Hypoxic induction of T-type Ca(2+) channels in rat cardiac myocytes: role of HIF-1α and RhoA/ROCK signalling

T-type Ca(2+) channels are expressed in the ventricular myocytes of the fetal and perinatal heart, but are normally downregulated as development progresses. Interestingly, however, these channels are re-expressed in adult cardiomyocytes under pathological conditions. We investigated low voltage-activated T-type Ca(2+) channel regulation in hypoxia in rat cardiomyocytes. Molecular studies revealed that hypoxia induces the upregulation of Cav 3.2 mRNA, whereas Cav 3.1 mRNA is not significantly altered. The effect of hypoxia on Cav 3.2 mRNA was time- and dose-dependent, and required hypoxia inducible factor-1α (HIF-1α) stabilization. Patch-clamp recordings confirmed that T-type Ca(2+) channel currents were upregulated in hypoxic conditions, and the addition of 50 μm NiCl2 (a T-type channel blocker) demonstrated that the Cav 3.2 channel is responsible for this upregulation. This increase in current density was not accompanied by significant changes in the Cav 3.2 channel electrophysiological properties. The small monomeric G-protein RhoA and its effector Rho-associated kinase I (ROCKI), which are known to play important roles in cardiovascular physiology, were also upregulated in neonatal rat ventricular myocytes subjected to hypoxia. Pharmacological experiments indicated that both proteins were involved in the observed upregulation of the Cav 3.2 channel and the stabilization of HIF-1α that occurred in response to hypoxia. These results suggest a possible role for Cav 3.2 channels in the increased probability of developing arrhythmias observed in ischaemic situations, and in the pathogenesis of diseases associated with hypoxic Ca(2+) overload.

How to unfasten the Spanish Stroke Belt? Andalusia chooses research

Andalusia in southern Spain, one of the largest regions in the European Union, has made a profound economic and social transformation that has led to establishment of a modern universal public health care system. However, due to its high stroke mortality rates, Andalusia is still known as the 'Spanish Stroke Belt'. To fight these figures, successive initiatives culminated in the launch of the Andalusian Plan for Stroke Care, to be developed during the period of 2011 to 2014. In addition, involved professionals have hypothesized that clinical and experimental research may contribute to improving stroke care in our community. To that end, one of the leading institutes of biomedical research in Andalusia, the Institute of Biomedicine of Seville (IBiS), has selected stroke as a flagship project in the region. Moreover, Seville, the capital of Andalusia, is now conducting a fusion process of its two largest hospitals, with the potential to generate a stroke alliance that will make it one of the main stroke hospitals in Europe (>2000 cases per year). It is anticipated that this will be an excellent platform to facilitate acute-phase clinical trials and speed the translation process from basic research in IBiS laboratories to the clinical setting. Furthermore, the recently created Andalusian Neurovascular Group is ready to develop prospective, collaborative, multicenter research projects that will evaluate interventions in areas of stroke care uncertainty. If we succeed in forging a link between research and health care quality, we may succeed in lowering the incidence of stroke and related mortality in the region in a short period of time.

Heat shock protein 70 kDa over-expression and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced nigrostriatal degeneration in mice

Oxidative damage in the dopaminergic neurons of substantia nigra pars compacta (SNpc) plays an important role in the pathogenesis of Parkinson's disease (PD). Heat shock proteins 70 kDa (HSP70s) are a sub-family of molecular chaperones involved in not only protein folding and degradation but also antioxidant defense and anti-apoptotic pathways. Here, a transgenic mice over-expressing an inducible form of Hsp70 was used to determine whether HSP70 affects 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced nigrostriatal degeneration, an experimental model of PD. The Hsp70 transgenic animals exhibited a high level of expression of HSP70 protein in ventral mesencephalon. Dopaminergic cell death in the SNpc was similar between wild-type and Hsp70 transgenic mice with either acute (40 mg/kg, single dose) or chronic (20 mg/kg, three times/week during 1 month) MPTP treatment. In addition, striatal dopamine loss was not different between wild-type and transgenic animals. Three months after the acute MPTP treatment, dopamine loss was partially recovered into a similar level between wild-type and transgenic groups. In conclusion, over-expression of Hsp70 does not suppress dopaminergic neuronal damage at either the somata or the axon terminals of dopaminergic neurons. Hsp70 over-expression does not help axon terminal regeneration either. These results indicate that HSP70 alone is not sufficient to reduce MPTP-induced dopaminergic neuronal damage.

First aid kit for hypoxic survival: sensors and strategies

Survival success under conditions of acute oxygen deprivation depends on efficiency of the central and peripheral chemoreception, optimization of oxygen extraction from the hypoxic environment and its delivery to the periphery, and adjustments of energy production and consumption. This article uses a comparative approach to assess the efficiency of adaptive strategies used by anoxia-tolerant and hypoxia-sensitive species to support survival during the first minutes to 1 h of oxygen deprivation. An aquatic environment is much more demanding in terms of diurnal and seasonal variations of the ambient oxygen availability from anoxia to hyperoxia than is an air environment. Therefore, fishes and aquatic turtles have developed a number of adaptive responses, which are lacking in most of the terrestrial mammals, to cope with these extreme conditions. These include efficient central and peripheral chemoreception, acute changes in respiratory rate and amplitude, and acute increase of the gas-exchange interface. A special set of adaptive mechanisms are engaged in reduction of the energy expenditure of the major oxygen-consuming organs: the brain and the heart. Both reduction of ATP consumption and a switch to alterative energy sources contribute to the maintenance of ATP and ion balance in hypoxia-tolerant animals. Hypoxia and hyperoxia are conditions favoring development of oxidative stress. Efficient protection from oxidation in anoxia-tolerant species includes reduction in the glutamate levels in the brain, stabilization of the mitochondrial function, and maintenance of nitric oxide production under conditions of oxygen deprivation. We give an overview of the current state of knowledge on some selected molecular and cellular acute adaptive mechanisms. These include the mechanisms of chemoreception in adult and neonatal mammals and in fishes, acute metabolic adaptive responses in the brain, and the role of nitrite in the preservation of heart function under hypoxic conditions.

Brain-derived neurotrophic factor G196A polymorphism and clinical features in Parkinson's disease

OBJECTIVES

Parkinson's disease (PD) is characterized by the dopaminergic neuronal death in substantia nigra, and genetic factors appear to be involved in the pathophysiology of this disease. Brain-derived neurotrophic factor (BDNF) is widely expressed in the central nervous system and is necessary for the survival of dopaminergic neurons in substantia nigra. G196A, a common polymorphism of the BDNF gene, not only affects cognitive and motor processes, but also is associated with various psychiatric disorders. We evaluated whether G196A polymorphism is associated with PD and/or modifies clinical manifestations in PD patients.

METHODS

We included 193 PD patients and 300 control subjects. G196A polymorphism was screened by restriction fragment length polymorphism analysis. Clinical features of each patient were examined in detail. The possible association between genotype and clinical characteristics were determined by bivariate and multivariate analyses.

