Hypoxia-induced catecholamine release and intracellular Ca2+ increase via suppression of K+ channels in cultured rat adrenal chromaffin cells

Forty years of the adrenal chromaffin cell through ISCCB meetings around the world

This historical review focuses on the evolution of the knowledge accumulated during the last two centuries on the biology of the adrenal medulla gland and its chromaffin cells (CCs). The review emerged in the context of a series of meetings that started on the Spanish island of Ibiza in 1982 with the name of the International Symposium on Chromaffin Cell Biology (ISCCB). Hence, the review is divided into two periods namely, before 1982 and from this year to 2022, when the 21st ISCCB meeting was just held in Hamburg, Germany. The first historical period extends back to 1852 when Albert Kölliker first described the fine structure and function of the adrenal medulla. Subsequently, the adrenal staining with chromate salts identified the CCs; this was followed by the establishment of the embryological origin of the adrenal medulla, and the identification of adrenaline-storing vesicles. By the end of the nineteenth century, the basic morphology, histochemistry, and embryology of the adrenal gland were known. The twentieth century began with breakthrough findings namely, the experiment of Elliott suggesting that adrenaline was the sympathetic neurotransmitter, the isolation of pure adrenaline, and the deciphering of its molecular structure and chemical synthesis in the laboratory. In the 1950s, Blaschko isolated the catecholamine-storing vesicles from adrenal medullary extracts. This switched the interest in CCs as models of sympathetic neurons with an explosion of studies concerning their functions, i.e., uptake of catecholamines by chromaffin vesicles through a specific coupled transport system; the identification of several vesicle components in addition to catecholamines including chromogranins, ATP, opioids, and other neuropeptides; the calcium-dependence of the release of catecholamines; the underlying mechanism of exocytosis of this release, as indicated by the co-release of proteins; the cross-talk between the adrenal cortex and the medulla; and the emission of neurite-like processes by CCs in culture, among other numerous findings. The 1980s began with the introduction of new high-resolution techniques such as patch-clamp, calcium probes, marine toxins-targeting ion channels and receptors, confocal microscopy, or amperometry. In this frame of technological advances at the Ibiza ISCCB meeting in 1982, 11 senior researchers in the field predicted a notable increase in our knowledge in the field of CCs and the adrenal medulla; this cumulative knowledge that occurred in the last 40 years of history of the CC is succinctly described in the second part of this historical review. It deals with cell excitability, ion channel currents, the exocytotic fusion pore, the handling of calcium ions by CCs, the kinetics of exocytosis and endocytosis, the exocytotic machinery, and the life cycle of secretory vesicles. These concepts together with studies on the dynamics of membrane fusion with super-resolution imaging techniques at the single-protein level were extensively reviewed by top scientists in the field at the 21st ISCCB meeting in Hamburg in the summer of 2022; this frontier topic is also briefly reviewed here. Many of the concepts arising from those studies contributed to our present understanding of synaptic transmission. This has been studied in physiological or pathophysiological conditions, in CCs from animal disease models. In conclusion, the lessons we have learned from CC biology as a peripheral model for brain and brain disease pertain more than ever to cutting-edge research in neurobiology. In the 22nd ISCCB meeting in Israel in 2024 that Uri Asheri is organizing, we will have the opportunity of seeing the progress of the questions posed in Ibiza, and on other questions that undoubtedly will arise.

Mitochondrial acute oxygen sensing and signaling

Oxygen (O₂) is essential for life and therefore the supply of sufficient O₂ to the tissues is a major physiological challenge. In mammals, a deficit of O₂ (hypoxia) triggers rapid cardiorespiratory reflexes (e.g. hyperventilation and increased heart output) that within a few seconds increase the uptake of O₂ by the lungs and its distribution throughout the body. The prototypical acute O₂-sensing organ is the carotid body (CB), which contains sensory glomus cells expressing O₂-regulated ion channels. In response to hypoxia, glomus cells depolarize and release transmitters which activate afferent fibers terminating at the brainstem respiratory and autonomic centers. In this review, we summarize the basic properties of CB chemoreceptor cells and the essential role played by their specialized mitochondria in acute O₂ sensing and signaling. We focus on recent data supporting a "mitochondria-to-membrane signaling" model of CB chemosensory transduction. The possibility that the differential expression of specific subunit isoforms and enzymes could allow mitochondria to play a generalized adaptive O₂-sensing and signaling role in a wide variety of cells is also discussed.

Oxygen-dependent regulation of ion channels: acute responses, post-translational modification, and response to chronic hypoxia

Oxygen is a vital element for the survival of cells in multicellular aerobic organisms such as mammals. Lack of O₂ availability caused by environmental or pathological conditions leads to hypoxia. Active oxygen distribution systems (pulmonary and circulatory) and their neural control mechanisms ensure that cells and tissues remain oxygenated. However, O₂-carrying blood cells as well as immune and various parenchymal cells experience wide variations in partial pressure of oxygen (P_O2) in vivo. Hence, the reactive modulation of the functions of the oxygen distribution systems and their ability to sense P_O2 are critical. Elucidating the physiological responses of cells to variations in P_O2 and determining the P_O2-sensing mechanisms at the biomolecular level have attracted considerable research interest in the field of physiology. Herein, we review the current knowledge regarding ion channel-dependent oxygen sensing and associated signalling pathways in mammals. First, we present the recent findings on O₂-sensing ion channels in representative chemoreceptor cells as well as in other types of cells such as immune cells. Furthermore, we highlight the transcriptional regulation of ion channels under chronic hypoxia and its physiological implications and summarize the findings of studies on the post-translational modification of ion channels under hypoxic or ischemic conditions.

Adrenal medulla development and medullary-cortical interactions

The mammalian adrenal gland is composed of two distinct tissue types in a bidirectional connection, the catecholamine-producing medulla derived from the neural crest and the mesoderm-derived cortex producing steroids. The medulla mainly consists of chromaffin cells derived from multipotent nerve-associated descendants of Schwann cell precursors. Already during adrenal organogenesis, close interactions between cortex and medulla are necessary for proper differentiation and morphogenesis of the gland. Moreover, communication between the cortex and the medulla ensures a regular function of the adult adrenal. In tumor development, interfaces between the two parts are also common. Here, we summarize the development of the mammalian adrenal medulla and the current understanding of the cortical-medullary interactions under development and in health and disease.

Stem cells, evolutionary aspects and pathology of the adrenal medulla: A new developmental paradigm

The mammalian adrenal gland is composed of two main components; the catecholaminergic neural crest-derived medulla, found in the center of the gland, and the mesoderm-derived cortex producing steroidogenic hormones. The medulla is composed of neuroendocrine chromaffin cells with oxygen-sensing properties and is dependent on tissue interactions with the overlying cortex, both during development and in adulthood. Other relevant organs include the Zuckerkandl organ containing extra-adrenal chromaffin cells, and carotid oxygen-sensing bodies containing glomus cells. Chromaffin and glomus cells reveal a number of important similarities and are derived from the multipotent nerve-associated descendants of the neural crest, or Schwann cell precursors. Abnormalities in complex developmental processes during differentiation of nerve-associated and other progenitors into chromaffin and oxygen-sensing populations may result in different subtypes of paraganglioma, neuroblastoma and pheochromocytoma. Here, we summarize recent findings explaining the development of chromaffin and oxygen-sensing cells, as well as the potential mechanisms driving neuroendocrine tumor initiation.

