Ca2+ current in rabbit carotid body glomus cells is conducted by multiple types of high-voltage-activated Ca2+ channels

Hypoxia releases S-nitrosocysteine from carotid body glomus cells-relevance to expression of the hypoxic ventilatory response

We have provided indirect pharmacological evidence that hypoxia may trigger release of the S-nitrosothiol, S-nitroso-L-cysteine (L-CSNO), from primary carotid body glomus cells (PGCs) of rats that then activates chemosensory afferents of the carotid sinus nerve to elicit the hypoxic ventilatory response (HVR). The objective of this study was to provide direct evidence, using our capacitive S-nitrosothiol sensor, that L-CSNO is stored and released from PGCs extracted from male Sprague Dawley rat carotid bodies, and thus further pharmacological evidence for the role of S-nitrosothiols in mediating the HVR. Key findings of this study were that 1) lysates of PGCs contained an S-nitrosothiol with physico-chemical properties similar to L-CSNO rather than S-nitroso-L-glutathione (L-GSNO), 2) exposure of PGCs to a hypoxic challenge caused a significant increase in S-nitrosothiol concentrations in the perfusate to levels approaching 100 fM via mechanisms that required extracellular Ca2+, 3) the dose-dependent increases in minute ventilation elicited by arterial injections of L-CSNO and L-GSNO were likely due to activation of small diameter unmyelinated C-fiber carotid body chemoafferents, 4) L-CSNO, but not L-GSNO, responses were markedly reduced in rats receiving continuous infusion (10 μmol/kg/min, IV) of both S-methyl-L-cysteine (L-SMC) and S-ethyl-L-cysteine (L-SEC), 5) ventilatory responses to hypoxic gas challenge (10% O2, 90% N2) were also due to the activation of small diameter unmyelinated C-fiber carotid body chemoafferents, and 6) the HVR was markedly diminished in rats receiving L-SMC plus L-SEC. This data provides evidence that rat PGCs synthesize an S-nitrosothiol with similar properties to L-CSNO that is released in an extracellular Ca2+-dependent manner by hypoxia.

Carotid body chemoreceptors: physiology, pathology, and implications for health and disease

The carotid body (CB) is the main peripheral chemoreceptor for arterial respiratory gases O2 and CO2 and pH, eliciting reflex ventilatory, cardiovascular, and humoral responses to maintain homeostasis. This review examines the fundamental biology underlying CB chemoreceptor function, its contribution to integrated physiological responses, and its role in maintaining health and potentiating disease. Emphasis is placed on 1) transduction mechanisms in chemoreceptor (type I) cells, highlighting the role played by the hypoxic inhibition of O2-dependent K+ channels and mitochondrial oxidative metabolism, and their modification by intracellular molecules and other ion channels; 2) synaptic mechanisms linking type I cells and petrosal nerve terminals, focusing on the role played by the main proposed transmitters and modulatory gases, and the participation of glial cells in regulation of the chemosensory process; 3) integrated reflex responses to CB activation, emphasizing that the responses differ dramatically depending on the nature of the physiological, pathological, or environmental challenges, and the interactions of the chemoreceptor reflex with other reflexes in optimizing oxygen delivery to the tissues; and 4) the contribution of enhanced CB chemosensory discharge to autonomic and cardiorespiratory pathophysiology in obstructive sleep apnea, congestive heart failure, resistant hypertension, and metabolic diseases and how modulation of enhanced CB reactivity in disease conditions may attenuate pathophysiology.

Influence of propofol on isolated neonatal rat carotid body glomus cell response to hypoxia and hypercapnia

•

The intravenous anaesthetic propofol acts directly on carotid body glomus cells to inhibit their response to hypoxia.

•

Propofol acts via novel mechanisms, as we excluded action via its known target receptors (nicotinic, GABA-ergic, or K+ channel).

•

Inhibition of the hypoxic response is clinically relevant in anaesthesia.

In humans the intravenous anaesthetic propofol depresses ventilatory responses to hypoxia and CO₂. Animal studies suggest that this may in part be due to inhibition of synaptic transmission between chemoreceptor glomus cells of the carotid body and the afferent carotid sinus nerve. It is however unknown if propofol can also act directly on the glomus cell. Here we report that propofol can indeed inhibit intracellular Ca²⁺ responses to hypoxia and hypercapnia in isolated rat glomus cells. Neither this propofol effect, nor the glomus cell response to hypoxia in the absence of propofol, were influenced by GABA receptor activation (using GABA, muscimol and baclofen) or inhibition (using bicuculline and 5-aminovaleric acid). Suggesting that these effects of propofol are not mediated through GABA receptors. Propofol inhibited calcium responses to nicotine in glomus cells but the nicotinic antagonists vecuronium and methyllycaconitine did not inhibit calcium responses to hypoxia. TASK channel activity was not altered by propofol. The glomus cell Ca²⁺ response to depolarisation with 30 mM K⁺ was however modestly inhibited by propofol. In summary we conclude that propofol does have a direct effect upon hypoxia signalling in isolated type-1 cells and that this may be partially due to its ability to inhibit voltage gated Ca²⁺_v channels. We also note that propofol has the capacity to supress glomus cell excitation via nicotinic receptors and may therefore also interfere with paracrine/autocrine cholinergic signalling in the intact organ. The effects of propofol on chemoreceptor function are however clearly complex and require further investigation.

Carotid Body Type-I Cells Under Chronic Sustained Hypoxia: Focus on Metabolism and Membrane Excitability

Chronic sustained hypoxia (CSH) evokes ventilatory acclimatization characterized by a progressive hyperventilation due to a potentiation of the carotid body (CB) chemosensory response to hypoxia. The transduction of the hypoxic stimulus in the CB begins with the inhibition of K+ currents in the chemosensory (type-I) cells, which in turn leads to membrane depolarization, Ca2+ entry and the subsequent release of one- or more-excitatory neurotransmitters. Several studies have shown that CSH modifies both the level of transmitters and chemoreceptor cell metabolism within the CB. Most of these studies have been focused on the role played by such putative transmitters and modulators of CB chemoreception, but less is known about the effect of CSH on metabolism and membrane excitability of type-I cells. In this mini-review, we will examine the effects of CSH on the ion channels activity and excitability of type-I cell, with a particular focus on the effects of CSH on the TASK-like background K+ channel. We propose that changes on TASK-like channel activity induced by CSH may contribute to explain the potentiation of CB chemosensory activity.

Hypoxia Tolerance in Teleosts: Implications of Cardiac Nitrosative Signals

Changes in environmental oxygen (O₂) are naturally occurring phenomena which ectotherms have to face on. Many species exhibit a striking capacity to survive and remain active for long periods under hypoxia, even tolerating anoxia. Some fundamental adaptations contribute to this capacity: metabolic suppression, tolerance of pH and ionic unbalance, avoidance and/or repair of free-radical-induced cell injury during reoxygenation. A remarkable feature of these species is their ability to preserve a normal cardiovascular performance during hypoxia/anoxia to match peripheral (tissue pO₂) requirements. In this review, we will refer to paradigms of hypoxia- and anoxia-tolerant teleost fish to illustrate cardiac physiological strategies that, by involving nitric oxide and its metabolites, play a critical role in the adaptive responses to O₂ limitation. The information here reported may contribute to clarify the molecular and cellular mechanisms underlying heart vulnerability vs. resistance in relation to O₂ availability.

Single cell transcriptome analysis of mouse carotid body glomus cells

KEY POINTS

Carotid body (CB) glomus cells mediate acute oxygen sensing and the initiation of the hypoxic ventilatory response, yet the gene expression profile of these cells is not available. We demonstrate that the single cell RNA-Seq method is a powerful tool for identifying highly expressed genes in CB glomus cells. Our single cell RNA-Seq results characterized novel CB glomus cell genes, including members of the G protein-coupled receptor signalling pathway, ion channels and atypical mitochondrial electron transport chain subunits. A heterologous cell-based screening identified acetate (which is known to affect CB glomus cell activity) as an agonist for the most highly abundant G protein-coupled receptor (Olfr78) in CB glomus cells. These data established the first transcriptome profile of CB glomus cells, highlighting genes with potential implications in CB chemosensory function.

ABSTRACT

The carotid body (CB) is a major arterial chemoreceptor containing glomus cells whose activities are regulated by changes in arterial blood content, including oxygen. Despite significant advancements in the characterization of their physiological properties, our understanding of the underlying molecular machinery and signalling pathway in CB glomus cells is still limited. To overcome this, we employed the single cell RNA-Seq method by performing next-generation sequencing on single glomus cell-derived cDNAs to eliminate contamination of genes derived from other cell types present in the CB. Using this method, we identified a set of genes abundantly expressed in glomus cells, which contained novel glomus cell-specific genes. Transcriptome and subsequent in situ hybridization and immunohistochemistry analyses identified abundant G protein-coupled receptor signalling pathway components and various types of ion channels, as well as members of the hypoxia-inducible factors pathway. A short-chain fatty acid olfactory receptor Olfr78, recently implicated in CB function, was the most abundant G protein-coupled receptor. Two atypical mitochondrial electron transport chain subunits (Ndufa4l2 and Cox4i2) were among the most specifically expressed genes in CB glomus cells, highlighting their potential roles in mitochondria-mediated oxygen sensing. The wealth of information provided by the present study offers a valuable foundation for identifying molecules functioning in the CB.