RESULTS

The distribution of G196A allele and genotype frequency was similar between PD and control subjects. Clinical characteristics, including Hoehn-Yahr stage, motor symptoms, non-motor symptoms (depression, cognitive dysfunction, psychiatric dysfunctions, and sleep behavior disorder), and dyskinesias, were not associated with this polymorphism.

CONCLUSIONS

G196A polymorphism is not a risk factor for PD and does not seem to modify clinical features in PD patients studied here.

The carotid body, a neurogenic niche in the adult peripheral nervous system

We have described a new population of adult neural stem cells residing in the carotid body, a chemoreceptor organ in the peripheral nervous system. These progenitor cells support neurogenesis in vivo in response to physiological stimuli like hypoxemia, and give rise to multipotent neurospheres in culture. Studying the biology of CB stem cells helps to understand the physiological adaptations of the organ, and might shed light on the pathogenesis of CB tumors. Understanding proliferation and differentiation of these cells will enable their use for cell therapy against neurodegenerative diseases.

Carotid body oxygen sensing

The carotid body (CB) is a neural crest-derived organ whose major function is to sense changes in arterial oxygen tension to elicit hyperventilation in hypoxia. The CB is composed of clusters of neuron-like glomus, or type-I, cells enveloped by glia-like sustentacular, or type-II, cells. Responsiveness of CB to acute hypoxia relies on the inhibition of O(2)-sensitive K(+) channels in glomus cells, which leads to cell depolarisation, Ca(2+) entry and release of transmitters that activate afferent nerve fibres. Although this model of O(2) sensing is generally accepted, the molecular mechanisms underlying K(+) channel modulation by O(2) tension are unknown. Among the putative hypoxia-sensing mechanisms there are: the production of oxygen radicals, either in mitochondria or reduced nicotinamide adenine dinucleotide phosphate oxidases; metabolic mitochondrial inhibition and decrease of intracellular ATP; disruption of the prolylhydroxylase/hypoxia inducible factor pathway; or decrease of carbon monoxide production by haemoxygenase-2. In chronic hypoxia, the CB grows with increasing glomus cell number. The current authors have identified, in the CB, neural stem cells, which can differentiate into glomus cells. Cell fate experiments suggest that the CB progenitors are the glia-like sustentacular cells. The CB appears to be involved in the pathophysiology of several prevalent human diseases.

Tyrosine phosphorylation of the inactivating peptide of the shaker B potassium channel: a structural-functional correlate

A synthetic peptide patterned after the sequence of the inactivating "ball" domain of the Shaker B K(+) channel restores fast (N-type) inactivation in mutant deletion channels lacking their constitutive ball domains, as well as in K(+) channels that do not normally inactivate. We now report on the effect of phosphorylation at a single tyrosine in position 8 of the inactivating peptide both on its ability to restore fast channel inactivation in deletion mutant channels and on the conformation adopted by the phosphorylated peptide when challenged by anionic lipid vesicles, a model target mimicking features of the inactivation site in the channel protein. We find that the inactivating peptide phosphorylated at Y8 behaves functionally as well as structurally as the noninactivating mutant carrying the mutation L7E. Moreover, it is observed that the inactivating peptide can be phosphorylated by the Src tyrosine kinase either as a free peptide in solution or when forming part of the membrane-bound protein channel as the constitutive inactivating domain. These findings suggest that tyrosine phosphorylation-dephosphorylation of this inactivating ball domain could be of physiological relevance to rapidly interconvert fast-inactivating channels into delayed rectifiers and vice versa.

Feedback Inhibition of Ca2+ Currents by Dopamine in Glomus Cells of the Carotid Body

The effect of dopamine on the voltage-dependent ionic channels of enzymatically dispersed glomus cells from rabbit carotid bodies was studied. Whole-cell currents were recorded on isolation with patch electrodes and dopamine applied to the bath solution. Dopamine at nanomolar concentrations produced a reversible attenuation of the calcium current whereas sodium and potassium currents remained unaltered. Dopamine inhibition of Ca2+ current was observed in all cells tested (n=48) and at a saturating concentration (1 microM) the average reduction was of 40 +/- 6.5% (n=8). The effect of dopamine was probably caused by a decrease in the number of channels activatable on depolarization since it did not modify the voltage-dependent parameters of the current. These results indicate that dopamine, which is the major transmitter secreted by glomus cells, regulates further transmitter release by feedback inhibition of Ca2+ channels.

Reduction of Ca(2+) channel activity by hypoxia in human and porcine coronary myocytes

OBJECTIVE

Oxygen (O(2)) tension is a major regulator of blood flow in the coronary circulation. Hypoxia can produce vasodilation through activation of ATP regulated K(+) (K(ATP)) channels in the myocyte membrane, which leads to hyperpolarization and closure of voltage-gated Ca(2+) channels. However, there are other O(2)-sensitive mechanisms intrinsic to the vascular smooth muscle since hypoxia can relax vessels precontracted with high extracellular K(+), a condition that prevents hyperpolarization following opening of K(+) channels. The objective of the present study was to determine whether inhibition of Ca(2+) influx through voltage-dependent channels participates in the response of coronary myocytes to hypoxia.

METHODS

Experiments were performed on porcine anterior descendent coronary arterial rings and on enzymatically dispersed human and porcine myocytes of the same artery. Cytosolic [Ca(2+)] was measured by microfluorimetry and whole-cell currents were recorded with the patch clamp technique.

RESULTS

Hypoxia (O(2) tension approximately 20 mmHg) dilated endothelium-denuded porcine coronary arterial rings precontracted with high K(+) in the presence of glibenclamide (5 microM), a blocker of K(ATP) channels. In dispersed human and porcine myocytes, low O(2) tension decreased basal cytosolic [Ca(2+)] and transmembrane Ca(2+) influx independently of K(+) channel activation. In patch clamped cells, hypoxia reversibly inhibited L-type Ca(2+) channels. RT-PCR indicated that rHT is the predominant mRNA variant of the alpha(1C) Ca(2+) channel subunit in human coronary myocytes.

CONCLUSION

Our study demonstrates, for the first time in a human preparation, that voltage-gated Ca(2+)channels in coronary myocytes are under control of O(2) tension.

Differential segmental activation of Ca2+-dependent CI−and K+channels in pulmonary arterial myocytes

Differential segmental distribution of electrophysiologically distinct myocytes helps to explain the variability of the pulmonary arteries to vasoactive agents. We have studied whether Ca2+ -dependent CI- (CICa) and K+ (KCa) channels are activated differentially in enzymatically dispersed conduit and resistance myocytes. We measured cytosolic [Ca2+] and the changes of membrane current and potential elicited by spontaneous or agonist-induced Ca2+ oscillations. Conduit arteries contained a heterogeneous cell population with a variable mixture of KCa and CICa conductances. Resistance arteries contained a more homogeneous cell population with predominance of CICa channel activation. The relation between KCa and CICa conductances in a given conduit myocyte determines the size of the V(m)change in response to a rise of cytosolic [Ca2+]. Conduit myocytes tend to hyperpolarize towards the K+ equilibrium potential (approximately - 90 m V). In resistance myocytes, release of Ca2+ from stores activates CI Cachannels and brings Vm to a value close to the chloride equilibrium potential (approximately - 20 or - 30 m V) thus favouring opening of Ca2+ channels and Ca2+ influx. In resistance vessels CICachannels contribute to link agonist-induced Ca2+ release from stores and membrane depolarization, thus permitting protracted vasoconstriction.

