Swelling- and cAMP-activated Cl- currents in isolated rat carotid body type I cells

Cell swelling enhances ligand-driven β-adrenergic signaling

G protein-coupled receptors' conformational landscape can be affected by their local, microscopic interactions within the cell plasma membrane. We employ here a pleiotropic stimulus, namely osmotic swelling, to alter the cortical environment within intact cells and monitor the response in terms of receptor function and downstream signaling. We observe that in osmotically swollen cells the β2-adrenergic receptor, a prototypical GPCR, favors an active conformation, resulting in cAMP transient responses to adrenergic stimulation that have increased amplitude. The results are validated in primary cell types such as adult cardiomyocytes, a model system where swelling occurs upon ischemia-reperfusion injury. Our results suggest that receptors' function is finely modulated by their biophysical context, and specifically that osmotic swelling acts as a potentiator of downstream signaling, not only for the β2-adrenergic receptor, but also for other receptors, hinting at a more general regulatory mechanism.

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.

Neurobiology of the carotid body

The carotid body (CB) is a bilateral arterial chemoreceptor located in the carotid artery bifurcation with an essential role in cardiorespiratory homeostasis. It is composed of highly perfused cell clusters, or glomeruli, innervated by sensory fibers. Glomus cells, the most abundant in each glomerulus, are neuron-like multimodal sensory elements able to detect and integrate changes in several physical and chemical parameters of the blood, in particular O₂ tension, CO₂ and pH, as well as glucose, lactate, or blood flow. Activation of glomus cells (e.g., during hypoxia or hypercapnia) stimulates the afferent fibers which impinge on brainstem neurons to elicit rapid compensatory responses (hyperventilation and sympathetic activation). This chapter presents an updated view of the structural organization of the CB and the mechanisms underlying the chemosensory responses of glomus cells, with special emphasis on the molecular processes responsible for acute O₂ sensing. The properties of the glomus cell-sensory fiber synapse as well as the organization of CB output are discussed. The chapter includes the description of recently discovered CB stem cells and progenitor cells, and their role in CB growth during acclimatization to hypoxemia. Finally, the participation of the CB in the mechanisms of disease is briefly discussed.

A 30-year journey from volume-regulated anion currents to molecular structure of the LRRC8 channel

Strange et al. review recent advances in our understanding of the molecular and structural basis of volume-regulated anion channel function within the framework of classical biophysical and physiological studies.

The swelling-activated anion channel VRAC has fascinated and frustrated physiologists since it was first described in 1988. Multiple laboratories have defined VRAC’s biophysical properties and have shown that it plays a central role in cell volume regulation and possibly other fundamental physiological processes. However, confusion and intense controversy surrounding the channel’s molecular identity greatly hindered progress in the field for >15 yr. A major breakthrough came in 2014 with the demonstration that VRAC is a heteromeric channel encoded by five members of the Lrrc8 gene family, Lrrc8A–E. A mere 4 yr later, four laboratories described cryo-EM structures of LRRC8A homomeric channels. As the melee of structure/function and physiology studies begins, it is critical that this work be framed by a clear understanding of VRAC biophysics, regulation, and cellular physiology as well as by the field’s past confusion and controversies. That understanding is essential for the design and interpretation of structure/function studies, studies of VRAC physiology, and studies aimed at addressing the vexing problem of how the channel detects cell volume changes. In this review we discuss key aspects of VRAC biophysics, regulation, and function and integrate these into our emerging understanding of LRRC8 protein structure/function.

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.

ATP release during cell swelling activates a Ca2+-dependent Cl- current by autocrine mechanism in mouse hippocampal microglia

Microglia cells, resident immune cells of the brain, survey brain parenchyma by dynamically extending and retracting their processes. Cl⁻ channels, activated in the cellular response to stretch/swelling, take part in several functions deeply connected with microglia physiology, including cell shape changes, proliferation, differentiation and migration. However, the molecular identity and functional properties of these Cl⁻ channels are largely unknown. We investigated the properties of swelling-activated currents in microglial from acute hippocampal slices of Cx3cr1^+/GFP mice by whole-cell patch-clamp and imaging techniques. The exposure of cells to a mild hypotonic medium, caused an outward rectifying current, developing in 5–10 minutes and reverting upon stimulus washout. This current, required for microglia ability to extend processes towards a damage signal, was carried mainly by Cl⁻ ions and dependent on intracellular Ca²⁺. Moreover, it involved swelling-induced ATP release. We identified a purine-dependent mechanism, likely constituting an amplification pathway of current activation: under hypotonic conditions, ATP release triggered the Ca²⁺-dependent activation of anionic channels by autocrine purine receptors stimulation. Our study on native microglia describes for the first time the functional properties of stretch/swelling-activated currents, representing a key element in microglia ability to monitor the brain parenchyma.

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.

Revisiting cAMP signaling in the carotid body

Chronic carotid body (CB) activation is now recognized as being essential in the development of hypertension and promoting insulin resistance; thus, it is imperative to characterize the chemotransduction mechanisms of this organ in order to modulate its activity and improve patient outcomes. For several years, and although controversial, cyclic adenosine monophosphate (cAMP) was considered an important player in initiating the activation of the CB. However, its relevance was partially displaced in the 90s by the emerging role of the mitochondria and molecules such as AMP-activated protein kinase and O₂-sensitive K⁺ channels. Neurotransmitters/neuromodulators binding to metabotropic receptors are essential to chemotransmission in the CB, and cAMP is central to this process. cAMP also contributes to raise intracellular Ca²⁺ levels, and is intimately related to the cellular energetic status (AMP/ATP ratio). Furthermore, cAMP signaling is a target of multiple current pharmacological agents used in clinical practice. This review (1) provides an outline on the classical view of the cAMP-signaling pathway in the CB that originally supported its role in the O₂/CO₂ sensing mechanism, (2) presents recent evidence on CB cAMP neuromodulation and (3) discusses how CB activity is affected by current clinical therapies that modify cAMP-signaling, namely dopaminergic drugs, caffeine (modulation of A_2A/A_2B receptors) and roflumilast (PDE4 inhibitors). cAMP is key to any process that involves metabotropic receptors and the intracellular pathways involved in CB disease states are likely to involve this classical second messenger. Research examining the potential modification of cAMP levels and/or interactions with molecules associated with CB hyperactivity is currently in its beginning and this review will open doors for future explorations.