CaV3.2 T-type Ca²⁺ channels in H₂S-mediated hypoxic response of the carotid body

Arterial blood O2 levels are detected by specialized sensory organs called carotid bodies. Voltage-gated Ca(2+) channels (VGCCs) are important for carotid body O2 sensing. Given that T-type VGCCs contribute to nociceptive sensation, we hypothesized that they participate in carotid body O2 sensing. The rat carotid body expresses high levels of mRNA encoding the α1H-subunit, and α1H protein is localized to glomus cells, the primary O2-sensing cells in the chemoreceptor tissue, suggesting that CaV3.2 is the major T-type VGCC isoform expressed in the carotid body. Mibefradil and TTA-A2, selective blockers of the T-type VGCC, markedly attenuated elevation of hypoxia-evoked intracellular Ca(2+) concentration, secretion of catecholamines from glomus cells, and sensory excitation of the rat carotid body. Similar results were obtained in the carotid body and glomus cells from CaV3.2 knockout (Cacna1h(-/-)) mice. Since cystathionine-γ-lyase (CSE)-derived H2S is a critical mediator of the carotid body response to hypoxia, the role of T-type VGCCs in H2S-mediated O2 sensing was examined. Like hypoxia, NaHS, a H2S donor, increased intracellular Ca(2+) concentration and augmented carotid body sensory nerve activity in wild-type mice, and these effects were markedly attenuated in Cacna1h(-/-) mice. In wild-type mice, TTA-A2 markedly attenuated glomus cell and carotid body sensory nerve responses to hypoxia, and these effects were absent in CSE knockout mice. These results demonstrate that CaV3.2 T-type VGCCs contribute to the H2S-mediated carotid body response to hypoxia.

Heme oxygenase-1 (HO-1) mediated respiratory responses to hypoxia in the goldfish, Carassius auratus

In this study we investigated the role of heme oxygenase-1 (HO-1) in modulating the hypoxic and hyperoxic ventilatory responses of goldfish (Carassius auratus) acclimated to 7 and 25°C. HO-1 was present in the neuroepithelial cells (NECs; putative branchial O2 chemoreceptors) of fish acclimated to 7°C only. Hypoxia exposure increased gill HO-1 activity in 7°C fish (14.0±1.4 to 42.5±3.2pmolbilirubinmin(-1)mgprotein(-1)). Inhibition of HO-1 activity with zinc protophorphyrin IX (ZnPPIX) increased the ventilation frequency response to acute hypoxia (30mmHg); frequency increased from 48.3±5.1 to 137.4±16.0 breaths per min (BPM) in hypoxic 7°C fish treated with ZnPPIX compared to 46.2±4.2 to 77.9±5.3 BPM in control fish. Unlike in the control (untreated) 7°C fish exposed to hyperoxia, fish injected with ZnPPIX did not significantly decrease breathing frequency. Inhibiting HO-1 activity was without effect on the hypoxic or hyperoxic ventilatory responses of fish acclimated to 25°C. Based on these observations, we suggest that HO-1 plays an inhibitory role in regulating breathing frequency but only in goldfish acclimated to 7°C.

Stress peptide PACAP engages multiple signaling pathways within the carotid body to initiate excitatory responses in respiratory and sympathetic chemosensory afferents

Consistent with a critical role in respiratory and autonomic stress responses, the carotid bodies are strongly excited by pituitary adenylate cyclase-activating polypeptide (PACAP), a neuropeptide implicated in stress responses throughout the sympathetic nervous system. PACAP excites isolated carotid body glomus cells via activation of PAC1 receptors, with one study suggesting PAC1-induced excitation is due entirely to protein kinase A (PKA)-mediated inhibition of TASK channels. However, in other systems, PAC1 is known to be coupled to multiple intracellular signaling pathways, including PKA, phospholipase C (PLC), phospholipase D (PLD), and protein kinase C (PKC), that trigger multiple downstream effectors including increased Ca²⁺ mobilization, inhibition of various K⁺ channels, and activation of nonselective cation channels. This study tests if non-PKA/TASK channel signaling helps mediate the stimulatory effects of PACAP on the carotid body. Using an ex vivo arterially perfused rat carotid body preparation, we show that PACAP-38 stimulates carotid sinus nerve activity in a biphasic manner (peak response, falling to plateau). PKA blocker H-89 only reduced the plateau response (~41%), whereas the TASK-1-like K⁺ channel blocker/transient receptor potential vanilloid 1 channel agonist anandamide only inhibited the peak response (~48%), suggesting involvement of additional pathways. The PLD blocker CAY10594 significantly inhibited both peak and plateau responses. The PLC blocker U73122 decimated both peak and plateau responses. Brefeldin A, a blocker of Epac (cAMP-activated guanine exchange factor, reported to link Gs-coupled receptors with PLC/PLD), also reduced both phases of the response, as did blocking signaling downstream of PLC/PLD with the PKC inhibitors chelerythrine chloride and GF109203X. Suggesting the involvement of non-TASK ion channels in the effects of PACAP, the A-type K⁺ channel blocker 4-aminopyridine, and the putative transient receptor potential channel (TRPC)/T-type calcium channel blocker SKF96365 each significantly inhibited the peak and steady-state responses. These data suggest the stimulatory effect of PACAP-38 on carotid body sensory activity is mediated through multiple signaling pathways: the PLC-PKC pathways predominates, with TRPC and/or T-type channel activation and Kv channel inactivation; only partial involvement is attributable to PKA and PLD activation.

Carbon monoxide (CO) and hydrogen sulfide (H(2)S) in hypoxic sensing by the carotid body

Carotid bodies are sensory organs for monitoring arterial blood oxygen (O(2)) levels, and the ensuing reflexes maintain cardio-respiratory homeostasis during hypoxia. This article provides a brief update of the role of carbon monoxide (CO) and hydrogen sulfide (H(2)S) in hypoxic sensing by the carotid body. Glomus cells, the primary site of O(2) sensing in the carotid body express heme oxygenase-2 (HO-2), a CO catalyzing enzyme. HO-2 is a heme containing enzyme and has high affinity for O(2). Hypoxia inhibits HO-2 activity and reduces CO generation. Pharmacological and genetic approaches suggest that CO inhibits carotid body sensory activity. Stimulation of carotid body activity by hypoxia may reflect reduced formation of CO. Glomus cells also express cystathionine γ-lyase (CSE), an H(2)S generating enzyme. Exogenous application of H(2)S donors, like hypoxia, stimulate the carotid body activity and CSE knockout mice exhibit severely impaired sensory excitation by hypoxia, suggesting that CSE catalyzed H(2)S is an excitatory gas messenger. Hypoxia increases H(2)S generation in the carotid body, and this response was attenuated or absent in CSE knockout mice. HO inhibitor increased and CO donor inhibited H(2)S generation. It is proposed that carotid body response to hypoxia requires interactions between HO-2-CO and CSE-H(2)S systems.

Peripheral chemoreceptors: function and plasticity of the carotid body

The discovery of the sensory nature of the carotid body dates back to the beginning of the 20th century. Following these seminal discoveries, research into carotid body mechanisms moved forward progressively through the 20th century, with many descriptions of the ultrastructure of the organ and stimulus-response measurements at the level of the whole organ. The later part of 20th century witnessed the first descriptions of the cellular responses and electrophysiology of isolated and cultured type I and type II cells, and there now exist a number of testable hypotheses of chemotransduction. The goal of this article is to provide a comprehensive review of current concepts on sensory transduction and transmission of the hypoxic stimulus at the carotid body with an emphasis on integrating cellular mechanisms with the whole organ responses and highlighting the gaps or discrepancies in our knowledge. It is increasingly evident that in addition to hypoxia, the carotid body responds to a wide variety of blood-borne stimuli, including reduced glucose and immune-related cytokines and we therefore also consider the evidence for a polymodal function of the carotid body and its implications. It is clear that the sensory function of the carotid body exhibits considerable plasticity in response to the chronic perturbations in environmental O2 that is associated with many physiological and pathological conditions. The mechanisms and consequences of carotid body plasticity in health and disease are discussed in the final sections of this article.