A small domain in the N terminus of the regulatory alpha-subunit Kv2. 3 modulates Kv2.1 potassium channel gating

Recent work has demonstrated the existence of regulatory K(+) channel alpha-subunits that are electrically silent but capable of forming heterotetramers with other pore-forming subunits to modify their function. We have investigated the molecular determinant of the modulatory effects of Kv2.3, a silent K(+) channel alpha-subunit specific of brain. This subunit induces on Kv2.1 channels a marked deceleration of activation, inactivation, and closing kinetics. We constructed chimeras of the Kv2.1 and Kv2.3 proteins and analyzed the K(+) currents resulting from the coexpression of the chimeras with Kv2.1. The data indicate that a region of 59 amino acids in the N terminus, adjacent to the first transmembrane segment, is the major structural element responsible for the regulatory function of Kv2.3. The sequence of this domain of Kv2.3 is highly divergent compared with the same region in the other channels of the Kv2 family. Replacement of the regulatory fragment of Kv2.3 by the equivalent of Kv2.1 leads to loss of modulatory function, whereas gain of modulatory function is observed when the Kv2.3 fragment is transferred to Kv2.1. Thus, this study identifies a N-terminus domain involved in Kv2.1 channel gating and in the modulation of this channel by a regulatory alpha-subunit.

Recovery of chronic parkinsonian monkeys by autotransplants of carotid body cell aggregates into putamen

We have studied the effect of unilateral autografts of carotid body cell aggregates into the putamen of MPTP-treated monkeys with chronic parkinsonism. Two to four weeks after transplantation, the monkeys initiated a progressive recovery of mobility with reduction of tremor and bradykinesia and restoration of fine motor abilities on the contralateral side. Apomorphine injections induced rotations toward the side of the transplant. Functional recovery was accompanied by the survival of tyrosine hydroxylase-positive (TH-positive) grafted glomus cells. A high density of TH-immunoreactive fibers was seen reinnervating broad regions of the ipsilateral putamen and caudate nucleus. The nongrafted, contralateral striatum remained deafferented. Intrastriatal autografting of carotid body tissue is a feasible technique with beneficial effects on parkinsonian monkeys; thus, this therapeutic approach could also be applied to treat patients with Parkinson's disease.

K+ and Ca2+ channel activity and cytosolic [Ca2+] in oxygen-sensing tissues

Ion channels are known to participate in the secretory or mechanical responses of chemoreceptor cells to changes in oxygen tension (P(O2)). We review here the modifications of K+ and Ca2+ channel activity and the resulting changes in cytosolic [Ca2+] induced by low P(O2) in glomus cells and arterial smooth muscle which are well known examples of O2-sensitive cells. Glomus cells of the carotid body behave as presynaptic-like elements where hypoxia produces a reduction of K+ conductance leading to enhanced membrane excitability, Ca2+ entry and release of dopamine and other neurotransmitters. In arterial myocytes, hypoxia can inhibit or potentiate Ca2+ channel activity, thus regulating cytosolic [Ca2+] and contraction. Ca2+ channel inhibition is observed in systemic myocytes and most conduit pulmonary myocytes, whereas potentiation is seen in a population of resistance pulmonary myocytes. The mechanism whereby O2 modulates ion channel activity could depend on either the direct allosteric modulation by O2-sensing molecules or redox modification by reactive chemical species.

Cellular and functional recovery of Parkinsonian rats after intrastriatal transplantation of carotid body cell aggregates

We have tested the suitability of chromaffin-like carotid body glomus cells for dopamine cell replacement in Parkinsonian rats. Intrastriatal grafting of cell aggregates resulted in almost optimal abolishment of motor asymmetries and deficits of sensorimotor orientation. Recovery of transplanted animals was apparent 10 days after surgery and progressed throughout the 3 months of the study. The behavioral effects were correlated with the long survival of glomus cells in the host brain. In host tissue, glomus cells were organized into glomerulus-like structures and retained the ability to secrete dopamine. Several weeks after transplantation, dopaminergic fibers emerged from the graft, reinnervating the striatal gray matter. The special durability of grafted glomus cells in the conditions of brain parenchyma could be related to their sensitivity to hypoxia, which is known to induce cell growth, excitability, and dopamine synthesis. This work should stimulate research on the clinical applicability of carotid body autotransplants in Parkinson's disease.

NMDA-glutamate receptors regulate phosphorylation of dendritic cytoskeletal proteins in the hippocampus

Most forms of synaptic potentiation need the activation of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors which generate changes in dendritic morphology of postsynaptic neurons. Since microtubule proteins have an essential role in dendritic morphology, they may be involved and regulated during the modifications of dendritic morphology associated with synaptic potentiation. The phosphorylation of the microtubule-associated proteins (MAPs) has been analyzed in situ after activation or blockade of NMDA-glutamate receptors in hippocampal slices. The phosphorylation of MAP1B and MAP2 has been studied by using several antibodies raised against phosphorylation-sensitive epitopes. Whereas antibodies 125 and 305 recognize phosphorylated epitopes on MAP1B and MAP2, respectively, Ab 842 recognizes a phosphorylatable sequence on MAP1B only when it is dephosphorylated. NMDA treatment decreased the phosphorylation state of the epitope recognized by the antibody 305 on MAP2 and caused a slight dephosphorylation of MAP1B sequences recognized by Ab 125 and 842. Moreover, exposure to APV (an antagonist of NMDA-glutamate receptors) counteracted the effect of NMDA and induced an increase in the phosphorylation state of these sequences in MAP2. Since phosphorylation regulates the interaction of MAPs with cytoskeleton, the results suggest that the modulation of the phosphorylated state of MAP2 by NMDA-glutamate receptors may be implicated in dendritic plasticity.

Identification and functional characterization of a K+ channel alpha-subunit with regulatory properties specific to brain

The physiological diversity of K+ channels mainly depends on the expression of several genes encoding different alpha-subunits. We have cloned a new K+ channel alpha-subunit (Kv2.3r) that is unable to form functional channels on its own but that has a major regulatory function. Kv2.3r can coassemble selectively with other alpha-subunits to form functional heteromultimeric K+ channels with kinetic properties that differ from those of the parent channels. Kv2.3r is expressed exclusively in the brain, being concentrated particularly in neocortical neurons. The functional expression of this regulatory alpha-subunit represents a novel mechanism without precedents in voltage-gated channels, which might contribute to further increase the functional diversity of K+ channels necessary to specify the intrinsic electrical properties of individual neurons.