Acute oxygen sensing-Role of metabolic specifications in peripheral chemoreceptor cells

Acute oxygen sensing is essential for humans under hypoxic environments or pathologic conditions. This is achieved by the carotid body (CB), the key arterial chemoreceptor, along with other peripheral chemoreceptor organs, such as the adrenal medulla (AM). Although it is widely accepted that inhibition of K⁺ channels in the plasma membrane of CB cells during acute hypoxia results in the activation of cardiorespiratory reflexes, the molecular mechanisms by which the hypoxic signal is detected to modulate ion channel activity are not fully understood. Using conditional knockout mice lacking mitochondrial complex I (MCI) subunit NDUFS2, we have found that MCI generates reactive oxygen species and pyridine nucleotides, which signal K⁺ channels during acute hypoxia. Comparing the transcriptomes from CB and AM, which are O₂-sensitive, with superior cervical ganglion, which is practically O₂-insensitive, we have found that CB and AM contain unique metabolic gene expression profiles. The "signature metabolic profile" and their biophysical characteristics could be essential for acute O₂ sensing by chemoreceptor cells.

Inflammatory Signaling in Hypertension: Regulation of Adrenal Catecholamine Biosynthesis

The immune system is increasingly recognized for its role in the genesis and progression of hypertension. The adrenal gland is a major site that coordinates the stress response via the hypothalamic-pituitary-adrenal axis and the sympathetic-adrenal system. Catecholamines released from the adrenal medulla function in the neuro-hormonal regulation of blood pressure and have a well-established link to hypertension. The immune system has an active role in the progression of hypertension and cytokines are powerful modulators of adrenal cell function. Adrenal medullary cells integrate neural, hormonal, and immune signals. Changes in adrenal cytokines during the progression of hypertension may promote blood pressure elevation by influencing catecholamine biosynthesis. This review highlights the potential interactions of cytokine signaling networks with those of catecholamine biosynthesis within the adrenal, and discusses the role of cytokines in the coordination of blood pressure regulation and the stress response.

Imbalance of autonomic nervous systems involved in ventricular arrhythmia after splenectomy in dogs

The role of cardiac autonomic modulation on ventricular arrhythmia, known as ventricular premature complexes (VPC), after splenectomy was investigated. Twelve dogs undergoing splenectomy were divided into 2 groups: low VPC (<1,000/day, n=6) and high VPC groups (≥1,000/day, n=6). Electrocardiograph recording was performed prior to (D0), during the first three days (D1-3) and on day 9 (D9) after surgery. Arrhythmic indices, T_peak-T_end, corrected QT interval and short-term variability of QT interval as well as heart rate variability (HRV) were evaluated. Plasma concentrations of norepinephrine (NE) and epinephrine (E) were measured. In the high VPC group, the occurrences of VPC were significantly increased (P<0.05) after surgery, and reached the levels higher than those in the low VPC group. For the arrhythmic indices, only Tp-Te in the high VPC group increased significantly (P<0.05) after surgery. For HRV analysis, enhancement of both time and frequency domains were found postoperatively in both groups. On D2, however, the high VPC group showed significantly lower total power and high frequency with higher low to high frequency ratio (P<0.05) than the low VPC group. Plasma NE concentration significantly increased in the high VPC group after surgery. Dogs in the high VPC group had shorter survival time than those in the low VPC group. In conclusion, dogs with imbalance cardiac autonomic modulation accompanied with high circulating NE concentration after splenectomy are prone to ventricular arrhythmia, which leads to short survival time.

Hypoxia-regulated catecholamine secretion in chromaffin cells

Adrenal catecholamine (CAT) secretion is a general physiological response of animals to environmental stressors such as hypoxia. This represents an important adaptive mechanism to maintain homeostasis and protect vital organs such as the brain. In adult mammals, CAT secretory responses are triggered by activation of the sympathetic nervous system that supplies cholinergic innervation of adrenomedullary chromaffin cells (AMC) via the splanchnic nerve. In the neonate, the splanchnic innervation of AMC is immature or absent, yet hypoxia stimulates a non-neurogenic CAT secretion that is critical for adaptation to extra-uterine life. This non-neurogenic, hypoxia-sensing mechanism in AMC is gradually lost or suppressed postnatally along a time course that parallels the development of splanchnic innervation. Moreover, denervation of adult AMC results in a gradual return of the direct hypoxia-sensing mechanism. The signaling pathways by which neonatal AMC sense acute hypoxia leading to non-neurogenic CAT secretion and the mechanisms that underlie the re-acquisition of hypoxia-sensing properties by denervated adult AMC, are beginning to be understood. This review will focus on current views concerning the mechanisms responsible for direct acute hypoxia sensing and CAT secretion in perinatal AMC and how they are regulated by innervation during postnatal development. It will also briefly discuss plasticity mechanisms likely to contribute to CAT secretion during exposures to chronic and intermittent hypoxia.

Gene expression analyses reveal metabolic specifications in acute O2 -sensing chemoreceptor cells

KEY POINTS

Glomus cells in the carotid body (CB) and chromaffin cells in the adrenal medulla (AM) are essential for reflex cardiorespiratory adaptation to hypoxia. However, the mechanisms whereby these cells detect changes in O₂ tension are poorly understood. The metabolic properties of acute O₂ -sensing cells have been investigated by comparing the transcriptomes of CB and AM cells, which are O₂ -sensitive, with superior cervical ganglion neurons, which are practically O₂ -insensitive. In O₂ -sensitive cells, we found a characteristic prolyl hydroxylase 3 down-regulation and hypoxia inducible factor 2α up-regulation, as well as overexpression of genes coding for three atypical mitochondrial electron transport subunits and pyruvate carboxylase, an enzyme that replenishes tricarboxylic acid cycle intermediates. In agreement with this observation, the inhibition of succinate dehydrogenase impairs CB acute O₂ sensing. The responsiveness of peripheral chemoreceptor cells to acute hypoxia depends on a 'signature metabolic profile'.

ABSTRACT

Acute O₂ sensing is a fundamental property of cells in the peripheral chemoreceptors, e.g. glomus cells in the carotid body (CB) and chromaffin cells in the adrenal medulla (AM), and is necessary for adaptation to hypoxia. These cells contain O₂ -sensitive ion channels, which mediate membrane depolarization and transmitter release upon exposure to hypoxia. However, the mechanisms underlying the detection of changes in O₂ tension by cells are still poorly understood. Recently, we suggested that CB glomus cells have specific metabolic features that favour the accumulation of reduced quinone and the production of mitochondrial NADH and reactive oxygen species during hypoxia. These signals alter membrane ion channel activity. To investigate the metabolic profile characteristic of acute O₂ -sensing cells, we used adult mice to compare the transcriptomes of three cell types derived from common sympathoadrenal progenitors, but exhibiting variable responsiveness to acute hypoxia: CB and AM cells, which are O₂ -sensitive (glomus cells > chromaffin cells), and superior cervical ganglion neurons, which are practically O₂ -insensitive. In the O₂ -sensitive cells, we found a characteristic mRNA expression pattern of prolyl hydroxylase 3/hypoxia inducible factor 2α and up-regulation of several genes, in particular three atypical mitochondrial electron transport subunits and some ion channels. In addition, we found that pyruvate carboxylase, an enzyme fundamental to tricarboxylic acid cycle anaplerosis, is overexpressed in CB glomus cells. We also observed that the inhibition of succinate dehydrogenase impairs CB acute O₂ sensing. Our data suggest that responsiveness to acute hypoxia depends on a 'signature metabolic profile' in chemoreceptor cells.