Expression of neuronal nitric oxide synthase in rabbit carotid body glomus cells regulates large-conductance Ca2+-activated potassium currents

Our previous studies show that a decrease in endogenous nitric oxide (NO) is involved in the blunted outward K(+) currents in carotid body (CB) glomus cells from chronic heart failure (CHF) rabbits. In the present study, we measured the effects of the neuronal nitric oxide synthase (nNOS) transgene on the K(+) currents in CB glomus cells from pacing-induced CHF rabbits. Using single-cell real-time RT-PCR and immunofluorescent techniques, we found that nNOS mRNA and protein are expressed in the rabbit CB glomus cells and CHF decreased the expression of nNOS mRNA and protein in CB glomus cells. After 3 days of an adenoviral nNOS (Ad.nNOS) gene transfection, the expression of nNOS protein was increased to the level found in sham CB glomus cells. In whole cell patch-clamp experiments, Ad.nNOS markedly reversed the attenuated K(+) currents in CB glomus cells from CHF rabbits. The specific nNOS inhibitor (S-methyl-l-thiocitrulline [SMTC]) and large-conductance Ca(2+)-activated K(+) (BK) channel blocker (iberiotoxin) fully abolished the effect of Ad.nNOS on the K(+) currents in the CB glomus cells from CHF rabbits. However, neither CHF nor Ad.nNOS altered the protein expression of BK channel alpha-subunit. These results suggest that a decrease of NO induced by an attenuated nNOS activity lowers the activation of the BK channels but not the protein expression of the BK channel alpha-subunit in the CB glomus cells during CHF.

Halothane and sevoflurane exert different degrees of inhibition on carotid body glomus cell intracellular Ca2+ response to hypoxia

The purpose of this study was to ascertain if effects of halothane and sevoflurane (0.18-1.45 MAC) on the magnitude of the rise in intracellular calcium ([Ca(2+)]i with approximately 90s hypoxia (measured using indo-1 dye) in rat pup carotid body type I glomus cells. paralleled their known effects on the human hypoxic ventilatory response, where halothane is more depressive. We also assessed these agents' effect on [Ca(2+)]i response to 100 mM K(+). Halothane depressed the [Ca(2+])i transient in hypoxia more than sevoflurane (p = 0.036). Both agents also depressed the [Ca(2+)]i response to K+ - halothane more than sevoflurane (p = 0.004). These actions reflect their known influence on human hypoxic ventilatory response, consistent with the notion that the cellular process underlies the whole-body effect. The responses to K(+), which depolarises the cell membrane, indicates that in addition to a putative effect on K(+) channels, voltage-activated Ca(2+) channels may also be involved in the anaesthetic effect.

Long-term regulation of carotid body function: acclimatization and adaptation--invited article

Physiological responses to hypoxia either continuous (CH) or intermittent (IH) depend on the O(2)-sensing ability of the peripheral arterial chemoreceptors, especially the carotid bodies, and the ensuing reflexes play important roles in maintaining homeostasis. The purpose of this article is to summarize the effects of CH and IH on carotid body function and the underlying mechanisms. CH increases baseline carotid body activity and sensitizes the response to acute hypoxia. These effects are associated with hyperplasia of glomus cells and neovascularization. Enhanced hypoxic sensitivity is due to alterations in ion current densities as well as changes in neurotransmitter dynamics and recruitment of additional neuromodulators (endothelin-1, ET-1) in glomus cells. Morphological alterations are in part due to up-regulation of growth factors (e.g. VEGF). Hypoxia-inducible factor-1 (HIF-1), a transcriptional activator might underlie the remodeling of carotid body structure and function by CH. Chronic IH, on the other hand, is associated with recurrent apneas in adults and premature infants. Two major effects of chronic IH on the adult carotid body are sensitization of the hypoxic sensory response and long-lasting increase in baseline activity i.e., sensory long-term facilitation (LTF) which involve reactive oxygen species (ROS) and HIF-1. In neonates, chronic IH leads to sensitization of the hypoxic response but does not induce sensory LTF. Chronic IH-induced sensitization of the carotid body response to hypoxia increases the likelihood of unstable breathing perpetuating in more number of apneas, whereas sensory LTF may contribute to increased sympathetic tone and systemic hypertension associated with recurrent apneas.

Voltage-activated calcium currents in octopus cells of the mouse cochlear nucleus

Octopus cells, neurons in the most posterior and dorsal part of the mammalian ventral cochlear nucleus, convey the timing of synchronous firing of auditory nerve fibers to targets in the contralateral superior paraolivary nucleus and ventral nucleus of the lateral lemniscus. The low input resistances and short time constants at rest that arise from the partial activation of a large, low-voltage-activated K(+) conductance (g(KL)) and a large mixed-cation, hyperpolarization-activated conductance (g(h)) enable octopus cells to detect coincident firing of auditory nerve fibers with exceptional temporal precision. Octopus cells fire conventional, Na(+) action potentials but a voltage-sensitive Ca(2+) conductance was also detected. In this study, we explore the nature of that calcium conductance under voltage-clamp. Currents, carried by Ca(2+) or Ba(2+) and blocked by 0.4 mM Cd(2+), were activated by depolarizations positive to -50 mV and peaked at -23 mV. At -23 mV they reached 1.1 +/- 0.1 nA in the presence of 5 mM Ca(2+) and 1.6 +/- 0.1 nA in 5 mM Ba(2+). Ten micromolar BAY K 8644, an agonist of high-voltage-activated L-type channels, enhanced I(Ba) by 63 +/- 11% (n = 8) and 150 microM nifedipine, an antagonist of L-type channels, reduced the I(Ba) by 65 +/- 5% (n = 5). Meanwhile, 0.5 microM omega-Agatoxin IVA, an antagonist of P/Q-type channels, or 1 microM omega-conotoxin GVIA, an antagonist of N-type channels, suppressed I(Ba) by 15 +/- 4% (n = 5) and 9 +/- 4% (n = 5), respectively. On average 16% of the current remained in the presence of the cocktail of blockers, indicative of the presence of R-type channels. Together these experiments show that octopus cells have a depolarization-sensitive g(Ca) that is largely formed from L-type Ca(2+) channels and that P/Q-, N-, and R-type channels are expressed at lower levels in octopus cells.

Pulmonary expression of voltage-gated calcium channels: special reference to sensory airway receptors

Studying depolarisation induced calcium entry in our recently developed in situ lung slice model for molecular live cell imaging of selectively visualised pulmonary neuroepithelial bodies (NEBs), exemplified the need for information on the localisation of voltage-gated calcium channels (Ca(v)) in lungs in general, and related to sensory airway receptors more specifically. The present study therefore aimed at identifying the expression pattern of all major classes and subtypes of Ca(v) channels, using multiple immunostaining of rat lung cryosections. Ca(v) channel antibodies were combined with antibodies that selectively label NEBs, nerve fibre populations, smooth muscle, endothelium and Clara cells. Ca(v)2.1 (P/Q-type) was the only Ca(v) channel expressed in NEB cell membranes, and appeared to be restricted to the apical membrane of the slender NEB cell processes that reach the airway lumen. Subpopulations of the vagal but not the spinal sensory nerve fibres that contact NEBs showed immunoreactivity (IR) for Ca(v)1.2 (L-type) and Ca(v)2.1. Ca(v)2.3 (R-type) was selectively expressed by the so-called Clara-like cells that cover NEBs only, and appears to be a unique marker to discriminate this epithelial cell type from the much more extensive group of Clara cells in rat airways. The laminar nerve endings of smooth muscle-associated airway receptors (SMARs) revealed IR for both Ca(v)2.1 and Ca(v)2.2 (N-type). More generally, Ca(v)1.2 was seen to be expressed in vascular smooth muscle, Ca(v)2.3 and Ca(v)3.1 (T-type) in bronchial smooth muscle, Ca(v)3.1 and Ca(v)3.2 (T-type) in endothelial cells, and Ca(v)1.3 (L-type) in a limited number of epithelial cells. In conclusion, the present immunocytochemical study has demonstrated that the various subtypes of Ca(v) channels have distinct expression patterns in rat lungs. Special focus on morphologically/neurochemically characterised sensory airway receptors learned us that both NEBs and SMARs present Ca(v) channels. Knowledge of the identification and localisation of Ca(v) channels in airway receptors and surrounding tissues provides a solid basis for interpretation of the calcium mediated activation studied in our ex vivo lung slice model.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

TASK-like potassium channels and oxygen sensing in the carotid body

Chemosensing by type-1 cells of the carotid body involves a series of events which culminate in the calcium-dependent secretion of neurotransmitter substances which then excite afferent nerves. This response is mediated via membrane depolarisation and voltage-gated calcium entry. Studies utilising isolated cells indicates that the membrane depolarisation in response to hypoxia, and acidosis, appears to be primarily mediated via the inhibition of a background K(+)-current. The pharmacological and biophysical characteristics of these channels suggest that they are probably closely related to the TASK subfamily of tandem-P-domain K(+)-channels. Indeed they show greatest similarity to TASK-1 and -3. In addition to being sensitive to hypoxia and acidosis, the background K(+)-channels of the type-1 cell are also remarkably sensitive to inhibition of mitochondrial energy metabolism. Metabolic poisons are known potent stimulants of the carotid body and cause membrane depolarisation of type-1 cells. In the presence of metabolic inhibitors hypoxic sensitivity is lost suggesting that oxygen sensing may itself be mediated via depression of mitochondrial energy production. Thus these TASK-like background channels play a central role in mediating the chemotransduction of several different stimuli within the type-1 cell. The mechanisms by which metabolic/oxygen sensitivity might be conferred upon these channels are briefly discussed.