Recombinant Ca2+ channels get O2 sensitive

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Pore mutations in Shaker K+ channels distinguish between the sites of tetraethylammonium blockade and C-type inactivation

1. We have studied the effect of external K+ and tetraethylammonium (TEA) on several mutants of Shaker B K+ channels with amino acid substitutions in the pore which alter TEA affinity and the rate of C-type inactivation. In all channels studied high external K+ makes C-type inactivation slower. 2. In the wild-type channel, TEA blockade is voltage dependent and produces slowing of the inactivation time course. However, in the double mutant channel (T449Y, D447E) TEA blockade, although of higher affinity, is voltage independent and does not affect the rate of C-type inactivation. 3. Mutants with a charged amino acid at position 449 (T449K and T449E) are resistant to TEA block. In these channels, C-type inactivation is also unaffected by TEA. 4. These results indicate that the sites where TEA blocks and competes with C-type inactivation can be segregated. To modulate inactivation, TEA must enter deeply into the channel mouth. These results suggest that C-type inactivation is not due to a large molecular rearrangement in the outer channel vestibule, but it is essentially produced by a conformational change restricted to a local site in the pore.

Low PO2 inhibits calcium channel activity in arterial smooth muscle cells

We studied the effect of O2 tension (PO2) on the activity of voltage-gated Ca2+ channels recorded in whole cell patch-clamped smooth muscle cells enzymatically dispersed from rabbit cerebral, celiac, femoral, and main pulmonary arteries, as well as from the porcine coronary artery. In all myocyte classes examined, a reduction of PO2 (hypoxia) produced a rapid and reversible inhibition of the macroscopic L-type Ca2+ current of similar general characteristics. The hypoxic inhibition of Ca2+ channel activity closely followed the time course of bath exchange, first becoming apparent at below approximately 80 mmHg PO2. The interaction of O2 with the Ca2+ channels was strongly voltage dependent. At -30 mV the average extent of current inhibition was approximately 80%; however, no effect or even potentiation of current amplitude was observed at potentials more positive than +30 mV. Hypoxia selectively slowed activation kinetics (approximately 1.5 times at -20 mV); however, channel deactivation and inactivation were unaltered by low PO2. In addition, hypoxia produced a reversible shift (8.1 +/- 1.0 mV, n = 12) of the Ca2+ conductance-voltage curve toward positive membrane potentials. We propose that the O2 sensitivity of Ca2+ channels may contribute to the well-known hypoxic dilatation of systemic and the main pulmonary arteries.

Synthesis of a photoaffinity labeling analogue of the inactivating peptide of the Shaker B potassium channel

A photoactivatable derivative of the inactivating peptide of the Shaker B potassium channel (ShB peptide) has been synthesized from ShB peptide containing an added cysteine residue at the peptide carboxy-terminus and 1-(p-azidosalicylamido)-4-(iodoacetamido)butane. The peptide derivative restores rapid inactivation in the deletion mutant Shaker Bdelta6-46 potassium channel in a manner indistinguishable from that of the wild-type ShB peptide. Also, both peptides display similar conformational behavior when challenged in vitro by an artificial model target that partly imitates the properties of the putative receptor site for the inactivating peptide in the Shaker B potassium channel. Therefore, we conclude that both functionally and conformationally the photoreactive peptide derivative is an adequate analogue of the wild-type ShB peptide, suitable for photoaffinity labeling of its binding site in the Shaker B potassium channel. Moreover, because the ShB peptide also serves as an efficient inactivating peptide for a large variety of other potassium channels, it appears that the photoreactive analogue may be useful to explore homologous sites in many different channel proteins.

Oxygen-sensing by ion channels and the regulation of cellular functions

From bacteria to mammals, ambient O2 tension influences such diverse cellular functions as gene expression, secretion, contraction and the patterns of electrical activity. Some of the effects of O2 are attributed to its interaction with various classes of voltage-dependent ion channels. In glomus cells of the carotid body, the differential properties of O2-sensitive K+ and Ca2+ channels help us to understand the basic features of O2 chemoreception. Modifications of ion-channel activity in response to changes in the partial pressure of O2 are also involved in the adjustments of vascular tone to hypoxia as well as in the response of chemoreceptors in pulmonary airways. Direct O2-sensing by ion channels might also help to explain the alterations of brain function by low O2 tension. The O2-sensitivity of ion-channel activity appears to be a broadly distributed phenomenon contributing to a wide variety of cellular responses to hypoxia.

Contrasting effects of hypoxia on cytosolic Ca2+ spikes in conduit and resistance myocytes of the rabbit pulmonary artery

1. The effects of hypoxia on cytosolic Ca2+ ([Ca2+]i) and spontaneous cytosolic Ca2+ spikes were examined in fura 2-loaded myocytes isolated from conduit and resistance branches of the rabbit pulmonary artery. In all myocyte classes, generation of the Ca2+ spikes was modulated by basal [Ca2+]i which, in turn, was influenced by the influx of Ca2+ through L-type Ca2+ channels of the plasmalemma. 2. Conduit and resistance myocytes responded distinctly to hypoxia. In most conduit myocytes (approximately 82% of total; n = 23) exposure to hypoxia reduced basal [Ca2+]i. This effect was often associated with the abolition of the Ca2+ spikes. Hypoxia gave rise to two main responses in resistance myocytes. In a subset of resistance myocytes (41 % of total; n = 34) hypoxia incremented basal [Ca2+]i but reduced Ca2+ spike amplitude. This response mimicked the effect of membrane depolarization with K+ and was reverted by nifedipine or the removal of extracellular Ca2+. In a second subset of resistance myocytes (59% of total; n = 34) hypoxia decreased basal [Ca2+]i and, in most cases, increased spike amplitude; a response counteracted by depolarization with K+. 3. These results indicate that hypoxia can differentially modulate [Ca2+]i in smooth muscle cells from large and small diameter pulmonary vessels through a dual effect on transmembrane Ca2+ influx. Our observations further demonstrate the longitudinal heterogeneity of myocytes along the pulmonary arterial tree and help to explain the hypoxic vasomotor responses in the pulmonary circulation.

Differential oxygen sensitivity of calcium channels in rabbit smooth muscle cells of conduit and resistance pulmonary arteries

1. Calcium currents were recorded from smooth muscle cells dispersed from conduit and resistance rabbit pulmonary arteries. We tested the hypothesis that Ca2+ channel activity was regulated by environmental O2 tension. 2. Conduit (proximal) and resistance (distal) myocytes differ in their Ca2+ channel density and responses to low PO2. Ca2+ current density in distal myocytes (20.7 +/- 7.4 pA pF-1, n = 10) is almost twice the value in proximal myocytes (12.6 +/- 5.5 pA pF-1, n = 39). In proximal myocytes, the predominant response to reductions in PO2 is inhibition of the calcium current (n = 12) at membrane potentials below 0 mV, whereas potentiation of current amplitude is observed in distal myocytes (n = 24). 3. Hypoxia also produces opposite shifts in the conductance-voltage relationships along the voltage axis. The average displacements induced by low PO2 are +5.05 +/- 2.98 mV (n = 5) in proximal myocytes and -6.06 +/- 2.45 (n = 10) in distal myocytes. 4. These findings demonstrate longitudinal differences in Ca2+ channel density and O2 sensitivity in myocytes along the pulmonary arterial tree. These results may help to understand the differential reactivity to hypoxia of the pulmonary vasculature: vasodilatation in conduit arteries and vasoconstriction in resistance vessels.

