Catecholamine secretion from rat foetal adrenal chromaffin cells and hypoxia sensitivity

Gambierol Blocks a K+ Current Fraction without Affecting Catecholamine Release in Rat Fetal Adrenomedullary Cultured Chromaffin Cells

Gambierol inhibits voltage-gated K⁺ (K_V) channels in various excitable and non-excitable cells. The purpose of this work was to study the effects of gambierol on single rat fetal (F19–F20) adrenomedullary cultured chromaffin cells. These excitable cells have different types of K_V channels and release catecholamines. Perforated whole-cell voltage-clamp recordings revealed that gambierol (100 nM) blocked only a fraction of the total outward K⁺ current and slowed the kinetics of K⁺ current activation. The use of selective channel blockers disclosed that gambierol did not affect calcium-activated K⁺ (K_Ca) and ATP-sensitive K⁺ (K_ATP) channels. The gambierol concentration necessary to inhibit 50% of the K⁺ current-component sensitive to the polyether (IC₅₀) was 5.8 nM. Simultaneous whole-cell current-clamp and single-cell amperometry recordings revealed that gambierol did not modify the membrane potential following 11s depolarizing current-steps, in both quiescent and active cells displaying repetitive firing of action potentials, and it did not increase the number of exocytotic catecholamine release events, with respect to controls. The subsequent addition of apamin and iberiotoxin, which selectively block the K_Ca channels, both depolarized the membrane and enhanced by 2.7 and 3.5-fold the exocytotic event frequency in quiescent and active cells, respectively. These results highlight the important modulatory role played by K_Ca channels in the control of exocytosis from fetal (F19–F20) adrenomedullary chromaffin cells.

Prenatal Hypoxia Affects Foetal Cardiovascular Regulatory Mechanisms in a Sex- and Circadian-Dependent Manner: A Review

Prenatal hypoxia during the prenatal period can interfere with the developmental trajectory and lead to developing hypertension in adulthood. Prenatal hypoxia is often associated with intrauterine growth restriction that interferes with metabolism and can lead to multilevel changes. Therefore, we analysed the effects of prenatal hypoxia predominantly not associated with intrauterine growth restriction using publications up to September 2021. We focused on: (1) The response of cardiovascular regulatory mechanisms, such as the chemoreflex, adenosine, nitric oxide, and angiotensin II on prenatal hypoxia. (2) The role of the placenta in causing and attenuating the effects of hypoxia. (3) Environmental conditions and the mother's health contribution to the development of prenatal hypoxia. (4) The sex-dependent effects of prenatal hypoxia on cardiovascular regulatory mechanisms and the connection between hypoxia-inducible factors and circadian variability. We identified that the possible relationship between the effects of prenatal hypoxia on the cardiovascular regulatory mechanism may vary depending on circadian variability and phase of the days. In summary, even short-term prenatal hypoxia significantly affects cardiovascular regulatory mechanisms and programs hypertension in adulthood, while prenatal programming effects are not only dependent on the critical period, and sensitivity can change within circadian oscillations.

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.

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.

Role of K₂p channels in stimulus-secretion coupling

Two-pore domain K(+) (K2P) channels are involved in a variety of physiological processes by virtue of their high basal activity and sensitivity to various biological stimuli. One of these processes is secretion of hormones and transmitters in response to stimuli such as hypoxia, acidosis, and receptor agonists. The rise in intracellular [Ca(2+)] ([Ca(2+)]i) that is critical for the secretory event can be achieved by several mechanisms: (a) inhibition of resting (background) K(+) channels, (b) activation of Na(+)/Ca(2+)-permeable channels, and (c) release of Ca(2+) from intracellular stores. Here, we discuss the role of TASK and TREK in stimulus-secretion mechanisms in carotid body chemoreceptor cells and adrenal medullary/cortical cells. Studies show that stimuli such as hypoxia and acidosis cause cell depolarization and transmitter/hormone secretion by inhibition of TASK or TREK. Subsequent elevation of [Ca(2+)]i produced by opening of voltage-dependent Ca(2+) channels then activates a Na(+)-permeable cation channel, presumably to help sustain the depolarization and [Ca(2+)]i. Agonists such as angiotensin II may elevate [Ca(2+)]i via multiple mechanisms involving both inhibition of TASK/TREK and Ca(2+) release from internal stores to cause aldosterone secretion. Thus, inhibition of resting (background) K(+) channels and subsequent activation of voltage-gated Ca(2+) channels and Na(+)-permeable non-selective cation channels may be a common ionic mechanism that lead to hormone and transmitter secretion.