Redox signaling in acute oxygen sensing

Acute oxygen (O₂) sensing is essential for individuals to survive under hypoxic conditions. The carotid body (CB) is the main peripheral chemoreceptor, which contains excitable and O₂-sensitive glomus cells with O₂-regulated ion channels. Upon exposure to acute hypoxia, inhibition of K⁺ channels is the signal that triggers cell depolarization, transmitter release and activation of sensory fibers that stimulate the brainstem respiratory center to produce hyperventilation. The molecular mechanisms underlying O₂ sensing by glomus cells have, however, remained elusive. Here we discuss recent data demonstrating that ablation of mitochondrial Ndufs2 gene selectively abolishes sensitivity of glomus cells to hypoxia, maintaining responsiveness to hypercapnia or hypoglycemia. These data suggest that reactive oxygen species and NADH generated in mitochondrial complex I during hypoxia are signaling molecules that modulate membrane K⁺ channels. We propose that the structural substrates for acute O₂ sensing in CB glomus cells are “O₂-sensing microdomains” formed by mitochondria and neighboring K⁺ channels in the plasma membrane.

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Acute O₂ sensing by peripheral chemoreceptors depends on K⁺ channels.

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Mitochondrial complex I function is required for acute O₂ sensing.

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Reactive oxygen species inhibits background K⁺ channels during acute hypoxia.

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Pyridine nucleotides may signal voltage-gated K⁺ channels during acute hypoxia.

Oxygen sensing by the carotid body: mechanisms and role in adaptation to hypoxia

Oxygen (O2) is fundamental for cell and whole-body homeostasis. Our understanding of the adaptive processes that take place in response to a lack of O2(hypoxia) has progressed significantly in recent years. The carotid body (CB) is the main arterial chemoreceptor that mediates the acute cardiorespiratory reflexes (hyperventilation and sympathetic activation) triggered by hypoxia. The CB is composed of clusters of cells (glomeruli) in close contact with blood vessels and nerve fibers. Glomus cells, the O2-sensitive elements in the CB, are neuron-like cells that contain O2-sensitive K(+)channels, which are inhibited by hypoxia. This leads to cell depolarization, Ca(2+)entry, and the release of transmitters to activate sensory fibers terminating at the respiratory center. The mechanism whereby O2modulates K(+)channels has remained elusive, although several appealing hypotheses have been postulated. Recent data suggest that mitochondria complex I signaling to membrane K(+)channels plays a fundamental role in acute O2sensing. CB activation during exposure to low Po2is also necessary for acclimatization to chronic hypoxia. CB growth during sustained hypoxia depends on the activation of a resident population of stem cells, which are also activated by transmitters released from the O2-sensitive glomus cells. These advances should foster further studies on the role of CB dysfunction in the pathogenesis of highly prevalent human diseases.

Ontogeny of O2 and CO2//H+ chemosensitivity in adrenal chromaffin cells: role of innervation

The adrenal medulla plays a key role in the physiological responses of developing and mature mammals by releasing catecholamines (CAT) during stress. In rodents and humans, the innervation of CAT-producing, adrenomedullary chromaffin cells (AMCs) is immature or absent during early postnatal life, when these cells possess 'direct' hypoxia- and CO2/H(+)-chemosensing mechanisms. During asphyxial stressors at birth, these mechanisms contribute to a CAT surge that is critical for adaptation to extra-uterine life. These direct chemosensing mechanisms regress postnatally, in parallel with maturation of splanchnic innervation. Here, we review the evidence that neurotransmitters released from the splanchnic nerve during innervation activate signaling cascades that ultimately cause regression of direct AMC chemosensitivity to hypoxia and hypercapnia. In particular, we consider the roles of cholinergic and opioid receptor signaling, given that splanchnic nerves release acetylcholine and opiate peptides onto their respective postsynaptic nicotinic and opioid receptors on AMCs. Recent in vivo and in vitro studies in the rat suggest that interactions involving α7 nicotinic acetylcholine receptors (nAChRs), the hypoxia inducible factor (HIF)-2α signaling pathway, protein kinases and ATP-sensitive K(+) (KATP) channels contribute to the selective suppression of hypoxic chemosensitivity. In contrast, interactions involving μ- and/or δ-opiod receptor signaling pathways contribute to the suppression of both hypoxic and hypercapnic chemosensitivity, via regulation of the expression of KATP channels and carbonic anhydrase (CA I and II), respectively. These data suggest that the ontogeny of O2 and CO2/H(+) chemosensitivity in chromaffin cells can be regulated by the tonic release of presynaptic neurotransmitters.

Chronic exposure of neonatal rat adrenomedullary chromaffin cells to opioids in vitro blunts both hypoxia and hypercapnia chemosensitivity

At birth, rat adrenomedullary chromaffin cells (AMCs) respond directly to asphyxial stressors such as hypoxia and hypercapnia by triggering catecholamine secretion, which is critical for proper transition to extrauterine life. These non-neurogenic responses are suppressed postnatally in parallel with the development of splanchnic innervation, and reappear following denervation of the adult adrenal gland. To test whether neural factors released from the splanchnic nerve may regulate AMC chemosensitivity, we previously showed that nicotinic agonists in utero and in vitro suppressed hypoxia, but not hypercapnia, sensitivity. Here, we considered the potential role of opiate peptides which are also released from the splanchnic nerve and act via postsynaptic μ-, δ- and -opioid receptors. Treatment of neonatal rat AMC cultures for ∼1 week with μ- and/or δ- (but not ) opioid agonists (2 μm) led to a marked suppression of both hypoxia and hypercapnia sensitivity, as measured by K(+) current inhibition and membrane depolarization; co-incubation with naloxone prevented the effects of combined opioids. The suppression of hypoxia sensitivity was attributable to upregulation of K(ATP) current density and the K(ATP) channel subunit Kir6.2, and was reversed by the K(ATP) channel blocker, glibenclamide. By contrast, suppression of hypercapnia sensitivity was associated with down-regulation of two key mediators of CO(2) sensing, i.e. carbonic anhydrase I and II. Collectively, these studies point to a novel role for opioid receptor signalling in the developmental regulation of chromaffin cell chemosensitivity, and suggest that prenatal exposure to opioid drugs could lead to impaired arousal responses in the neonate.

Low glucose sensitivity and polymodal chemosensing in neonatal rat adrenomedullary chromaffin cells

Glucose is the primary metabolic fuel in mammalian fetuses, yet mammals are incapable of endogenous glucose production until several hours after birth. Thus, when the maternal supply of glucose ceases at birth there is a transient hypoglycemia that elicits a counterregulatory surge in circulating catecholamines. Because the innervation of adrenomedullary chromaffin cells (AMCs) is immature at birth, we hypothesized that neonatal AMCs act as direct glucosensors, a property that could complement their previously established roles as hypoxia and acid hypercapnia sensors. During perforated-patch, whole cell recordings, low glucose depolarized and/or excited a subpopulation of neonatal AMCs; in addition, aglycemia (0 mM glucose) caused inhibition of outward K+ current, blunted by the simultaneous activation of glibenclamide-sensitive KATP channels. Some cells were excited by each of the three metabolic stimuli, i.e., aglycemia, hypoxia (Po2 ∼30 mmHg), and isohydric hypercapnia (10% CO2; pH = 7.4). Using carbon fiber amperometry, aglycemia and hypoglycemia (3 mM glucose) induced robust catecholamine secretion that was sensitive to nickel (50 μM and 2 mM) and the L-type Ca2+ channel blocker nifedipine (10 μM), suggesting involvement of both T-type and L-type voltage-gated Ca2+ channels. Fura-2 measurements of intracellular Ca2+ ([Ca2+] i) revealed that ∼42% of neonatal AMCs responded to aglycemia with a significant rise in [Ca2+] i. Approximately 40% of these cells responded to hypoxia, whereas ∼25% cells responded to both aglycemia and hypoxia. These data suggest that together with hypoxia and acid hypercapnia, low glucose is another important metabolic stimulus that contributes to the vital asphyxia-induced catecholamine surge from AMCs at birth.

