Properties of ionic currents from isolated adult rat carotid body chemoreceptor cells: effect of hypoxia

Mechanisms of Chemosensory Transduction in the Carotid Body

The mammalian carotid body (CB) is a polymodal chemoreceptor, which is activated by blood-borne stimuli, most notably hypoxia, hypercapnia and acidosis, thus ensuring an appropriate cellular response to changes in physical and chemical parameters of the blood. The glomus cells are considered the CB chemosensory cells and the initial site of chemoreceptor transduction. However, the molecular mechanisms by which they detect changes in blood chemical levels and how these changes lead to transmitter release are not yet well understood. Chemotransduction mechanisms are by far best described for oxygen and acid/carbon dioxide sensing. A few testable hypotheses have been postulated including a direct interaction of oxygen with ion channels in the glomus cells (membrane hypothesis), an indirect interface by a reversible ligand like a heme (metabolic hypothesis), or even a functional interaction between putative oxygen sensors (chemosome hypothesis) or the interaction of lactate with a highly expressed in the CB atypical olfactory receptor, Olfr78, (endocrine model). It is also suggested that sensory transduction in the CB is uniquely dependent on the actions and interactions of gaseous transmitters. Apparently, oxygen sensing does not utilize a single mechanism, and later observations have given strong support to a unified membrane model of chemotransduction.

Neurochemical Anatomy of the Mammalian Carotid Body

Carotid body (CB) glomus cells in most mammals, including humans, contain a broad diversity of classical neurotransmitters, neuropeptides and gaseous signaling molecules as well as their cognate receptors. Among them, acetylcholine, adenosine triphosphate and dopamine have been proposed to be the main excitatory transmitters in the mammalian CB, although subsequently dopamine has been considered an inhibitory neuromodulator in almost all mammalian species except the rabbit. In addition, co-existence of biogenic amines and neuropeptides has been reported in the glomus cells, thus suggesting that they store and release more than one transmitter in response to natural stimuli. Furthermore, certain metabolic and transmitter-degrading enzymes are involved in the chemotransduction and chemotransmission in various mammals. However, the presence of the corresponding biosynthetic enzyme for some transmitter candidates has not been confirmed, and neuroactive substances like serotonin, gamma-aminobutyric acid and adenosine, neuropeptides including opioids, substance P and endothelin, and gaseous molecules such as nitric oxide have been shown to modulate the chemosensory process through direct actions on glomus cells and/or by producing tonic effects on CB blood vessels. It is likely that the fine balance between excitatory and inhibitory transmitters and their complex interactions might play a more important than suggested role in CB plasticity.

Activation of voltage-dependent K+ channels strongly limits hypoxia-induced elevation of [Ca2+ ]i in rat carotid body glomus cells

KEY POINTS

We studied the role of the large-conductance Ca²⁺ -activated K⁺ channel (BK) and voltage-dependent K⁺ channels (Kv) on [Ca²⁺ ]_i responses to a wide range of hypoxia at different resting cell membrane potential (E_m ). BK/Kv were mostly closed at rest in normoxia. BK/Kv became basally active when cells were depolarized by elevated [KCl]_o (>12 mm). Regardless of whether BK/Kv were closed or basally open, hypoxia-induced elevation of [Ca²⁺ ]_i was enhanced 2- to 3-fold by inhibitors of BK/Kv. Hypoxia-induced elevation of [Ca²⁺ ]_i was enhanced ∼2-fold by an inhibitor of Kv2, a major Kv in rat glomus cells. Hypoxia did not inhibit BK in inside-out patches. Our study supports a scheme in which activation of BK/Kv strongly limits the magnitude of hypoxia-induced [Ca²⁺ ]_i rise, with Kv having a much greater effect than BK.

ABSTRACT

Large-conductance K_Ca (BK) and other voltage-dependent K⁺ channels (Kv) are highly expressed in carotid body (CB) glomus cells, but their role in hypoxia-induced excitation is still not well defined and remains controversial. We addressed this issue by studying the effects of inhibitors of BK (IBTX) and BK/Kv (TEA/4-AP) on [Ca²⁺ ]_i responses to a wide range of hypoxia at different levels of resting cell membrane potential (E_m ). IBTX and TEA/4-AP did not affect the basal [Ca²⁺ ]_i in isolated glomus cells bathed in 5 mm KCl_o , but elicited transient increases in [Ca²⁺ ]_i in cells that were moderately depolarized (11-20 mV) by elevation of [KCl]_o (12-20 mm). Thus, BK and Kv were mostly closed at rest and activated by depolarization. Four different levels of hypoxia (mild, moderate, severe, anoxia) were used to produce a wide range of [Ca²⁺ ]_i elevation (0-700 nm). IBTX did not affect the rise in [Ca²⁺ ]_i , but TEA/4-AP strongly (∼3-fold) enhanced [Ca²⁺ ]_i rise by moderate and severe levels of hypoxia. Guangxitoxin, a Kv2 blocker, inhibited the whole-cell current by ∼50%, and enhanced 2-fold the [Ca²⁺ ]_i rise elicited by moderate and severe levels of hypoxia. Anoxia did not directly affect BK, but activated BK via depolarization. Our findings do not support the view that hypoxia inhibits BK/Kv to initiate or maintain the hypoxic response. Rather, our results show that BK/Kv are activated as glomus cells depolarize in response to hypoxia, which then limits the rise in [Ca²⁺ ]_i . Inhibition of Kv may provide a mechanism to enhance the chemosensory activity of the CB and ventilation.

Characterization of ion channels and O2 sensitivity in gill neuroepithelial cells of the anoxia-tolerant goldfish (Carassius auratus)

The neuroepithelial cell (NEC) of the fish gill is an important model for O₂ sensing in vertebrates; however, a complete picture of the chemosensory mechanisms in NECs is lacking, and O₂ chemoreception in vertebrates that are tolerant to anoxia has not yet been explored. Using whole cell patch-clamp recording, we characterized four types of ion channels in NECs isolated from the anoxia-tolerant goldfish. A Ca²⁺-dependent K⁺ current (I_KCa) peaked at ~20 mV, was potentiated by increased intracellular Ca²⁺, and was reduced by 100 μM Cd²⁺ A voltage-dependent inward current in Ba²⁺ solution, with peak at 0 mV, confirmed the presence of Ca²⁺ channels. A voltage-dependent K⁺ current (I_KV) was inhibited by 20 mM tetraethylammonium and 5 mM 4-aminopyridine, revealing a background K⁺ current (I_KB) with open rectification. Mean resting membrane potential of -45.2 ± 11.6 mV did not change upon administration of hypoxia (Po₂ = 11 mmHg), nor were any of the K⁺ currents sensitive to changes in Po₂ during whole cell recording. By contrast, when the membrane and cytosol were left undisturbed during fura-2 or FM 1-43 imaging experiments, hypoxia increased intracellular Ca²⁺ concentration and initiated synaptic vesicle activity. 100 μM Cd²⁺ and 50 μM nifedipine eliminated uptake of FM 1-43. We conclude that Ca²⁺ influx via L-type Ca²⁺ channels is correlated with vesicular activity during hypoxic stimulation. In addition, we suggest that expression of I_KCa in gill NECs is species specific and, in goldfish, may contribute to an attenuated response to acute hypoxia.NEW & NOTEWORTHY This study provides the first physiological characterization of oxygen chemoreceptors from an anoxia-tolerant vertebrate. Neuroepithelial cells (NECs) from the gills of goldfish displayed L-type Ca²⁺ channels and three types of K⁺ channels, one of which was dependent upon intracellular Ca²⁺ Although membrane currents were not inhibited by hypoxia during patch-clamp recording, this study is the first to show that NECs with an undisturbed cytosol responded to hypoxia with increased intracellular Ca²⁺ and synaptic vesicle activity.

