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

María Teresa Miras Portugal: a pioneer in the study of purinoceptors in chromaffin cells

María Teresa Miras Portugal devoted most of her scientific life to the study of purinergic signalling. In an important part of her work, she used a model system: the chromaffin cells of the adrenal medulla. It was in these cells that she identified diadenosine polyphosphates, from which she proceeded to the study of adrenomedullary purinome: nucleotide synthesis and degradation, adenosine transport, nucleotide uptake into chromaffin granules, exocytotic release of nucleotides and autocrine regulation of chromaffin cell function via purinoceptors. This short review will focus on the current state of knowledge of the purinoceptors of adrenal chromaffin cells, a subject to which María Teresa made seminal contributions and which she continued to study until the end of her scientific life.

Adrenal G Protein-Coupled Receptors and the Failing Heart: A Long-distance, Yet Intimate Affair

ABSTRACT

Systolic heart failure (HF) is a chronic clinical syndrome characterized by the reduction in cardiac function and still remains the disease with the highest mortality worldwide. Despite considerable advances in pharmacological treatment, HF represents a severe clinical and social burden. Chronic human HF is characterized by several important neurohormonal perturbations, emanating from both the autonomic nervous system and the adrenal glands. Circulating catecholamines (norepinephrine and epinephrine) and aldosterone elevations are among the salient alterations that confer significant hormonal burden on the already compromised function of the failing heart. This is why sympatholytic treatments (such as β-blockers) and renin-angiotensin system inhibitors or mineralocorticoid receptor antagonists, which block the effects of angiotensin II (AngII) and aldosterone on the failing heart, are part of the mainstay HF pharmacotherapy presently. The adrenal gland plays an important role in the modulation of cardiac neurohormonal stress because it is the source of almost all aldosterone, of all epinephrine, and of a significant amount of norepinephrine reaching the failing myocardium from the blood circulation. Synthesis and release of these hormones in the adrenals is tightly regulated by adrenal G protein-coupled receptors (GPCRs), such as adrenergic receptors and AngII receptors. In this review, we discuss important aspects of adrenal GPCR signaling and regulation, as they pertain to modulation of cardiac function in the context of chronic HF, by focusing on the 2 best studied adrenal GPCR types in that context, adrenergic receptors and AngII receptors (AT 1 Rs). Particular emphasis is given to findings from the past decade and a half that highlight the emerging roles of the GPCR-kinases and the β-arrestins in the adrenals, 2 protein families that regulate the signaling and functioning of GPCRs in all tissues, including the myocardium and the adrenal gland.

Mice overexpressing chromogranin A display hypergranulogenic adrenal glands with attenuated ATP levels contributing to the hypertensive phenotype

OBJECTIVE

Elevated circulating chromogranin A (CHGA) is observed in human hypertension. CHGA is critical for granulogenesis and exocytosis of catecholamine stores from secretory large dense core vesicles (LDCV). This study aims to understand the morphological, molecular and phenotypic changes because of excess CHGA and the mechanistic link eventuating in hyper-adrenergic hypertension.

METHODS

Blood pressure and heart rate was monitored in mouse models expressing normal and elevated level of CHGA by telemetry. Catecholamine and oxidative stress radicals were measured. Adrenal ultrastructure, LDCV content and mitochondrial abundance were compared and respiration analyzed by Seahorse assay. Effect of CHGA dosage on adrenal ATP content, electron transport chain components and uncoupling protein 2 (UCP-2) were compared in vivo and in vitro.

RESULTS

Mice with excess-CHGA displayed hypertensive phenotype, higher heart rate and increased sympathetic tone. They had elevated plasma catecholamine and adrenal ROS levels. Excess-CHGA caused an increase in size and abundance of LDCV and adrenal mitochondria. Nonetheless, they had attenuated levels of ATP. Isolated adrenal mitochondria from mice with elevated CHGA showed higher maximal respiration rates in the presence of protonophore, which uncouples oxidative phosphorylation. Elevated CHGA resulted in overexpression of UCP2 and diminished ATP. In vitro in chromaffin cells overexpressing CHGA, concomitant increase in UCP2 protein and decreased ATP was detected.

CONCLUSION

Elevated CHGA expression resulted in underlying bioenergetic dysfunction in ATP production despite higher mitochondrial mass. The outcome was unregulated negative feedback of LDCV exocytosis and secretion, resulting in elevated levels of circulating catecholamine and consequently the hypertensive phenotype.