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.

A role for the volume regulated anion channel in volume regulation in the murine CNS cell line, CAD

AIM

The role of the volume regulated anion channel (VRAC) in a model CNS neuronal cell line, CAD, was investigated.

METHODS

Changes in cell volume following hypotonic challenges were measured using a video-imaging technique. The effect of the Cl(-) channel antagonists tamoxifen (10 microm) and 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS; 100 microm) on regulatory volume decrease (RVD) were measured. The whole-cell voltage-clamp technique was used to characterize ICl(swell), the current underlying the VRAC.

RESULTS

Using the video-imaging technique, CAD cells were found to swell and subsequently exhibit RVD when subjected to a sustained hypotonic challenge from 300 mOsmol kg(-1) H(2)O to 210 mOsmol kg(-1) H(2)O. In the presence of tamoxifen (10 microm) or DIDS (100 microm) RVD was abolished, suggesting a role for the VRAC. A hypotonic solution (230 mOsmol kg(-1) H(2)O) evoked ICl(swell), an outwardly rectifying current displaying time-independent activation, which reversed upon return to isotonic conditions. The reversal potential (E(rev)) for ICl(swell) was -14.7 + or - 1.4 mV, similar to the theoretical E(rev) for a selective Cl(-) conductance. ICl(swell) was inhibited in the presence of DIDS (100 microm) and tamoxifen (10 microm), the DIDS inhibition being voltage dependent.

CONCLUSIONS

Osmotic swelling elicits an outwardly rectifying Cl(-) conductance in CAD cells. The ICl(swell) observed in these cells is similar to that observed in other cells, and is likely to provide a pathway for the loss of Cl(-) which leads to water loss and RVD. As ischaemia, brain trauma, hypoxia and other brain pathologies can cause cell swelling, CAD cells represent a model cell line for the study of neuronal cell volume regulation.

AQP1 mediates water transport in the carotid body

In this study, we explored the presence of aquaporins (AQPs), a family of membrane water channel proteins, in carotid body (CB) type I chemoreceptor cells. The CB is a polymodal chemoreceptor whose major function is to detect changes in arterial O2 tension to elicit hyperventilation during hypoxia. The CB has also been proposed to function as a systemic osmoreceptor, thus we hypothesized that the presence of AQPs in type I cell membrane may confer higher sensitivity to osmolarity changes and hence accelerate the activation of chemoreceptor cells. We detected the expression of AQP1, AQP7, and AQP8 in the CB and confirmed the location of AQP1 in type I cells. We have also shown that inhibition of AQP1 expression clearly reduced type I cell swelling after a hyposmotic shock, demonstrating that AQP1 has a major contribution in transmembrane water movement in these chemoreceptor cells. Interestingly, CB AQP1 expression levels change during postnatal development, increasing during the first postnatal weeks as the organ matures. In conclusion, in this study, we report the novel observation that AQPs are expressed in the CB. We also show that AQP1 mediates water transport across the cell membrane of type I cells, supporting the contribution of this protein to the osmoreception function of the CB.

EPAC signalling pathways are involved in low PO2 chemoreception in carotid body chemoreceptor cells

Chemoreceptor cells of the carotid bodies (CB) are activated by hypoxia and acidosis, responding with an increase in their rate of neurotransmitter release, which in turn increases the electrical activity in the carotid sinus nerve and evokes a homeostatic hyperventilation. Studies in isolated chemoreceptor cells have shown that moderate hypoxias ( 46 mmHg) produces smaller depolarisations and comparable Ca(2+) transients but a much higher catecholamine (CA) release response in intact CBs than intense acidic/hypercapnic stimuli (20% CO(2), pH 6.6). Similarly, intense hypoxia ( 20 mmHg) produces smaller depolarizations and Ca(2+) transients in isolated chemoreceptor cells but a higher CA release response in intact CBs than a pure depolarizing stimulus (30-35 mm external K(+)). Studying the mechanisms responsible for these differences we have found the following. (1) Acidic hypercapnia inhibited I(Ca) (60%; whole cell) and CA release (45%; intact CB) elicited by ionomycin and high K(+). (2) Adenylate cyclase inhibition (SQ-22536; 80 microm) inhibited the hypoxic release response (>50%) and did not affect acidic/hypercapnic release, evidencing that the high gain of hypoxia to elicit neurotransmitter release is cAMP dependent. (3) The last effect was independent of PKA activation, as three kinase inhibitors (H-89, KT 5720 and Rp-cAMP; 10 x IC(50)) did not alter the hypoxic release response. (4) The Epac (exchange protein activated by cAMP) activator (8-pCPT-2-O-Me-cAMP, 100 microm) reversed the effects of the cyclase inhibitor. (5) The Epac inhibitor brefeldin A (100 microm) inhibited (54%) hypoxic induced release. Our findings show for the first time that an Epac-mediated pathway mediates O(2) sensing/transduction in chemoreceptor cells.

