Acetylcholine-induced calcium signalling in adrenaline- and noradrenaline-containing adrenal chromaffin cells

Effects of reversible SERCA inhibition on catecholamine exocytosis and intracellular [Ca2+] signaling in chromaffin cells from normotensive Wistar Kyoto rats and spontaneously hypertensive rats

Intracellular Ca2+ ([Ca2+]i) signaling and catecholamine (CA) exocytosis from adrenal chromaffin cells (CCs) differ between mammalian species. These differences partly result from the different contributions of Ca2+-induced Ca2+-release (CICR) from internal stores, which boosts intracellular Ca2+ signals. Transient inhibition of the sarcoendoplasmic reticulum (SERCA) Ca2+ pump with cyclopiazonic acid (CPA) reduces CICR. Recently, Martínez-Ramírez et al. found that CPA had contrasting effects on catecholamine secretion and intracellular Ca2+ signals in mouse and bovine CCs, where it enhanced and inhibited exocytosis, respectively. After CPA withdrawal, exocytosis diminished in mouse CCs and increased in bovine CCs. These differences can be explained if mouse CCs have weak CICR and strong Ca2+ uptake, and the reverse is true for bovine CCs. Surprisingly, CPA slightly reduced the amplitude of Ca2+ signals in both mouse and bovine CCs. Here we examined the effects of CPA on stimulated CA exocytosis and Ca2+ signaling in rat CCs and investigated if it alters differently the responses of CCs from normotensive (WKY) or hypertensive (SHR) rats, which differ in the gain of CICR. Our results demonstrate that CPA application strongly inhibits voltage-gated exocytosis and Ca2+ transients in rat CCs, regardless of strain (SHR or WKY). Thus, despite the greater phylogenetic distance from the most recent common ancestors, suppression of endoplasmic reticulum (ER) Ca2+ uptake through CPA inhibits the CA secretion in rat CCs more similarly to bovine than mouse CCs, unveiling divergent evolutionary relationships in the mechanism of CA exocytosis of CCs between rodents. Agents that inhibit the SERCA pump, such as CPA, suppress catecholamine secretion equally well in WKY and SHR CCs and are not potential therapeutic agents for hypertension. Rat CCs display Ca2+ signals of varying widths. Some even show early and late Ca2+ components. Narrowing the Ca2+ transients by CPA and ryanodine suggests that the late component is mainly due to CICR. Simultaneous recordings of Ca2+ signaling and amperometry in CCs revealed the existence of a robust and predictable correlation between the kinetics of the whole-cell intracellular Ca2+ signal and the rate of exocytosis at the single-cell level.

Adaptive remodeling of the stimulus-secretion coupling: Lessons from the 'stressed' adrenal medulla

Stress is part of our daily lives and good health in the modern world is offset by unhealthy lifestyle factors, including the deleterious consequences of stress and associated pathologies. Repeated and/or prolonged stress may disrupt the body homeostasis and thus threatens our lives. Adaptive processes that allow the organism to adapt to new environmental conditions and maintain its homeostasis are therefore crucial. The adrenal glands are major endocrine/neuroendocrine organs involved in the adaptive response of the body facing stressful situations. Upon stress episodes and in response to activation of the sympathetic nervous system, the first adrenal cells to be activated are the neuroendocrine chromaffin cells located in the medullary tissue of the adrenal gland. By releasing catecholamines (mainly epinephrine and to a lesser extent norepinephrine), adrenal chromaffin cells actively contribute to the development of adaptive mechanisms, in particular targeting the cardiovascular system and leading to appropriate adjustments of blood pressure and heart rate, as well as energy metabolism. Specifically, this chapter covers the current knowledge as to how the adrenal medullary tissue remodels in response to stress episodes, with special attention paid to chromaffin cell stimulus-secretion coupling. Adrenal stimulus-secretion coupling encompasses various elements taking place at both the molecular/cellular and tissular levels. Here, I focus on stress-driven changes in catecholamine biosynthesis, chromaffin cell excitability, synaptic neurotransmission and gap junctional communication. These signaling pathways undergo a collective and finely-tuned remodeling, contributing to appropriate catecholamine secretion and maintenance of body homeostasis in response to stress.

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.