Extracellular ATP Regulates the Vesicular Pore Opening in Chromaffin Cells and Increases the Fraction Released During Individual Exocytosis Events

Adenosine triphosphate (ATP) is the main energy source for cellular metabolism. Besides that, ATP is a neurotransmitter and a cotransmitter that acts on purinergic receptors present either pre- or postsynaptically. Almost all types of secretory vesicles from any neuron or animal species contain high concentrations of ATP, being an essential factor in the accumulation of neurotransmitters. In this work, we studied the effects of ATP on quantum catecholamine release and vesicular storage in chromaffin cells. We combined three electrochemical methods: conventional amperometry with intracellular vesicle impact electrochemical cytometry and vesicle impact electrochemical cytometry. We found that extracellular ATP increased the released quantal fraction of catecholamine but not its vesicular content. Studying the dynamics of exocytosis events in ATP treated cells showed that ATP affects the release fusion pore. To elucidate the mechanisms of the observed ATP effects, cells and vesicles were pharmacologically treated with suramin (a purinergic blocker) and ARL-67156 (an antagonist of ecto-ATPases). The data indicate that the catecholamine content of vesicles increased compared to control after these drugs. Our data suggest that ATP acting on purinergic receptors increases the quantum releasable size through an increased fusion pore opening and that ARL-67156 and/or suramin protect the vesicle from neurotransmitter leakage by functioning as competitive inhibitors to ATP.

Serotonin and Serotonin Transporters in the Adrenal Medulla: A Potential Hub for Modulation of the Sympathetic Stress Response

Serotonin (5-HT) is an important neurotransmitter in the central nervous system where it modulates circuits involved in mood, cognition, movement, arousal, and autonomic function. The 5-HT transporter (SERT; SLC6A4) is a key regulator of 5-HT signaling, and genetic variations in SERT are associated with various disorders including depression, anxiety, and autism. This review focuses on the role of SERT in the sympathetic nervous system. Autonomic/sympathetic dysfunction is evident in patients with depression, anxiety, and other diseases linked to serotonergic signaling. Experimentally, loss of SERT function (SERT knockout mice or chronic pharmacological block) has been reported to augment the sympathetic stress response. Alterations to serotonergic signaling in the CNS and thus central drive to the peripheral sympathetic nervous system are presumed to underlie this augmentation. Although less widely recognized, SERT is robustly expressed in chromaffin cells of the adrenal medulla, the neuroendocrine arm of the sympathetic nervous system. Adrenal chromaffin cells do not synthesize 5-HT but accumulate small amounts by SERT-mediated uptake. Recent evidence demonstrated that 5-HT1A receptors inhibit catecholamine secretion from adrenal chromaffin cells via an atypical mechanism that does not involve modulation of cellular excitability or voltage-gated Ca2+ channels. This raises the possibility that the adrenal medulla is a previously unrecognized peripheral hub for serotonergic control of the sympathetic stress response. As a framework for future investigation, a model is proposed in which stress-evoked adrenal catecholamine secretion is fine-tuned by SERT-modulated autocrine 5-HT signaling.

GPCRs of adrenal chromaffin cells & catecholamines: The plot thickens

The circulating catecholamines (CAs) epinephrine (Epi) and norepinephrine (NE) derive from two major sources in the whole organism: the sympathetic nerve endings, which release NE on effector organs, and the chromaffin cells of the adrenal medulla, which are cells that synthesize, store and release Epi (mainly) and NE. All of the Epi in the body and a significant amount of circulating NE derive from the adrenal medulla. The secretion of CAs from adrenal chromaffin cells is regulated in a complex way by a variety of membrane receptors, the vast majority of which are G protein-coupled receptors (GPCRs), including adrenergic receptors (ARs), which act as "presynaptic autoreceptors" in this regard. There is a plethora of CA-secretagogue signals acting on these receptors but some of them, most notably the α2ARs, inhibit CA secretion. Over the past few years, however, a few new proteins present in chromaffin cells have been uncovered to participate in CA secretion regulation. Most prominent among these are GRK2 and β-arrestin1, which are known to interact with GPCRs regulating receptor signaling and function. The present review will discuss the molecular and signaling mechanisms by which adrenal chromaffin cell-residing GPCRs and their regulatory proteins modulate CA synthesis and secretion. Particular emphasis will be given to the newly discovered roles of GRK2 and β-arrestins in these processes and particular points of focus for future research will be highlighted, as well.