Volume-activated chloride channels in mice Leydig cells

Production and secretion of testosterone in Leydig cells are mainly controlled by the luteinizing hormone (LH). Biochemical evidences suggest that the activity of Cl(-) ions can modulate the steroidogenic process, but the specific ion channels involved are not known. Here, we extend the characterization of Cl(-) channels in mice Leydig cells (50-60 days old) by describing volume-activated Cl(-) currents (I(Cl,swell)). The amplitude of I(Cl,swell) is dependent on the osmotic gradient across the cell membrane, with an apparent EC(50) of approximately 75 mOsm. These currents display the typical biophysical signature of volume-activated anion channels (VRAC): dependence on intracellular ATP, outward rectification, inactivation at positive potentials, and selectivity sequence (I(- )> Cl(- )> F(-)). Staurosporine (200 nM) did not block the activation of I(Cl,swell). The block induced by 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB; 128 microM), SITS (200 microM), ATP (500 microM), pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate (PPADS; 100 miccroM), and Suramin (10 microM) were described by the permeant blocker model with apparent dissociation constant at 0 mV K(do) and fractional distance of the binding site (delta) of 334 microM and 47 %, 880 microM and 35 %, 2,100 microM and 49%, 188 microM and 27%, and 66.5 microM and 49%, respectively. These numbers were derived from the peak value of the currents. We conclude that I(Cl,swell) in Leydig cells are activated independently of purinergic stimulation, that Suramin and PPADS block these currents by a direct interaction with VRAC and that ATP is able to permeate this channel.

Adequate stimuli of the carotid body: more than an oxygen sensor?

The past 10-20 years has seen a significant increase in the number of studies aimed at elucidating the mechanism of action of the carotid body and this has led to an increased knowledge of how this sensory organ transduces hypoxaemia into afferent chemodischarge. Whilst hypoxia is often considered as the most significant, peripheral chemostimulus, the carotid body is able to transduce many other physico-chemical stimuli, including not only arterial P(CO2) and pH but also blood potassium concentration, temperature and osmolarity as well as, potentially, blood glucose levels and all with appropriate physiological sensitivity. Although it is difficult to be definitive, these other stimuli appear to be sensed independently of the hypoxia transduction process, albeit converging at the point of type I cell membrane depolarisation or Ca(2+) -dependent neurosecretion. We suggest, therefore, that the carotid body might better be viewed as a polymodal receptor with its multiple adequate stimuli interacting to provide additive or greater than additive effects upon chemoafferent discharge for the purpose of cardiorespiratory homeostasis during periods of stress.

Prostanoid receptors regulate the volume-sensitive efflux of osmolytes from murine fibroblasts via a cyclic AMP-dependent mechanism

The ability of prostanoid receptors to regulate the volume-dependent efflux of the organic osmolyte taurine from murine fibroblasts (L cells) via a cAMP-dependent mechanism has been examined. Incubation of L cells under hypoosmotic conditions resulted in a time-dependent efflux of taurine, the threshold of release occurring at 250 mOsM. Addition of prostaglandin E(1) (PGE(1)) potently (EC(50) = 2.5 nM) enhanced the volume-dependent efflux of taurine at all time points examined and increased the threshold for osmolyte release to 290 mOsM. Maximal PGE(1) stimulation (250-300% of basal) of taurine release was observed at 250 mOsM. Of the PGE analogs tested, only the EP(2)-selective agonist butaprost (9-oxo-11alpha,16S-dihydroxy-17-cyclobutyl-prost-13E-en-1-oic acid) was able to enhance taurine efflux. Inclusion of 1,9-dideoxyfoskolin, 5-nitro-2-(3-phenylpropylamino) benzoic acid, or 4-[(2-butyl-6,7-dicloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden-5-yl)oxy]-butanoic acid blocked the ability of PGE(1) to enhance taurine release, indicating the mediation of a volume-sensitive organic osmolyte and anion channel. The ability of PGE(1) to increase osmolyte release from L cells was mimicked by the addition of agents that inhibit cAMP breakdown, directly activate adenylyl cyclase, or are cell-permeant analogs of cAMP. Taurine release elicited by either PGE(1) or 8-(4-chlorophenylthio)-cAMP was attenuated by >70% in L cells that had been stably transfected with a mutant regulatory subunit of cAMP-dependent protein kinase (PKA). PGE(1) stimulation of taurine efflux was not attenuated by either depletion of intracellular calcium or inhibition of protein kinase C. The results indicate that activation of prostanoid receptors on murine fibroblasts enhances osmolyte release via a cAMP and PKA-dependent mechanism.

Volume-sensitive chloride channels in mouse cortical neurons: characterization and role in volume regulation

Because persistent swelling causes cell damage and often results in cell death, volume regulation is an important physiological function in both neuronal and non-neuronal cells. Brain cell swelling has been observed not only in various pathological conditions but also during physiological synaptic transmissions. Volume-sensitive anion channels have been reported to play an important role in the regulatory volume decrease occurring after osmotic swelling in many cell types. In this study, using a two-photon laser scanning microscope and patch-clamp techniques, we found that mouse cortical neurons in primary culture exhibit regulatory volume decrease after transient swelling and activation of Cl- currents during exposure to a hypotonic solution. The regulatory volume decrease was inhibited by Cl- channel blockers or K+ channel blockers. Swelling-activated Cl- currents exhibited outward rectification, time-dependent inactivation at large positive potentials, a low-field anion permeability sequence, an intermediate unitary conductance and sensitivity to known blockers of volume-sensitive Cl- channels. Thus, it is concluded that the activity of the volume-sensitive outwardly rectifying Cl- channel plays a role in the control of cell volume in cortical neurons.