Gap junction communication between chromaffin cells: the hidden face of adrenal stimulus-secretion coupling

From birth to death, catecholamine secretion undergoes continuous adjustments, allowing the organism to adapt to homeostasis changes. To cope with these stressful conditions, the neuroendocrine cells of the adrenal medulla play an immediate and crucial role. Chromaffin cell-driven catecholamine release is chiefly controlled by a neurogenic command that arises from the sympathetic nervous system, which releases acetylcholine at the splanchnic nerve terminal-chromaffin cell synapses. In addition to receiving several synaptic inputs individually, chromaffin cells are coupled by gap junctions. This raises interesting questions about the usefulness and the role of the gap junctional coupling within the chromaffin tissue, considering that secretory function is efficiently completed by the neurogenic pathway. The findings that gap junctions contribute to catecholamine secretion, both ex vivo and in vivo, provide some early answers, but their involvement in other cellular functions still remains unexplored. This review summarizes the molecular and physiological evidence that gap junctions can act either as an accelerator or a brake of stimulus-secretion coupling and discusses this functional plasticity in the context of specific needs in circulating catecholamine levels. It introduces the concept of gap junctions as sympathetic activity sensors and guardians of the functional integrity of the chromaffin tissue.

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.

Nicotinic receptor Alpha7 expression during mouse adrenal gland development

The nicotinic acetylcholine receptor alpha 7 (α7) is a ligand-activated ion channel that contributes to a diversity of cellular processes involved in development, neurotransmission and inflammation. In this report the expression of α7 was examined in the mouse developing and adult adrenal gland that expresses a green fluorescent protein (GFP) reporter as a bi-cistronic extension of the endogenous α7 transcript (α7(G)). At embryonic day 12.5 (E12.5) α7(G) expression was associated with the suprarenal ganglion and precursor cells of the adrenal gland. The α7(G) cells are catecholaminergic chromaffin cells as reflected by their progressive increase in the co-expression of tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DBH) that is complete by E18.5. In the adult, α7(G) expression is limited to a subset of chromaffin cells in the adrenal medulla that cluster near the border with the adrenal cortex. These chromaffin cells co-express α7(G), TH and DBH, but they lack phenylethanolamine N-methyltransferase (PNMT) consistent with only norepinephrine (NE) synthesis. These cell groups appear to be preferentially innervated by pre-ganglionic afferents identified by the neurotrophin receptor p75. No afferents identified by beta-III tubulin, neurofilament proteins or p75 co-expressed α7(G). Occasional α7(G) cells in the pre-E14.5 embryos express neuronal markers consistent with intrinsic ganglion cells and in the adult some α7(G) cells co-express glutamic acid decarboxylase. The transient expression of α7 during adrenal gland development and its prominent co-expression by a subset of NE chromaffin cells in the adult suggests that the α7 receptor contributes to multiple aspects of adrenal gland development and function that persist into adulthood.

Adrenal adrenoceptors in heart failure

Heart failure (HF) is a chronic clinical syndrome characterized by the reduction in left ventricular (LV) function and it represents one of the most important causes of morbidity and mortality worldwide. Despite considerable advances in pharmacological treatment, HF represents a severe clinical and social burden. Sympathetic outflow, characterized by increased circulating catecholamines (CA) biosynthesis and secretion, is peculiar in HF and sympatholytic treatments (as β-blockers) are presently being used for the treatment of this disease. Adrenal gland secretes Epinephrine (80%) and Norepinephrine (20%) in response to acetylcholine stimulation of nicotinic cholinergic receptors on the chromaffin cell membranes. This process is regulated by adrenergic receptors (ARs): α2ARs inhibit CA release through coupling to inhibitory Gi-proteins, and β ARs (mainly β2ARs) stimulate CA release through coupling to stimulatory Gs-proteins. All ARs are G-protein-coupled receptors (GPCRs) and GPCR kinases (GRKs) regulate their signaling and function. Adrenal GRK2-mediated α2AR desensitization and downregulation are increased in HF and seem to be a fundamental regulator of CA secretion from the adrenal gland. Consequently, restoration of adrenal α2AR signaling through the inhibition of GRK2 is a fascinating sympatholytic therapeutic strategy for chronic HF. This strategy could have several significant advantages over existing HF pharmacotherapies minimizing side-effects on extra-cardiac tissues and reducing the chronic activation of the renin–angiotensin–aldosterone and endothelin systems. The role of adrenal ARs in regulation of sympathetic hyperactivity opens interesting perspectives in understanding HF pathophysiology and in the identification of new therapeutic targets.