Carotid body function in heart failure

In this review, we summarize the present state of knowledge of the functional characteristics of the carotid body (CB) chemoreflex with respect to control of sympathetic nerve activity (SNA) in chronic heart failure (CHF). Evidence from both CHF patients and animal models of CHF has clearly established that the CB chemoreflex is enhanced in CHF and contributes to the tonic elevation in SNA. This adaptive change derives from altered function at the level of both the afferent and central nervous system (CNS) pathways of the reflex arc. At the level of the CB, an elevation in basal afferent discharge occurs under normoxic conditions in CHF rabbits, and the discharge responsiveness to hypoxia is enhanced. Outward voltage-gated K(+) currents (I(K)) are suppressed in CB glomus cells from CHF rabbits, and their sensitivity to hypoxic inhibition is enhanced. These changes in I(K) derive partly from downregulation of nitric oxide synthase (NOS)/NO signaling and upregulation of angiotensin II (Ang II)/Ang II receptor (AT(1)R) signaling in glomus cells. At the level of the CNS, interactions of the enhanced input from CB chemoreceptors with altered input from baroreceptor and cardiac afferent pathways and from central Ang II further enhance sympathetic drive. In addition, impaired function of NO in the paraventricular nucleus of the hypothalamus participates in the increased SNA response to CB chemoreceptor activation. These results underscore the principle that multiple mechanisms involving Ang II and NO at the level of both the CB and CNS represent complementary and perhaps redundant adaptive mechanisms to enhance CB chemoreflex function in CHF.

Role of acetylcholine in neurotransmission of the carotid body

Acetylcholine (ACh) has been considered an important excitatory neurotransmitter in the carotid body (CB). Its physiological and pharmacological effects, metabolism, release, and receptors have been well documented in several species. Various nicotinic and muscarinic ACh receptors are present in both afferent nerve endings and glomus cells. Therefore, ACh can depolarize or hyperpolarize the cell membrane depending on the available receptor type in the vicinity. Binding of ACh to its receptor can create a wide variety of cellular responses including opening cation channels (nicotinic ACh receptor activation), releasing Ca(2+) from intracellular storage sites (via muscarinic ACh receptors), and modulating activities of K(+) and Ca(2+) channels. Interactions between ACh and other neurotransmitters (dopamine, adenosine, nitric oxide) have been known, and they may induce complicated responses. Cholinergic biology in the CB differs among species and even within the same species due to different genetic composition. Development and environment influence cholinergic biology. We discuss these issues in light of current knowledge of neuroscience.

Enhanced sensitivity of Kv channels to hypoxia in the rabbit carotid body in heart failure: role of angiotensin II

Angiotensin II (Ang II) plays an important role in the enhanced chemoreflex function that occurs in congestive heart failure (CHF), but the mechanism of this effect within the carotid body (CB) is not known. We investigated the sensitivity of Ca2+-independent, voltage-gated K+ (Kv) channels to hypoxia in CB glomus cells from CHF rabbits, and whether endogenous angiotensin II (Ang II) modulates this action. Using the conventional whole-cell patch clamp technique, we found that Kv currents (IK) under normoxic conditions were blunted in the CB glomus cells from CHF rabbits compared with sham rabbits. In addition, the inhibition of IK and the decrease of resting membrane potential (RMP) induced by hypoxia were greater in CHF versus sham glomus cells. Ang II, at 100 pM, had no direct effect on IK at constant normoxic PO2, but increased the sensitivity of IK and RMP to hypoxia in sham glomus cells. In CHF glomus cells, an AT1 receptor (AT1R) antagonist, L-158 809 (1 microM), alone did not affect IK at normoxia, but it decreased the sensitivity of IK and RMP to hypoxia. At higher concentrations, Ang II dose dependently (0.1-100 nM) reduced IK under constant normoxic conditions in sham and CHF glomus cells, with threshold concentrations of about 900 and 600 pM, respectively. Immunocytochemical and Western blot assessments demonstrated the down-expression of Kv3.4 but not Kv4.3 channels in CHF glomus cells. These results indicate that: (1) Ang II/AT1R signalling increases the sensitivity of Kv channels to hypoxia in CB glomus cells from CHF rabbits; (2) high concentrations of Ang II (> 1 nM) directly inhibit IK in CB glomus cells from sham and CHF rabbits; (3) changes in Kv channel protein expression (Kv3.4 versus Kv4.3) in the CB glomus cell may contribute to the suppression of IK and enhanced sensitivity of IK to hypoxia in CHF.

Cellular and Molecular Mechanisms Associated with Carotid Body Adaptations to Chronic Hypoxia

Chronic hypoxia leads to adaptations in the respiratory system manifested as a persistent increase in resting ventilation, termed ventilatory acclimatization to hypoxia (VAH). Increased afferent nerve activity from carotid bodies and the ensuing reflex activation of ventilation are critical for eliciting VAH. In this review we highlight recent information on the cellular and molecular mechanisms associated with chronic hypoxia-induced functional and structural changes in the carotid body. Chronic hypoxia leads to hypersensitivity of the carotid bodies and induces morphological changes, including enlargement of the organ, hyperplasia of glomus cells, and neovascularization. Enhanced hypoxic sensitivity is due to alterations in ion current densities, as well as changes in neurotransmitter dynamics and recruitment of additional neuromodulators (endothelin- 1, ET-1) in glomus cells. Morphological alterations are in part due to upregulation of growth factors (e.g., VEGF). VAH is markedly attenuated in mice partially deficient in HIF-1 transcription factor, which regulates several downstream genes, including VEGF, ET-1, and Ca(2+) channels. The finding that VAH is also blunted in mice deficient in the fosB gene led to the suggestion that the magnitude and time course of VAH depend on complex interactions between more than one transcription factor, resulting in coordinated regulation of several downstream genes and their protein products.

Calcium channels of cultured rat glomus cells in normoxia and acute hypoxia

Glomus cells harvested from Wistar rat carotid bodies were cultured for 4 to 7 days. Inward calcium currents elicited by voltage ramps (0.24 V/s) or pulses were recorded during voltage-clamping in the whole-cell and perforated patch configurations. Currents were enhanced by an excess of [Ca(2+)](o), barium and BayK 8644, and depressed or eliminated by cobalt or nifedipine. Single calcium channels were studied by patch-clamping in the cell-attached configuration with voltage clamp pulses ranging from 0.5 to 50 s. Channel conductances (g) decreased and open times (OT) increased as clamp pulses increased in duration. For comparisons, conductances and OTs obtained with short (0.5-1 s) and long (6-12 s) pulses were grouped as SVH and LVH, respectively. SVH conductances were higher and OTs shorter when compared to LVH. BayK 8644 increased conductances and OT during SVH but this agonist decreased g during LVH. Nifedipine either eliminated channel activity, had no effects or depressed g and OT. Hypoxia (pO(2) 30 Torr) induced by 100% N(2) significantly increased calcium currents in normal bathing solutions and during exposure to 110 mM BaCl(2) in whole-cell and perforated patch recordings. Sodium dithionite (Na(2)S(2)O(4)), lowering pO(2) to 10 Torr, also increased the amplitude of calcium currents, but shifted to more positive voltages the onset and trough (maximum) of calcium currents. N(2)-induced hypoxia increased g and reduced OT during SVH but had opposite effects with longer pulses: conductance decreased and open times increased. N(2)-induced hypoxia increased the numbers of active channels (from 1 to 35) over a mean normoxic level of 47 per cell. It is suggested that increased calcium currents accompany calcium inflow in glomus cells, but calcium influx may not depend exclusively on this mechanism.

Role of voltage-dependent calcium channels in stimulus-secretion coupling in rabbit carotid body chemoreceptor cells

We have defined Ca2+ channel subtypes expressed in rabbit carotid body (CB) chemoreceptor cells and their participation in the stimulus-evoked catecholamine (CA) release. Ca2+ currents (I(Ca)) activated at -30 mV, peaked at +10 mV and were fully blocked by 200 microm Cd2+. L-type channels (sensitive to 2 microm nisoldipine) activated at -30 mV and carried 21 +/- 2% of total I(Ca). Non-L-type channels activated at potentials positive to -10 mV and carried: N channels (sensitive to 1 microM omega-conotoxin-GVIA) 16 +/- 1% of total I(Ca), P/Q channels (sensitive to 3 microM omega-conotoxin-MVIIC after nisoldipine plus GVIA) 23 +/- 3% of total I(Ca) and R channels (resistant to all blockers combined) 40 +/- 3% of total I(Ca). CA release induced by hypoxia, hypercapnic acidosis, dinitrophenol (DNP) and high K(+)(o) in the intact CB was inhibited by 79-98% by 200 microm Cd2+. Hypoxia, hypercapnic acidosis and DNP, depolarized chemoreceptor cells and eventually generated repetitive action potential discharge. Nisoldipine plus MVIIC nearly abolished the release of CAs induced by hypoxia and hypercapnic acidosis and reduced by 74% that induced by DNP. All these secretory responses were insensitive to GVIA. 30 and 100 mm K(+)(o) brought resting membrane potential (E(m)) of chemoreceptor cells (-48.1 +/- 1.2 mV) to -22.5 and +7.2 mV, respectively. Thirty millimolar K(+)(o)-evoked release was abolished by nisoldipine but that induced by 100 mm K(+)(o) was mediated by activation of L, N, and P/Q channels. Data show that tested stimuli depolarize rabbit CB chemoreceptor cells and elicit CA release through Ca2+ entry via voltage-activated channels. Only L and P/Q channels are tightly coupled to the secretion of CA.