Selective block of swelling-activated Cl- channels over cAMP-dependent Cl- channels in ventricular myocytes

The objective of this study on guinea-pig and rabbit ventricular myocytes was to evaluate the sensitivities of swelling-activated Cl- current (ICl(swell)) and cAMP-dependent cystic fibrosis transmembrane regulator (CFTR) Cl- current (ICl(CFTR)) to block by dideoxyforskolin and verapamil. The currents were recorded from whole-cell configured myocytes that were dialysed with a Cs+-rich pipette solution and superfused with either isosmotic Na+-, K+-, Ca2+-free solution that contained 140 mM sucrose or hyposmotic sucrose-free solution. Forskolin-activated ICl(CFTR) was inhibited by reference blocker anthracene-9-carboxylic acid but unaffected by < or = 200 microM dideoxyforskolin and verapamil. However, dideoxyforskolin and verapamil had strong inhibitory effects on outwardly-rectifying, inactivating, distilbene-sensitive ICl(swell); IC50 values were approximately 30 microM, and blocks were voltage-independent and reversible. The results establish that dideoxyforskolin and verapamil can be used to distinguish between ICl(CFTR) and ICl(swell) in heart cells, and expand the pharmacological characterization of cardiac ICl(swell).

Volume-sensitive Cl- current in bovine adrenocortical cells: comparison with the ACTH-induced Cl- current

In a previous study performed on zona fasciculata (ZF) cells isolated from calf adrenal glands, we identified an ACTH-induced Cl- current involved in cell membrane depolarization. In the present work, we describe a volume-sensitive Cl- current and compare it with the ACTH-activated Cl- current. Experiments were performed using the whole-cell patch-clamp recording method, video microscopy and cortisol-secretion measurements. In current-clamp experiments, hypotonic solutions induced a membrane depolarization to -22 mV. This depolarization, correlated with an increase in the membrane conductance, was sensitive to different Cl- channel inhibitors. In voltage-clamp experiments, hypotonic solution induced a membrane current that slowly decayed and reversed at -21 mV. This ionic current displayed no time dependence and showed a slight outward rectification. It was blocked to variable extent by different conventional Cl- channel inhibitors. Under hypotonic conditions, membrane depolarizations were preceded by an increase in cell volume that was not detected under ACTH stimulation. It was concluded that hypotonic solution induced cell swelling, which activated a Cl- current involved in membrane depolarization. Although cell volume change was not observed in the presence of ACTH, biophysical properties and pharmacological profile of the volume-sensitive Cl- current present obvious similarities with the ACTH-activated Cl- current. As compared to ACTH, hypotonic solutions failed to trigger cortisol production that was weakly stimulated in the presence of high-K+ solution. This shows that in ZF cells, membrane depolarization is not a sufficient condition to fully activate secretory activities.

Prostaglandin E2 activates outwardly rectifying Cl(-) channels via a cAMP-dependent pathway and reduces cell motility in rat osteoclasts

We examined changes in electrical and morphological properties of rat osteoclasts in response to prostaglandin (PG)E(2). PGE(2) (>10 nM) stimulated an outwardly rectifying Cl(-) current in a concentration-dependent manner and caused a long-lasting depolarization of cell membrane. This PGE(2)-induced Cl(-) current was reversibly inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB), and tamoxifen. The anion permeability sequence of this current was I(-) > Br(-) approximately Cl(-) > gluconate(-). When outwardly rectifying Cl(-) current was induced by hyposmotic extracellular solution, no further stimulatory effect of PGE(2) was seen. Forskolin and dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP) mimicked the effect of PGE(2). The PGE(2)-induced Cl(-) current was inhibited by pretreatment with guanosine 5'-O-2-(thiodiphosphate) (GDPbetaS), Rp-adenosine 3',5'-cyclic monophosphorothioate (Rp-cAMPS), N-(2-[p-bromocinnamylamino]ethyl)-5-isoquinolinesulfonamide dihydrochloride (H-89), and protein kinase A inhibitors. Even in the absence of nonosteoclastic cells, PGE(2) (1 microM) reduced cell surface area and suppressed motility of osteoclasts, and these effects were abolished by Rp-cAMPS or H-89. PGE(2) is known to exert its effects through four subtypes of PGE receptors (EP1-EP4). EP2 and EP4 agonists (ONO-AE1-259 and ONO-AE1-329, respectively), but not EP1 and EP3 agonists (ONO-DI-004 and ONO-AE-248, respectively), mimicked the electrical and morphological actions of PGE(2) on osteoclasts. Our results show that PGE(2) stimulates rat osteoclast Cl(-) current by activation of a cAMP-dependent pathway through EP2 and, to a lesser degree, EP4 receptors and reduces osteoclast motility. This effect is likely to reduce bone resorption.

Effects of osmotic changes on the chemoreceptor cell of rat carotid body

The carotid body plays a crucial role in cardiorespiratory regulation. In the present study we investigated the effect of osmotic changes on cytoplasmic calcium concentration ([Ca(2+)](c)) and pH (pH(i)) of isolated chemoreceptor cells of the rat carotid body. In CO(2)/HCO(3)(-)-buffered medium, reduction of osmolality from the control level of 300 mosmol kg(-1) to 250-285 mosmol kg(-1) resulted in a rise in [Ca(2+)](c), as measured with Indo-1, whereas elevation of osmolality to 350 mosmol kg(-1) had no effect. The Ca(2+) response required extracellular Ca(2+) and was reduced by application of the L-type Ca(2+) channel antagonist nifedipine (10 microM). The hyposmosis-induced Ca(2+) response could be prevented by application of niflumic acid (300 microM), an inhibitor of the swelling-activated Cl(-) channel. In whole-cell patch-clamp experiments niflumic acid abolished the swelling-activated Cl(-) current but only slightly depressed the Ca(2+) current. The inhibition of Ca(2+) current by niflumic acid does not account for its action in preventing of hyposmosis-induced Ca(2+) response, which seems to be initiated by Cl(-)-mediated depolarisation. Withdrawal of CO(2)/HCO(3)(-) also prevented the Ca(2+) response. Reduction of the osmotic concentration by 50 mosmol kg(-1) induced a small but sustained decrease in pH(i), while elevation by 50 mosmol kg(-1) had an inverse effect, as measured fluorimetrically with carboxy SNARF-1. Our conclusion is that in the rat chemoreceptor cell the activation of Cl(-) channels, e.g. by hyposmotic challenge, induces depolarisation, which, in turn, activates voltage-gated Ca(2+) channels.