Guinea Pig Oxygen-Sensing and Carotid Body Functional Properties

Mammals have developed different mechanisms to maintain oxygen supply to cells in response to hypoxia. One of those mechanisms, the carotid body (CB) chemoreceptors, is able to detect physiological hypoxia and generate homeostatic reflex responses, mainly ventilatory and cardiovascular. It has been reported that guinea pigs, originally from the Andes, have a reduced ventilatory response to hypoxia compared to other mammals, implying that CB are not completely functional, which has been related to genetically/epigenetically determined poor hypoxia-driven CB reflex. This study was performed to check the guinea pig CB response to hypoxia compared to the well-known rat hypoxic response. These experiments have explored ventilatory parameters breathing different gases mixtures, cardiovascular responses to acute hypoxia, in vitro CB response to hypoxia and other stimuli and isolated guinea pig chemoreceptor cells properties. Our findings show that guinea pigs are hypotensive and have lower arterial pO₂ than rats, probably related to a low sympathetic tone and high hemoglobin affinity. Those characteristics could represent a higher tolerance to hypoxic environment than other rodents. We also find that although CB are hypo-functional not showing chronic hypoxia sensitization, a small percentage of isolated carotid body chemoreceptor cells contain tyrosine hydroxylase enzyme and voltage-dependent K⁺ currents and therefore can be depolarized. However hypoxia does not modify intracellular Ca²⁺ levels or catecholamine secretion. Guinea pigs are able to hyperventilate only in response to intense acute hypoxic stimulus, but hypercapnic response is similar to rats. Whether other brain areas are also activated by hypoxia in guinea pigs remains to be studied.

Functional Properties of Mitochondria in the Type-1 Cell and Their Role in Oxygen Sensing

The identity of the oxygen sensor in arterial chemoreceptors has been the subject of much speculation. One of the oldest hypotheses is that oxygen is sensed through oxidative phosphorylation. There is a wealth of data demonstrating that arterial chemoreceptors are excited by inhibitors of oxidative phosphorylation. These compounds mimic the effects of hypoxia inhibiting TASK1/3 potassium channels causing membrane depolarisation calcium influx and neurosecretion. The TASK channels of Type-I cells are also sensitive to cytosolic MgATP. The existence of a metabolic signalling pathway in Type-1 cells is thus established; the contentious issue is whether this pathway is also used for acute oxygen sensing. The main criticism is that because cytochrome oxidase has a high affinity for oxygen (P50 ≈ 0.2 mmHg) mitochondrial metabolism should be insensitive to physiological hypoxia. This argument is however predicated on the assumption that chemoreceptor mitochondria are analogous to those of other tissues. We have however obtained new evidence to support the hypothesis that type-1 cell mitochondria are not like those of other cells in that they have an unusually low affinity for oxygen (Mills E, Jobsis FF, J Neurophysiol 35(4):405-428, 1972; Duchen MR, Biscoe TJ, J Physiol 450:13-31, 1992a). Our data confirm that mitochondrial membrane potential, NADH, electron transport and cytochrome oxidase activity in the Type-1 cell are all highly sensitive to hypoxia. These observations not only provide exceptionally strong support for the metabolic hypothesis but also reveal an unknown side of mitochondrial behaviour.

TASK channels in arterial chemoreceptors and their role in oxygen and acid sensing

Arterial chemoreceptors play a vital role in cardiorespiratory control by providing the brain with information regarding blood oxygen, carbon dioxide, and pH. The main chemoreceptor, the carotid body, is composed of sensory (type 1) cells which respond to hypoxia or acidosis with a depolarising receptor potential which in turn activates voltage-gated calcium entry, neurosecretion and excitation of adjacent afferent nerves. The receptor potential is generated by inhibition of Twik-related acid-sensitive K(+) channel 1 and 3 (TASK1/TASK3) heterodimeric channels which normally maintain the cells' resting membrane potential. These channels are thought to be directly inhibited by acidosis. Oxygen sensitivity, however, probably derives from a metabolic signalling pathway. The carotid body, isolated type 1 cells, and all forms of TASK channel found in the type 1 cell, are highly sensitive to inhibitors of mitochondrial metabolism. Moreover, type1 cell TASK channels are activated by millimolar levels of MgATP. In addition to their role in the transduction of chemostimuli, type 1 cell TASK channels have also been implicated in the modulation of chemoreceptor function by a number of neurocrine/paracrine signalling molecules including adenosine, GABA, and serotonin. They may also be instrumental in mediating the depression of the acute hypoxic ventilatory response that occurs with some general anaesthetics. Modulation of TASK channel activity is therefore a key mechanism by which the excitability of chemoreceptors can be controlled. This is not only of physiological importance but may also offer a therapeutic strategy for the treatment of cardiorespiratory disorders that are associated with chemoreceptor dysfunction.

Increase in cytosolic Ca2+ produced by hypoxia and other depolarizing stimuli activates a non-selective cation channel in chemoreceptor cells of rat carotid body

The current model of O2 sensing by carotid body chemoreceptor (glomus) cells is that hypoxia inhibits the outward K(+) current and causes cell depolarization, Ca(2+) influx via voltage-dependent Ca(2+) channels and a rise in intracellular [Ca(2+)] ([Ca(2+)]i). Here we show that hypoxia (<5% O2), in addition to inhibiting the two-pore domain K(+) channels TASK-1/3 (TASK), indirectly activates an ∼20 pS channel in isolated glomus cells. The 20 pS channel was permeable to K(+), Na(+) and Cs(+) but not to Cl(-) or Ca(2+). The 20 pS channel was not sensitive to voltage. Inhibition of TASK by external acid, depolarization of glomus cells with high external KCl (20 mm) or opening of the Ca(2+) channel with FPL64176 activated the 20 pS channel when 1 mm Ca(2+) was present in the external solution. Ca(2+) (10 μm) applied to the cytosolic side of inside-out patches activated the 20 pS channel. The threshold [Ca(2+)]i for activation of the 20 pS channel in cell-attached patches was ∼200 nm. The reversal potential of the 20 pS channel was estimated to be -28 mV. Our results reveal a sequential mechanism in which hypoxia (<5% O2) first inhibits the K(+) conductance and then activates a Na(+)-permeable, non-selective cation channel via depolarization-induced rise in [Ca(2+)]i. Our results suggest that inhibition of K(+) efflux and stimulation of Na(+) influx both contribute to the depolarization of glomus cells during moderate to severe hypoxia.

Cellular properties and chemosensory responses of the human carotid body

The carotid body (CB) is the major peripheral arterial chemoreceptor in mammals that mediates the acute hyperventilatory response to hypoxia. The CB grows in response to sustained hypoxia and also participates in acclimatisation to chronic hypoxaemia. Knowledge of CB physiology at the cellular level has increased considerably in recent times thanks to studies performed on lower mammals, and rodents in particular. However, the functional characteristics of human CB cells remain practically unknown. Herein, we use tissue slices or enzymatically dispersed cells to determine the characteristics of human CB cells. The adult human CB parenchyma contains clusters of chemosensitive glomus (type I) and sustentacular (type II) cells as well as nestin-positive progenitor cells. This organ also expresses high levels of the dopaminotrophic glial cell line-derived neurotrophic factor (GDNF). We found that GDNF production and the number of progenitor and glomus cells were preserved in the CBs of human subjects of advanced age. Moreover, glomus cells exhibited voltage-dependent Na(+), Ca(2+) and K(+) currents that were qualitatively similar to those reported in lower mammals. These cells responded to hypoxia with an external Ca(2+)-dependent increase of cytosolic Ca(2+) and quantal catecholamine secretion, as reported for other mammalian species. Interestingly, human glomus cells are also responsive to hypoglycaemia and together these two stimuli can potentiate each other's effects. The chemosensory responses of glomus cells are also preserved at an advanced age. These new data on the cellular and molecular physiology of the CB pave the way for future pathophysiological studies involving this organ in humans.