Crosstalk between purinergic receptors and canonical signaling pathways in the mouse salivary gland

Isolated clusters of mouse parotid acinar cells in combination with live cell imaging were used to explore the crosstalk in molecular signaling between purinergic, cholinergic and adrenergic pathways that integrate to control fluid and protein secretion. This crosstalk was manifested by (1) β-adrenergic receptor activation and amplification of P2X4R evoked Ca(2+) signals, (2) β-adrenergic-induced amplification of P2X7R-evoked Ca(2+) signals and (3) muscarinic receptor induced activation of P2X7Rs via exocytotic activity. The findings from our study reveal that purinoceptor-mediated Ca(2+) signaling is modulated by crosstalk with canonical signaling pathways in parotid acinar cells. Integration of these signals are likely important for dynamic control of saliva secretion to match physiological demand in the parotid gland.

Impairment of vesicular ATP release affects glucose metabolism and increases insulin sensitivity

Neuroendocrine cells store ATP in secretory granules and release it along with hormones that may trigger a variety of cellular responses in a process called purinergic chemical transmission. Although the vesicular nucleotide transporter (VNUT) has been shown to be involved in vesicular storage and release of ATP, its physiological relevance in vivo is far less well understood. In Vnut knockout (Vnut(-/-)) mice, we found that the loss of functional VNUT in adrenal chromaffin granules and insulin granules in the islets of Langerhans led to several significant effects. Vesicular ATP accumulation and depolarization-dependent ATP release were absent in the chromaffin granules of Vnut(-/-) mice. Glucose-responsive ATP release was also absent in pancreatic β-cells in Vnut(-/-) mice, while glucose-responsive insulin secretion was enhanced to a greater extent than that in wild-type tissue. Vnut(-/-) mice exhibited improved glucose tolerance and low blood glucose upon fasting due to increased insulin sensitivity. These results demonstrated an essential role of VNUT in vesicular storage and release of ATP in neuroendocrine cells in vivo and suggest that vesicular ATP and/or its degradation products act as feedback regulators in catecholamine and insulin secretion, thereby regulating blood glucose homeostasis.

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.

Autocrine and paracrine actions of ATP in rat carotid body

Carotid bodies are peripheral chemoreceptors that detect lowering of arterial blood O(2) level. The carotid body comprises clusters of glomus (type I) cells surrounded by glial-like sustentacular (type II) cells. Hypoxia triggers depolarization and cytosolic [Ca(2+)] ([Ca(2+)](i)) elevation in glomus cells, resulting in the release of multiple transmitters, including ATP. While ATP has been shown to be an important excitatory transmitter in the stimulation of carotid sinus nerve, there is considerable evidence that ATP exerts autocrine and paracrine actions in carotid body. ATP acting via P2Y(1) receptors, causes hyperpolarization in glomus cells and inhibits the hypoxia-mediated [Ca(2+)](i) rise. In contrast, adenosine (an ATP metabolite) triggers depolarization and [Ca(2+)](i) rise in glomus cells via A(2A) receptors. We suggest that during prolonged hypoxia, the negative and positive feedback actions of ATP and adenosine may result in an oscillatory Ca(2+) signal in glomus cells. Such mechanisms may allow cyclic release of transmitters from glomus cells during prolonged hypoxia without causing cellular damage from a persistent [Ca(2+)](i) rise. ATP also stimulates intracellular Ca(2+) release in sustentacular cells via P2Y(2) receptors. The autocine and paracrine actions of ATP suggest that ATP has important roles in coordinating chemosensory transmission in the carotid body.

Inhibition of Ca2+ channels and adrenal catecholamine release by G protein coupled receptors

Catecholamines and other transmitters released from adrenal chromaffin cells play central roles in the "fight-or-flight" response and exert profound effects on cardiovascular, endocrine, immune, and nervous system function. As such, precise regulation of chromaffin cell exocytosis is key to maintaining normal physiological function and appropriate responsiveness to acute stress. Chromaffin cells express a number of different G protein coupled receptors (GPCRs) that sense the local environment and orchestrate this precise control of transmitter release. The primary trigger for catecholamine release is Ca2+ entry through voltage-gated Ca2+ channels, so it makes sense that these channels are subject to complex regulation by GPCRs. In particular G protein βγ heterodimers (Gbc) bind to and inhibit Ca2+ channels. Here I review the mechanisms by which GPCRs inhibit Ca2+ channels in chromaffin cells and how this might be altered by cellular context. This is related to the potent autocrine inhibition of Ca2+ entry and transmitter release seen in chromaffin cells. Recent data that implicate an additional inhibitory target of Gβγ on the exocytotic machinery and how this might fine tune neuroendocrine secretion are also discussed.