Ca(2+) signaling mechanisms in bovine adrenal chromaffin cells

Calcium (Ca(2+)) is a crucial intracellular messenger in physiological aspects of cell signaling. Adrenal chromaffin cells are the secretory cells from the adrenal gland medulla that secrete catecholamines, which include epinephrine and norepinephrine important in the 'fight or flight' response. Bovine adrenal chromaffin cells have long been used as an important model for secretion -(exocytosis) not only due to their importance in the short-term stress response, but also as a neuroendocrine model of neurotransmtter release, as they have all the same exocytotic proteins as neurons but are easier to prepare, culture and use in functional assays. The components of the Ca(2+) signal transduction cascade and it role in secretion has been extensively characterized in bovine adrenal chromaffin cells. The Ca(2+) sources, signaling molecules and how this relates to the short-term stress response are reviewed in this book chapter in an endeavor to generally -overview these mechanisms in a concise and uncomplicated manner.

Muscarinic excitation-secretion coupling in chromaffin cells

Excitation-secretion coupling in adrenomedullary chromaffin cells physiologically commences when acetylcholine molecules released from splanchnic nerve terminals bind to cholinergic receptors located at the cell's plasma membrane. While nicotinic acetylcholine receptors ensure a rapid and efficacious transmission of preganglionic impulses, muscarinic acetylcholine receptors are considered to play a subsidiary role mostly by facilitating the nicotinic responses. Nevertheless, the variety of effects brought about by muscarinic stimulation in chromaffin cells (release of intracellular Ca2+, activation of Ca2+ entry through non-selective cation channels and voltage-dependent Ca2+ channels, impairment and/or enhancement of action potential firing, etc.) and the long-lasting nature of many of them suggests that muscarinic receptors might contribute to the fine tuning of the catecholamine secretory response upon graded preganglionic stimulation and prolonged periods of time. Such a variety of effects probably reflects not only the diversity of muscarinic receptors expressed in chromaffin cells but also the existence of differences among the animal species employed in the reported investigations. Accordingly, we first review on an animal species-based approach the most relevant features of the muscarinic response in chromaffin cells from a set of mammals, and finally present a unified picture of the mechanisms of muscarinic excitation-secretion coupling in chromaffin cells.

Key role of the nicotinic receptor in neurotransmitter exocytosis in human chromaffin cells

The whole-cell secretory response evoked by acetylcholine (ACh) in human chromaffin cells was examined using a new protocol based on quickly switching from the voltage-clamp to the current-clamp (CC) configuration of the patch-clamp technique. Our experiments revealed that Ca(2+) entry through the nicotinic receptor at hyperpolarized membrane potentials contributed as much to the exocytosis (100.4 +/- 27.3 fF) evoked by 200 ms pulses of ACh, as Ca(2+) flux through voltage-dependent Ca(2+) channels at depolarized membrane potentials. The nicotinic current triggered a depolarization event with a peak at +49.3 mV and a 'plateau' phase that ended at -23.9 mV, which was blocked by 10 mumol/L mecamylamine. When a long ACh stimulus (15 s) was applied, the nicotinic current at the end of the pulse reached a value of 15.45 +/- 3.6 pA, but the membrane potential depolarization still remained at the 'plateau' stage until withdrawal of the agonist. Perfusion with 200 mumol/L Cd(2+) during the 15 s ACh pulse completely abolished the plasma membrane depolarization at the end of the pulse, indicating that Ca(2+) entry through Ca(2+) channels contributed to the membrane potential depolarization provoked by prolonged ACh pulses. These findings also reflect that voltage-dependent Ca(2+) channels were recruited by the small current flowing through the desensitized nicotinic receptor to maintain the depolarization. Finally, muscarinic receptor activation triggered a delayed exocytotic process after prolonged ACh stimulation, dependent on Ca(2+) mobilization from the endoplasmic reticulum. In summary, we show here that nicotinic and muscarinic receptors contribute to the exocytosis of neurotransmitters in human chromaffin cells, and that the nicotinic receptor plays a key role in several stages of the stimulus-secretion coupling process in these cells.