Oxygen and mitochondrial inhibitors modulate both monomeric and heteromeric TASK-1 and TASK-3 channels in mouse carotid body type-1 cells

In rat arterial chemoreceptors, background potassium channels play an important role in maintaining resting membrane potential and promoting depolarization and excitation in response to hypoxia or acidosis. It has been suggested that these channels are a heterodimer of TASK-1 and TASK-3 based on their similarity to heterologously expressed TASK-1/3 fusion proteins. In this study, we sought to confirm the identity of these channels through germline ablation of Task-1 (Kcnk3) and Task-3 (Kcnk9) in mice. Background K-channels were abundant in carotid body type-1 cells from wild-type mice and comparable to those previously described in rat type-1 cells with a main conductance state of 33 pS. This channel was absent from both Task-1(-/-) and Task-3(-/-) cells. In its place we observed a larger (38 pS) K(+)-channel in Task-1(-/-) cells and a smaller (18 pS) K(+)-channel in Task-3(-/-) cells. None of these channels were observed in Task-1(-/-)/Task-3(-/-) double knock-out mice. We therefore conclude that the predominant background K-channel in wild-type mice is a TASK-1/TASK-3 heterodimer, whereas that in Task-1(-/-) mice is TASK-3 and, conversely, that in Task-3(-/-) mice is TASK-1. All three forms of TASK channel in type-1 cells were inhibited by hypoxia, cyanide and the uncoupler FCCP, but the greatest sensitivity was seen in TASK-1 and TASK-1/TASK-3 channels. In summary, the background K-channel in type-1 cells is predominantly a TASK-1/TASK-3 heterodimer. Although both TASK-1 and TASK-3 are able to couple to the oxygen and metabolism sensing pathways present in type-1 cells, channels containing TASK-1 appear to be more sensitive.

Acute Hypoxia Activates an ENaC-like Channel in Rat Pheochromocytoma (PC12) Cells

Cells can resist and even recover from stress induced by acute hypoxia, whereas chronic hypoxia often leads to irreversible damage and eventually death. Although little is known about the response(s) to acute hypoxia in neuronal cells, alterations in ion channel activity could be preferential. This study aimed to elucidate which channel type is involved in the response to acute hypoxia in rat pheochromocytomal (PC12) cells as a neuronal cell model. Using perfusing solution saturated with 95% N(2) and 5% CO(2), induction of cell hypoxia was confirmed based on increased intracellular Ca(2+) with diminished oxygen content in the perfusate. During acute hypoxia, one channel type with a conductance of about 30 pS (2.5 pA at -80 mV) was activated within the first 2~3 min following onset of hypoxia and was long-lived for more than 300 ms with high open probability (P(o), up to 0.8). This channel was permeable to Na(+) ions, but not to K(+), Ca(+), and Cl(-) ions, and was sensitively blocked by amiloride (200 nM). These characteristics and behaviors were quite similar to those of epithelial sodium channel (ENaC). RT-PCR and Western blot analyses confirmed that ENaC channel was endogenously expressed in PC12 cells. Taken together, a 30-pS ENaC-like channel was activated in response to acute hypoxia in PC12 cells. This is the first evidence of an acute hypoxia-activated Na(+) channel that can contribute to depolarization of the cell.

Oxygen-sensitive reduction in Ca²⁺-activated K⁺ channel open probability in turtle cerebrocortex

In response to low ambient oxygen levels the western painted turtle brain undergoes a large depression in metabolic rate which includes a decrease in neuronal action potential frequency. This involves the arrest of N-methyl-D-aspartate receptor (NMDAR) and α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR) currents and paradoxically an increase in γ-aminobutyric acid receptor (GABAR) currents in turtle cortical neurons. In a search for other oxygen-sensitive channels we discovered a Ca(2+)-activated K(+) channel (K(Ca)) that exhibited a decrease in open time in response to anoxia. Single-channel recordings of K(Ca) activity were obtained in cell-attached and excised inside-out patch configurations from neurons in cortical brain sheets bathed in either normoxic or anoxic artificial cerebrospinal fluid (aCSF). The channel has a slope conductance of 223pS, is activated in response to membrane depolarization, and is controlled in a reversible manner by free [Ca(2+)] at the intracellular membrane surface. In the excised patch configuration anoxia had no effect on K(Ca) channel open probability (P(open)); however, in cell-attached mode, there was a reversible fivefold reduction in P(open) (from 0.5 ± 0.05 to 0.1 ± 0.03) in response to 30-min anoxia. The inclusion of the potent protein kinase C (PKC) inhibitor chelerythrine prevented the anoxia-mediated decrease in P(open) while drip application of a phorbol ester PKC activator decreased P(open) during normoxia (from normoxic 0.4 ± 0.05 to phorbol-12-myristate-13-acetate (PMA) 0.1 ± 0.02). Anoxia results in a slight depolarization of turtle pyramidal neurons (∼8 mV) and an increase in cytosolic [Ca(2+)]; therefore, K(Ca) arrest is likely important to prevent Ca(2+) activation during anoxia and to reduce the energetic cost of maintaining ion gradients. We conclude that turtle pyramidal cell Ca(2+)-activated K(+) channels are oxygen-sensitive channels regulated by cytosolic factors and are likely the reptilian analog of the mammalian large conductance Ca(2+)-activated K(+) channels (BK channels).

Responses of glomus cells to hypoxia and acidosis are uncoupled, reciprocal and linked to ASIC3 expression: selectivity of chemosensory transduction

Carotid body glomus cells are the primary sites of chemotransduction of hypoxaemia and acidosis in peripheral arterial chemoreceptors. They exhibit pronounced morphological heterogeneity. A quantitative assessment of their functional capacity to differentiate between these two major chemical signals has remained undefined. We tested the hypothesis that there is a differential sensory transduction of hypoxia and acidosis at the level of glomus cells. We measured cytoplasmic Ca(2+) concentration in individual glomus cells, isolated in clusters from rat carotid bodies, in response to hypoxia ( mmHg) and to acidosis at pH 6.8. More than two-thirds (68%) were sensitive to both hypoxia and acidosis, 19% were exclusively sensitive to hypoxia and 13% exclusively sensitive to acidosis. Those sensitive to both revealed significant preferential sensitivity to either hypoxia or to acidosis. This uncoupling and reciprocity was recapitulated in a mouse model by altering the expression of the acid-sensing ion channel 3 (ASIC3) which we had identified earlier in glomus cells. Increased expression of ASIC3 in transgenic mice increased pH sensitivity while reducing cyanide sensitivity. Conversely, deletion of ASIC3 in the knockout mouse reduced pH sensitivity while the relative sensitivity to cyanide or to hypoxia was increased. In this work, we quantify functional differences among glomus cells and show reciprocal sensitivity to acidosis and hypoxia in most glomus cells. We speculate that this selective chemotransduction of glomus cells by either stimulus may result in the activation of different afferents that are preferentially more sensitive to either hypoxia or acidosis, and thus may evoke different and more specific autonomic adjustments to either stimulus.