Regulation of calcium channels and exocytosis in mouse adrenal chromaffin cells by prostaglandin EP3 receptors

Prostaglandin (PG) E(2) controls numerous physiological functions through a family of cognate G protein-coupled receptors (EP1-EP4). Targeting specific EP receptors might be therapeutically useful and reduce side effects associated with nonsteroidal anti-inflammatory drugs and selective cyclooxygenase-2 inhibitors that block prostanoid synthesis. Systemic immune challenge and inflammatory cytokines have been shown to increase expression of the synthetic enzymes for PGE(2) in the adrenal gland. Catecholamines and other hormones, released from adrenal chromaffin cells in response to Ca(2+) influx through voltage-gated Ca(2+) channels, play central roles in homeostatic function and the coordinated stress response. However, long-term elevation of circulating catecholamines contributes to the pathogenesis of hypertension and heart failure. Here, we investigated the EP receptor(s) and cellular mechanisms by which PGE(2) might modulate chromaffin cell function. PGE(2) did not alter resting intracellular [Ca(2+)] or the peak amplitude of nicotinic acetylcholine receptor currents, but it did inhibit Ca(V)2 voltage-gated Ca(2+) channel currents (I(Ca)). This inhibition was voltage-dependent and mediated by pertussis toxin-sensitive G proteins, consistent with a direct Gβγ subunit-mediated mechanism common to other G(i/o)-coupled receptors. mRNA for all four EP receptors was detected, but using selective pharmacological tools and EP receptor knockout mice, we demonstrated that EP3 receptors mediate the inhibition of I(Ca). Finally, changes in membrane capacitance showed that Ca(2+)-dependent exocytosis was reduced in parallel with I(Ca). To our knowledge, this is the first study of EP receptor signaling in mouse chromaffin cells and identifies a molecular mechanism for paracrine regulation of neuroendocrine function by PGE(2).

G protein betagamma subunits modulate the number and nature of exocytotic fusion events in adrenal chromaffin cells independent of calcium entry

G-protein-coupled receptors (GPCR) play important roles in controlling neurotransmitter and hormone release. Inhibition of voltage-gated Ca(2+) channels (Ca(2+) channels) by G protein betagamma subunits (Gbetagamma) is one prominent mechanism, but there is evidence for additional effects distinct from those on calcium entry. However, relatively few studies have investigated the Ca(2+)-channel-independent effects of Gbetagamma on transmitter release, so the impact of this mechanism remains unclear. We used carbon fiber amperometry to analyze catecholamine release from individual vesicles in bovine adrenal chromaffin cells, a widely used neurosecretory model. To bypass the effects of Gbetagamma on Ca(2+) entry, we stimulated secretion using ionomycin (a Ca(2+) ionophore) or direct intracellular application of Ca(2+) through a patch pipette. Activation of endogenous GPCR or transient transfection with exogenous Gbetagamma significantly reduced the number of amperometric spikes (the number of vesicular fusion events). The charge ("quantal size") and amplitude of the amperometric spikes were also significantly reduced by GPCR/Gbetagamma. We conclude that independent from effects on calcium entry, Gbetagamma can regulate both the number of vesicles that undergo exocytosis and the amount of catecholamine released per fusion event. We discuss possible mechanisms by which Gbetagamma might exert these novel effects including interaction with the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex.

Measuring secretion in chromaffin cells using electrophysiological and electrochemical methods

Our present understanding of exocytosis of catecholamines has benefited tremendously from the arrival of single-cell electrochemical methods (amperometry and voltammetry), electrophysiological techniques (whole-cell and patch capacitance) and from the combination of both techniques (patch amperometry). In this brief review, we will outline the strengths and limitations of amperometric and electrophysiological methods and highlight the major contribution obtained with the use of these techniques in chromaffin cells.