Oxygen sensing by ion channels and chemotransduction in single glomus cells

We have monitored cytosolic [Ca2+] and dopamine release in intact fura-2-loaded glomus cells with microfluoroimetry and a polarized carbon fiber electrode. Exposure to low PO2 produced a rise of cytosolic [Ca2+] with two distinguishable phases: an initial period (with PO2 values between 150 and approximately 70 mm Hg) during which the increase of [Ca2+] is very small and never exceeds 150-200 nM, and a second phase (with PO2 below approximately 70 mm Hg) characterized by a sharp rise of cytosolic [Ca2+]. Secretion occurs once cytosolic [Ca2+] reaches a threshold value of 180 +/- 43 nM. The results demonstrate a characteristic relationship between PO2 and transmitter secretion at the cellular level that is comparable with the relation described for the input (O2 tension)output (afferent neural discharges) variables in the carotid body. Thus, the properties of single glomus cells can explain the sensory functions of the entire organ. In whole-cell, patch-clamped cells, we have found that in addition to O2-sensitive K+ channels, there are Ca2+ channels whose activity is also regulated by PO2. Ca2+ channel activity is inhibited by hpoxia, although in a strongly voltage-dependent manner. The average hypoxic inhibition of the calcium current in 30% +/- 10% at -20 mV but only 2% +/- 2% at +30 mV. The differential inhibition of K+ and Ca2+ channels by hypoxia helps to explain why the secretory response of the cells is displaced toward PO2 values (below approximately 70 mm Hg) within the range of those normally existing in arterial blood. These data provide a conceptual framework for understanding the cellular mechanisms of O2 chemotransduction in the carotid body.

Oxygen-sensitive calcium channels in vascular smooth muscle and their possible role in hypoxic arterial relaxation

We have investigated the modifications of cytosolic [Ca2+] and the activity of Ca2+ channels in freshly dispersed arterial myocytes to test whether lowering O2 tension (PO2) directly influences Ca2+ homeostasis in these cells. Unclamped cells loaded with fura-2 AM exhibit oscillations of cytosolic Ca2+ whose frequency depends on extracellular Ca2+ influx. Switching from a PO2 of 150 to 20 mmHg leads to a reversible attenuation of the Ca2+ oscillations. In voltage-clamped cells, hypoxia reversibly reduces the influx of Ca2+ through voltage-dependent channels, which can account for the inhibition of the Ca2+ oscillations. Low PO2 selectively inhibits L-type Ca2+ channel activity, whereas the current mediated by T-type channels is unaltered by hypoxia. The effect of low PO2 on the L-type channels is markedly voltage dependent, being more apparent with moderate depolarizations. These findings demonstrate the existence of O2-sensitive, voltage-dependent, Ca2+ channels in vascular smooth muscle that may critically contribute to the local regulation of circulation.

Hypoxia induces voltage-dependent Ca2+ entry and quantal dopamine secretion in carotid body glomus cells

We have investigated the changes of cytosolic [Ca2+] and the secretory activity in single glomus cells dispersed from rabbit carotid bodies during exposure to solutions with variable O2 tension (Po2). In normoxic conditions (Po2 = 145 mmHg; 1 mmHg = 133 Pa), intracellular [Ca2+] was 58 +/- 29 nM, and switching to low Po2 (between 10 and 60 mmHg) led to a reversible increase of [Ca2+] up to 800 nM. The response to hypoxia completely disappeared after removal of external Ca2+ or with the addition of 0.2 mM Cd2+ to the external solution. These same solutions also abolished both the Ca2+ current of the cells and the increase of internal [Ca2+] elicited by high external K+. Elevations of cytosolic [Ca2+] in response to hypoxia or to direct membrane depolarization elicited the release of dopamine, which was detected by amperometric techniques. Dopamine secretion occurred in episodes of spike-like activity that appear to represent the release from single secretory vesicles. From the mean charge of well-resolved secretory events, we estimated the average number of dopamine molecules per vesicle to be approximately 140,000, a value about 15 times smaller than a previous estimate in chromaffin granules of adrenomedullary cells. These results directly demonstrate in a single-cell preparation the secretory response of glomus cells to hypoxia. The data indicate that the enhancement of cellular excitability upon exposure to low Po2 results in Ca2+ entry through voltage-gated channels, which leads to an increase in intracellular [Ca2+] and exocytotic transmitter release.

Dual modulation of K+ currents and cytosolic Ca2+ by the peptide TRH and its derivatives in guinea-pig septal neurones

1. We describe a dual effect of the peptide TRH (thyrotrophin-releasing hormone) and its derivatives at concentrations between 0.1 and 1 microM on the K+ currents and cytosolic Ca2+ concentration in enzymatically dispersed septal neurones. 2. In response to membrane depolarization, septal neurones recorded under whole-cell patch clamp can generate two major K+ currents: (i) a fast and transient K+ current (I(t)), that after a maximum at 2-5 ms inactivates completely at all membrane potentials in less than 50 ms; and (ii) a slowly activating current (I(s)), which reaches a maximum in 15-20 ms and does not exhibit appreciable inactivation during short-lasting voltage pulses. 3. In about 70% of the neurones tested (n = 48) TRH induced a reversible, and often transient, increase of I(t), I(s) or both K+ conductaNces. In approximately 10% of the cells the peptide had an opposite effect and caused a more protracted and partially reversible attenuation of the amplitude of I(t) and I(s). 4. The dual action of TRH on the K+ currents was mimicked by its derivatives but the effects varied depending on their structural relationship with the precursor neuropeptide. The physiological metabolite cyclo-His-Pro and the synthetic analogue methyl-TRH, in which the carboxyl terminus of the molecule is conserved, increased the K+ currents, whereas depression of the K+ conductances was predominantly observed in the presence of TRH-OH, in which the amino end of TRH is maintained intact. 5. In fura-2-loaded unclamped cells, TRH induced either release of Ca2+ from internal stores, Ca2+ entry, or both. With TRH-OH we never observed mobilization of internal Ca2+ but this peptide evoked a large Ca2+ influx. 6. The results demonstrate that the physiological metabolites of brain TRH (cyclo-His-Pro and TRH-OH) have biological activity. TRH and its derivatives exert two types of regulatory actions on the voltage-gated K+ channels and cytosolic Ca2+ concentration in central neurones, which can be explained assuming that TRH and TRH-derived products interact with different subtypes of brain receptors recognizing preferentially either the amino or the carboxyl termini of the TRH molecule.

High external potassium induces an increase in the phosphorylation of the cytoskeletal protein MAP2 in rat hippocampal slices

Depolarization induced in rat hippocampal slices by a high concentration of extracellular K+ leads to an increase in the phosphorylation of microtubule-associated protein MAP2. The comparison of the major phosphopeptides derived from in situ and in vitro phosphorylated MAP2 suggests the implication of calcium-dependent protein kinases, including calcium/calmodulin-dependent protein kinase type II and protein kinase C, in the up-phosphorylation of MAP2. In particular, a peptide containing the tubulin-binding domain of the MAP2 molecule may be phosphorylated by protein kinase C. As the association of MAP2 with the cytoskeleton may be regulated by phosphorylation, we suggest that changes in the phosphorylation level of MAP2 might be involved in synaptic remodelling in hippocampal neurons.