K(+) channels in O(2) sensing and postnatal development of carotid body glomus cell response to hypoxia

The sensitivity of carotid body chemoreceptors to hypoxia is low just after birth and increases over the first few weeks of the postnatal period. At present, it is believed that the hypoxia-induced excitation of carotid body glomus cells begins with the inhibition of the outward K(+) current via one or more O(2) sensors. Although the nature of the O(2) sensors and their signals that inhibit the K(+) current are not well defined, studies suggest that the postnatal maturation of the glomus cell response to hypoxia is largely due to the increased sensitivity of K(+) channels to hypoxia. As K(V), BK and TASK channels that are O(2)-sensitive contribute to the K(+) current, it is important to identify the O(2) sensor and the signaling molecule for each of these K(+) channels. Various O(2) sensors (mitochondrial hemeprotein, hemeoxygenase-2, NADPH oxidase) and associated signals have been proposed to mediate the inhibition of K(+) channels by hypoxia. Studies suggest that a mitochondrial hemeprotein is likely to serve as an O(2) sensor for K(+) channels, particularly for TASK, and that multiple signals may be involved. Thus, changes in the sensitivity of the mitochondrial O(2) sensor to hypoxia, the sensitivity of K(+) channels to signals generated by mitochondria, and/or the expression levels of K(+) channels are likely to account for the postnatal maturation of O(2) sensing by glomus cells.

Chemoreceptor hypersensitivity, sympathetic excitation, and overexpression of ASIC and TASK channels before the onset of hypertension in SHR

RATIONALE

Increased sympathetic nerve activity has been linked to the pathogenesis of hypertension in humans and animal models. Enhanced peripheral chemoreceptor sensitivity which increases sympathetic nerve activity has been observed in established hypertension but has not been identified as a possible mechanism for initiating an increase in sympathetic nerve activity before the onset of hypertension.

OBJECTIVE

We tested this hypothesis by measuring the pH sensitivity of isolated carotid body glomus cells from young spontaneously hypertensive rats (SHR) before the onset of hypertension and their control normotensive Wistar-Kyoto (WKY) rats.

METHODS AND RESULTS

We found a significant increase in the depolarizing effect of low pH in SHR versus WKY glomus cells which was caused by overexpression of 2 acid-sensing non-voltage-gated channels. One is the amiloride-sensitive acid-sensing sodium channel (ASIC3), which is activated by low pH and the other is the 2-pore domain acid-sensing K(+) channel (TASK1), which is inhibited by low pH and blocked by quinidine. Moreover, we found that the increase in sympathetic nerve activity in response to stimulation of chemoreceptors with sodium cyanide was markedly enhanced in the still normotensive young SHR compared to control WKY rats.

CONCLUSIONS

Our results establish a novel molecular basis for increased chemotransduction that contributes to excessive sympathetic activity before the onset of hypertension.

Ion channel regulation by AMPK: the route of hypoxia-response coupling in thecarotid body and pulmonary artery

Vital homeostatic mechanisms monitor O2 supply and adjust respiratory and circulatory function to meet demand. The pulmonary arteries and carotid bodies are key systems in this respect. Hypoxic pulmonary vasoconstriction (HPV) aids ventilation-perfusion matching in the lung by diverting blood flow from areas with an O2 deficit to those rich in O2, while a fall in arterial pO2 increases sensory afferent discharge from the carotid body to elicit corrective changes in breathing patterns. We discuss here the new concept that hypoxia, by inhibiting oxidative phosphorylation, activates AMP-activated protein kinase (AMPK) leading to consequent phosphorylation of target proteins, such as ion channels, which initiate pulmonary artery constriction and carotid body activation. Consistent with this view, AMPK knockout mice exhibit an impaired ventilatory response to hypoxia. Thus, AMPK may be sufficient and necessary for hypoxia-response coupling and may regulate O2 and thereby energy (ATP) supply at the whole body as well as the cellular level.

MaxiK potassium channels in the function of chemoreceptor cells of the rat carotid body

Hypoxia activates chemoreceptor cells of the carotid body (CB) promoting an increase in their normoxic release of neurotransmitters. Catecholamine (CA) release rate parallels the intensity of hypoxia. Coupling of hypoxia to CA release requires cell depolarization, produced by inhibition of O(2)-regulated K(+) channels, and Ca(2+) entering the cells via voltage-operated channels. In rat chemoreceptor cells hypoxia inhibits large-conductance, calcium-sensitive K channels (maxiK) and a two-pore domain weakly inward rectifying K(+) channel (TWIK)-like acid-sensitive K(+) channel (TASK)-like channel, but the significance of maxiK is controversial. A proposal envisions maxiK contributing to set the membrane potential (E(m)) and the hypoxic response, but the proposal is denied by authors finding that maxiK inhibition does not depolarize chemoreceptor cells or alters intracellular Ca(2+) concentration or CA release in normoxia or hypoxia. We found that maxiK channel blockers (tetraethylammonium and iberiotoxin) did not modify CA release in rat chemoreceptor cells, in either normoxia or hypoxia, and iberiotoxin did not alter the Ca(2+) transients elicited by hypoxia. On the contrary, both maxiK blockers increased the responses elicited by dinitrophenol, a stimulus we demonstrate does not affect maxiK channels in isolated patches of rat chemoreceptor cells. We conclude that in rat chemoreceptor cells maxiK channels do not contribute to the genesis of the E(m), and that their full inhibition by hypoxia, preclude further inhibition by maxiK channel blockers. We suggest that full inhibition of this channel is required to generate the spiking behavior of the cells in acute hypoxia.

DPPX modifies TEA sensitivity of the Kv4 channels in rabbit carotid body chemoreceptor cells

Chemoreceptor cells from rabbit carotid body (CB) exhibit transient outward currents reversibly inhibited by low P(o2). Molecular and functional dissection of the components of these outward currents indicates that at least two different channels (Kv4.3 and Kv3.4) contribute to this current. Furthermore, several lines of evidence support the conclusion that Kv4 channel subfamily members (either Kv4.3 alone or Kv4.3/Kv4.1 heteromultimers) are the oxygen sensitive K channels (K(o2)) in rabbit CB chemoreceptor cells. However, the pharmacological characterization of these currents shows that they are almost completely blocked by high external TEA concentrations, while Kv4 channels have been shown to be TEA-insensitive. We hypothesized that the expression of regulatory subunits in chemoreceptor cells could modify TEA sensitivity of Kv4 channels. Here, we explore the presence and functional contribution of DPPX to K(o2) currents in rabbit CB chemoreceptor cells by using DPPX functional knockdown with siRNA. Our data suggest that DPPX proteins are integral components of K(o2) currents, and that their association with Kv4 subunits modulate the pharmacological profile of the heteromultimers.