N- and P/Q-type Ca2+ channels in adrenal chromaffin cells

Ca2+ is the most ubiquitous second messenger found in all cells. Alterations in [Ca2+]i contribute to a wide variety of cellular responses including neurotransmitter release, muscle contraction, synaptogenesis and gene expression. Voltage-dependent Ca2+ channels, found in all excitable cells (Hille 1992), mediate the entry of Ca2+ into cells following depolarization. Ca2+ channels are composed of a large pore-forming subunit, called the alpha1 subunit, and several accessory subunits. Ten different alpha1 subunit genes have been identified and classified into three families, Ca(v1-3) (Dunlap et al. 1995, Catterall 2000). Each alpha1 gene produces a unique Ca2+ channel. Although chromaffin cells express several different types of Ca2+ channels, this review will focus on the Cav(2.1) and Cav(2.2) channels, also known as P/Q- and N-type respectively (Nowycky et al. 1985, Llinas et al. 1989b, Wheeler et al. 1994). These channels exhibit physiological and pharmacological properties similar to their neuronal counterparts. N-, P/Q and to a lesser extent R-type Ca2+ channels are known to regulate neurotransmitter release (Hirning et al. 1988, Horne & Kemp 1991, Uchitel et al. 1992, Luebke et al. 1993, Takahashi & Momiyama 1993, Turner et al. 1993, Regehr & Mintz 1994, Wheeler et al. 1994, Wu & Saggau 1994, Waterman 1996, Wright & Angus 1996, Reid et al. 1997). N- and P/Q-type Ca2+ channels are abundant in nerve terminals where they colocalize with synaptic vesicles. Similarly, these channels play a role in neurotransmitter release in chromaffin cells (Garcia et al. 2006). N- and P/Q-type channels are subject to many forms of regulation (Ikeda & Dunlap 1999). This review pays particular attention to the regulation of N- and P/Q-type channels by heterotrimeric G-proteins, interaction with SNARE proteins, and channel inactivation in the context of stimulus-secretion coupling in adrenal chromaffin cells.

Selective stimulation of catecholamine release from bovine adrenal chromaffin cells by an ionotropic purinergic receptor sensitive to 2-methylthio ATP

Background

2-Methylthioadenosine 5'-triphosphate (2-MeSATP), formerly regarded as a specific P2Y (metabotropic) purinergic receptor agonist, stimulates Ca²⁺ influx and evokes catecholamine release from adrenal chromaffin cells. These cells express P2Y and P2X (ionotropic) purinoceptors, with the latter providing an important Ca²⁺ influx pathway. Using single cell calcium imaging techniques, we have determined whether 2-MeSATP might be a specific P2X receptor agonist in bovine chromaffin cells and assessed the relative role of P2X and P2Y receptors on catecholamine secretion from these cells.

Results

ATP raised the [Ca²⁺]_i in ~50% of the cells. Removing extracellular Ca²⁺ suppressed the [Ca²⁺]_i-raising ability of 2-MeSATP, observed in ~40% of the ATP-sensitive cells. This indicates that 2-MeSATP behaves as a specific ionotropic purinoceptor agonist in bovine chromaffin cells. The 2-MeSATP-induced [Ca²⁺]_i-rises were suppressed by PPADS. UTP raised the [Ca²⁺]_i in ~40% of the ATP-sensitive cells, indicating that these expressed Ca²⁺-mobilizing P2Y receptors. UTP-sensitive receptors may not be the only P2Y receptors present, as suggested by the observation that ~20% of the ATP-sensitive pool did not respond to either 2-MeSATP or UTP. The average sizes of the ATP- and 2-MeSATP-evoked [Ca²⁺]_i responses were identical in UTP-insensitive cells. 2-MeSATP stimulated Ca²⁺ influx and evoked catecholamine release, whereas UTP elicited Ca²⁺ release from intracellular stores but did not evoke secretion. 2-MeSATP-induced secretion was strongly inhibited by Cd²⁺ and suppressed by extracellular Ca²⁺ or Na⁺ removal. TTX inhibited 2-MeSATP-evoked secretion by ~20%.

Conclusion

2-MeSATP is a specific P2X purinoceptor agonist and a potent secretagogue in bovine chromaffin cells. Activation of 2-MeSATP-sensitive receptors stimulates Ca²⁺ influx mainly via voltage-sensitive Ca²⁺ channels. For the most part, these are activated by the depolarization brought about by Na⁺ influx across P2X receptor pores.