Gap junction-mediated intercellular communication in the adrenal medulla: an additional ingredient of stimulus-secretion coupling regulation

The traditional understanding of stimulus-secretion coupling in adrenal neuroendocrine chromaffin cells states that catecholamines are released upon trans-synaptic sympathetic stimulation mediated by acetylcholine released from the splanchnic nerve terminals. Although this statement remains largely true, it deserves to be tempered. In addition to its neurogenic control, catecholamine secretion also depends on a local gap junction-mediated communication between chromaffin cells. We review here the insights gained since the first description of gap junctions in the adrenal medullary tissue. Adrenal stimulus-secretion coupling now appears far more intricate than was previously envisioned and its deciphering represents a challenge for neurobiologists engaged in the study of the regulation of neuroendocrine secretion. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Developmental and stress-induced remodeling of cell–cell communication in the adrenal medullary tissue

The adrenal medullary tissue contributes to maintain body homeostasis in reaction to stressful environmental changes via the release of catecholamines into the blood circulation in response to splanchnic nerve activation. Accordingly, chromaffin cell stimulus-secretion coupling undergoes temporally restricted periods of anatomo- functional remodeling in response to prevailing hormonal requirements of the organism. The postnatal development of the adrenal medulla and response to stress are remarkable physiological situations in which the stimulus- secretion coupling is critically affected. Catecholamine secretion from rat chromaffin cells is under a dual control involving an incoming initial command arising from the sympathetic nervous system that releases acetylcholine at the splanchnic nerve terminal-chromaffin cell synapses and a local gap junction-mediated intercellular communication. Interestingly, these two communication pathways are functionally interconnected within the gland and exhibit coordinated plasticity mechanisms. This article reviews the physiological and molecular evidence that the adrenal medullary tissue displays anatomical and functional adaptative remodeling of cell–cell communications upon physiological (postnatal development) and/or physiopathological (stress) situations associated with specific needs in circulating catecholamine levels.

Divergent roles of reactive oxygen species in the responses of perinatal adrenal chromaffin cells to hypoxic challenges

The fetus and neonate experience variable patterns of low P(O)₂(hypoxia) ranging from acute, sustained, and intermittent. Adaptation to hypoxia involves activation of key transcription factors, known as hypoxia-inducible factors (e.g. HIF-1α, HIF-2α), which regulate a number of genes in different cell types. This review focuses on the signaling pathways that mediate proper physiological responses of perinatal adrenomedullary chromaffin cells (AMC) to varying patterns of hypoxic challenges, and particularly on the controversial role of reactive oxygen species (ROS). At birth, acute hypoxia (seconds to minutes) directly stimulates catecholamine release from AMC via K+ channel inhibition, mediated by a decrease in mitochondrial-derived ROS. By contrast, exposure to chronic sustained hypoxia (CSH) induces HIF-2α in a fetal-derived chromaffin cell line independently of changes in ROS. Exposure to chronic intermittent hypoxia (CIH) activates antioxidant responses via the regulator Nrf-2, in association with an increase in ROS and the induction of HIF-1α. We propose that the physiological responses of perinatal AMC to hypoxia and the ensuing directional changes in ROS are dependent on the pattern and duration of the hypoxic exposure.

NADPH oxidase-dependent regulation of T-type Ca2+ channels and ryanodine receptors mediate the augmented exocytosis of catecholamines from intermittent hypoxia-treated neonatal rat chromaffin cells

Nearly 90% of premature infants experience the stress of intermittent hypoxia (IH) as a consequence of recurrent apneas (periodic cessation of breathing). In neonates, catecholamine secretion from the adrenal medulla is critical for maintaining homeostasis under hypoxic stress. We recently reported that IH treatment enhanced hypoxia-evoked catecholamine secretion and [Ca2+]i responses in neonatal rat adrenal chromaffin cells and involves reactive oxygen species (ROS). The purpose of the present study was to identify the source(s) of ROS generation and examine the mechanisms underlying the enhanced catecholamine secretion by IH. Neonatal rats of either sex (postal day 0-5) were exposed to either IH or normoxia. IH treatment increased NADPH oxidase (NOX) activity, upregulated NOX2 and NOX4 transcription in adrenal medullae, and a NOX inhibitor prevented the effects of IH on hypoxia-evoked chromaffin cell secretion. IH upregulated Cav3.1 and Cav3.2 T-type Ca2+ channel mRNAs via NOX/ROS signaling and augmented T-type Ca2+ current in IH-treated chromaffin cells. Mibefradil, a blocker of T-type Ca2+ channels attenuated the effects of hypoxia on [Ca2+]i and catecholamine secretion in IH-treated cells. In Ca2+-free medium, IH-treated cells exhibited higher basal [Ca2+]i levels and more pronounced [Ca2+]i responses to hypoxia compared with controls, and blockade of ryanodine receptors (RyRs) prevented these effects. RyR2 and RyR3 mRNAs were upregulated, RyR2 was S-glutathionylated in IH-treated adrenal medullae, and NOX/ROS inhibitors prevented these effects. These results demonstrate that neonatal IH treatment leads to NOX/ROS-dependent recruitment of T-type Ca2+ channels and RyRs, resulting in augmented [Ca2+]i mobilization and catecholamine secretion.

Neonatal intermittent hypoxia impairs neuronal nicotinic receptor expression and function in adrenal chromaffin cells

We recently reported that adrenomedullary chromaffin cells (AMC) from neonatal rats treated with intermittent hypoxia (IH) exhibit enhanced catecholamine secretion by hypoxia (Souvannakitti D, Kumar GK, Fox A, Prabhakar NR. J Neurophysiol 101: 2837-2846, 2009). In the present study, we examined whether neonatal IH also facilitate AMC responses to nicotine, a potent stimulus to chromaffin cells. Experiments were performed on rats exposed to either IH (15-s hypoxia-5-min normoxia; 8 h/day) or to room air (normoxia; controls) from ages postnatal day 0 (P0) to P5. Quantitative RT-PCR analysis revealed expression of mRNAs alpha(3-), alpha(5-), alpha(7-), and beta(2-) and beta(4-)nicotinic acetylcholine receptor (nAChR) subunits in adrenal medullae from control P5 rats. Nicotine-elevated intracellular Ca(2+) concentration ([Ca(2+)](i)) in AMC and nAChR antagonists prevented this response, suggesting that nAChRs are functional in neonatal AMC. In IH-treated rats, nAChR mRNAs were downregulated in AMC, which resulted in a markedly attenuated nicotine-evoked elevation in [Ca(2+)](i) and subsequent catecholamine secretion. Systemic administration of antioxidant prevented IH-evoked downregulation of nAChR expression and function. P35 rats treated with neonatal IH exhibited reduced nAChR mRNA expression in adrenal medullae, attenuated AMC responses to nicotine, and impaired neurogenic catecholamine secretion. Thus the response to neonatal IH lasts for at least 30 days. These observations demonstrate that neonatal IH downregulates nAChR expression and function in AMC via reactive oxygen species signaling, and the effects of neonatal IH persist at least into juvenile life, leading to impaired neurogenic catecholamine secretion from AMC.