N-methyl-D-aspartate stimulates the dephosphorylation of the microtubule-associated protein 2 and potentiates excitatory synaptic pathways in the rat hippocampus

We have studied the effect of brief (50-150 s) applications of N-methyl-D-aspartate (10-100 microM) on the phosphorylated state of the microtubule-associated protein 2 in slices of rat hippocampus. Following a similar experimental protocol we also studied the pattern of excitatory postsynaptic potentials intracellularly recorded in CA1 pyramidal cells elicited by stimulation of the Schaffer collateral-commissural pathway. N-Methyl-D-aspartate treatment produced a marked and specific dephosphorylation of the cytoskeletal microtubule-associated protein 2, which was not due to enhanced proteolytic activity. Dephosphorylation of the microtubule-associated protein 2 affects mainly the tubulin-binding domain of the molecule and seems to be a consequence of the activation of the Ca2+/calmodulin-dependent phosphatase calcineurin, as it is partially inhibited by calmidazolium but not by okadaic acid. A few minutes after N-methyl-D-aspartate treatment we observed a 23 +/- 17% increase in the amplitude of the monosynaptic excitatory postsynaptic potential recorded in the cells and the appearance of a large polysynaptic excitatory postsynaptic potential. Both effects lasted for several tens of minutes. The late polysynaptic potential was not observed when the CA3 and CA1 subfields were surgically separated. Our results indicate that the N-methyl-D-aspartate receptor activation leads to the dephosphorylation of the microtubule-associated protein 2 via a Ca2+/calmodulin phosphatase, probably calcineurine. This may, in turn, participate in the potentiation of synaptic efficacy.

Excitatory and inhibitory synaptic currents and receptors in rat medial septal neurones

1. A thin-slice preparation was used to study the postsynaptic potentials and the underlying currents of visually identified rat medial septal (MS) neurones under tight-seal voltage- and current-clamp conditions. 2. Upon stimulation of the afferent fibres, all MS neurones exhibited a sequence of excitatory-inhibitory postsynaptic potentials (EPSP-IPSP). Under voltage clamp, with potassium glutamate as internal solution and at negative holding potentials (Vh), this synaptic pattern appeared as an initial inward current followed by a longer lasting outward current. 3. The inward postsynaptic current was completely abolished by 5 microM-6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) whereas the outward current disappeared in the presence of 10 microM-bicuculline. Thus the major excitatory and inhibitory synaptic inputs were identified as being due to activation of quisqualate/kainate glutamatergic and gamma-aminobutyric acid (GABAA) receptors, respectively. 4. At positive Vh a CNQX-resistant component of the excitatory postsynaptic current (EPSC) was revealed. This component was slower than the one mediated by the quisqualate receptor and was abolished by 3-3(2-carboxypiperazine-4-yl)propyl-1-phosphonate (CPP), indicating that N-methyl-D-aspartate (NMDA) receptors are involved in excitatory synaptic transmission in MS cells. The existence of the two main subtypes (NMDA and non-NMDA) of glutamatergic receptors in MS neurones was also confirmed by the responses of the neurones to bath application of the different agonists (glutamate, quisqualate, kainate and NMDA). 5. The CNQX-sensitive EPSC had a reversal potential near 0 mV. The fast rise time (approximately 0.7 ms) indicates a somatic location of the excitatory synapses. The relaxation kinetics of the fast EPSC were fitted by a single exponential function with a time constant of 1.13 +/- 0.1 ms. This parameter was independent of Vh. Fast EPSCs were blocked by CNQX in a dose-dependent manner (dissociation constant, KD = 0.2 microM). 6. Inhibitory postsynaptic currents (IPSCs) were studied in symmetrical chloride solutions after blockade of the excitatory receptors. The current-voltage relation was linear and reversed at 0 mV. The IPSCs had a fast rise time and their decay was best fitted by the sum of two exponentials with time constant of approximately 20 and 50 ms (Vh = -60 mV). The IPSCs were abolished by bicuculline (KD = 1 microM), a selective antagonist of GABAA receptors. As expected, bath application of GABA produced large whole-cell currents. 7. In many cells, in addition to the usual EPSP-IPSP sequence, failures of either the EPSP or the IPSP were frequently observed during the experimental protocol.(ABSTRACT TRUNCATED AT 400 WORDS)

Gating of O2-sensitive K+ channels of arterial chemoreceptor cells and kinetic modifications induced by low PO2

We have studied the kinetic properties of the O2-sensitive K+ channels (KO2 channels) of dissociated glomus cells from rabbit carotid bodies exposed to variable O2 tension (PO2). Experiments were done using single-channel and whole-cell recording techniques. The major gating properties of KO2 channels in excised membrane patches can be explained by a minimal kinetic scheme that includes several closed states (C0 to C4), an open state (O), and two inactivated states (I0 and I1). At negative membrane potentials most channels are distributed between the left-most closed states (C0 and C1), but membrane depolarization displaces the equilibrium toward the open state. After opening, channels undergo reversible transitions to a short-living closed state (C4). These transitions configure a burst, which terminates by channels either returning to a closed state in the activation pathway (C3) or entering a reversible inactivated conformation (I0). Burst duration increases with membrane depolarization. During a maintained depolarization, KO2 channels make several bursts before ending at a nonreversible, absorbing, inactivated state (I1). On moderate depolarizations, KO2 channels inactivate very often from a closed state. Exposure to low PO2 reversibly induces an increase in the first latency, a decrease in the number of bursts per trace, and a higher occurrence of closed-state inactivation. The open state and the transitions to adjacent closed or inactivated states seem to be unaltered by hypoxia. Thus, at low PO2 the number of channels that open in response to a depolarization decreases, and those channels that follow the activation pathway open more slowly and inactivate faster. At the macroscopic level, these changes are paralleled by a reduction in the peak current amplitude, slowing down of the activation kinetics, and acceleration of the inactivation time course. The effects of low PO2 can be explained by assuming that under this condition the closed state C0 is stabilized and the transitions to the absorbing inactivated state I1 are favored. The fact that hypoxia modifies kinetically defined conformational states of the channels suggests that O2 levels determine the structure of specific domains of the KO2 channel molecule. These results help to understand the molecular mechanisms underlying the enhancement of the excitability of glomus cells in response to hypoxia.