Functional distribution of Ca2+-coupled P2 purinergic receptors among adrenergic and noradrenergic bovine adrenal chromaffin cells

BACKGROUND

Adrenal chromaffin cells mediate acute responses to stress through the release of epinephrine. Chromaffin cell function is regulated by several receptors, present both in adrenergic (AD) and noradrenergic (NA) cells. Extracellular ATP exerts excitatory and inhibitory actions on chromaffin cells via ionotropic (P2X) and metabotropic (P2Y) receptors. We have taken advantage of the actions of the purinergic agonists ATP and UTP on cytosolic free Ca2+ concentration ([Ca2+]i) to determine whether P2X and P2Y receptors might be asymmetrically distributed among AD and NA chromaffin cells.

RESULTS

The [Ca2+]i and the [Na+]i were recorded from immunolabeled bovine chromaffin cells by single-cell fluorescence imaging. Among the ATP-sensitive cells ~40% did not yield [Ca2+]i responses to ATP in the absence of extracellular Ca2+ (Ca2+o), indicating that they expressed P2X receptors and did not express Ca2+- mobilizing P2Y receptors; the remainder expressed Ca2+-mobilizing P2Y receptors. Relative to AD-cells approximately twice as many NA-cells expressed P2X receptors while not expressing Ca2+- mobilizing P2Y receptors, as indicated by the proportion of cells lacking [Ca2+]i responses and exhibiting [Na+]i responses to ATP in the absence and presence of Ca2+o, respectively. The density of P2X receptors in NA-cells appeared to be 30-50% larger, as suggested by comparing the average size of the [Na+]i and [Ca2+]i responses to ATP. Conversely, approximately twice as many AD-cells expressed Ca2+-mobilizing P2Y receptors, and they appeared to exhibit a higher (~20%) receptor density. UTP raised the [Ca2+]i in a fraction of the cells and did not raise the [Na+]i in any of the cells tested, confirming its specificity as a P2Y agonist. The cell density of UTP-sensitive P2Y receptors did not appear to vary among AD- and NA-cells.

CONCLUSION

Although neither of the major purinoceptor types can be ascribed to a particular cell phenotype, P2X and Ca2+-mobilizing P2Y receptors are preferentially located to noradrenergic and adrenergic chromaffin cells, respectively. ATP might, in addition to an UTP-sensitive P2Y receptor, activate an UTP-insensitive P2Y receptor subtype. A model for a short-loop feedback interaction is presented whereby locally released ATP acts upon P2Y receptors in adrenergic cells, inhibiting Ca2+ influx and contributing to terminate evoked epinephrine secretion.

Changes in P2Y 2 receptor localization on adrenaline‐ and noradrenaline‐containing chromaffin cells in the rat adrenal gland during development and aging

Using immunohistochemistry, the occurrence and age-related changes of the P2Y2 receptor was investigated in the adrenal gland of rat at different ages, ranging from embryonic day E16 to 22 months. Immunoreactivity for the P2Y2 receptor was present in chromaffin cells and nerve fibres at all ages examined. Double labeling with the antibody against phenyl ethanolamine-N-methyltransferase, which marks adrenaline-producing chromaffin cells, revealed that only a few of the P2Y2-immunoreactive cells were adrenaline producing at embryonic day E16, the vast majority being noradrenaline-containing cells. However, immunoreactivity for adrenaline-containing cells in the P2Y2 receptor-labeled chromaffin cells increased with increasing age and at 1 week post-natal almost all chromaffin cells were positive for both P2Y2 and phenyl ethanolamine-N-methyltransferase, while noradrenaline-containing cells were minimal. At 2 weeks, there was a dramatic drop in P2Y2-immunoreactive chromaffin cells and this was maintained in adult rats, noradrenaline-containing cells dominating. In the aging rat adrenals, P2Y2 receptor-immunoreactivity was localized in subpopulations of both adrenaline and noradrenaline-producing cells. Intrinsic neurones were also visible that were positively labeled with the P2Y2 receptor antibody in the adrenals of both adult and aging rats. P2Y2-immunoreactive nerve fibres formed a plexus around the adrenal cortical cells of zona glomerulosa in the post-natal, but not in adult or aging rats. In conclusion, this study suggests that ATP, acting through P2Y2 receptors, may influence the phenotypic expression of chromaffin cells during the development and aging of the rat adrenal gland. However, during early development, when the chromaffin cells are actively dividing and during aging, when the adrenal medullary cells are known to show hyperplastic lesions, ATP acting through P2Y2 receptors may be involved in other physiological activities, such as proliferation and/or differentiation of the chromaffin cells associated with their adrenaline or noradrenaline phenotype.

