptors plays an important role in catecholamine release from chromaffin cells. In these studies, short depolarizations were used to elicit transmitter release measured by membrane capacitance. We find that stimulation of chromaffin cells results in the release of endogenous ATP which may suppress Ca(2+) channel currents and secretion. In the presence of a maximal concentration of ATP, both the amount of secretion and the maximal rate of release are about half that observed in the absence of ATP. ATP-mediated inhibition of secretion was blocked by Reactive Blue-2 suggesting the involvement of P(2Y) purinergic receptors. Prepulses to positive potentials that relieve the Ca(2+) channel block largely relieve the inhibition of secretion. Furthermore, when secretion is plotted as a function of Ca(2+) influx there is no apparent change in the relationship between control cells and those stimulated in the presence of ATP and prepulses. These results suggest that ATP diminishes secretion by inhibiting Ca(2+) influx into the cells. Our results indicate that feedback inhibition by ATP, mediated primarily by Ca(2+) channels, may be an important regulator of catecholamine release in chromaffin cells.

Intercellular communication in the anterior pituitary

In addition to hypothalamic and feedback inputs, the secretory cells of the anterior pituitary are influenced by the activity of factors secreted within the gland. The list of putative intrapituitary factors has been expanding steadily over the past decade, although until recently much of the work was limited to descriptions of potential interactions. This took the form of evidence of production within the pituitary of factors already known to influence activity of secretory cells, or further descriptions of actions on pituitary cells by such factors when added exogenously. A new phase of discovery has been entered, with extensive efforts being made to delineate the control of the synthesis and secretion of the pituitary factors within the gland, regulation of the receptors and response mechanisms for the factors in pituitary cells, and measurements of the endogenous actions of the factors through the use of specific immunoneutralization, receptor blockade, tissue from transgenic animals, and other means. Taken together, these findings are producing blueprints of the intrapituitary interactions that influence each of the individual types of secretory cells, leading toward an understanding of the physiological significance of the interactions. The purpose of this article is to review the recent literature on many of the factors acting as intrapituitary signals and to present such finding in the context of the physiology of the secretory cells.

ATP-induced [Ca(2+)](i) changes and depolarization in GH3 cells

Extracellular ATP is a neurotransmitter and mediates a variety of responses. In the endocrine system, there are data suggesting a physiological role for ATP in Ca(2+) signalling and hormone secretion. However, the ATP receptor subtype involved has not been clearly elucidated in GH3 cells, a rat anterior pituitary cell line. BzATP- and ATP-induced [Ca(2+)](i) responses had EC(50) values of 18 and 651 microM, respectively. The maximal response to ATP was only 59+/-8% of that for BzATP. The BzATP-induced [Ca(2+)](i) increase was dependent upon the extracellular Ca(2+) concentration. Preincubation with oxidized ATP (oATP) nearly abolished the ATP- and BzATP-induced [Ca(2+)](i) increases. Both BzATP and ATP induced depolarization in GH3 cells, with EC(50) values of 31 microM and 1 mM, respectively. The maximal depolarization to BzATP and ATP were 152+/-21 and 146+/-16% of that elicited by 30 mM KCl. The rank order of agonist potency for [Ca(2+)](i) and depolarization responses was BzATP > > ATP >2-MeSATP and purine derivatives such as ADP, AMP, adenosine were ineffective. Neither UTP nor alpha, beta-methylene ATP showed any effect. In low-divalent conditions BzATP evoked non-desensitizing inward currents, which were reversed at approximately 0 mV. This nonselective cationic conductance was increased by repeated applications of BzATP and the cells became very permeable to NMDG. Longer applications (30 min) of BzATP stimulated ethidium bromide influx in low divalent conditions, suggesting increased permeability to larger molecules. We also identified the existence of P2X(7) mRNA on GH3 cells by using reverse transcriptase (RT)-polymerase chain reaction (PCR). These results suggest that the GH3 cells have an endogenous P2X(7) receptor and purinergic stimulation may play a potential role in neuroendocrine modulation on these cells.