Impaired activity of volume-sensitive anion channel during lactacidosis-induced swelling in neuronally differentiated NG108-15 cells

Acidosis coupled to lactate accumulation, called lactacidosis, occurs in cerebral ischemia or trauma and is known to cause persistent swelling in neuronal and glial cells. It is therefore possible that mechanisms of cell volume regulation are impaired during lactacidosis. Here we tested this possibility using neuronally differentiated NG108-15 cells. These cells responded to a hypotonic challenge with osmotic swelling followed by a regulatory volume decrease (RVD) under physiological pH conditions in the absence of lactate. Under normotonic conditions, sustained cell swelling without subsequent RVD was induced by exposure to lactate-containing solution with acidic pH (6.4 or 6.2), but not with physiological pH (7.4). Under whole-cell patch-clamp, osmotic swelling was found to activate outwardly rectifying Cl(-) currents in cells exposed to control hypotonic solution. A Cl(-) channel blocker, NPPB, inhibited both RVD and the swelling-activated Cl(-) current. RVD and the volume-sensitive Cl(-) current were also markedly inhibited by lactacidosis (pH 6.4 or 6.2), but neither by application of lactate with physiological pH (7.4) nor by acidification without lactate (pH 6.2). RT-PCR analysis showed mRNA expression of two isoforms of proton-coupled monocarboxylate transporters, MCT1 and MCT8, in differentiated NG108-15 cells. Thus, we conclude that persistence of neuronal cell swelling under lactacidosis is coupled to an impairment of the activity of the volume-sensitive Cl(-) channel and to dysfunction of RVD. It is also suggested that the volume-sensitive Cl(-) channel is inhibited by intracellular acidification induced by MCT-mediated proton influx under lactacidosis.

GABA and chloride permeate via the same channels across single plasma membranes microdissected from rabbit Deiters' vestibular neurones

The permeation of labelled gamma-aminobutyric acid (GABA) across single microdissected Deiters' membranes has been studied in a microchamber system. The GABA permeation is via pores which are blocked by 4,4'-diisothiocyanato stilbene-2-2'disulphonic acid (DIDS). As this substance blocks as well chloride permeation across these membranes we tested whether GABA and chloride permeate across the same pores. Membrane pre-treatment with different doses of corticotropin releasing factor (CRF), a membrane permeant cyclic AMP analogue and phalloidin parallelly block the permeation of the two substances. Thus, it is most probable that GABA and chloride pass across the same pores. These pores may be swelling activated ones, opened by the mechanical stress on the membranes in the microchamber system. The passage of GABA across these pores may be of physiological importance in the termination of GABA inhibitory action on the vestibular Deiters' neurones.

A pH-sensitive chloride current in the chemoreceptor cell of rat carotid body

1. Cardiorespiratory response to acidosis is initiated by the carotid body. 2. The direct effect of extracellular pH (pH(o)) on the chloride currents of isolated chemoreceptor cells of the rat carotid body was investigated using the whole-cell patch-clamp technique. 3. On applying intra- and extracellular solutions with a symmetrical high-Cl(-) content and with the monovalent cations replaced with membrane-impermeant ones, an inwardly rectifying Cl(-) current was found. 4. The current activated slowly and did not display any time-dependent inactivation. Current activation was present at membrane potentials negative to 0 mV (pH(o) = 7.0). 5. The current was activated by extracellular acidosis and inhibited by alkalosis in the physiologically relevant pH range of 7.0-7.8. 6. The current was reduced by 0.1 mM Cd2+ to the level of the leak current and by 1 mM anthracene-9-carboxylic acid (9-AC) to about 40 %, while 0.1 mM Ba2+ had no effect. 7. Application of 1 mM 9-AC caused a slow but statistically significant increase in the resting pH(i) (from a mean of 7.29 to 7.37 in 5 min) in clusters of chemoreceptor cells in CO(2)/HCO3(-)-buffered media as measured with carboxy-SNARF-1. 8. When membrane potential changes were estimated in the cell-attached mode, 1 mM 9-AC hyperpolarized three out of five tested cells (by 14 mV in average) incubated in CO(2)/HCO3(-)-buffered media. 9. In summary, chemoreceptor cells express an inwardly rectifying Cl(-) current, which is directly regulated by pH(o). The current may participate in intracellular acidification and membrane depolarization during acidic challenge.

Na+-alanine uptake activates a Cl- conductance in frog renal proximal tubule cells via nonconventional PKC

Hyposmotically induced swelling of frog renal proximal tubule cells activates a DIDS-sensitive, outwardly rectifying Cl- conductance via a conventional protein kinase C (PKC). This study examines whether Na+-alanine cotransport similarly activates a DIDS-sensitive Cl- conductance in frog renal proximal tubule cells. On stimulation of Na+-alanine cotransport, the DIDS-sensitive current (I(DIDS-Ala)) increased markedly over time. I(DIDS-Ala) exhibited outward rectification, a Na+/Cl- selectivity ratio of 0.19 +/- 0.03, and an anion selectivity sequence Br- = Cl- > I- > gluconate-. Activation of I(DIDS-Ala) was dependent on ATP hydrolysis and PKC-mediated phosphorylation and was inhibited by hyperosmotic conditions. Activation could be not ascribed to a conventional PKC isoform, as I(DIDS-Ala) was not affected by removing Ca2+ or by phorbol ester treatment, suggesting a role for a nonconventional PKC isoform, either novel or atypical. We conclude that Na+-alanine cotransport activates a DIDS-sensitive Cl- conductance via a nonconventional PKC isoform. This contrasts with the hyposmotically activated Cl- conductance, which requires conventional PKC activation.