Attenuated outward potassium currents in carotid body glomus cells of heart failure rabbit: involvement of nitric oxide

It has been shown that peripheral chemoreceptor sensitivity is enhanced in both clinical and experimental heart failure (HF) and that impairment of nitric oxide (NO) production contributes to this enhancement. In order to understand the cellular mechanisms associated with the alterations of chemoreceptor function and the actions of NO in the carotid body (CB), we compared the outward K+ currents (IK) of glomus cells in sham rabbits with that in HF rabbits and monitored the effects of NO on these currents. Ik was measured in glomus cells using conventional and perforated whole-cell configurations. IK was attenuated in glomus cells of HF rabbits, and their resting membrane potentials (-34.7 +/- 1.0 mV) were depolarized as compared with those in sham rabbits (-47.2 +/- 1.9 mV). The selective Ca(2+)-dependent K+ channel (KCa) blocker iberiotoxin (IbTx, 100 nm) reduced IK in glomus cells from sham rabbits, but had no effect on IK from HF rabbits. In perforated whole-cell mode, the NO donor SNAP (100 microm) increased IK in glomus cells from HF rabbits to a greater extent than that in sham rabbits (P < 0.01), and IbTx inhibited the effects of SNAP. However, in conventional whole-cell mode, SNAP had no effect. N omega-nitro-L-arginine (L-NNA, NO synthase inhibitor) decreased Ik in sham rabbits but not in HF rabbits. The guanylate cyclase inhibitor 1H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one (ODQ) inhibited the effect of SNAP on Ik. These results demonstrate that IK is reduced in CB glomus cells from HF rabbits. This effect is due mainly to the suppression of KCa channel activity caused by decreased availability of NO. In addition, intracellular cGMP is necessary for the KCa channel modulation by NO.

Acetylcholine release from the carotid body by hypoxia: evidence for the involvement of autoinhibitory receptors

The purpose of the present study was to investigate whether hypoxia influences acetylcholine (ACh) release from the rabbit carotid body and, if so, to determine the mechanism(s) associated with this response. ACh is expressed in the rabbit carotid body (5.6 +/- 1.3 pmol/carotid body) as evidenced by electrochemical analysis. Immunocytochemical analysis of the primary cultures of the carotid body with antibody specific to ACh further showed that ACh-like immunoreactivity is localized to many glomus cells. The effect of hypoxia on ACh release was examined in ex vivo carotid bodies harvested from anesthetized rabbits. The basal release of ACh during normoxia ( approximately 150 Torr) averaged 5.9 +/- 0.5 fmol.min-1.carotid body-1. Lowering the Po2 to 90 and 20 Torr progressively decreased ACh release by approximately 15 and approximately 68%, respectively. ACh release returned to the basal value on reoxygenation. Simultaneous monitoring of dopamine showed a sixfold increase in dopamine release during hypoxia. Hypercapnia (21% O2 + 10% CO2) as well as high K+ (100 mM) facilitated ACh release from the carotid body, suggesting that hypoxia-induced inhibition of ACh release is not due to deterioration of the carotid body. Hypoxia had no significant effect on acetylcholinesterase activity in the medium, implying that increased hydrolysis of ACh does not account for hypoxia-induced inhibition of ACh release. In the presence of either atropine (10 microM) or domperidone (10 microM), hypoxia stimulated ACh release. These results demonstrate that glomus cells of the rabbit carotid body express ACh and that hypoxia overall inhibits ACh release via activation of muscarinic and dopaminergic autoinhibitory receptors in the carotid body.

CO(2) and pH independently modulate L-type Ca(2+) current in rabbit carotid body glomus cells

The carotid bodies respond to changes in arterial O(2), CO(2), and pH, and Ca(2+) influx via voltage-gated Ca(2+) channels is an important step in the chemoreception process. The objectives of the present study were as follows: 1) to determine whether hypercapnia modulates Ca(2+) current in glomus cells, and if so, to determine if this modulation is secondary to changes in pH; 2) to examine the mechanism of CO(2) modulation of the Ca(2+) current; and 3) to determine whether the effects of hypercapnia and hypoxia on Ca(2+) channel activity in glomus cells are synergistic. The effects of CO(2) on Ca(2+) current were monitored in glomus cells isolated from rabbit carotid bodies using both perforated and conventional patch-clamp techniques. Raising CO(2) in the extracellular solution from 5 to 10% (hypercapnia) reversibly augmented the whole-cell Ca(2+) current. This augmentation was rapid and increased the whole-cell Ca(2+) current similarly in both the perforated and the conventional patch configurations by 16 +/- 2% (n = 5) and 15 +/- 1% (n = 32), respectively. The following observations suggest that the effects of CO(2) are not secondary to changes in pH: 1) isohydric hypercapnia (pH maintained at 7.4) augmented the Ca(2+) current by 24 +/- 2% (n = 6); 2) decreasing the pH of the extra- or intracellular solutions decreased the Ca(2+) current by 43 +/- 4% (n = 8) and 13 +/- 1% (n = 5), respectively; and 3) hypercapnia did not shift the half-maximal activation voltage (V(1/2)), whereas intracellular and extracellular acidosis alone caused shifts in V(1/2). Furthermore, 100 nM of a membrane-permeable protein kinase A inhibitor prevented the augmentation by CO(2), and 500 microM 8-Br-cAMP mimicked the effect of CO(2) by augmenting the Ca(2+) current by 10 +/- 2% (n = 6). Also, cyclic AMP levels in carotid bodies increased from 1.98 +/- 0.6 to 9.0 +/- 2 pmol/microg protein in response to hypercapnia. In contrast, decreasing pH in the nominal absence of CO(2) did not affect cAMP levels in rabbit carotid bodies. Further, nisoldipine, but not omega-conotoxin MVIIC, prevented augmentation of the Ca(2+) current by CO(2). In addition, when combined, hypercapnia and hypoxia augmented the Ca(2+) current by 26 +/- 4% (n = 7), which is greater than either stimulus alone, suggesting the effects are additive. Taken together, these results indicate that L-type Ca(2+) current is augmented by hypercapnia. The effect of CO(2) is not secondary to changes in pH and seems to be mediated by a protein kinase A-dependent mechanism. Furthermore, hypercapnia and hypoxia act additively in stimulating Ca(2+) current in glomus cells.

Release of substance P by low oxygen in the rabbit carotid body: evidence for the involvement of calcium channels

Carotid bodies from diverse species contain substance P (SP), an 11-residue peptide that belongs to the tachykinin peptide family. Previous studies indicated that SP is excitatory to the carotid body and is associated with sensory response to hypoxia. However, release of SP from the carotid body during hypoxia has not been documented. In the present study, we determined whether hypoxia releases SP from the carotid body and further characterized the mechanism(s) associated with SP release by low oxygen. The release of SP from superfused rabbit carotid body was determined by an enzyme immunoassay (EIA). SP-like immunoreactivity was localized to many glomus cells and nerve fibers and the concentration of SP in the rabbit carotid body was 1.5+/-0.1 ng/mg protein. For release studies, carotid bodies (n=56) were superfused with a modified Tyrode medium containing Hepes buffer, pH 7.4, saturated with either room air (normoxia) or hypoxic gas mixtures. The basal release of SP during normoxia was 51.0+/-1.5 fmol/min per mg protein. Hypoxia increased SP release from the carotid body and the magnitude of release is dependent on the severity of hypoxic stimulus. Moderate hypoxia (pO2, 79+/-4 mmHg) stimulated SP release by approximately 50%, whereas SP release during severe hypoxia (pO2, 11+/-6 mmHg) was 2-fold higher than the normoxic control. A similar pattern of SP release was also observed when superfusion medium containing CO2-HCO3 buffer, pH 7.4, was used for release studies. To examine the mechanism(s) associated with hypoxia-induced SP release from the carotid body, moderate level of hypoxia (12% O2+N2) was used. Omission of calcium in the superfusion medium markedly attenuated hypoxia-induced SP release (>95%), whereas the basal release of SP was unaffected. Cd2+ (100 microM), a voltage-dependent Ca2+ channel blocker, abolished hypoxia-induced SP release. About 85% of SP release by hypoxia was inhibited by omega-conotoxin GVIA (1 microM), an N-type Ca2+ channel blocker, whereas nitrendipine (1.5 microM), an inhibitor of L-type Ca2+ channel partially attenuated ( approximately 65%) hypoxia-induced SP release. By contrast, omega-agatoxin TK (50 nM), a P/Q-type Ca2+ channel inhibitor, had no significant effect (P>0.05, n=6). These results suggest that SP is released from the rabbit carotid body by hypoxia that depends on the severity of the hypoxic stimulus. Further, SP release by hypoxia is a calcium-dependent process and is primarily mediated by N- and L-type Ca2+ channels.