A role for DPPX modulating external TEA sensitivity of Kv4 channels

Shal-type (Kv4) channels are expressed in a large variety of tissues, where they contribute to transient voltage-dependent K⁺ currents. Kv4 are the molecular correlate of the A-type current of neurons (I_SA), the fast component of I_TO current in the heart, and also of the oxygen-sensitive K⁺ current (K_O2) in rabbit carotid body (CB) chemoreceptor cells. The enormous degree of variability in the physiological properties of Kv4-mediated currents can be attributable to the complexity of their regulation together with the large number of ancillary subunits and scaffolding proteins that associate with Kv4 proteins to modify their trafficking and their kinetic properties. Among those, KChIPs and DPPX proteins have been demonstrated to be integral components of I_SA and I_TO currents, as their coexpression with Kv4 subunits recapitulates the kinetics of native currents. Here, we explore the presence and functional contribution of DPPX to K_O2 currents in rabbit CB chemoreceptor cells by using DPPX functional knockdown with siRNA. Additionally, we investigate if the presence of DPPX endows Kv4 channels with new pharmacological properties, as we have observed anomalous tetraethylammonium (TEA) sensitivity in the native K_O2 currents. DPPX association with Kv4 channels induced an increased TEA sensitivity both in heterologous expression systems and in CB chemoreceptor cells. Moreover, TEA application to Kv4-DPPX heteromultimers leads to marked kinetic effects that could be explained by an augmented closed-state inactivation. Our data suggest that DPPX proteins are integral components of K_O2 currents, and that their association with Kv4 subunits modulate the pharmacological profile of the heteromultimers.

Acid-sensing ion channels contribute to transduction of extracellular acidosis in rat carotid body glomus cells

Carotid body chemoreceptors sense hypoxemia, hypercapnia, and acidosis and play an important role in cardiorespiratory regulation. The molecular mechanism of pH sensing by chemoreceptors is not clear, although it has been proposed to be mediated by a drop in intracellular pH of carotid body glomus cells, which inhibits a K+ current. Recently, pH-sensitive ion channels have been described in glomus cells that respond directly to extracellular acidosis. In this study, we investigated the possible molecular mechanisms of carotid body pH sensing by recording the responses of glomus cells isolated from rat carotid body to rapid changes in extracellular pH using the whole-cell patch-clamping technique. Extracellular acidosis evoked transient inward current in glomus cells that was inhibited by the acid-sensing ion channel (ASIC) blocker amiloride, absent in Na+-free bathing solution, and enhanced by either Ca2+-free buffer or addition of lactate. In addition, ASIC1 and ASIC3 were shown to be expressed in rat carotid body by quantitative PCR and immunohistochemistry. In the current-clamp mode, extracellular acidosis evoked both a transient and sustained depolarizations. The initial transient component of depolarization was blocked by amiloride, whereas the sustained component was eliminated by removal of K+ from the pipette solution and partially blocked by the TASK (tandem-p-domain, acid-sensitive K+ channel) blockers anandamide and quinidine. The results provide the first evidence that ASICs may contribute to chemotransduction of low pH by carotid body chemoreceptors and that extracellular acidosis directly activates carotid body chemoreceptors through both ASIC and TASK channels.

Molecular identification and functional role of voltage-gated sodium channels in rat carotid body chemoreceptor cells. Regulation of expression by chronic hypoxia in vivo

We have assessed the expression, molecular identification and functional role of Na+ channels (Na(v)) in carotid bodies (CB) obtained from normoxic and chronically hypoxic adult rats. Veratridine evoked release of catecholamines (CA) from an in vitro preparation of intact CBs obtained from normoxic animals, the response being Ca2+ and Na+-dependent and sensitive to tetrodotoxin (TTX). TTX inhibited by 25-50% the CA release response evoked by graded hypoxia. Immunoblot assays demonstrated the presence of Na(v)alpha-subunit (c. 220 kDa) in crude homogenates from rat CBs, being evident an up-regulation (60%) of this protein in the CBs obtained from chronically hypoxic rats (10% O2; 7 days). This up-regulation was accompanied by an enhanced TTX-sensitive release response to veratridine, and by an enhanced ventilatory response to acute hypoxic stimuli. RT-PCR studies demonstrated the expression of mRNA for Na(v)1.1, Na(v)1.2, Na(v)1.3, Na(v)1.6 and Na(v)1.7 isoforms. At least three isoforms, Na(v)1.1, Na(v)1.3 and Na(v)1.6 co-localized with tyrosine hydroxylase in all chemoreceptor cells. RT-PCR and immunocytochemistry indicated that Na(v)1.1 isoform was up-regulated by chronic hypoxia in chemoreceptor cells. We conclude that Na(v) up-regulation represents an adaptive mechanism to increase chemoreceptor sensitivity during acclimatization to sustained hypoxia as evidenced by enhanced ventilatory responses to acute hypoxic tests.

Oxygen sensitive Kv channels in the carotid body

Hypoxic inhibition of K(+) channels has been documented in many native chemoreceptor cells, and is crucial to initiate reflexes directed to improve tissue O(2) supply. In the carotid body (CB) chemoreceptors, there is a general consensus regarding the facts that a decrease in P(O2) leads to membrane depolarization, increase of Ca(2+) entry trough voltage-dependent Ca(2+) channels and Ca(2+)-dependent release of neurotransmitters. Central to this pathway is the modulation by hypoxia of K(+) channels that triggers depolarization. However, the details of this process are still controversial, and even the molecular nature of these oxygen-sensitive K(+) (K(O2)) channels in the CB is hotly debated. Clearly there are inter-species differences, and even in the same preparation more that one K(O2) may be present. Here we recapitulate our present knowledge of the role of voltage dependent K(+) channels as K(O2) in the CB from different species, and their functional contribution to cell excitability in response to acute and chronic exposure to hypoxia.

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.

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

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

Purines, the carotid body and respiration

The carotid body is essential to detecting levels of oxygen in the blood and initiating the compensatory response. Increasing evidence suggests that the purines ATP and adenosine make a key contribution to this signaling by the carotid body. The glomus cells release ATP in response to hypoxia. This released ATP can stimulate P2X receptors on the carotid body to elevate intracellular Ca(2+) and to produce an excitatory response. This released ATP can be dephosphorylated to adenosine by a series of extracellular enzymes, which in turn can stimulate A(1), A(2A) and A(2B) adenosine receptors. Levels of extracellular adenosine can also be altered by membrane transporters. Endogenous adenosine stimulates these receptors to increase the ventilation rate and may modulate the catecholamine release from the carotid sinus nerve. Prolonged hypoxic challenge can alter the expression of purinergic receptors, suggesting a role in the adaptation. This review discusses evidence for a key role of ATP and adenosine in the hypoxic response of the carotid body, and emphasizes areas of new contributions likely to be important in the future.

The role of maxiK channels in carotid body chemotransduction

MaxiK channels are a unique class of K(+) channels activated by both voltage and intracellular Ca(2+). Derived from a single gene, their diversity arises from extensive splicing, and their wide distribution has led to their implication in a large variety of cellular functions. In the carotid body, they have been proposed to contribute to the resting membrane potential of type I cells, and also to be O(2) sensitive. Thus, they have been suggested to have an important role in hypoxic chemotransduction. Their O(2) sensitivity is preserved when the channels are expressed in HEK 293 cells, permitting detailed studies of candidate mechanisms underlying hypoxic inhibition of maxiK channels. In this article, we review evidence for and against an important role for maxiK channels in chemotransduction. We also consider different mechanisms proposed to account for hypoxic channel inhibition and suggest that, although our understanding of this important physiological process has advanced significantly in recent years, there remain important, unanswered questions as to the importance of maxiK in carotid body chemoreception.