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.

Electrophysiological changes in laterodorsal tegmental neurons associated with prenatal nicotine exposure: implications for heightened susceptibility to addict to drugs of abuse

Prenatal nicotine exposure (PNE) is a risk factor for developing an addiction to nicotine at a later stage in life. Understanding the neurobiological changes in reward related circuitry induced by exposure to nicotine prenatally is vital if we are to combat the heightened addiction liability in these vulnerable individuals. The laterodorsal tegmental nucleus (LDT), which is comprised of cholinergic, GABAergic and glutamatergic neurons, is importantly involved in reward mediation via demonstrated excitatory projections to dopamine-containing ventral tegmental neurons. PNE could lead to alterations in LDT neurons that would be expected to alter responses to later-life nicotine exposure. To examine this issue, we monitored nicotine-induced responses of LDT neurons in brain slices of PNE and drug naive mice using calcium imaging and whole-cell patch clamping. Nicotine was found to induce rises in calcium in a smaller proportion of LDT cells in PNE mice aged 7-15 days and smaller rises in calcium in PNE animals from postnatal ages 11-21 days when compared with age-matched control animals. While inward currents induced by nicotine were not found to be different, nicotine did induce larger amplitude excitatory postsynaptic currents in PNE animals in the oldest age group when compared with amplitudes induced in similar-aged control animals. Immunohistochemically identified cholinergic LDT cells from PNE animals exhibited slower spike rise and decay slopes, which likely contributed to the wider action potential observed. Further, PNE was associated with a more negative action potential afterhyperpolarization in cholinergic cells. Interestingly, the changes found in these parameters in animals exposed prenatally to nicotine were age related, in that they were not apparent in animals from the oldest age group examined. Taken together, our data suggest that PNE induces changes in cholinergic LDT cells that would be expected to alter cellular excitability. As the changes are age related, these PNE-associated alterations could contribute differentially across ontogeny to nicotine-mediated reward and may contribute to the particular susceptibility of in utero nicotine exposed individuals to addict to nicotine upon nicotine exposure in the juvenile period.

Hypoxia-elicited catecholamine release is controlled by L-type as well as N/PQ types of calcium channels in rat embryo chromaffin cells

At early life, the adrenal chromaffin cells respond with a catecholamine surge under hypoxic conditions. This response depends on Ca(2+) entry through voltage-activated calcium channels (VACCs). We have investigated here three unresolved questions that concern this response in rat embryo chromaffin cells (ECCs): 1) the relative contribution of L (α1D, Cav1.3), N (α1B, Cav2.2), and PQ (α1A, Cav2.1) to the whole cell Ca(2+) current (ICa); 2) the relative contribution of L and N/PQ channels to the cytosolic Ca(2+) elevations triggered by hypoxia (Δ[Ca(2+)]c); and 3) the role of L and non-L high-VACCs in the regulation of the catecholamine surge occurring during prolonged (1 min) hypoxia exposure of ECCs. Nimodipine halved peak ICa and blocked 60% the total Ca(2+) entry during a 50-ms depolarizing pulse to 0 mV (QCa). Combined ω-agatoxin IVA plus ω-conotoxin GVIA (Aga/GVIA) blocked 30% of both ICa peak and QCa. This relative proportion of L- and non-L VACCs was corroborated by Western blot that indicated 55, 23, and 25% relative expression of L, N, and PQ VACCs. Exposure of ECCs to hypoxia elicited a mild but sustained Δ[Ca(2+)]c; the area of Δ[Ca(2+)]c was blocked 50% by nifedipine and 10% by Aga/GVIA. Exposure of ECCs to 1-min hypoxia elicited an initial transient burst of amperometric secretory spikes followed by scattered spikes along the time of cell exposure to hypoxia. This bulk response was blocked 85% by nimodipine and 35% by Aga/GVIA. Histograms on secretory spike frequency vs. time indicated a faster initial inactivation when Ca(2+) entry took place through N/PQ channels; more sustained secretion but at a lower rate was associated to Ca(2+) entry through L channels. The results suggest that the HIS response may initially be controlled by L and P/Q channels, but later on, N/PQ channels inactivate and the delayed HIS response is maintained at lower rate by slow-inactivating L channels.