Chloride channels regulate HIT cell volume but cannot fully account for swelling-induced insulin secretion

Insulin-secreting pancreatic islet beta-cells possess anion-permeable Cl- channels (I(Cl,islet)) that are swelling-activated, but the role of these channels in the cells is unclear. The Cl- channel blockers 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) and niflumic acid were evaluated for their ability to inhibit I(Cl,islet) in clonal beta-cells (HIT cells). Both drugs blocked the channel, but the blockade due to niflumic acid was less voltage-dependent than the blockade due to DIDS. HIT cell volume initially increased in hypotonic solution and was followed by a regulatory volume decrease (RVD). The addition of niflumic acid and, to a lesser extent, DIDS to the hypotonic solution potentiated swelling and blocked the RVD. In isotonic solution, niflumic acid produced swelling, suggesting that islet Cl- channels are activated under basal conditions. The channel blockers glyburide, gadolinium, or tetraethylammonium-Cl did not alter hypotonic-induced swelling or volume regulation. The Na/K/2Cl transport blocker furosemide produced cell shrinkage in isotonic solution and blocked cell swelling normally induced by hypotonic solution. Perifused HIT cells secreted insulin when challenged with hypotonic solutions. However, this could not be completely attributed to I(Cl,islet)-mediated depolarization, because secretion persisted even when Cl- channels were fully blocked. To test whether blocker-resistant secretion occurred via a distal pathway, distal secretion was isolated using 50 mmol/l potassium and diazoxide. Under these conditions, glucose-dependent secretion was blunted, but hypotonically induced secretion persisted, even with Cl- channel blockers present. These results suggest that beta-cell swelling stimulates insulin secretion primarily via a distal I(Cl,islet)-independent mechanism, as has been proposed for K(ATP)-independent glucose- and sulfonylurea-stimulated insulin secretion. Reverse transcriptase-polymerase chain reaction of HIT cell mRNA identified a CLC-3 transcript in HIT cells. In other systems, CLC-3 is believed to mediate swelling-induced outwardly rectifying Cl- channels. This suggests that the proximal effects of swelling to regulate cell volume may be mediated by CLC-3 or a closely related Cl- channel.

Receptor-mediated control of regulatory volume decrease (RVD) and apoptotic volume decrease (AVD)

A fundamental property of animal cells is the ability to regulate their own cell volume. Even under hypotonic stress imposed by either decreased extracellular or increased intracellular osmolarity, the cells can re-adjust their volume after transient osmotic swelling by a mechanism known as regulatory volume decrease (RVD). In most cell types, RVD is accomplished mainly by KCl efflux induced by parallel activation of K+ and Cl- channels. We have studied the molecular mechanism of RVD in a human epithelial cell line (Intestine 407). Osmotic swelling results in a significant increase in the cytosolic Ca2+ concentration and thereby activates intermediate-conductance Ca2+-dependent K+ (IK) channels. Osmotic swelling also induces ATP release from the cells to the extracellular compartment. Released ATP stimulates purinergic ATP (P2Y2) receptors, thereby inducing phospholipase C-mediated Ca2+ mobilization. Thus, RVD is facilitated by stimulation of P2Y2 receptors due to augmentation of IK channels. In contrast, stimulation of another G protein-coupled Ca2+-sensing receptor (CaR) enhances the activity of volume-sensitive outwardly rectifying Cl- channels, thereby facilitating RVD. Therefore, it is possible that Ca2+ efflux stimulated by swelling-induced and P2Y2 receptor-mediated intracellular Ca2+ mobilization activates the CaR, thereby secondarily upregulating the volume-regulatory Cl- conductance. On the other hand, the initial process towards apoptotic cell death is coupled to normotonic cell shrinkage, called apoptotic volume decrease (AVD). Stimulation of death receptors, such as TNF receptor and Fas, induces AVD and thereafter biochemical apoptotic events in human lymphoid (U937), human epithelial (HeLa), mouse neuroblastoma x rat glioma hybrid (NG108-15) and rat phaeochromocytoma (PC12) cells. In those cells exhibiting AVD, facilitation of RVD is always observed. Both AVD induction and RVD facilitation as well as succeeding apoptotic events can be abolished by prior treatment with a blocker of volume-regulatory K+ or Cl- channels, suggesting that AVD is caused by normotonic activation of ion channels that are normally involved in RVD under hypotonic conditions. Therefore, it is likely that G protein-coupled receptors involved in RVD regulation and death receptors triggering AVD may share common downstream signals which should give us key clues to the detailed mechanisms of volume regulation and survival of animal cells. In this Topical Review, we look at the physiological ionic mechanisms of cell volume regulation and cell death-associated volume changes from the facet of receptor-mediated cellular processes.

Activation of volume-regulated Cl(-) channels by ACh and ATP in Xenopus follicles

Osmolarity-dependent ionic currents from follicle-enclosed Xenopus oocytes (follicles) were studied using electrophysiological techniques. Whole follicle currents were monitored using a two-electrode voltage clamp and single-channel activity was measured using the patch-clamp technique. In follicles held at -60 mV two chloride currents were activated in external hyposmotic solutions. One was the habitual volume-regulated current elicited by external hyposmolarity (ICl,swell), and the second was a slow and smooth current (Sin) generated by ACh or ATP application. In follicles, the permeability ratios for different anions with respect to Cl- were similar for both ICl,swell and Sin, with a sequence of: SCN- > I- > Br- >= NO3- >= Cl- > gluconate >= cyclamate > acetate > SO42-. Extracellular ATP blocked the outward component of Sin. Also, extracellular pH modulated the inactivation kinetics of Sin elicited by ACh; e.g. inactivation at +80 mV was approximately 100 % slower at pH 8.0 compared with that at pH 6.0. Lanthanides inhibited ICl, swell and Sin. La3+ completely inhibited ICl,swell with a half-maximal inhibitory concentration (IC50) of 17 +/- 1.9 microM, while Sin was blocked up to 55 % with an apparent IC50 of 36 +/- 2.6 microM. Patch-clamp recordings in follicular cells showed that hyposmotic challenge opened inward single-channel currents. The single channel conductance (4.7 +/- 0.4 pS) had a linear current-voltage relationship with a reversal membrane potential close to -20 mV. This single-channel activity was increased by application of ACh or ATP. The ICl,swell generation was not affected by pirenzepine or metoctramine, and did not affect the purinergic activation of the chloride current named Fin. Thus, ICl,swell was not generated via neurotransmitters released during cellular swelling. All together, equal discrimination for different anions, similar modulatory effects by extracellular pH, the blocking effects by ATP and La3+, and the same single-channel activity, strongly suggest that ICl,swell and Sin currents depend on the opening of the same type or a closely related class of volume-regulated chloride channels.