Oxygen sensing in the body

This review is divided into three parts: (a) The primary site of oxygen sensing is the carotid body which instantaneously respond to hypoxia without involving new protein synthesis, and is historically known as the first oxygen sensor and is therefore placed in the first section (Lahiri, Roy, Baby and Hoshi). The carotid body senses oxygen in acute hypoxia, and produces appropriate responses such as increases in breathing, replenishing oxygen from air. How this oxygen is sensed at a relatively high level (arterial PO2 approximately 50 Torr) which would not be perceptible by other cells in the body, is a mystery. This response is seen in afferent nerves which are connected synaptically to type I or glomus cells of the carotid body. The major effect of oxygen sensing is the increase in cytosolic calcium, ultimately by influx from extracellular calcium whose concentration is 2 x 10(4) times greater. There are several contesting hypotheses for this response: one, the mitochondrial hypothesis which states that the electron transport from the substrate to oxygen through the respiratory chain is retarded as the oxygen pressure falls, and the mitochondrial membrane is depolarized leading to the calcium release from the complex of mitochondria-endoplasmic reticulum. This is followed by influx of calcium. Also, the inhibitors of the respiratory chain result in mitochondrial depolarization and calcium release. The other hypothesis (membrane model) states that K(+) channels are suppressed by hypoxia which depolarizes the membrane leading to calcium influx and cytosolic calcium increase. Evidence supports both the hypotheses. Hypoxia also inhibits prolyl hydroxylases which are present in all the cells. This inhibition results in membrane K(+) current suppression which is followed by cell depolarization. The theme of this section covers first what and where the oxygen sensors are; second, what are the effectors; third, what couples oxygen sensors and the effectors. (b) All oxygen consuming cells have a built-in mechanism, the transcription factor HIF-1, the discovery of which has led to the delineation of oxygen-regulated gene expression. This response to chronic hypoxia needs new protein synthesis, and the proteins of these genes mediate the adaptive physiological responses. HIF-1alpha, which is a part of HIF-1, has come to be known as master regulator for oxygen homeostasis, and is precisely regulated by the cellular oxygen concentration. Thus, the HIF-1 encompasses the chronic responses (gene expression in all cells of the body). The molecular biology of oxygen sensing is reviewed in this section (Semenza). (c) Once oxygen is sensed and Ca(2+) is released, the neurotransmittesr will be elaborated from the glomus cells of the carotid body. Currently it is believed that hypoxia facilitates release of one or more excitatory transmitters from glomus cells, which by depolarizing the nearby afferent terminals, leads to increases in the sensory discharge. The transmitters expressed in the carotid body can be classified into two major categories: conventional and unconventional. The conventional neurotransmitters include those stored in synaptic vesicles and mediate their action via activation of specific membrane bound receptors often coupled to G-proteins. Unconventional neurotransmitters are those that are not stored in synaptic vesicles, but spontaneously generated by enzymatic reactions and exert their biological responses either by interacting with cytosolic enzymes or by direct modifications of proteins. The gas molecules such as NO and CO belong to this latter category of neurotransmitters and have unique functions. Co-localization and co-release of neurotransmitters have also been described. Often interactions between excitatory and inhibitory messenger molecules also occur. Carotid body contains all kinds of transmitters, and an interplay between them must occur. But very little has come to be known as yet. Glimpses of these interactions are evident in the discussion in the last section (Prabhakar).

Postnatal development of carotid body glomus cell O2 sensitivity

In mammals, the main sensors of arterial oxygen level are the carotid chemoreceptors, which exhibit low sensitivity to hypoxia at birth and become more sensitive over the first few days or weeks of life. This postnatal increase in hypoxia sensitivity of the arterial chemoreceptors, termed "resetting", remains poorly understood. In the carotid body, hypoxia is transduced by glomus cells, which are secretory sensory neurons that respond to hypoxia at higher P(O2) levels than non-chemoreceptor cell types. Maturation or resetting of carotid body O2 sensitivity potentially involves numerous aspects of the O2 transduction cascade at the glomus cell level, including glomus cell neurotransmitter secretion, neuromodulator function, neurotransmitter receptor expression, glomus cell depolarization in response to hypoxia, [Ca2+]i responses to hypoxia, K+ and Ca2+ channel O2 sensitivity and K+ channel expression. However, although progress has been made in the understanding of carotid body development, the precise mechanisms underlying postnatal maturation of these numerous aspects of chemotransduction remain obscure.

ATP inhibits the hypoxia response in type I cells of rat carotid bodies

Summary During hypoxia, ATP was released from type I (glomus) cells in the carotid bodies. We studied the action of ATP on the intracellular Ca(2+) concentration ([Ca(2+)](i)) of type I cells dissociated from rat carotid bodies using a Ca(2+) imaging technique. ATP did not affect the resting [Ca(2+)](i) but strongly suppressed the hypoxia-induced [Ca(2+)](i) elevations in type I cells. The order of purinoreceptor agonist potency in inhibiting the hypoxia response was 2-methylthioATP > ATP > ADP >> alpha, beta-methylene ATP > UTP, implicating the involvement of P2Y(1) receptors. Simultaneous measurements of membrane potential and [Ca(2+)](i) show that ATP inhibited the hypoxia-induced Ca(2+) signal by reversing the hypoxia-triggered depolarization. However, ATP did not oppose the hypoxia-mediated inhibition of the oxygen-sensitive TASK-like K(+) background current. Neither the inhibition of the large-conductance Ca(2+)-activated K(+) (maxi-K) channels nor the removal of extracellular Na(+) could affect the inhibitory action of ATP. Under normoxic condition, ATP caused hyperpolarization and increase in cell input resistance. These results suggest that the inhibitory action of ATP is mediated via the closure of background conductance(s) other than the TASK-like K(+), maxi-K or Na(+) channels. In summary, ATP exerts strong negative feedback regulation on hypoxia signaling in rat carotid type I cells.

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.

The mitochondrial SDHD gene is required for early embryogenesis, and its partial deficiency results in persistent carotid body glomus cell activation with full responsiveness to hypoxia

The SDHD gene encodes one of the two membrane-anchoring proteins of the succinate dehydrogenase (complex II) of the mitochondrial electron transport chain. This gene has recently been proposed to be involved in oxygen sensing because mutations that cause loss of its function produce hereditary familiar paraganglioma, a tumor of the carotid body (CB), the main arterial chemoreceptor that senses oxygen levels in the blood. Here, we report the generation of a SDHD knockout mouse, which to our knowledge is the first mammalian model lacking a protein of the electron transport chain. Homozygous SDHD(-/-) animals die at early embryonic stages. Heterozygous SDHD(+/-) mice show a general, noncompensated deficiency of succinate dehydrogenase activity without alterations in body weight or major physiological dysfunction. The responsiveness to hypoxia of CBs from SDHD(+/-) mice remains intact, although the loss of an SDHD allele results in abnormal enhancement of resting CB activity due to a decrease of K(+) conductance and persistent Ca(2+) influx into glomus cells. This CB overactivity is linked to a subtle glomus cell hypertrophy and hyperplasia. These observations indicate that constitutive activation of SDHD(+/-) glomus cells precedes CB tumor transformation. They also suggest that, contrary to previous beliefs, mitochondrial complex II is not directly involved in CB oxygen sensing.