Evidence for paracrine modulation of voltage-dependent calcium channels by amperometric analysis in cultured porcine adrenal chromaffin cells

We investigated the endogenous control through vesicular contents of voltage-dependent Ca2+ channels (VDCCs) in cultured porcine adrenal chromaffin cells. To examine paracrine regulation of VDCCs, catecholamine release was monitored amperometrically together with patch-clamp recording under culture conditions at different cell densities. A depolarizing pulse evoked Ca(2+)- (ICa) and Ba(2+)-currents (IBa) in Ca(2+)- and Ba(2+)-containing solutions, respectively. In cells cultured at high density, stop-flow of the external solution decreased the I(Ba) concomitant with a sustained increase of amperometric current (Iamp), but not in cells at low density, suggesting the endogenous modulation of VDCCs in a paracine fashion. The degree of the prepulse facilitation was similar regardless of the flow condition. Application of noradrenaline (NA), ATP, methionine-enkephalin (ENK) or protons decreased IBa. The extent of the prepulse facilitation of the endogenous VDCC inhibition was similar to those induced by NA and ATP. GDPbetaS, pertussis toxin (PTX), blockers for alpha-adrenoceptors and P2-purinoceptors significantly reduced the endogenous VDCC inhibition. These results suggest that VDCCs are regulated by vesicular substances in a paracrine fashion, at least by noradrenaline and ATP, through activation of alpha-adrenoceptors and P2-purinoceptors, respectively, in porcine adrenal chromaffin cells.

Neurotransmitter modulation of neuronal calcium channels

There are many different calcium channels expressed in the mammalian nervous system, but N-type and P/Q-type calcium channels appear to dominate the presynaptic terminals of central and peripheral neurons. The neurotransmitter-induced modulation of these channels can result in alteration of synaptic transmission. This review highlights the mechanisms by which neurotransmitters affect the activity of N-type and P/Q-type calcium channels. The inhibition of these channels by voltage-dependent and voltage-independent mechanisms is emphasized because of the wealth of information available on the intracellular mediators and on the effect of these pathways on the single-channel gating.

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.

Adenosine inhibition via A(1) receptor of N-type Ca(2+) current and peptide release from isolated neurohypophysial terminals of the rat

Effects of adenosine on voltage-gated Ca(2+) channel currents and on arginine vasopressin (AVP) and oxytocin (OT) release from isolated neurohypophysial (NH) terminals of the rat were investigated using perforated-patch clamp recordings and hormone-specific radioimmunoassays. Adenosine, but not adenosine 5'-triphosphate (ATP), dose-dependently and reversibly inhibited the transient component of the whole-terminal Ba(2+) currents, with an IC(50) of 0.875 microM. Adenosine strongly inhibited, in a dose-dependent manner (IC(50) = 2.67 microM), depolarization-triggered AVP and OT release from isolated NH terminals. Adenosine and the N-type Ca(2+) channel blocker omega-conotoxin GVIA, but not other Ca(2+) channel-type antagonists, inhibited the same transient component of the Ba(2+) current. Other components such as the L-, Q- and R-type channels, however, were insensitive to adenosine. Similarly, only adenosine and omega-conotoxin GVIA were able to inhibit the same component of AVP release. A(1) receptor agonists, but not other purinoceptor-type agonists, inhibited the same transient component of the Ba(2+) current as adenosine. Furthermore, the A(1) receptor antagonist 8-cyclopentyltheophylline (CPT), but not the A(2) receptor antagonist 3, 7-dimethyl-1-propargylxanthine (DMPGX), reversed inhibition of this current component by adenosine. The inhibition of AVP and OT release also appeared to be via the A(1) receptor, since it was reversed by CPT. We therefore conclude that adenosine, acting via A(1) receptors, specifically blocks the terminal N-type Ca(2+) channel thus leading to inhibition of the release of both AVP and OT.