Effects of hypoxia and dithionite on catecholamine release from isolated type I cells of the rat carotid body

1. Amperometric recordings were conducted to investigate the ability of hypoxia and anoxia to evoke quantal catecholamine secretion from isolated type I cells of the rat carotid body. 2. Hypoxia (PO2 8-14 mmHg) consistently failed to evoke catecholamine secretion from type I cells, when cells were perfused either at room temperature (21-24 C) or at 35-37 C, and regardless of whether Hepes- or HCO3-/CO2-buffered solutions were used. 3. Elevating extracellular [K+] caused concentration-dependent secretion from individual type I cells, with a threshold concentration of approximately 25 mM. In the presence of this level of extracellular K+, hypoxia (PO2 8-14 mmHg) caused a marked enhancement of secretion which was fully blocked by 200 microM Cd2+, a non-specific blocker of voltage-gated Ca2+ channels. 4. Anoxia (N2-equilibrated solution containing 0.5 mM dithionite) evoked exocytosis from type I cells when extracellular [K+] was 5 mM. This secretion was completely inhibited by removal of extracellular Ca2+, but was not significantly affected by Cd2+ (200 microM), Ni2+ (2 mM), Zn2+ (1 mM) or nifedipine (2 microM). Secretion was also observed when 0.5 mM dithionite was added to air-equilibrated solutions. 5. Anoxia also evoked secretion from chemoreceptive phaeochromocytoma (PC12) cells, which was wholly Ca2+ dependent, but unaffected by Cd2+ (200 microM). 6. Our results suggest that hypoxia can evoke catecholamine secretion from isolated type I cells, but only in the presence of elevated extracellular [K+]. This may be due to the cells being relatively hyperpolarized following dissociation. In addition, we have shown that dithionite evokes catecholamine release regardless of PO2 levels, and this release is due mainly to an artefactual Ca2+ influx pathway activated in the presence of dithionite.

Intracellular ATP depletion inhibits swelling-induced D-[3H]aspartate release from primary astrocyte cultures

Volume expansion-sensing outward rectifier (VSOR) anion channel, also referred to as volume-sensitive organic osmolyte-anion channel (VSOAC), appears to be responsible for cell swelling-induced amino acid release in a variety of cells. One prominent feature of the VSOR/VSOAC is that non-hydrolyzed intracellular ATP binding to the channel or an accessory protein is required for its activation. In this study, the effect of intracellular ATP depletion on the swelling-induced release of D-[3H]aspartate from rat primary astrocyte cultures due to exposure to either high K(+) or hypotonic media was studied. When the cells were pretreated for 10 min with a combination of the metabolic inhibitors 2-deoxyglucose and rotenone, 100 mM K(+) media- or hypotonic media-induced D-[3H]aspartate release was completely suppressed. Added separately, each inhibitor showed only partial or no inhibition of D-[3H]aspartate release, which correlated with its relative effectiveness in decreasing intracellular ATP levels. These data are consistent with the view that during high [K(+)](o) or hypotonic media-induced swelling of primary astrocyte cultures an ATP-dependent swelling-activated VSOAC channel is responsible for D-[3H]aspartate release and close to normal ATP is required for full channel activation.

ATP dependence of the ICl,swell channel varies with rate of cell swelling. Evidence for two modes of channel activation

Swelling-induced activation of the outwardly rectifying anion current, ICl, swell, is modulated by intracellular ATP. The mechanisms by which ATP controls channel activation, however, are unknown. Whole cell patch clamp was employed to begin addressing this issue. Endogenous ATP production was inhibited by dialyzing N1E115 neuroblastoma cells for 4-5 min with solutions containing (microM): 40 oligomycin, 5 iodoacetate, and 20 rotenone. The effect of ATP on current activation was observed in the absence of intracellular Mg2+, in cells exposed to extracellular metabolic inhibitors for 25-35 min followed by intracellular dialysis with oligomycin, iodoacetate, and rotenone, after substitution of ATP with the nonhydrolyzable analogue AMP-PNP, and in the presence of AMP-PNP and alkaline phosphatase to dephosphorylate intracellular proteins. These results demonstrate that the ATP dependence of the channel requires ATP binding rather than hydrolysis and/or phosphorylation reactions. When cells were swollen at 15-55%/min in the absence of intracellular ATP, current activation was slow (0.3-0.8 pA/pF per min). ATP concentration increased the rate of current activation up to maximal values of 4-6 pA/pF per min, but had no effect on the sensitivity of the channel to cell swelling. Rate of current activation was a saturable, hyperbolic function of ATP concentration. The EC50 for ATP varied inversely with the rate of cell swelling. Activation of current was rapid (4-6 pA/pF per min) in the absence of ATP when cells were swollen at rates >/=65%/min. Intracellular ATP concentration had no effect on current activation induced by high rates of swelling. Current activation was transient when endogenous ATP was dialyzed out of the cytoplasm of cells swollen at 15%/min. Rundown of the current was reversed by increasing the rate of swelling to 65%/min. These results indicate that the channel and/or associated regulatory proteins are capable of sensing the rate of cell volume increase. We suggest that channel activation occurs via ATP-dependent and -independent mechanisms. Increasing the rate of cell swelling appears to increase the proportion of channels activating via the ATP-independent pathway. These findings have important physiological implications for understanding ICl, swell regulation, the mechanisms by which cells sense volume changes, and volume homeostasis under conditions where cell metabolism is compromised.