Postnatal development of E-4031-sensitive potassium current in rat carotid chemoreceptor cells

The O2 sensitivity of dissociated type I cells from rat carotid body increases with age until approximately 14-16 days. Hypoxia-induced depolarization appears to be mediated by an O2-sensitive K+ current, but other K+ currents may modulate depolarization. We hypothesized that membrane potential may be stabilized in newborn type I cells by human ether-a-go-go-related gene (HERG)-like K+ currents that inhibit hypoxia-induced depolarization and that a decrease in this current with age could underlie, in part, the developmental increase in type I cell depolarization response to hypoxia. In dissociated type I cells from 0- to 1- and 11- to 16-day-old rats, using perforated patch-clamp and 70 mM K+ extracellular solution, we measured repolarization-induced inward K+ tail currents in the absence and presence of E-4031, a specific HERG channel blocker. This allowed isolation of the E-4031-sensitive HERG-like current. E-4031-sensitive peak currents in type I cells from 0- to- 1-day-old rats were 2.5-fold larger than in cells from 11- to 16-day-old rats. E-4031-sensitive current density in newborn type I cells was twofold greater than in cells from 11- to 16-day-old rats. Under current clamp conditions, E-4031 enhanced hypoxia-induced depolarization in type I cells from 0- to- 1-day-old but not 11- to 16-day-old rats. With use of fura 2 to measure intracellular Ca2+, E-4031 increased the cytosolic Ca2+ concentration response to anoxia in cells from 0- to- 1-day-old but not cells from 11- to 16-day-old rats. E-4031-sensitive K+ currents are present in newborn carotid body type I cells and decline with age. These findings are consistent with a role for E-4031-sensitive K+ current, and possibly HERG-like K+ currents, in the type I cell hypoxia response maturation.

Characterization of the Kv channels of mouse carotid body chemoreceptor cells and their role in oxygen sensing

As there are wide interspecies variations in the molecular nature of the O(2)-sensitive Kv channels in arterial chemoreceptors, we have characterized the expression of these channels and their hypoxic sensitivity in the mouse carotid body (CB). CB chemoreceptor cells were obtained from a transgenic mouse expressing green fluorescent protein (GFP) under the control of tyrosine hydroxylase (TH) promoter. Immunocytochemical identification of TH in CB cell cultures reveals a good match with GFP-positive cells. Furthermore, these cells show an increase in [Ca(2+)](i) in response to low P(O(2)), demonstrating their ability to engender a physiological response. Whole-cell experiments demonstrated slow-inactivating K(+) currents with activation threshold around -30 mV and a bi-exponential kinetic of deactivation (tau of 6.24 +/- 0.52 and 32.85 +/- 4.14 ms). TEA sensitivity of the currents identified also two different components (IC(50) of 17.8 +/- 2.8 and 940.0 +/- 14.7 microm). Current amplitude decreased reversibly in response to hypoxia, which selectively affected the fast deactivating component. Hypoxic inhibition was also abolished in the presence of low (10-50 microm) concentrations of TEA, suggesting that O(2) interacts with the component of the current most sensitive to TEA. The kinetic and pharmacological profile of the currents suggested the presence of Kv2 and Kv3 channels as their molecular correlates, and we have identified several members of these two subfamilies by single-cell PCR and immunocytochemistry. This report represents the first functional and molecular characterization of Kv channels in mouse CB chemoreceptor cells, and strongly suggests that O(2)-sensitive Kv channels in this preparation belong to the Kv3 subfamily.

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.

Mechanisms for Hypoxia Detection in O2-Sensitive Cells

Since O(2) is the bare necessity for multicellular organisms, they develop multiple protective mechanisms against hypoxia. Mammals will adapt to hypoxia in short and long terms. The short-term responses include enhancement of the respiratory and cardiac functions, adrenaline secretion from adrenal medullary cells, and pulmonary vasoconstriction, whereas the long-term response is the increase in erythropoietin production with the consequent increase in red blood cells. Although much work has been done to elucidate molecular mechanisms for O(2)-sensing for the last ten years, the majority of the mechanisms remain unclear. We will review mechanisms proposed for hypoxia detection in carotid body type I cells, pulmonary artery smooth muscle, adrenal medullary cells, and liver cells, with the special focus on adrenal medullary cells.

ATP triggers intracellular Ca2+ release in type II cells of the rat carotid body

Using a Ca2+-imaging technique, we studied the action of ATP on the intracellular Ca2+ concentration ([Ca2+]i) of fura-2-loaded mixtures of type I and type II cells dissociated from rat carotid bodies. ATP (100 micro M) triggered a transient rise in [Ca2+]i in the spindle-shaped type II (sustentacular) cells, but not the ovoid type I (glomus) cells. When challenged with ionomycin (1 micro M), no amperometry signal could be detected from the ATP-responsive type II cells, suggesting that these cells lacked catecholamine-containing granules. In contrast, KCl depolarization triggered robust quantal catecholamine release from type I cells that were not responsive to ATP. In type II cells voltage clamped at -70 mV, the ATP-induced [Ca2+]i rise was not accompanied by any current change, suggesting that P2X receptors are not involved. The ATP-induced Ca2+ signal could be observed in the presence of Ni2+ (a blocker of voltage-gated Ca2+ channels) or in the absence of extracellular Ca2+, indicating that Ca2+ release from intracellular stores was the dominant mechanism. The order of purinoreceptor agonist potency in triggering the [Ca2+]i rise was UTP > ATP > 2-methylthioATP >> alpha,beta-methyleneATP, implicating the involvement of P2Y2 receptors. In carotid body sections, immunofluorescence revealed localization of P2Y2 receptors on spindle-shaped type II cells that partially enveloped ovoid type I cells. Since ATP is released from type I cells during hypoxia, we suggest that the ATP-induced Ca2+ signal in type II cells can mediate paracrine interactions within the carotid bodies.

Carotid body thin slices: responses of glomus cells to hypoxia and K(+)-channel blockers

We describe the rat carotid body thin slice preparation, which allows to perform patch-clamp recording of membrane ionic currents and to monitor catecholamine secretion by amperometry in single glomus cells under direct visual control. We observed several electrophysiologically distinct cell classes within the same glomerulus. A voltage- and Ca(2+)-dependent component of the whole cell K(+) current was reversibly inhibited by low P(O(2)) (20 mmHg). Exposure of the cells to hypoxia elicited the appearance of spike-like exocytotic events. This response to hypoxia was reversible and required extracellular Ca(2+) influx. Addition of tetraethylammonium (TEA, 2-5 mM) to the extracellular solution induced in most (>95%) cells tested a secretory response similar to that elicited by low P(O(2)). Cells non-responsive to hypoxia but activated by exposure to high external K(+) were also stimulated by TEA. A secretory response similar to that of hypoxia or TEA was also observed after treatment of the cells with iberiotoxin to block selectively maxi-K(+) channels. Our data further support the view that membrane ion channels are critically involved in sensory transduction in the carotid body. We demonstrate that in intact glomus cells inhibition of voltage-dependent K(+) channels can contribute to initiate the secretory response to low P(O(2)).

Significance of ROS in oxygen sensing in cell systems with sensitivity to physiological hypoxia

Reactive oxygen species (ROS) are oxygen-containing molecular entities which are more potent and effective oxidizing agents than is molecular oxygen itself. With the exception of phagocytic cells, where ROS play an important physiological role in defense reactions, ROS have classically been considered undesirable byproducts of cell metabolism, existing several cellular mechanisms aimed to dispose them. Recently, however, ROS have been considered important intracellular signaling molecules, which may act as mediators or second messengers in many cell functions. This is the proposed role for ROS in oxygen sensing in systems, such as carotid body chemoreceptor cells, pulmonary artery smooth muscle cells, and erythropoietin-producing cells. These unique cells comprise essential parts of homeostatic loops directed to maintain oxygen levels in multicellular organisms in situations of hypoxia. The present article examines the possible significance of ROS in these three cell systems, and proposes a set of criteria that ROS should satisfy for their consideration as mediators in hypoxic transduction cascades. In none of the three cell types do ROS satisfy these criteria, and thus it appears that alternative mechanisms are responsible for the transduction cascades linking hypoxia to the release of neurotransmitters in chemoreceptor cells, contraction in pulmonary artery smooth muscle cells and erythropoietin secretion in erythropoietin producing cells.