Differences in the H2S-induced quantal release of catecholamine in adrenal chromaffin cells of neonatal and adult rats

Both catecholamine (CA) released from adrenal chromaffin cells and hydrogen sulfide (H2S) have been shown to play critical roles in the regulation of hypoxic stress response. Our previous study has demonstrated that exogenous H2S directly induced quantal CA released from adult rat adrenal chromaffin cells (ARACCs) by inhibiting Ca(2+)-activated K(+) current [IK(Ca) current]. However, it is not clear now whether H2S can also directly induce quantal CA released from neonatal rat adrenal chromaffin cells (NRACCs). In the present study, we investigated whether exogenous H2S can stimulate quantal CA released from NRACCs, and whether there were differences in the kinetics of H2S-induced quantal CA released between ARACCs and NRACCs. Using carbon-fiber amperometry and whole-cell patch clamping techniques, our experimental results showed: (1) H2S can directly induce quantal CA released from NRACCs; (2) H2S induced the depolarization of membrane potential and inhibited IK(Ca) current; (3) compared with ARACCs, much smaller quantal size and faster quantal release were showed in NRACCs through the kinetic analysis of the single-vesicle secretion induced by H2S. Our results may not only help to further understand the H2S-induced CA released from adrenal chromaffin cells in the aspect of development, but also provide the insights for the clinical prevention and therapy for hypoxic stress-induced injury in neonates at birth.

Role of hypoxia and HIF2α in development of the sympathoadrenal cell lineage and chromaffin cell tumors with distinct catecholamine phenotypic features

Hypoxia has wide-ranging impact in normal physiology and disease processes. This stimulus evokes changes in gene expression mediated by transcription factors termed hypoxia-inducible factors (HIFs) that affect numerous processes: angiogenesis, cell survival, cellular metabolism, stem cell self-renewal and multipotency, migration, invasiveness, and metastatic progression in tumor cells. Over the past decade, increasing numbers of reports have emerged documenting differential roles of HIF1α and HIF2α in these processes. In cells of the sympathoadrenal lineage, both HIFs differentially mediate influences of hypoxia on catecholamine synthesis and secretion, but HIF2α signaling has particularly prominent functions in regulating developmental processes of growth and differentiation. This chapter discusses the role of HIF2α and HIF1α in the context of the development, phenotypic features, and functions of chromaffin cells. Moreover, current knowledge about tumor formation in cells of the sympathoadrenal lineage, leading to catecholamine-producing pheochromocytomas and paragangliomas, is analyzed in the light of the HIF2α signaling network.

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.

Adenosine A₂a receptors and O₂ sensing in development

Reduced mitochondrial oxidative phosphorylation, via activation of adenylate kinase and the resulting exponential rise in the cellular AMP/ATP ratio, appears to be a critical factor underlying O₂ sensing in many chemoreceptive tissues in mammals. The elevated AMP/ATP ratio, in turn, activates key enzymes that are involved in physiologic adjustments that tend to balance ATP supply and demand. An example is the conversion of AMP to adenosine via 5'-nucleotidase and the resulting activation of adenosine A(₂A) receptors, which are involved in acute oxygen sensing by both carotid bodies and the brain. In fetal sheep, A(₂A) receptors associated with carotid bodies trigger hypoxic cardiovascular chemoreflexes, while central A(₂A) receptors mediate hypoxic inhibition of breathing and rapid eye movements. A(₂A) receptors are also involved in hypoxic regulation of fetal endocrine systems, metabolism, and vascular tone. In developing lambs, A(₂A) receptors play virtually no role in O₂ sensing by the carotid bodies, but brain A(₂A) receptors remain critically involved in the roll-off ventilatory response to hypoxia. In adult mammals, A(₂A) receptors have been implicated in O₂ sensing by carotid glomus cells, while central A(₂A) receptors likely blunt hypoxic hyperventilation. In conclusion, A(₂A) receptors are crucially involved in the transduction mechanisms of O₂ sensing in fetal carotid bodies and brains. Postnatally, central A(₂A) receptors remain key mediators of hypoxic respiratory depression, but they are less critical for O₂ sensing in carotid chemoreceptors, particularly in developing lambs.