Do monoamine-synthesizing cells constitute a complex network of oxygen sensors?

Oxygen represents an essential molecule for organisms. Because of this, sophisticated systems of sensors have evolved to monitor oxygenation of tissues. We propose that monoamine-synthesizing cells represent an important part of this system. It is well known that the carotid body, which contains chromaffin cells, serves as a chemical sensor of blood oxygenation. Similarly, the activity of adrenal medullary chromaffin cells is increased during hypoxia. Moreover, neurons located in the central nervous system containing catecholamines, serotonin, and histamine are also sensitive to hypoxia. On the basis of this common sensitivity of monoamine-synthesizing cells to changes in oxygenation we propose the hypothesis that these cells constitute a widely distributed network of sensors that monitor oxygen levels. The role of monoamine-synthesizing cells in monitoring tissue oxygen supply during both physiological and pathological conditions is also discussed.

Chronic nicotine blunts hypoxic sensitivity in perinatal rat adrenal chromaffin cells via upregulation of KATP channels: role of alpha7 nicotinic acetylcholine receptor and hypoxia-inducible factor-2alpha

Fetal nicotine exposure blunts hypoxia-induced catecholamine secretion from neonatal adrenomedullary chromaffin cells (AMCs), providing a link between maternal smoking, abnormal arousal responses, and risk of sudden infant death syndrome. Here, we show that the mechanism is attributable to upregulation of K(ATP) channels via stimulation of alpha7 nicotinic ACh receptors (AChRs). These K(ATP) channels open during hypoxia, thereby suppressing membrane excitability. After in utero exposure to chronic nicotine, neonatal AMCs show a blunted hypoxic sensitivity as determined by inhibition of outward K(+) current, membrane depolarization, rise in cytosolic Ca(2+), and catecholamine secretion. However, hypoxic sensitivity could be unmasked in nicotine-exposed AMCs when glibenclamide, a blocker of K(ATP) channels, was present. Both K(ATP) current density and K(ATP) channel subunit (Kir 6.2) expression were significantly enhanced in nicotine-exposed cells relative to controls. The entire sequence could be reproduced in culture by exposing neonatal rat AMCs or immortalized fetal chromaffin (MAH) cells to nicotine for approximately 1 week, and was prevented by coincubation with selective blockers of alpha7 nicotinic AChRs. Additionally, coincubation with inhibitors of protein kinase C and CaM kinase, but not protein kinase A, prevented the effects of chronic nicotine in vitro. Interestingly, chronic nicotine failed to blunt hypoxia-evoked responses in MAH cells bearing short hairpin knockdown (>90%) of the transcription factor, hypoxia-inducible factor-2alpha (HIF-2alpha), suggesting involvement of the HIF pathway. The therapeutic potential of K(ATP) channel blockers was validated in experiments in which hypoxia-induced neonatal mortality in nicotine-exposed pups was significantly reduced after pretreatment with glibenclamide.

Developmental change of T-type Ca2+ channel expression and its role in rat chromaffin cell responsiveness to acute hypoxia

Neonatal chromaffin cells of the adrenal medulla (AM) are intrinsic chemoreceptors that secrete catecholamines in response to hypoxia, thus contributing to fetal adaptation to extrauterine life. In most mammals studied, oxygen sensitivity of AM cells disappears a few days after birth, possibly due to innervation of the adrenal gland by the cholinergic fibres of the splanchnic nerve (approximately postnatal day 7 in the rat). The mechanisms underlying these homeostatic changes in chromaffin cells are unknown. Low voltage-activated, T-type, Ca(2+) channels regulate cell excitability and their expression is up-regulated by hypoxia. Hence, we hypothesized that these channels contribute to the developmental changes in the chemoreceptive properties of AM chromaffin cells. Using electrophysiological, immunocytochemical and molecular biology methodologies we show here that neonatal AM chromaffin cells express T-type Ca(2+) channels (of alpha1H or Ca(v)3.2 sub-type) and that the function of these channels is necessary for catecholamine release in response to acute hypoxia. T-type Ca(2+) channel expression, as well as chromaffin cell responsiveness to hypoxia, decrease with postnatal maturation. Adult chromaffin cell sensitivity to hypoxia reappears after AM denervation in parallel with the recruitment of T-type Ca(2+) channels. These observations indicate that T-type Ca(2+) channels are essential for the acute response of chromaffin cells to hypoxia and help explain the disappearance of O(2) sensitivity in adult AM chromaffin cells. Our results may also be relevant for understanding the pathogenesis of disorders associated with chronic hypoxia or maternal nicotine consumption.

Glucosensing in an immortalized adrenomedullary chromaffin cell line: role of ATP-sensitive K+ channels

Using an immortalized adrenal chromaffin cell line (MAH cells), we investigated the cellular mechanisms underlying sensitivity to glucose-free solution (aglycemia) using ratiometric Ca2+ imaging and whole-cell recording. Though few cells (< 15%) responded to aglycemia with an increase in intracellular-free Ca2+ concentration ([Ca2+]i), in most cells (approximately 75%), aglycemia caused > 50% suppression of the Delta[Ca2+]i induced by the depolarizing stimulus, high (10 mM) K+. Moreover, in normal K+, the average aglycemia-induced rise in Cai2+ as well as the proportion of aglycemia-responsive cells increased in the presence of the K(ATP) channel blocker, glibenclamide. During membrane potential (Vm) measurements, aglycemia induced either hyperpolarization (6/20), depolarization (4/20) or no change in Vm. RT-PCR and Western blotting confirmed the presence of K(ATP) channel subunits Kir6.2 and SUR1 in MAH cells. These findings suggest a dual inhibitory and excitatory action of aglycemia in MAH cells, where activation of K(ATP) channels effectively inhibits or blunts the Delta[Ca2+]i due to the excitatory effect. Thus, this cell line appears as an attractive model for studying the molecular mechanisms of glucosensing.

High oxygen prevents fetal lethality due to lack of catecholamines

The catecholamine norepinephrine is required for fetal survival, but its essential function is unknown. When catecholamine-deficient [tyrosine hydroxylase (Th) null] mouse fetuses die at embryonic day (E)13.5-14.5, they resemble wild-type (wt) fetuses exposed to hypoxia. They exhibit bradycardia (28% reduction in heart rate), thin ventricular myocardium (20% reduction in tissue), epicardial detachment, and death with vascular congestion, hemorrhage, and edema. At E12.5, before the appearance of morphological deficits, catecholamine-deficient fetuses are preferentially killed by experimentally induced hypoxia and have lower tissue Po(2) levels than wt siblings. By microarray analysis (http://www.ncbi.nlm.nih.gov/geo; accession no. GSE10341), hypoxia-inducible factor-1 target genes are induced to a greater extent in null fetuses than in wt siblings, supporting the notion that mutants experience lower oxygen tension or have an enhanced response to hypoxia. Hypoxia induces a 13-fold increase in plasma norepinephrine levels, which would be expected to increase heart rate, thereby improving oxygen delivery in wt mice. Surprisingly, increasing maternal oxygen (inspired O(2) 33 or 63%) prevents the effects of catecholamine deficiency, restoring heart rate, myocardial tissue, and survival of Th null fetuses to wt levels. We suggest that norepinephrine mediates fetal survival by maintaining oxygen homeostasis.