Molecular identification of Kvalpha subunits that contribute to the oxygen-sensitive K+ current of chemoreceptor cells of the rabbit carotid body

Rabbit carotid body (CB) chemoreceptor cells possess a fast-inactivating K+ current that is specifically inhibited by hypoxia. We have studied the expression of Kvalpha subunits, which might be responsible for this current. RT-PCR experiments identified the expression of Kv1.4, Kv3.4, Kv4.1 and Kv4.3 mRNAs in the rabbit CB. There was no expression of Kv3.3 or Kv4.2 transcripts. Immunocytochemistry with antibodies to tyrosine hydroxylase (anti-TH) and to specific Kv subunits revealed the expression of Kv3.4 and Kv4.3 in chemoreceptor cells, while Kv1.4 was only found in nerve fibres. Kv4.1 mRNA was also found in chemoreceptor cells following in situ hybridization combined with anti-TH antibody labelling. Kv4.1 and Kv4.3 appeared to be present in all chemoreceptor cells, but Kv3.4 was only expressed in a population of them. Electrophysiological experiments applying specific toxins or antibodies demonstrated that both Kv3.4 and Kv4.3 participate in the oxygen-sensitive K+ current of chemoreceptor cells. However, toxin application experiments confirmed a larger contribution of members of the Kv4 subfamily. [Ca2+]i measurements under hypoxic conditions and immunocytochemistry experiments in dispersed CB cells demonstrated the expression of Kv3.4 and Kv4.3 in oxygen-sensitive cells; the presence of Kv3.4 in the chemoreceptor cell membrane was not required for the response to low PO2. In summary, three Kv subunits (Kv3.4, Kv4.1 and Kv4.3) may be involved in the fast-inactivating outward K+ current of rabbit CB chemoreceptor cells. The homogeneous distribution of the Kv4 subunits in chemoreceptor cells, along with their electrophysiological properties, suggest that Kv4.1, Kv4.3, or their heteromultimers, are the molecular correlate of the oxygen-sensitive K+ channel.

Dopaminergic cells of the carotid body: physiological significance and possible therapeutic applications in Parkinson's disease

Parkinson's disease is a neurodegenerative disorder characterized by the progressive loss of dopaminergic neurons in the substantia nigra projecting to the striatum. One therapeutic approach to this disease has been the intrastriatal transplantation of dopamine-secreting cells. We have investigated the suitability of glomus cells of the carotid body for dopamine-cell replacement in animal models of Parkinson's disease. Carotid body glomus cells are physiologic arterial oxygen sensors that release large amounts of dopamine in response to hypoxia. We have used hemi-Parkinsonian rats, induced by injection of 6-hydroxydopamine into the substantia nigra, and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treated monkeys with chronic Parkinsonism. In both cases we made transplants of carotid body cell aggregates into the putamen. Functional recovery of the grafted animals was observed after the surgery and was stable for several months. Although the study was more detailed in the rat, in the two animal models the amelioration of the motor deficits was paralleled by striatal dopaminergic reinnervation and survival of grafted glomus cells. Our results suggest that intrastriatal autotransplants of carotid body tissue could be a feasible technique to treat some cases of Parkinson's disease in humans.

Role of ATP decrease in secretion induced by mitochondrial dysfunction in guinea-pig adrenal chromaffin cells

The mechanism related to mitochondrial dysfunction-induced catecholamine (CA) secretion in dispersed guinea-pig adrenal chromaffin cells was investigated using amperometry and confocal laser microscopy. Application of CCCP, which does not stimulate generation of reactive oxygen species (ROS), reversibly induced CA secretion, whereas application of either cyanide or oligomycin (OL), a stimulator for ROS, enhanced CA secretion to a smaller extent. The CCCP-induced secretion was abolished by removal of external Ca2+ ions and was markedly diminished by D600. The mitochondrial membrane potential, measured using rhodamine 123, was rapidly lost in response to CCCP, but did not change noticeably during a 3 min exposure to OL. Prior exposure to OL markedly facilitated depolarization of the mitochondrial membrane potential in response to cyanide. The mitochondrial inhibitors rapidly produced an increase in Magnesium Green (MgG) fluorescence in the absence of external Ca2+ and Mg2+ ions, an increase that was larger in the cytoplasm than in the nucleus. The rank order of potency in increasing MgG fluorescence among the inhibitors was similar to that in increasing secretion. Thus, mitochondrial inhibition rapidly decreases [ATP] and the mitochondrial dysfunction-induced secretion is not due to ROS generation or to mitochondrial depolarization, but is possibly mediated by a decrease in ATP.

O(2) modulates large-conductance Ca(2+)-dependent K(+) channels of rat chemoreceptor cells by a membrane-restricted and CO-sensitive mechanism

Hypoxic inhibition of large-conductance Ca(2+)-dependent K(+) channels (maxiK) of rat carotid body type I cells is a well-established fact. However, the molecular mechanisms of such inhibition and the role of these channels in the process of hypoxic transduction remain unclear. We have examined the mechanisms of interaction of O(2) with maxiK channels exploring the effect of hypoxia on maxiK currents recorded with the whole-cell and the inside-out configuration of the patch-clamp technique. Hypoxia inhibits channel activity both in whole-cell and in excised membrane patches. This effect is strongly voltage- and Ca(2+)-dependent, being maximal at low [Ca(2+)] and low membrane potential. The analysis of single-channel kinetics reveals a gating scheme comprising three open and five closed states. Hypoxia inhibits channel activity increasing the time the channel spends in the longest closed states, an effect that could be explained by a decrease in the Ca(2+) sensitivity of those closed states. Reducing maxiK channels with dithiothreitol (DTT) increases channel open probability, whereas oxidizing the channels with 2,2'-dithiopyridine (DTDP) has the opposite effect. These results suggest that hypoxic inhibition is not related with a reduction of channel thiol groups. However, CO, a competitive inhibitor of O(2) binding to hemoproteins, fully reverts hypoxic inhibition, both at the whole-cell and the single-channel level. We conclude that O(2) interaction with maxiK channels does not require cytoplasmic mediators. Such interaction could be mediated by a membrane hemoprotein that, as an O(2) sensor, would modulate channel activity.

Cellular mechanism of oxygen sensing

O2 sensing is a fundamental biological process necessary for adaptation of living organisms to variable habitats and physiological situations. Cellular responses to hypoxia can be acute or chronic. Acute responses rely mainly on O2-regulated ion channels, which mediate adaptive changes in cell excitability, contractility, and secretory activity. Chronic responses depend on the modulation of hypoxia-inducible transcription factors, which determine the expression of numerous genes encoding enzymes, transporters and growth factors. O2-regulated ion channels and transcription factors are part of a widely operating signaling system that helps provide sufficient O2 to the tissues and protect the cells against damage due to O2 deficiency. Despite recent advances in the molecular characterization of O2-regulated ion channels and hypoxia-inducible factors, several unanswered questions remain regarding the nature of the O2 sensor molecules and the mechanisms of interaction between the sensors and the effectors. Current models of O2 sensing are based on either a heme protein capable of reversibly binding O2 or the production of oxygen reactive species by NAD(P)H oxidases and mitochondria. Complete molecular characterization of the hypoxia signaling pathways will help elucidate the differential sensitivity to hypoxia of the various cell types and the gradation of the cellular responses to variable levels of PO2. A deeper understanding of the cellular mechanisms of O2 sensing will facilitate the development of new pharmacological tools effective in the treatment of diseases such as stroke or myocardial ischemia caused by localized deficits of O2.