Adrenal adrenoceptors in heart failure: fine-tuning cardiac stimulation

Chronic heart failure (HF) is characterized by sympathetic hyperactivity reflected by increased circulating catecholamines (CAs), which contributes significantly to its morbidity and mortality. Therefore, sympatholytic treatments, that is, treatments that reduce sympathetic hyperactivity, are being pursued currently for the treatment of HF. Secretion of CAs from the adrenal gland, which is a major source of CAs, is regulated by alpha(2)-adrenoceptors (alpha(2)ARs), which inhibit, and by beta-adrenoceptors (betaARs), which enhance CA secretion. All ARs are G-protein-coupled receptors (GPCRs), whose signaling and function are regulated tightly by the family of GPCR kinases (GRKs). Despite the enormous potential of adrenal ARs for the regulation of sympathetic outflow, elucidation of their properties has only begun recently. Here, recent advances regarding the roles of adrenal ARs in the regulation of sympathetic outflow in HF and the regulatory properties of ARs are discussed, along with the potential benefits and challenges of harnessing their function for HF therapy.

Blockade of nicotinic receptors of bovine adrenal chromaffin cells by nanomolar concentrations of atropine

Nanomolar concentrations of atropine have been considered up to now to be selective for blockade of muscarinic receptors for acetylcholine. A collateral finding indicated to us that these low concentrations of atropine could also target the neuronal nicotinic receptors. We report here a detailed study on this novel property of atropine. Catecholamine release, measured on-line with amperometry in chromaffin cells stimulated with acetylcholine pulses was blocked by atropine in a competitive manner. To corroborate a direct action of atropine on nicotinic receptors, we have employed N,N-dimethyl-N'-phenyl-piperazinium (DMPP), a pure nicotinic receptor agonist; atropine blocked its secretory responses with an IC50 of 2.04 nM. Nicotinic currents, recorded with the whole cell configuration of the patch-clamp technique were blocked by atropine in a concentration-dependent manner (IC50 of 11 nM), also showing a competitive nature. Nicotinic receptor currents in oocytes expressing bovine alpha7 and alpha3beta4 nicotinic receptors were blocked by atropine with an IC50 of 11.2 and 46.8 nM, respectively. Atropine (30 nM) also decreased the increment of the cytosolic calcium concentrations after stimulation with 30 microM DMPP in bovine chromaffin cells. However, action potentials evoked by DMPP were not modified by atropine. Our results demonstrate that nicotinic currents and their downstream consequences (i.e. cytosolic calcium elevations and catecholamine release) were blocked by nanomolar concentrations of atropine; although the blockade was partial, it must be considered when using atropine to study cholinergic neurotransmission, particularly at synapses where both nicotinic and muscarinic receptors are present i.e., the adrenal medulla and autonomic ganglia.

Different Signaling Pathways Involved in Glucose- and Cell Swelling-Induced Insulin Secretion by Rat Pancreatic Islets in Vitro

BACKGROUND

The objective was to compare signal transduction pathways exploited by glucose and cell swelling in stimulating insulin secretion.

METHODS

Isolated rat (Wistar) pancreatic islets were stimulated in vitro by 20 mmol/l glucose or 30% hypotonic medium (202 mOsm/kg) in various experimental conditions.

RESULTS

Glucose did not stimulate insulin release in calcium free medium. Cell swelling-induced insulin release in calcium free medium, even in the presence of the membrane permeable calcium chelator BAPTA/AM (10 micromol/l). Protein kinase C (PKC) inhibitor bisindolylmaleimide VIII (1 micromol/l) abolished the stimulation of insulin secretion by glucose but did not affect the swelling-induced insulin release. PKC activator phorbol 12-13-dibutyrate (1 micromol/l) stimulated insulin secretion in medium containing Ca2+ and did not potentiate insulin secretion stimulated by hypotonic extracellular fluid. Dilution of the medium (10-30%) had an additive effect on the glucose-induced insulin secretion. Noradrenaline (1 micromol/l) abolished glucose-induced insulin secretion but did not inhibit hypotonic stimulation either in presence or absence of Ca2+.

CONCLUSION

Glucose- and swelling-induce insulin secretion through separate signal transduction pathways. Hyposmotic stimulation is independent from both the extracellular and intracellular Ca2+, does not involve PKC activation, and could not be inhibited by noradrenaline. These data indicate a novel signaling pathway for stimulation of insulin secretion exploited by cell swelling.