Functional mitochondria are required for O2but not CO2sensing in immortalized adrenomedullary chromaffin cells

Catecholamine (CAT) release from adrenomedullary chromaffin cells (AMC) in response to stressors such as low O2(hypoxia) and elevated CO2/H+is critical during adaptation of the newborn to extrauterine life. Using a surrogate model based on a v -myc immortalized adrenal chromaffin cell line (i.e., MAH cells), combined with genetic perturbation of mitochondrial function, we tested the hypothesis that functional mitochondria are required for O2sensing. Wild-type MAH cells responded to both hypoxia and increased CO2(hypercapnia) with K+current inhibition and membrane depolarization. Additionally, these stimuli caused a rise in cytosolic Ca2+and CAT secretion, determined by fura-2 spectrofluorimetry and carbon fiber amperometry, respectively. In contrast, mitochondria-deficient (ρ0) MAH cells were hypoxia insensitive, although responses to hypercapnia and expression of several markers, including carbonic anhydrase II, remained intact. Rotenone (1 μM), a mitochondrial complex I blocker known to mimic and occlude the effects of hypoxia in primary AMC, was effective in wild-type but not ρ0MAH cells. These data demonstrate that functional mitochondria are involved in hypoxia-sensing by adrenal chromaffin cells. We also show for the first time that, like their neonatal chromaffin cell counterparts, MAH cells are CO2sensors; however, this property is independent of functional mitochondria.

Chronic nicotine in utero selectively suppresses hypoxic sensitivity in neonatal rat adrenal chromaffin cells

Nicotine in cigarette smoke has been linked to several deleterious side effects on the offspring of smoking mothers, including impaired development of the sympathoadrenal system, abnormal arousal reflexes, and sudden infant death syndrome. Catecholamine (CA) release from adrenomedullary chromaffin cells (AMCs) in response to asphyxial stressors, e.g., low O(2) (hypoxia) and elevated CO(2) (hypercapnia), is critical for adaptation to extrauterine life and occurs before splanchnic innervation. Here, we investigated the effects of prenatal nicotine bitartrate exposure on the ability of neonatal (P0) rat AMCs to respond appropriately to asphyxial stressors. Control AMCs isolated from pups born to saline-treated dams displayed typical responses to hypoxia and hypercapnia, including inhibition of outward K(+) current, membrane depolarization, increased cytosolic calcium, and CA secretion. In contrast, P0 AMCs from pups born to nicotine-treated dams showed a marked suppression or loss of hypoxic sensitivity, although hypercapnic sensitivity and the expression of CO(2) markers (i.e., carbonic anhydrase I and II) appeared normal. Moreover, isolated saline-treated P0 AMCs lost their hypoxic sensitivity when grown in culture for approximately 1 wk in the presence of a subsaturating concentration of nicotine base (50 microM), and this effect was abolished by the nicotinic acetylcholine receptor (nAChR) blocker mecamylamine (100 microM). Taken together, these data suggest that the adverse effects of maternal smoking on sympathoadrenal function in the offspring are due in part to a loss or suppression of acute hypoxic sensitivity in adrenal chromaffin cells, triggered by the direct action of nicotine on endogenous nicotinic acetylcholine receptors.

Mechanisms of acute oxygen sensing by the carotid body: Lessons from genetically modified animals

We have studied carotid body (CB) glomus cell sensitivity to changes in O(2) tension in three different genetically engineered animals models using thin CB slices and monitoring the secretory response to hypoxia by amperometry. Glomus cells from partially HIF-1alpha deficient mice exhibited a normal sensitivity to hypoxia. Animals with complete deletion of the small membrane anchoring subunit of succinate dehydrogenase (SDHD) died during embryonic life but heterozygous SDHD +/- mice showed a normal CB response to low O(2) tension. SDHD +/- mice had, however, a clear CB phenotype characterized by a decrease of K(+) current amplitude, an increase of basal catecholamine release from glomus cells, and a slight organ growth. The lack of hemeoxygenase-2 (HO-2), a ubiquitous powerful antioxidant enzyme, produces a notable CB phenotype, characterized by hypertrophy and alteration in the level of CB expression of some stress-dependent genes (including down-regulation of the maxi-K(+) channel alpha-subunit). Nevertheless, in HO-2 deficient mice the exquisite intrinsic O(2) responsiveness of CB glomus cells remains unaltered. Therefore, HO-2 is not absolutely necessary for acute CB O(2) sensing. Although the nature of the CB acute O(2) sensor(s) is yet unknown, studies similar to those summarized here serve to test the existing hypothesis and help to distinguish between those that need to be explored further and those that definitively lack experimental support.

Developmental changes of chromaffin cell secretory response to hypoxia studied in thin adrenal slices

Adrenomedullary chromaffin (AMC) cells are sensitive to hypoxia in the newborn, but whether this property is lost during postnatal maturation is a matter of controversy. We have developed a rat adrenal slice preparation that allows the study of neonatal and adult AMC cell sensitivity to hypoxia in almost optimal physiological conditions. Responses to secretagogues can be quantitatively and noninvasively monitored in intact cells by amperometry. We have found hypoxia "responsive" (R) and "non-responsive" AMC cells in both neonatal (P0-P8) and juvenile/adult (P12-P60) adrenal glands. However, in the neonate, the proportion of R cells and the magnitude of the response to hypoxia were larger than in the adult. This developmental change of hypoxia responsiveness did not seem to depend on a decrease of the AMC cell's excitability. Spontaneous secretory activity in slices from adult rats was even increased with respect to neonatal animals. The analysis of the secretory events suggests that changes in spike frequency, rather than in vesicle size, account for the increased basal secretion rate in adult AMC cells. Thus, we report a major, but not complete, loss of direct hypoxia sensitivity in adult AMC cells. The adrenal slice appears to be a valuable technique to study acute O(2) sensing and its modifications in pathophysiological states.

Translational control of collagen prolyl 4-hydroxylase-alpha(I) gene expression under hypoxia

Hypoxia is a pro-fibrotic stimulus, which is associated with enhanced collagen synthesis, as well as with augmented collagen prolyl 4-hydroxylase (C-P4H) activity. C-P4H activity is controlled mainly by regulated expression of the alpha C-P4H subunit. In this study we demonstrate that the increased synthesis of C-P4H-alpha(I) protein in human HT1080 fibroblasts under long term hypoxia (36 h, 1% oxygen) is controlled at the translational level. This is mediated by an interaction of RNA-binding protein nucleolin (approximately 64 kDa form) at the 5'- and 3'-untranslated regions (UTR) of the mRNA. The 5'/3'-UTR-dependent mechanism elevates the C-P4H-alpha(I) expression rate 2.3-fold, and participates in a 5.3-fold increased protein level under long term hypoxia. The interaction of nucleolin at the 5'-UTR occurs directly and depends on the existence of an AU-rich element. Statistical evaluation of the approximately 64-kDa nucleolin/RNA interaction studies revealed a core binding sequence, corresponding to UAAAUC or AAAUCU. At the 3'-UTR, nucleolin assembles indirectly via protein/protein interaction, with the help of another 3'-UTR-binding protein, presumably annexin A2. The increased protein level of the approximately 64-kDa nucleolin under hypoxia can be attributed to an autocatalytic cleavage of a high molecular weight nucleolin form, without alterations in nucleolin mRNA concentration. Thus, the alteration of translational efficiency by nucleolin, which occurs through a hypoxia inducible factor independent pathway, is an important step in C-P4H-alpha(I) regulation under hypoxia.