Potassium channel types in arterial chemoreceptor cells and their selective modulation by oxygen

Single K+ channel currents were recorded in excised membrane patches from dispersed chemoreceptor cells of the rabbit carotid body under conditions that abolish current flow through Na+ and Ca2+ channels. We have found three classes of voltage-gated K+ channels that differ in their single-channel conductance (gamma), dependence on internal Ca2+ (Ca2+i), and sensitivity to changes in O2 tension (PO2). Ca(2+)-activated K+ channels (KCa channels) with gamma approximately 210 pS in symmetrical K+ solutions were observed when [Ca2+]i was greater than 0.1 microM. Small conductance channels with gamma = 16 pS were not affected by [Ca2+]i and they exhibited slow activation and inactivation time courses. In these two channel types open probability (P(open)) was unaffected when exposed to normoxic (PO2 = 140 mmHg) or hypoxic (PO2 approximately 5-10 mmHg) external solutions. A third channel type (referred to as KO2 channel), having an intermediate gamma(approximately 40 pS), was the most frequently recorded. KO2 channels are steeply voltage dependent and not affected by [Ca2+]i, they inactivate almost completely in less than 500 ms, and their P(open) reversibly decreases upon exposure to low PO2. The effect of low PO2 is voltage dependent, being more pronounced at moderately depolarized voltages. At 0 mV, for example, P(open) diminishes to approximately 40% of the control value. The time course of ensemble current averages of KO2 channels is remarkably similar to that of the O2-sensitive K+ current. In addition, ensemble average and macroscopic K+ currents are affected similarly by low PO2. These observations strongly suggest that KO2 channels are the main contributors to the macroscopic K+ current of glomus cells. The reversible inhibition of KO2 channel activity by low PO2 does not desensitize and is not related to the presence of F-, ATP, and GTP-gamma-S at the internal face of the membrane. These results indicate that KO2 channels confer upon glomus cells their unique chemoreceptor properties and that the O2-K+ channel interaction occurs either directly or through an O2 sensor intrinsic to the plasma membrane closely associated with the channel molecule.

Patch-clamp analysis of voltage-gated currents in intermediate lobe cells from rat pituitary thin slices

Ionic currents of hypophyseal intermediate lobe cells were studied using a thin-slice preparation of the rat pituitary in conjunction with conventional and perforated whole-cell patch-clamp recording techniques. A majority (89%) of the cells studied generated Na+, Ca2+ and K+ currents upon depolarizing voltage steps and responded to bath application of gamma-aminobutyric acid (GABA; 20-50 microM) with inward currents (in symmetrical chloride, holding potential -80 mV). A small percentage of cells (11%) did not display inward membrane currents upon depolarization and was unresponsive to GABA. In the first type of cells, Ca2+ and K+ currents were further studied in isolation. Ca2+ tail currents showed a biphasic time course upon repolarization, with time constants and amplitudes of 2.07 +/- 0.29 ms, 123 +/- 22 pA (for the slowly deactivating component) and 0.14 +/- 0.06 ms, 437 +/- 33 pA (for the fast-deactivating component; means +/- SD of n = 4 cells). Slowly and fast-deactivating conductances were half-maximally activated at around -10 mV and +10 mV respectively. Depolarizing voltage steps elicited two types of K+ current, which were separated using a prepulse protocol. A fast-activating, transient component showed half-maximal steady-state inactivation between -65 mV and -45 mV depending on the divalent cation composition of the external solution. Its decay was fitted by single-exponential functions with time constants of 36 +/- 11 ms and 3.9 +/- 0.9 ms at -20 mV and +40 mV respectively (mean +/- SD; n = 4 cells). Whereas the peak current amplitudes of the transient K+ current component remained stable, the amplitude of the second, delayed component increased progressively throughout the course of whole-cell experiments. In cells recorded with the perforated whole-cell technique, bath application of dopamine (10 nM-1 microM) induced large hyperpolarizations from a spontaneous membrane potential of -40 mV, but did not consistently affect the amplitude of the voltage-gated K+ conductances. These data are compared to previous studies using other preparations of the intermediate lobe, and differences are discussed, thus helping to extend our knowledge of electrical excitability of hypophyseal cells.

Sodium and calcium currents in dispersed mammalian septal neurons

Voltage-gated Na+ and Ca2+ conductances of freshly dissociated septal neurons were studied in the whole-cell configuration of the patch-clamp technique. All cells exhibited a large Na+ current with characteristic fast activation and inactivation time courses. Half-time to peak current at -20 mV was 0.44 +/- 0.18 ms and maximal activation of Na+ conductance occurred at 0 mV or more positive membrane potentials. The average value was 91 +/- 32 nS (approximately 11 mS cm-2). At all membrane voltages inactivation was well fitted by a single exponential that had a time constant of 0.44 +/- 0.09 ms at 0 mV. Recovery from inactivation was complete in approximately 900 ms at -80 mV but in only 50 ms at -120 mV. The decay of Na+ tail currents had a single time constant that at -80 mV was faster than 100 microseconds. Depolarization of septal neurons also elicited a Ca2+ current that peaked in approximately 6-8 ms. Maximal peak Ca2+ current was obtained at 20 mV, and with 10 mM external Ca2+ the amplitude was 0.35 +/- 0.22 nA. During a maintained depolarization this current partially inactivated in the course of 200-300 ms. The Ca2+ current was due to the activity of two types of conductances with different deactivation kinetics. At -80 mV the closing time constants of slow (SD) and fast (FD) deactivating channels were, respectively, 1.99 +/- 0.2 and 0.11 +/- 0.03 ms (25 degrees C). The two kinds of channels also differed in their activation voltage, inactivation time course, slope of the conductance-voltage curve, and resistance to intracellular dialysis. The proportion of SD and FD channels varied from cell to cell, which may explain the differential electrophysiological responses of intracellularly recorded septal neurons.

Thyrotropin-releasing-hormone (TRH) and its physiological metabolite TRH-OH inhibit Na+ channel activity in mammalian septal neurons

The interaction of thyrotropin-releasing hormone (TRH) and its physiological metabolite TRH-OH with Na+ channels was studied in enzymatically dissociated guinea pig septal neurons by using the whole-cell variant of the patch-clamp technique. In about 60% of the cells tested, the neuropeptides at concentrations between 0.01 and 2.5 microM produced a dose-dependent reversible attenuation of Na+ currents. With 2 microM TRH-OH, peak Na+ current amplitude was reduced by 20-50% (27 +/- 8%, mean +/- SD; n = 16), whereas at the same concentration TRH was approximately half as effective as TRH-OH. In the presence of the tripeptides, the voltage-dependent parameter of the Na+ current were unaltered. TRH-induced reduction of Na+ current amplitude was transient and recovered almost completely during maintained exposure to the peptides. In addition, the response to either TRH-OH or TRH decreased with repeated treatment. Our results demonstrate that neuronal Na+ channels can be modulated by naturally occurring neuropeptides.