Altered adrenal chromaffin cell function during experimental colitis

The sympathetic nervous system regulates visceral function through the release of catecholamines and cotransmitters from postganglionic sympathetic neurons and adrenal chromaffin cells (ACCs). Previous studies have shown that norepinephrine secretion is decreased during experimental colitis due to the inhibition of voltage-gated Ca(2+) current (I(Ca)) in postganglionic sympathetic neurons. The present study examined whether colonic inflammation causes a similar impairment in depolarization-induced Ca(2+) influx in ACCs using the dextran sulfate sodium (DSS) model of acute colitis in mice. Alterations in ACC function during colitis were assessed using fura 2-acetoxymethyl ester Ca(2+) imaging techniques and perforated patch-clamp electrophysiology. In ACCs isolated from mice with DSS-induced acute colitis, the high-K(+)-stimulated increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) was significantly reduced to 74% of the response of ACCs from control mice. Acute colitis caused a 10-mV hyperpolarization of ACC resting membrane potential, without a significant effect on cellular excitability. Delayed-rectifier K(+) and voltage-gated Na(+) current densities were significantly enhanced in ACCs from mice with DSS-induced acute colitis, with peak current densities of 154 and 144% that of controls, respectively. Importantly, acute colitis significantly inhibited I(Ca) in ACCs between -25 and +20 mV. Peak I(Ca) density in ACCs from mice with DSS-induced acute colitis was 61% that of controls. High-K(+)-induced increases in [Ca(2+)](i) were also reduced in ACCs from mice with 2,4,6-trinitrobenzene sulfonic acid-induced acute colitis and DSS-induced chronic colitis to 68 and 78% of the control responses, respectively. Our results suggest that, during colitis, voltage-dependent Ca(2+) influx is impaired in ACCs. Given the importance of Ca(2+) signaling in exocytosis, these alterations may decrease systemic catecholamine levels, which could play an important role in inflammatory bowel disease. This is the first demonstration of aberrant ACC function during experimental colitis.

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.

Intermittent hypoxia augments acute hypoxic sensing via HIF-mediated ROS

Carotid bodies and neonatal adrenal medullary chromaffin cells (AMC) respond rapidly to acute hypoxia before compromising cellular functions. Responses to acute hypoxia are dynamically altered by chronic perturbations in arterial blood O2 levels resulting from breathing disorders. Sleep disordered breathing with recurrent apneas cause periodic decreases in arterial blood O2 or intermittent hypoxia (IH). Recent studies suggest that reactive oxygen species (ROS) mediate cellular adaptations to prolonged hypoxia. In this article we discuss the evidence for ROS in mediating exaggerated carotid body and AMC responses to acute hypoxia by IH and the underlying cellular and molecular mechanisms. IH increases ROS levels, and anti-oxidants prevent IH-induced augmented responses of the carotid body and AMC to hypoxia. The enhanced hypoxic sensitivity by IH involves ROS-dependent recruitment of transmitters/modulators in the carotid body and Ca2+ signaling mechanisms in AMC. Mechanisms by which IH elevates ROS include activation of NADPH oxidases, inhibition of mitochondrial complex I activity and down-regulation of anti-oxidant enzymes. Transcriptional regulation of pro- and anti-oxidant enzymes by hypoxia-inducible factors 1 and 2 appears to be a major molecular mechanism regulating ROS generation by IH.

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.

Ultrastructural evidence of a vesicle-mediated mode of cell degranulation in chicken chromaffin cells during the late phase of embryonic development