Chronic intermittent hypoxia induces hypoxia-evoked catecholamine efflux in adult rat adrenal medulla via oxidative stress

Chronic intermittent hypoxia (CIH) augments physiological responses to low partial pressures of O2 in the arterial blood. Adrenal medullae from adult rats, however, are insensitive to direct effects of acute hypoxia. In the present study, we examined whether CIH induces hypoxic sensitivity in the adult rat adrenal medulla and, if so, by what mechanism(s). Experiments were performed on adult male rats exposed to CIH (15 s of 5% O2 followed by 5 min of 21% O2; 9 episodes h(-1); 8 h d(-1); for 3 or 10 days) or to comparable, cumulative durations of continuous hypoxia (CH; 4 h of 7% O2 followed by 20 h of 21% O2 for 1 or 10 days). Noradrenaline (NA) and adrenaline (ADR) effluxes were monitored from ex vivo adrenal medullae. In adrenal medullae of rats exposed to CIH, acute hypoxia evoked robust NA and ADR effluxes, whereas these responses were absent in control rats or in those exposed to CH for 1 or 10 days. Hypercapnia (10% CO2; either acidic, pH 6.8, or isohydric, pH 7.4) was ineffective in eliciting catecholamine (CA) efflux from control, CIH or CH rats. Nicotine (100 microM) evoked NA and ADR effluxes in control rats, and this response was abolished in CIH but not in CH rats. Systemic administration of 2-deoxyglucose depleted ADR content in control rats, and CIH attenuated this response, indicating downregulation of neurally regulated CA secretion. Cytosolic and mitochondrial aconitase enzyme activities decreased in CIH adrenal medullae, suggesting increased generation of superoxide anions. Systemic administration of antioxidants reversed the effect of CIH on the adrenal medulla. Rats exposed to CIH exhibited increased blood pressures and elevated plasma CA, and antioxidants abolished these responses. These observations demonstrate that CIH induces hypoxic sensing in the adult rat adrenal medulla via mechanisms involving increased generation of superoxide anions and suggest that hypoxia-evoked CA efflux from the adrenal medulla contributes, in part, to elevated blood pressure and plasma CA.

Calcium signaling stimulates translation of HIF-alpha during hypoxia

Hypoxia-inducible factors (HIFs) are ubiquitous transcription factors that mediate adaptation to hypoxia by inducing specific sets of target genes. It is well accepted that hypoxia induces accumulation and activity of HIFs by causing stabilization of their alpha subunits. We have demonstrated that hypoxia stimulates translation of HIF-1alpha and -2alpha proteins by distributing HIF-alpha mRNAs to larger polysome fractions. This requires influx of extracellular calcium, stimulation of classical protein kinase C-alpha (cPKC-alpha), and the activity of mammalian target of rapamycin, mTOR. The translational component contributes to approximately 40-50% of HIF-alpha proteins accumulation after 3 h of 1% O2. Hypoxia also inhibits general protein synthesis and mTOR activity; however, cPKC-alpha inhibitors or rapamycin reduce mTOR activity and total protein synthesis beyond the effects of hypoxia alone. These data show that during general inhibition of protein synthesis by hypoxia, cap-mediated translation of selected mRNAs is induced through the mTOR pathway. We propose that calcium-induced activation of cPKC-alpha hypoxia partially protects an activity of mTOR from hypoxic inhibition. These results provide an important physiologic insight into the mechanism by which hypoxia-stimulated influx of calcium selectively induces the translation of mRNAs necessary for adaptation to hypoxia under conditions repressing general protein synthesis.

Rat adrenal chromaffin cells are neonatal CO2 sensors

We studied the participation of adrenal medulla (AM) chromaffin cells in hypercapnic chemotransduction. Using amperometric recordings, we measured catecholamine (CAT) secretion from cells in AM slices of neonatal and adult rats perfused with solutions bubbled with different concentrations of CO2. The secretory activity augmented from 1.74 +/- 0.19 pC/min at 5% CO2 to 6.36 +/- 0.77 pC/min at 10% CO2. This response to CO2 was dose dependent and appeared without changes in extracellular pH, although it was paralleled by a drop in intracellular pH. Responsiveness to hypercapnia was higher in neonatal than in adult slices. The secretory response to hypercapnia required extracellular Ca2+ influx. Both the CO2-induced internal pH drop and increase in CAT secretion were markedly diminished by methazolamide (2 microm), a membrane-permeant carbonic anhydrase (CA) inhibitor. We detected the presence of two CA isoforms (CAI and CAII) in neonatal AM slices by in situ hybridization and real-time PCR. The expression of these enzymes decreased in adult AM together with the disappearance of responsiveness to CO2. In patch-clamped chromaffin cells, hypercapnia elicited a depolarizing receptor potential, which led to action potential firing, extracellular Ca2+ influx, and CAT secretion. This receptor potential (inhibited by methazolamide) was primarily attributable to activation of a resting cationic conductance. In addition, voltage-gated K+ current amplitude was also decreased by high CO2. The CO2-sensing properties of chromaffin cells may be of physiologic relevance, particularly for the adaptation of neonates to extrauterine life, before complete maturation of peripheral and central chemoreceptors.

Stupor and coma: pathophysiology of hypoxia--ontogenetic aspects

There are several levels of O2 deprivation with different possibilities of adaptation and compensation. All of them appear to be highly conservative in phylogenesis and are active early in ontogenetic development. The coordinated structural and functional responses to systemic and/or local hypoxia form the basis for individual adaptation to environmental requirements and pathological threat.

Hypoxia and acidosis increase the secretion of catecholamines in the neonatal rat adrenal medulla: an in vitro study

Hypoxia elicits catecholamine (CA) secretion from the adrenal medulla (AM) in perinatal animals by acting directly on chromaffin cells. However, whether innervation of the AM, which in the rat occurs in the second postnatal week, suppresses this direct hypoxic response is the subject of debate. Opioid peptides have been proposed as mediators of this suppression. To resolve these controversies, we have compared CA-secretory responses with high external concentrations of K+ ([K+]e) and hypoxia in the AM of neonatal (1- to 2-day-old) and juvenile (14- or 15- and 30-day-old) rats subjected to superfusion in vitro. In addition, we studied the effect of hypercapnic acidosis on the CA-secretory responses in the AM during postnatal development and the possible interaction between acidic and hypoxic stimuli. Responses to high [K+]e were comparable at all ages, but responses to hypoxia and hypercapnic acidosis were maximal in neonatal animals. Suppression of the hypoxic response in the rat AM was not mediated by opioids, because their agonists did not affect the hypoxic CA response. The association of hypercapnic acidosis and hypoxia, mimicking the episodes of asphyxia occurring during delivery, generates a more than additive secretory response in the neonatal rat AM. Our data confirm the loss of the direct sensitivity to hypoxia of the AM in the initial weeks of life and demonstrate a direct response of neonatal AM to hypercapnic acidosis. The synergistic effect of hypoxia and acidosis would explain the CA outburst crucial for adaptation to extrauterine life observed in naturally delivered mammals.