Ionic currents in dispersed chemoreceptor cells of the mammalian carotid body

Ionic currents of enzymatically dispersed type I and type II cells of the carotid body have been studied using the whole cell variant of the patch-clamp technique. Type II cells only have a tiny, slowly activating outward potassium current. By contrast, in every type I chemoreceptor cell studied we found (a) sodium, (b) calcium, and (c) potassium currents. (a) The sodium current has a fast activation time course and an activation threshold at approximately -40 mV. At all voltages inactivation follows a single exponential time course. The time constant of inactivation is 0.67 ms at 0 mV. Half steady state inactivation occurs at a membrane potential of approximately -50 mV. (b) The calcium current is almost totally abolished when most of the external calcium is replaced by magnesium. The activation threshold of this current is at approximately -40 mV and at 0 mV it reaches a peak amplitude in 6-8 ms. The calcium current inactivates very slowly and only decreases to 27% of the maximal value at the end of 300-ms pulses to 40 mV. The calcium current was about two times larger when barium ions were used as charge carriers instead of calcium ions. Barium ions also shifted 15-20 mV toward negative voltages the conductance vs. voltage curve. Deactivation kinetics of the calcium current follows a biphasic time course well fitted by the sum of two exponentials. At -80 mV the slow component has a time constant of 1.3 +/- 0.4 ms whereas the fast component, with an amplitude about 20 times larger than the slow component, has a time constant of 0.16 +/- 0.03 ms. These results suggest that type I cells have predominantly fast deactivating calcium channels. The slow component of the tails may represent the activity of a small population of slowly deactivating calcium channels, although other possibilities are considered. (c) Potassium current seems to be mainly due to the activity of voltage-dependent potassium channels, but a small percentage of calcium-activated channels may also exist. This current activates slowly, reaches a peak amplitude in 5-10 ms, and thereafter slowly inactivates. Inactivation is almost complete in 250-300 ms. The potassium current is reversibly blocked by tetraethylammonium. Under current-clamp conditions type I cells can spontaneously fire large action potentials. These results indicate that type I cells are excitable and have a variety of ionic conductances. We suggest a possible participation of these conductances in chemoreception.

Low pO2 selectively inhibits K channel activity in chemoreceptor cells of the mammalian carotid body

The hypothesis that changes in environmental O2 tension (pO2) could affect the ionic conductances of dissociated type I cells of the carotid body was tested. Cells were subjected to whole-cell patch clamp and ionic currents were recorded in a control solution with normal pO2 (pO2 = 150 mmHg) and 3-5 min after exposure to the same solution with a lower pO2. Na and Ca currents were unaffected by lowering pO2 to 10 mmHg, however, in all cells studied (n = 42) exposure to hypoxia produced a reversible reduction of the K current. In 14 cells exposed to a pO2 of 10 mmHg peak K current amplitude decreased to 35 +/- 8% of the control value. The effect of low pO2 was independent of the internal Ca2+ concentration and was observed in the absence of internal exogenous nucleotides. Inhibition of K channel activity by hypoxia is a graded phenomenon and in the range between 70 and 120 mmHg, which includes normal pO2 values in arterial blood, it is directly correlated with pO2 levels. Low pO2 appeared to slow down the activation time course of the K current but deactivation kinetics seemed to be unaltered. Type I cells subjected to current clamp generate large Na- and Ca-dependent action potentials repetitively. Exposure to low pO2 produces a 4-10 mV increase in the action potential amplitude and a faster depolarization rate of pacemaker potentials, which leads to an increase in the firing frequency. Repolarization rate of individual action potentials is, however, unaffected, or slightly increased. The selective inhibition of K channel activity by low pO2 is a phenomenon without precedents in the literature that explains the chemoreceptive properties of type I cells. The nature of the interaction of molecular O2 with the K channel protein is unknown, however, it is argued that a hemoglobin-like O2 sensor, perhaps coupled to a G protein, could be involved.

Potassium currents in dissociated cells of the rat pineal gland

The properties of K currents of pineal cells were studied using the whole-cell variant of the patch-clamp technique. The total K current could be separated in two distinct components: a fast, transient current (It) and a slow current (Is). The activation threshold of It was at -35 to -30 mV. On depolarization to +50 mV it reaches a peak in 2-3 ms and inactivates almost completely in 50 ms. Half steady state inactivation occurs at -45 mV. Inactivation of It is voltage-dependent and is well fitted by single exponentials with time constants between 17.2 ms at +50 mV and 27.2 ms at -10 mV. Inactivation is removed with time and the recovery period shortened by membrane hyperpolarization. The slow K current has a threshold at -20 to -15 mV. It reaches a maximum in about 30-40 ms and inactivates slightly, to about 80% of the peak value at the end of pulses lasting 200 ms. With 80 mM external K, tail currents recorded after short (1-2 ms) depolarizations were about 2.5 times faster than the tails recorded at the end of 50 ms pulses. The fast tails were removed by depolarizing prepulses but the slow tails remained unaltered. Thus, the fast and slow tails are probably a reflection of the closing of the transient and slow K channels. The transient K current of pineal cells has general characteristics similar to transient currents recorded in non-secretory cells, but also has particular kinetic properties.

Properties of calcium and potassium currents of clonal adrenocortical cells

The ionic currents of clonal Y-1 adrenocortical cells were studied using the whole-cell variant of the patch-clamp technique. These cells had two major current components: a large outward current carried by K ions, and a small inward Ca current. The Ca current depended on the activity of two populations of Ca channels, slow (SD) and fast (FD) deactivating, that could be separated by their different closing time constants (at -80 mV, SD, 3.8 ms, and FD, 0.13 ms). These two kinds of channels also differed in (a) activation threshold (SD, approximately -50 mV; FD, approximately -20 mV), (b) half-maximal activation (SD, between -15 and -10 mV; FD between +10 and +15 mV), and (c) inactivation time course (SD, fast; FD, slow). The total amplitude of the Ca current and the proportion of SD and FD channels varied from cell to cell. The amplitude of the K current was strongly dependent on the internal [Ca2+] and was almost abolished when internal [Ca2+] was less than 0.001 microM. The K current appeared to be independent, or only slightly dependent, of Ca influx. With an internal [Ca2+] of 0.1 microM, the activation threshold was -20 mV, and at +40 mV the half-time of activation was 9 ms. With 73 mM external K the closing time constant at -70 mV was approximately 3 ms. The outward current was also modulated by internal pH and Mg. At a constant pCa gamma a decrease of pH reduced the current amplitude, whereas the activation kinetics were not much altered. Removal of internal Mg produced a drastic decrease in the amplitude of the Ca-activated K current. It was also found that with internal [Ca2+] over 0.1 microM the K current underwent a time-dependent transformation characterized by a large increase in amplitude and in activation kinetics.

Differential burst firing modes in neurons of the mammalian visual cortex in vitro

The firing patterns of visual cortical neurons were studied by intracellular recording in in vitro guinea pig brain slices. On depolarization 57% of the cells exhibited tonic firing of action potentials while the remaining cells (43%) had a phasic component in their response. Phasic cells exhibited a large diversity in their burst characteristics as well as in the burst dependence on the membrane potential. Ionic conductances underlying burst generation appeared to be also diverse, thus bursting neurons in the visual cortex cannot be grouped in a single, homogeneous population.

Chemotransduction in the carotid body: K+ current modulated by PO2 in type I chemoreceptor cells

The ionic currents of carotid body type I cells and their possible involvement in the detection of oxygen tension (Po2) in arterial blood are unknown. The electrical properties of these cells were studied with the whole-cell patch clamp technique, and the hypothesis that ionic conductances can be altered by changes in PO2 was tested. The results show that type I cells have voltage-dependent sodium, calcium, and potassium channels. Sodium and calcium currents were unaffected by a decrease in PO2 from 150 to 10 millimeters of mercury, whereas, with the same experimental protocol, potassium currents were reversibly reduced by 25 to 50 percent. The effect of hypoxia was independent of internal adenosine triphosphate and calcium. Thus, ionic conductances, and particularly the O2-sensitive potassium current, play a key role in the transduction mechanism of arterial chemoreceptors.