Synergetic activation of outwardly rectifying Cl- currents by hypotonic stress and external Ca2+ in murine osteoclasts

1. An outwardly rectifying Cl- (ORCl) current of murine osteoclasts was activated by hypotonic stimulation. The current was characterized by rapid activation, little inactivation, strong outward rectification, blockage by DIDS and permeability to organic acids (pyruvate and glutamate). 2. The hypotonically activated ORCl current was inhibited by intracellular dialysis with an ATP-free pipette solution, but not by replacement of ATP with a poorly hydrolysable ATP analogue adenosine 5'-O-(3-thiotriphosphate). The current amplitude was reduced when intracellular alkalinity increased over the pH range 6.6-8.0. 3. Intracellular application of cytochalasin D occasionally activated the ORCl current without hypotonic stress, but inhibited activation of the ORCl current by hypotonic stimulation. The hypotonically activated ORCl current was unaffected by a non-actin-depolymerizing cytochalasin, chaetoglobosin C, but partially inhibited by deoxyribonuclease I. 4. Removal of extracellular Ca2+ inhibited activation of the ORCl current by hypotonic shock, but did not reduce the current once activated. The hypotonically activated ORCl current was partially decreased by intracellular dialysis with 20 mM EGTA. 5. With 10 mM Ca2+ in the extracellular medium, the ORCl current was activated in response to more minor decreases in osmolarity than with 1 mM Ca2+. The increased sensitivity to hypotonicity was mimicked by increasing the intracellular Ca2+ level (pCa 6.5). 6. These results suggest that hypotonic stimulation and a rise in the extracellular Ca2+ level synergistically activate the ORCl channel of murine osteoclasts, and that the activating process is modified by multiple intracellular factors (pH, ATP and actin cytoskeletal organization).

Ionic currents in carotid body type I cells isolated from normoxic and chronically hypoxic adult rats

Whole-cell recordings were used to investigate the effects of a 3-week period of hypoxia (10% O2) on the properties of K+ and Ca2+ currents in type I cells isolated from adult rat carotid bodies. Chronic hypoxia significantly increased whole-cell membrane capacitance. K+ current amplitudes were not affected by this period of hypoxia, but K+ current density was significantly reduced in cells from chronically hypoxic rats as compared with normoxically maintained, age-matched controls. K+ current density was separated into Ca2+-dependent and Ca2+-independent components by bath application of 200 microM Cd2+, which blocked Ca2+ currents and therefore, indirectly, Ca2+-dependent K+ currents. Ca2+-dependent K+ current density was not significantly different in control and chronically hypoxic type I cells. Cd2+-resistant (Ca2+-insensitive) K+ current densities were significantly reduced in type I cells from chronically hypoxic rats. Acute hypoxia (Po2 15-22 mmHg) caused reversible, selective inhibition of Ca2+-dependent K+ currents in both groups of cells and Ca2+-insensitive K+ currents were unaffected by acute hypoxia. Ca2+ channel current density was not significantly affected by chronic hypoxia, nor was the degree of Ca2+ channel current inhibition caused by nifedipine (5 microM). Acute hypoxia did not affect Ca2+ channel currents in either group. Our results indicate that adult rat type I cells undergo a selective suppression of Ca2+-insensitive, voltage-gated K+ currents in response to chronic hypoxia in vivo. These findings are discussed in relation to the known adaptations of the intact carotid body to chronic hypoxia.

Inhibition of Ca2+-dependent K+ channels in rat carotid body type I cells by protein kinase C

1. Whole-cell patch clamp recordings were used to investigate the effects of protein kinase C (PKC) activation on K+ and Ca2+ currents in type I cells isolated from the rat carotid body. 2. Pretreatment of cells for 10 min at 37 C with 4alpha-phorbol 12,13-didecanoate (4alpha-PDD, 200 nM), a phorbol ester which does not activate PKC, did not affect K+ current density as compared with cells pretreated with vehicle alone. By contrast, identical pretreatment with 200 nM 12-O-teradecanoylphorbol-13-acetate (TPA, a PKC activator) dramatically reduced K+ current density in type I cells. This effect was prevented by co-incubation of cells with the PKC inhibitor bisindolylmaleimide (BIM, 3 microM). 3. The sensitivity of K+ currents to inhibition by 200 microM Cd2+ (indicative of the presence of Ca2+-dependent K+ channels) was markedly reduced in TPA-treated cells as compared with sham-treated cells, cells treated with 4alpha-PDD, and cells treated with both TPA and BIM. Cd2+-resistant K+ current densities were of similar magnitude in all four groups of cells, as were the input resistances determined over the voltage range -100 mV to -50 mV. 4. Ca2+ channel current density was not significantly different in type I cells pretreated with 200 nM 4alpha-PDD as compared with cells treated with the same concentration of TPA. 5. The degree of inhibition of K+ currents caused by hypoxia (Po2 15-20 mmHg) was unaltered by pretreatment of cells with 3 microM BIM. 6. The resting membrane potential of cells pretreated with TPA was depolarized as compared with controls, and the Ca2+-dependent K+ channel inhibitor iberiotoxin (20 nM) failed to depolarize these cells further. 7. Our results suggest that activation of PKC causes a marked, selective inhibition of Ca2+-dependent K+ currents in type I carotid body cells, but that PKC activation is unlikely to account for inhibition of these channels by acute hypoxia.