Doxapram stimulates the carotid body via a different mechanism than hypoxic chemotransduction

To determine if doxapram stimulates the carotid body through the same mechanism as hypoxia, we compared the effects of doxapram and hypoxia on isolated-perfused carotid bodies in rabbits. Doxapram stimulated the carotid body in a dose-dependent manner. In Ca(2+)-free solution, neither doxapram nor hypoxia stimulated the carotid body. Although, doxapram had an additive effect on the carotid body chemosensory response to hypercapnia, a synergistic effect was not observed. Also, we investigated the various K(+) channel activators on the response to doxapram and hypoxia: pinacidil and levcromakalim as ATP-sensitive K(+) channel activators; NS-1619 as a Ca(2+)-sensitive K(+) channel activator; and halothane as a TASK-like background K(+) channel activator. The hypoxic response was partially reduced by halothane only, while pinacidil, levcromakalim and NS-1619 had no effect. Interestingly, the effect of doxapram was partially inhibited by NS-1619. Neither pinacidil nor levcromakalim affected the stimulatory effect of doxapram. We conclude that doxapram stimulates the carotid body via a different mechanism than hypoxic chemotransduction.

A new focus on interoceptive properties of adrenal medulla

The adrenal medulla is an important part of the sympathoadrenal system. Chromaffin cells of the adrenal medulla respond to a broad spectrum of stressful situations by releasing epinephrine and norepinephrine. Originally, it was accepted that this response is controlled exclusively by central nervous system structures. However, it was also demonstrated that a surgically denervated adrenal medulla can respond directly by secreting epinephrine and norepinephrine during an imbalance of internal environment (hypoglycemia, asphyxia). Published data had documented the innervation of the adrenal medulla by sensory neurons of spinal dorsal root ganglia. In addition, recent data showed that ganglion cells of the adrenal medulla project ascending axons. These data suggested potential transmission of information from the adrenal medulla to the central nervous system regarding metabolic changes in the blood. This paper presents an overview of possible involvement of adrenal medullary chromaffin cells in the detection of changes in the internal environment and in the transmission of this information to the central nervous system.

Acute hypoxia differentially regulates K+ channels. Implications with respect to cardiac arrhythmia

The first ion channels demonstrated to be sensitive to changes in oxygen tension were K(+) channels in glomus cells of the carotid body. Since then a number of hypoxia-sensitive ion channels have been identified. However, not all K(+) channels respond to hypoxia alike. This has raised some debate about how cells detect changes in oxygen tension. Because ion channels respond rapidly to hypoxia it has been proposed that the channel is itself an oxygen sensor. However, channel function can also be modified by thiol reducing and oxidizing agents, implicating reactive oxygen species as signals in hypoxic events. Cardiac ion channels can also be modified by hypoxia and redox agents. The rapid and slow components of the delayed rectifier K(+) channel are differentially regulated by hypoxia and beta-adrenergic receptor stimulation. Mutations in the genes that encode the subunits for the channel are associated with Long QT syndrome and sudden cardiac death. The implications with respect to effects of hypoxia on the channel and triggering of cardiac arrhythmia will be discussed.

Neuroendocrine properties of intrinsic cardiac adrenergic cells in fetal rat heart

Intrinsic cardiac adrenergic (ICA) cells in developing rat heart constitute a novel adrenergic signaling system involved in cardiac regulation. Regulatory mechanisms of ICA cells remain to be defined. Immunohistochemical study of fetal rat hearts demonstrated ICA cells with catecholamine biosynthetic enzyme tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT). The mRNA of TH and PNMP was also detected in fetal rat hearts before sympathetic innervation. Immunoreactivity of norepinephrine transporter (NET) was localized to ICA cells in rat heart tissue and primary cell culture. For the functional study, the activity of intracellular Ca2+concentration ([Ca2+]i) transients was quantified by a ratio fluorescent spectrometer in cultured ICA cells and myocytes. ICA cells generated spontaneous [Ca2+]itransients that were eliminated by tetrodotoxin or Ca2+-free solutions and showed greatly reduced amplitude with the addition of L-type Ca2+channel blocker nifedipine. [3H]norepinephrine studies demonstrate release and uptake of norepinephrine. Functional interaction between catecholamines produced by the ICA cells and cocultured myocytes was evident by the effect of the β-adrenergic blocker atenolol eliciting a dose-dependent reduction in the amplitude and frequency of [Ca2+]itransients of beating myocytes. Hypoxia inhibited [Ca2+]itransient activity of ICA cells, which subsequently produced a reoxygenation-mediated rebound augmentation of [Ca2+]itransients. We conclude that ICA cells are capable of catecholamine synthesis, release, and uptake. They generate spontaneous [Ca2+]itransient activity that can be regulated by oxygen tension. ICA cells may provide an alternative adrenergic supply to maintain cardiac contractile and pacemaker function at rest and during stress in the absence of sympathetic innervation.

Metabolic regulation of potassium channels

Potassium (K+) channels exist in all three domains of organisms: eubacteria, archaebacteria, and eukaryotes. In higher animals, these membrane proteins participate in a multitude of critical physiological processes, including food and fluid intake, locomotion, stress response, and cognitive functions. Metabolic regulatory factors such as O2, CO2/pH, redox equivalents, glucose/ATP/ADP, hormones, eicosanoids, cell volume, and electrolytes regulate a diverse group of K+ channels to maintain homeostasis.

ERG K+ channel blockade enhances firing and epinephrine secretion in rat chromaffin cells: the missing link to LQT2-related sudden death?

The ether-a-go-go-related genes (erg) are expressed in tissues other than heart and brain, in which human erg (HERG) K+ channels are known to regulate the repolarization of heart action potentials and neuronal spike-frequency accommodation. We provide evidence that erg1 transcripts and ERG proteins are present in rat chromaffin cells in which we could isolate a K+ current that was biophysically and pharmacologically similar to the ERG current. Firing frequency and catecholamine release were analyzed at the single-cell level by means of perforated patch-clamp and carbon fiber electrochemical detection. It was found that the blocking of ERG, KATP, and KCa channels led to hyperexcitability and an increase in catecholamine release. Combined immunocytochemical experiments with antibodies directed against phenylethanolamine N-methyltransferase and ERG channels suggested expression of these channels in epinephrine- but not in norepinephrine-containing cells. It is concluded that, in addition to being crucial in regulating the QT period in the heart, ERG channels play a role in modulating epinephrine, a fundamental neurotransmitter shaping cardiac function. This finding suggests that the sudden death phenotype associated with LQT2 syndrome mutations may be the result of an emotionally triggered increase in epinephrine in a long-QT running heart.

Nonneurogenic hypoxia sensitivity in rat adrenal slices

A change in the intracellular Ca(2+) ([Ca(2+)](i)) level induced by hypoxia was detected in rat adrenal slices by use of fura-2/AM. After hypoxic stress, an increase in [Ca(2+)](i) was observed only in the adrenal medulla. This increase was inhibited by nifedipine, but not modified by the cholinergic receptor blockers. The hypoxia-induced increase in [Ca(2+)](i) was observed in all postnatal developmental stages to a similar extent, whereas the nicotine and high K(+) sensitivities increased along with postnatal development. A 10 nM ryanodine enhanced the hypoxia-induced [Ca(2+)](i) increase in adult but not in neonatal rat slices. These results suggest the existence of an oxygen-sensing mechanism in adult rat adrenals even after sympathetic innervation. Hypoxic responses seemed to be similar both in neonate and in adult rat adrenals and were triggered by the influx of Ca(2+) via L-type voltage-sensitive Ca(2+) channels. However, the sustained [Ca(2+)](i) increase caused by hypoxia might depend on postnatal development and be triggered by Ca(2+)-induced Ca(2+) release (CICR).