Augmentation of calcium current by hypoxia in carotid body glomus cells

Several lines of evidence indicate that transduction of the hypoxic stimulus at the carotid body involves an increase in cytosolic Ca2+ ([Ca2+]i) via activation of voltage-gated Ca2+ channels in the glomus cells. However, reported responses to hypoxia include either no effect on or inhibition of Ca2+ current in glomus cells. The apparent discrepancy between the effects of hypoxia on [Ca2+]i and Ca2+ channel activity prompted us to re-examine the effects of low oxygen on Ca2+ currents in glomus cells. Experiments were performed on freshly dissociated glomus cells from rabbit carotid bodies. Ca2+ channel activity was monitored using the whole-cell configuration of the patch clamp technique with Ba2+ as the charge carrier. Hypoxia (pO2 = 40 mmHg) augmented the Ca2+ current by 24% (at 0 mV). This augmentation was seen in a CO2/HCO3- but not in a HEPES buffered extracellular solution. However, when the extracellular pH (pHo) of a HEPES buffered solution is lowered from 7.4 to 7.0, then the Ca2+ current in glomus cells is augmented by hypoxia by 20%. Nisoldipine, an L-type Ca2+ channel blocker (2 microM), prevented augmentation of the Ca2+ current by hypoxia. On the other hand, an N- and P-type Ca2+ channel blocker (2 microM omega-conotoxin MVIIC) did not prevent the augmentation of the Ca2+ current by hypoxia. Protein kinase C (PKC) inhibitors, staurosporine (100 nM) and bisindolylmaleimide (2 microM), prevented augmentation by hypoxia. Okadaic acid (100 nM), an inhibitor of serine/threonine phosphatases also prevented augmentation of Ca2+ current by hypoxia; whereas, norokadaone, an inactive analog of okadaic acid, had no effect. These results suggest that hypoxia augments Ca2+ current through L-type Ca2+ channels via a PKC and/or phosphatase-sensitive pathways.

Chemosensing at the carotid body. Involvement of a HERG-like potassium current in glomus cells

Currently, it is not clear what type of K+ channel(s) is active at the resting membrane potential (RMP) in glomus cells of the carotid body (CB). HERG channels produce currents that are known to contribute to the RMP in other neuronal cells. The goal of the present study was to determine whether CB glomus cells express HERG-like (HL) K+ current, and if so, to determine whether HL currents regulate the RMP. With high [K+]o, depolarizing voltage steps from -85 mV revealed a slowly deactivating inward tail current indicative of HL K+ current in whole-cell, voltage clamped glomus cells. The HL currents were blocked by dofetilide (DOF) in a concentration-dependent manner (IC50 = 13 nM) and high concentrations (1 and 10 mM) of Ba2+. The steady-state activation properties of the HL current (Vh = -45 mV) suggest that it is active at the RMP in glomus cells. Whole-cell, current clamped glomus cells exhibited a RMP of -48 mV. 150 nM DOF caused a significant (14 mV) depolarizing shift in the RMP. In isolated glomus cells, [Ca2+]i increased in response to DOF (1 microM). In an in-vitro CB preparation, DOF increased basal sensory discharge in a concentration-dependent manner and significantly attenuated the sensory response to hypoxia. These results suggest that the HERG-like current is responsible for controlling the RMP in glomus cells of the rabbit CB, and that it is involved in the chemosensory response to hypoxia of the CB.

Cellular mechanisms of oxygen sensing at the carotid body: heme proteins and ion channels

The purpose of this article is to highlight some recent concepts on oxygen sensing mechanisms at the carotid body chemoreceptors. Most available evidence suggests that glomus (type I) cells are the initial site of transduction and they release transmitters in response to hypoxia, which in turn depolarize the nearby afferent nerve ending, leading to an increase in sensory discharge. Two main hypotheses have been advanced to explain the initiation of the transduction process that triggers transmitter release. One hypothesis assumes that a biochemical event associated with a heme protein triggers the transduction cascade. Supporting this idea it has been shown that hypoxia affects mitochondrial cytochromes. In addition, there is a body of evidence implicating non-mitochondrial enzymes such as NADPH oxidases, NO synthases and heme oxygenases located in glomus cells. These proteins could contribute to transduction via generation of reactive oxygen species, nitric oxide and/or carbon monoxide. The other hypothesis suggests that a K(+) channel protein is the oxygen sensor and inhibition of this channel and the ensuing depolarization is the initial event in transduction. Several oxygen sensitive K(+) channels have been identified. However, their roles in initiation of the transduction cascade and/or cell excitability are unclear. In addition, recent studies indicate that molecular oxygen and a variety of neurotransmitters may also modulate Ca(2+) channels. Most importantly, it is possible that the carotid body response to oxygen requires multiple sensors, and they work together to shape the overall sensory response of the carotid body over a wide range of arterial oxygen tensions.

Augmentation of L-type calcium current by hypoxia in rabbit carotid body glomus cells: evidence for a PKC-sensitive pathway

Previous studies have suggested that voltage-gated Ca(2+) influx in glomus cells plays a critical role in sensory transduction at the carotid body chemoreceptors. The purpose of the present study was to determine the effects of hypoxia on the Ca(2+) current in glomus cells and to elucidate the underlying mechanism(s). Experiments were performed on freshly dissociated glomus cells from rabbit carotid bodies. Ca(2+) current was monitored using the whole cell configuration of the patch-clamp technique, with Ba(2+) as the charge carrier. Hypoxia (pO(2) = 40 mmHg) augmented the Ca(2+) current by 24 +/- 3% (n = 42, at 0 mV) in a voltage-independent manner. This effect was seen in a CO(2)/HCO(3)(-)-, but not in a HEPES-buffered extracellular solution at pH 7.4 (n = 6). When the pH of a HEPES-buffered extracellular solution was lowered from 7.4 to 7. 0, hypoxia augmented the Ca(2+) current by 20 +/- 5% (n = 4, at 0 mV). Nisoldipine, an L-type Ca(2+) channel blocker (2 microM, n = 6), prevented, whereas, omega-conotoxin MVIIC (2 microM, n = 6), an inhibitor of N and P/Q type Ca(2+) channels, did not prevent augmentation of the Ca(2+) current by hypoxia, implying that low oxygen affects L-type Ca(2+) channels in glomus cells. Protein kinase C (PKC) inhibitors, staurosporine (100 nM, n = 6) and bisindolylmaleimide (2 microM, n = 8, at 0 mV), prevented, whereas, a protein kinase A inhibitor (4 nM PKAi, n = 10) did not prevent the hypoxia-induced increase of the Ca(2+) current. Phorbol 12-myristate 13-acetate (PMA, 100 nM), a PKC activator, augmented the Ca(2+) current by 20 +/- 3% (n = 8, at 0 mV). In glomus cells treated with PMA overnight (100 nM), hypoxia did not augment the Ca(2+) current (-3 + 4%, n = 5, at 0 mV). Immunocytochemical analysis revealed PKCdelta-like immunoreactivity in the cytosol of the glomus cells. Following hypoxia (6% O(2) for 5 min), PKCdelta-like immunoreactivity translocated to the plasma membrane in 87 +/- 3% of the cells, indicating PKC activation. These results demonstrate that hypoxia augments Ca(2+) current through L-type Ca(2+) channels via a PKC-sensitive mechanism.

Cellular mechanisms involved in rabbit carotid body excitation elicited by endothelin peptides

The present study evaluated the effects of endothelin (ET) peptides on carotid sinus nerve (CSN) activity, catecholamine (CA) release, and second messenger signaling pathways in rabbit carotid bodies superfused in vitro, and in dissociated chemosensory type I cells. ET-1 (1.0 microM) and ET-3 (1.0 microM) did not alter basal CSN activity and CA release, but they potentiated nerve activity (P<0. 05) and CA release (P<0.05) evoked by hypoxia. Under basal conditions, ET-1 and ET-3 (1.0 microM each) elevated tissue cyclic AMP (cAMP) levels nearly 3-fold (P<0.001, ET-1; P<0.05, ET-3) and inositol phosphate (IP(n)) levels nearly 4-fold (P<0.01, ET-1). Hypoxia evoked an increase in carotid body cAMP, and this response was also potentiated in the presence of 1.0 microM ET-1 (P<0.01) or 1.0 microM ET-3 (P<0.001). Patch-clamp studies of isolated type I cells showed that 100 nM ET-1 elevated the peak amplitude of voltage-sensitive (L-type) Ca(2+)-currents by an average of 37.6% (P<0.001). Fluorescent Ca(2+)-imaging revealed that 100 nM ET-1 did not alter [Ca(2+)](i) under basal conditions, but that [Ca(2+)](i)-responses evoked by hypoxia were potentiated by 87% (P<0. 01). Our data indicate that ET augments chemoreceptor responses by activating second messenger signaling pathways which promote the phosphorylation of Ca(2+)-channel protein, thereby enhancing stimulus-evoked intracellular Ca(2+) levels.