In the present investigation, we attempted to determine whether ultrastructural features indicative of a vesicle-mediated mode of cell secretion were detectable in chick chromaffin cells during embryo development. The adrenal anlagen of domestic fowls were examined at embryonic days (E) 12, 15, 19 and 21 by electron microscopy quantitative analysis. Morphometric evaluation revealed a series of granule and cytoplasmic changes highly specific for piecemeal degranulation (PMD), a secretory process based on vesicular transport of cargoes from within granules for extracellular release. At E19 and E21 we found a significant peak in the percentage of granules exhibiting changes indicative of progressive release of secretory materials, i.e. granules with lucent areas in their cores, reduced electron density, disassembled matrices, residual cores and membrane empty containers. A dramatic raise in the density of 30-80-nm-diameter, membrane-bound, electron-dense and electron-lucent vesicles--which were located either next to granules or close to the plasma membrane--was recognizable at E19, that is, during the prehatching phase. The cytoplasmic burst of dense and clear vesicles was paralleled by the appearance of chromaffin granules showing outpouches or protrusions of their profiles ('budding features'). These ultrastructural data are indicative of an augmented vesicle-mediated transport of chromaffin granule products for extracellular release in chick embryo chromaffin cells during the prehatching stage. In conclusion, this study provides new data on the fine structure of chromaffin cell organelles during organ development and suggests that PMD may be part of an adrenomedullary secretory response that occurs towards the end of chicken embryogenesis. From an evolutionary point of view, this study lends support to the concept that PMD is a secretory mechanism highly conserved throughout vertebrate classes.

Fetal adrenal gland alterations in a rat model of adverse intrauterine environment

By feeding a low-sodium diet to dams over the last third of gestation, we have developed an animal model of intrauterine growth restriction (IUGR). Given that fetal adrenal development and maturation occur during late gestation in rats, the aim of this study was to evaluate the expression of proteins and enzymes involved in steroidogenesis and catecholamine synthesis in adrenal glands from IUGR fetuses. A gene microarray was performed to investigate for alteration in the pathways participating in hormone production. Results show that increased aldosterone serum concentrations in IUGR fetuses were associated with higher mRNA adrenal levels of angiotensin II receptor type 1 (AT(1)R) and cytochrome P450 aldosterone synthase in response to decreased serum sodium content. Conversely, reduced serum corticosterone concentrations in these fetuses appear to result from alterations in gene expression involved in cholesterol metabolism, such as the augmented apolipoprotein E levels, and in steroidogenesis, like the decreased levels of cytochrome P45011beta-hydroxylase. Furthermore, increased AT(2)R expression and the presence of hypoxia and oxidative stress may, in turn, explain the higher adrenal mRNA levels of enzymes involved in catecholamine synthesis. Despite this increase, catecholamine adrenal content was reduced in males and was similar in females compared with sex-matched controls, suggesting higher catecholamine secretion. This could be associated with the induction of genes involved in inflammation-related, acute-phase response in IUGR fetuses. All of these alterations could have long-lasting health effects and may, hence, be implicated in the pathogenesis of increased blood pressure and cardiac hypertrophy observed in IUGR adult animals from this model.

Differences in the quantal release of catecholamines in chromaffin cells of rat embryos and their mothers

Studies on the bulk catecholamine release from fetal and neonatal rat adrenals, adrenal slices, or isolated chromaffin cells stimulated with high K+, hypoxia, hypercapnia, or acidosis are available. However, a study analyzing the kinetics of quantal secretion is lacking. We report here such a study in which we compare the quantal release of catecholamines from immature rat embryo chromaffin cells (ECCs) and their mothers' (MCCs). Cell challenging with a strong depolarizing stimulus (75 mM K+) caused spike bursts having the following characteristics. ECCs released more multispike events and wave envelopes than MCCs. This, together with narrower single-spike events, a faster decay, and a threefold smaller quantal size suggest a faster secretory machinery in ECCs. Furthermore, with a milder stimulus (25 mM K+) enhanced Ca2+ entry by L-type Ca2+ channel activator BAY K 8644 did not change the kinetic parameters of single spikes in ECCs; in contrast, augmentation of Ca2+ entry increased spike amplitude and width, quantal size, and decay time in MCCs. This suggests that in mature MCCs, the last exocytotic steps are more tightly regulated than in immature ECCs. Finally, we found that quantal secretion was fully controlled by L-type voltage-dependent Ca2+ channels (VDCCs) in ECCs, whereas both L- and non-L VDCCs (N and PQ) contributed equally to secretion control in MCCs. Our results have the following physiological, pharmacological, and clinical relevance: 1) they may help to better understand the regulation of adrenal catecholamine release in response to stress during fetal life and delivery; 2) if clinically used, L-type Ca2+ channel blockers may augment the incidence of sudden infant death syndrome (SIDS); and 3) so-called Ca2+ promotors or activators of Ca2+ entry through L-type VDCCs may be useful to secure a healthy catecholamine surge upon violent stress during fetal life, at birth, or to prevent the SIDS in neonates at risk.

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.

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.