Oxygen sensing by the carotid body chemoreceptors

Carotid bodies are sensory organs that detect changes in arterial blood oxygen, and the ensuing reflexes are critical for maintaining homeostasis during hypoxemia. During the past decade, tremendous progress has been made toward understanding the cellular mechanisms underlying oxygen sensing at the carotid body. The purpose of this minireview is to highlight some recent concepts on sensory transduction and transmission at the carotid body. A bulk of evidence suggests that glomus (type I) cells are the initial site of transduction and that they release transmitters in response to hypoxia, which causes depolarization of nearby afferent nerve endings, leading to an increase in sensory discharge. There are two main hypotheses to explain the transduction process that triggers transmitter release. One hypothesis assumes that a biochemical event associated with a heme protein triggers the transduction cascade. The other hypothesis suggests that a K(+) channel protein is the oxygen sensor and that inhibition of this channel by hypoxia leading to depolarization is a seminal event in transduction. Although there is body of evidence supporting and questioning each of these, this review will try to point out that the truth lies somewhere in an interrelation between the two. Several transmitters have been identified in glomus cells, and they are released in response to hypoxia. However, their precise roles in sensory transmission remain uncertain. It is hoped that future studies involving transgenic animals with targeted disruption of genes encoding transmitters and their receptors may resolve some of the key issues surrounding the sensory transmission at the carotid body. Further studies are necessary to identify whether a single sensor or multiple oxygen sensors are needed for the transduction process.

L-type Ca(2+) channel activation regulates induction of c-fos transcription by hypoxia

In the present study we examined the intracellular pathways that link hypoxia to activation of c-fos gene expression. Experiments were performed on rat pheocromocytoma-12 (PC-12) cells. c-fos mRNA and promoter activities were analyzed by RT-PCR and reporter gene assays, respectively. BAPTA, a Ca(2+) chelator, inhibited c-fos mRNA and promoter activation by hypoxia. Nitrendipine, an L-type Ca(2+)-channel blocker, abolished, whereas BAY K 8644, an L-type channel agonist, enhanced c-fos activation by hypoxia. Ca(2+) currents were augmented reversibly by hypoxia, suggesting that Ca(2+) influx mediated by L-type Ca(2+) channels is essential for c-fos activation by hypoxia. We next determined downstream pathways activated by intracellular Ca(2+) concentration. Immunoblot analysis revealed Ca(2+)/calmodulin-dependent kinase II (CaMKII) protein in PC-12 cells and revealed that hypoxia increased the enzyme activity. KN-93, a CaMK inhibitor, blocked CaMKII activation and c-fos promoter stimulation by hypoxia. Ectopic expression of an active mutant of CaMKII (pCaMKII290) stimulated c-fos promoter activity under normoxia. Hypoxia increased phosphorylation of CREB at the serine residue 133 (Ser-133), and KN-93 attenuated this effect. Point mutations at the Ca(2+)/cAMP-responsive cis-element (Ca/CRE) attenuated, whereas point mutations in the serum-responsive cis-element (SRE) abolished transcriptional activation of c-fos by hypoxia. These results demonstrate that c-fos activation by hypoxia involves CaMK activation and CREB phosphorylation at Ser-133 and requires Ca/CRE and SRE. These observations demonstrate that Ca(2+)-dependent signaling pathways play a crucial role in induction of c-fos gene expression, which may underlie long-term adaptive responses to hypoxia.

HERG-Like potassium current regulates the resting membrane potential in glomus cells of the rabbit carotid body

Direct evidence for a specific K(+) channel underlying the resting membrane potential in glomus cells of the carotid body has been absent. The product of the human ether-a-go-go-related gene (HERG) produces inward rectifier currents that are known to contribute to the resting membrane potential in other neuronal cells. The goal of the present study was to determine whether carotid body glomus cells express HERG-like K(+) current, and if so, to determine whether a HERG-like current regulates the resting membrane potential. Freshly dissociated rabbit glomus cells under whole cell voltage clamp exhibited slowly decaying outward currents that activated 20-30 mV positive to the resting membrane potential. Raising extracellular K(+) revealed a slowly deactivating inward tail current indicative of HERG-like K(+) current. HERG-like currents were not found in cells resembling type II cells. The HERG-like current was blocked by dofetilide (DOF) in a concentration-dependent manner (IC(50) = 13 +/- 4 nM, mean +/- SE) and high concentrations of Ba(2+) (1 and 10 mM). The biophysical and pharmacological characteristics of this inward tail current suggest that it is conducted by a HERG-like channel. The steady-state activation properties of the HERG-like current (V(h) = -44 +/- 2 mV) suggest that it is active at the resting membrane potential in glomus cells. In whole cell, current-clamped glomus cells (average resting membrane potential, - 48 +/- 4 mV), DOF, but not tetraethylammonium, caused a significant (13 mV) depolarizing shift in the resting membrane potential. Using fluorescence imaging, DOF increased [Ca(2+)](i) in isolated glomus cells. In an in-vitro carotid body preparation, DOF increased basal sensory discharge in the carotid sinus nerve in a concentration-dependent manner. These results demonstrate that glomus cells express a HERG-like current that is active at, and responsible for controlling the resting membrane potential.

High-threshold calcium channel activity in rat hippocampal neurones during hypoxia

Whole-cell patch clamp recordings in combination with direct control and measurements of O2 tension (pO2) in bath solution were used to determine the sensitivity of Ca2+ channels of cultured hippocampal neurones to hypoxia in glucose free solution. In all tested neurones, a lowering of pO2 to 4/50 mmHg did not induce changes either in magnitude, kinetics or voltage-current relations of total Ca2+ currents, which composed mainly from two types, L-type (64%) and N-type (31%) components. Hypoxia only induced a delay of Ca2+ current run-down about 27.5% and 39% at 50 and 4 mmHg pO2 respectively that presumably depended on changes in cytoplasmic channel-modulatory metabolites. The obtained results demonstrate that Ca2+ channel molecules in cultured hippocampal neurones are themselves insensitive to short-lasting (10-20 min) oxygen and glucose deprivation, and that they are not a principal target for hypoxic influences on hippocampal function.

Chemosensory response to high pCO is blocked by cadmium, a voltage-sensitive calcium channel blocker

In the dark, during normocapnic (pCO2=35 Torr, pHo=7.4) normoxia (pO2=100 Torr), high pCO (>300 Torr) causes Ca2+-dependent photolabile excitation of chemosensors in the carotid body (CB). We previously proposed that the source of this Ca2+ was the [Ca2+]i stores because CO would react only intracellularly. However, influx of extracellular Ca2+ was not excluded. Now, using perfused rat CB (n=6) in the presence of normal extracellular [Ca2+] we show that chemosensory response to CO (pCO approximately 550 Torr) in normoxic (pO2 approximately 100 Torr) normocapnia (pCO2 approximately 30 Torr, pH approximately 7.4) is completely but reversibly inhibited by Cd2+ (200 microM), a voltage-gated Ca2+ channel blocker. Thus, extracellular Ca2+ is necessary for excitatory chemosensory response to high pCO. Cd2+ block occurs in spite of an enhanced [Ca2+]i rise. This shows that Ca2+ rise alone is unable to release neurotransmitter and to elicit a chemosensory response. Therefore, as a corollary, we conclude that Cd2+ blocks the Ca2+ flux that is needed for vesicle-membrane fusion for neurotransmitter release and neural discharge.

Effect of the removal of extracellular Ca2+ on the response of cytosolic concentrations of Ca2+ to ouabain in carotid body glomus cells of adult rabbits

Effect of the removal of extracellular Ca2+ on the response of cytosolic concentrations of Ca2+ ([Ca2+]i) to ouabain, an Na+/K+ exchanger antagonist, was examined in clusters of cultured carotid body glomus cells of adult rabbits using fura-2AM and microfluorometry. Application of ouabain (10 mM) induced a sustained increase in [Ca2+]i (mean+/-S.E.M.; 38+/-5% increase, n=16) in 55% of tested cells (n=29). The ouabain-induced [Ca2+]i increase was abolished by the removal of extracellular Na+. D600 (50 microM), an L-type voltage-gated Ca2+ channel antagonist, inhibited the [Ca2+]i increase by 57+/-7% (n=4). Removal of extracellular Ca2+ eliminated the [Ca2+]i increase, but subsequent washing out of ouabain in Ca2+-free solution produced a rise in [Ca2+]i (62+/-8% increase, n=6, P<0.05), referred to as a [Ca2+]i rise after Ca2+-free/ouabain. The magnitude of the [Ca2+]i rise was larger than that of ouabain-induced [Ca2+]i increase. D600 (5 microM) inhibited the [Ca2+]i rise after Ca2+-free/ouabain by 83+/-10% (n=4). These results suggest that ouabain-induced [Ca2+]i increase was due to Ca2+ entry involving L-type Ca2+ channels which could be activated by cytosolic Na+ accumulation. Ca2+ removal might modify the [Ca2+]i response, resulting in the occurrence of a rise in [Ca2+]i after Ca2+-free/ouabain which mostly involved L-type Ca2+ channels.

Nitric oxide inhibits L-type Ca2+ current in glomus cells of the rabbit carotid body via a cGMP-independent mechanism

Previous studies have shown that nitric oxide (NO) inhibits carotid body sensory activity. To begin to understand the cellular mechanisms associated with the actions of NO in the carotid body, we monitored the effects of NO donors on the macroscopic Ca2+ current in glomus cells isolated from rabbit carotid bodies. Experiments were performed on freshly dissociated glomus cells from adult rabbit carotid bodies using the whole cell configuration of the patch-clamp technique. The NO donors sodium nitroprusside (SNP; 600 microM, n = 7) and spermine nitric oxide (SNO; 100 microM, n = 7) inhibited the Ca2+ current in glomus cells in a voltage-independent manner. These effects of NO donors were rapid in onset and peaked within 1 or 2 min. In contrast, the outward K+ current was unaffected by SNP (600 microM, n = 6), indicating that the inhibition by SNP was not a nonspecific membrane effect. 2-(4-carboxyphenyl)-4,4,5, 5-tetramethyl-imidazoline-1-oxyl-3-oxide (carboxy-PTIO; 500 microM), an NO scavenger, prevented inhibition of the Ca2+ current by SNP (n = 7), whereas neither superoxide dismutase (SOD; 2,000 U/ml, n = 4), a superoxide scavenger, nor sodium hydrosulfite (SHS; 1 mM, n = 7), a reducing agent, prevented inhibition of the Ca2+ current by SNP. However, SNP inhibition of the Ca2+ current was reversible in the presence of either SOD or SHS. These results suggest that NO itself inhibits Ca2+ current in a reversible manner and that subsequent formation of peroxynitrites results in irreversible inhibition. SNP inhibition of the Ca2+ current was not affected by 30 microM LY 83, 583 (n = 7) nor was it mimicked by 600 microM 8-bromoguanosine 3':5'-cyclic monophosphate (8-Br-cGMP; n = 6), suggesting that the effects of NO on the Ca2+ current are mediated, in part, via a cGMP-independent mechanism. N-ethylmaleimide (NEM; 2.5 mM, n = 6) prevented the inhibition of the Ca2+ current by SNP, indicating that SNP is acting via a modification of sulfhydryl groups on Ca2+ channel proteins. Norepinephrine (NE; 10 microM) further inhibited the Ca2+ current in the presence of NEM (n = 7), implying that NEM did not nonspecifically eliminate Ca2+ current modulation. Nisoldipine, an L-type Ca2+ channel blocker (2 microM, n = 6), prevented the inhibition of Ca2+ current by SNP, whereas omega-conotoxin GVIA, an N-type Ca2+ channel blocker (1 microM, n = 9), did not prevent the inhibition of Ca2+ current by SNP. These results demonstrate that NO inhibits L-type Ca2+ channels in adult rabbit glomus cells, in part, due to a modification of calcium channel proteins. The inhibition might provide one plausible mechanism for efferent inhibition of carotid body activity by NO.

Calcium currents of rhythmic neurons recorded in the isolated respiratory network of neonatal mice

To obtain a quantitative characterization of voltage-activated calcium currents in respiratory neurons, we performed voltage-clamp recordings in the transverse brainstem slice of mice from neurons located within the ventral respiratory group. It is assumed that this medullary region contains the neuronal network responsible for generating the respiratory rhythm. This study represents one of the first attempts to analyze quantitatively the currents in respiratory neurons. The inward calcium currents of VRG neurons consisted of two components: a high voltage-activated (HVA) and a low voltage-activated (LVA) calcium current. The activation threshold of the HVA current was at -40 mV. It was fully activated (peak voltage) between -10 and 0 mV. The half-maximal activation (V50) was at -27. 29 mV +/- 1.15 (n = 24). The HVA current was inactivated completely at a holding potential of -35 mV and fully deinactivated at a holding potential of -65 mV (V50, -52.26 mV +/- 0.27; n = 18). The threshold for the activation of the LVA current was at -65 mV. This current had its peak voltage between -50 and -40 mV (mean, V50 = -59. 15 mV +/- 0.21; n = 15). The LVA current was inactivated completely at a holding potential of -65 mV and deinactivated fully at a holding potential of -95 mV (mean, V50 = -82.40 mV +/- 0.32; n = 38). These properties are consistent with other studies suggesting that the LVA current is a T-type current. The properties of these inward currents are discussed with respect to their role in generating Ca2+ potentials that may contribute to the generation of the mammalian respiratory rhythm.

Effects of hypoxia induced by Na2S2O4 on intracellular calcium and resting potential of mouse glomus cells

Isolated and cultured glomus cells, obtained from mouse carotid bodies, were superfused with Ham's F-12 equilibrated with air (mean PO2, 119 Torr; altitude 1350 m). [Ca2+]o was 3.0 mM. In one experimental series, dual cell penetrations with microelectrodes measured intracellular calcium ([Ca2+]i) and the resting potential (Em). In another series, [Ca2+]i was measured with Indo-1/AM, dissolved in DMSO. Normoxic cells had a mean Em of -42.4 mV and [Ca2+]i was about 80 nM (measured with both methods). The calculated calcium equilibrium potential (ECa) was 137+/-0.74 mV. Hypoxia, induced by Na2S2O4 1 mM, reduced pO2 to 10-14 Torr. This effect was accompanied by cell depolarization to -19.1 mV. Hypoxia increased [Ca2+]i to 231 nM when detected with Ca-sensitive microelectrodes, but only to 130.2 nM when measured with Indo-1/AM. Calcium increases were preceded by decreases in [Ca2+]i, which also were more pronounced with microelectrode measurements. CoCl2 1 mM blocked the hypoxic [Ca2+]i increase and exaggerated the decreases in [Ca2+]i. Correlations between DeltaEm and Delta[Ca2+]i during hypoxia were significant (p<0.05) in 19% of the cells. But, in 29% of them significance was at the p<0.1 level. In the rest (52%), there was no correlation between these parameters. Thus, voltage-gated calcium channels are rare in mouse glomus cells. Their activation by depolarization cannot explain the two to threefold increase in [Ca2+]i seen during hypoxia. More likely, [Ca2+]i increase may be due to hypoxic inactivation of a Ca-Mg ATPase transport system across the cell membrane. The blunting of hypoxic [Ca2+]i increase, seen in Indo-1/AM experiments, is probably due to its solvent (DMSO), which also depresses hypoxic cell depolarization.

Adenosine inhibits L-type Ca2+ current and catecholamine release in the rabbit carotid body chemoreceptor cells

In an in vitro preparation of the intact carotid body (CB) of the rabbit, adenosine (100 microM) inhibited hypoxia-induced catecholamine release by 25%. The specific A1 antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 1 microM) prevented the inhibition and increased the response to hypoxia further. In isolated chemoreceptor cells from the same species, adenosine inhibited voltage-dependent Ca2+ currents by 29% at 1 microM (concentration producing half-maximal inhibition, IC50 = 50 nM). This inhibition was mimicked by R(-)N6-(2-phenylisopropyl)-adenosine and 2-chloroadenosine (1 microM), two purinergic agonists poorly active at the intracellular ('P') site, and persisted in the presence of dipyridamole (a blocker of adenosine uptake; 1 microM) and was fully inhibited by 8-phenyltheophylline (10 microM). The A1 antagonists DPCPX (10 microM) and 8-cyclopentyl-1,3-dimethylxantine (0.1 microM) inhibited the effect of adenosine by 93% (IC50 = 0.14 microM) and 59%, respectively. The inhibition of the Ca2+ current (I(Ca)) was reduced by nisoldipine (an L-type Ca2+ channel antagonist) by nearly 50%, and was unaltered by omega-conotoxin GVIA, a blocker of N-type Ca2+ channels. Adenosine did not affect the voltage-dependent Na+ current (I(Na)) or K+ current (I(K)). We conclude that adenosine A1 receptors are located in chemoreceptor cells and mediate the inhibition of L-type Ca2+ channels and thereby the release of catecholamines produced by hypoxia. The data also indicate that endogenous adenosine acts as a physiological negative modulator of the chemoreceptor cell function. The previously reported excitatory action of adenosine on the activity of the sensory nerve of the CB is discussed in terms of a balance between the inhibition mediated by A1 receptors and the excitation mediated by A2 receptors.