Redox agents as a link between hypoxia and the responses of ionic channels in rabbit pulmonary vascular smooth muscle

A narrative review of research advances in hypoxic pulmonary hypertension

Background and Objective

Hypoxic pulmonary hypertension (HPH) is a pathological syndrome characterized by pulmonary vasoconstriction and pulmonary vascular remodeling caused by hypoxia, which eventually leads to right heart failure or death. There are 2 stages of onset of HPH: hypoxic pulmonary vasoconstriction (HPV) and hypoxic pulmonary vascular remodeling (HPVR). It is an important pathophysiological link in the pathogenesis of chronic obstructive pulmonary disease (COPD) and chronic mountain sickness (CMS), and its severity is closely related to the course and prognosis of COPD and CMS. However, there is a lack of systematic review on the diagnosis, pathogenesis and treatment of HPH. The objective of this paper is to review the diagnosis, pathogenesis, treatment of HPH.

Methods

In this paper, the method of literature review is adopted to obtain the information about HPH. Based on the literature, comprehensive and systematic review is made. The diagnosis, pathogenesis, treatment of HPH are summarized.

Key Content and Findings

Right heart catheterization is the gold standard for diagnosing HPH. Hypoxia-inducible factor, oxidative stress, metal metabolism, ion channel, inflammatory cytokines, cell apoptosis and vascular factors are the main pathogenesis of HPH. The treatment of HPH includes long-term oxygen therapy, statins, prostaglandins, phosphodiesterase inhibitor and ET receptor antagonists.

Conclusions

Although great progress has been made in the pathophysiology and molecular biology of HPH, it is still unclear which factors play a leading role in the pathogenesis of HPH, and no breakthrough has been made in the treatment of HPH. It is believed that the specific mechanism will be revealed as the research continues, and earlier diagnosis and the development of more effective targeted drugs will be the focus of future research.

Mitochondrial Complex IV Subunit 4 Isoform 2 Is Essential for Acute Pulmonary Oxygen Sensing

RATIONALE

Acute pulmonary oxygen sensing is essential to avoid life-threatening hypoxemia via hypoxic pulmonary vasoconstriction (HPV) which matches perfusion to ventilation. Hypoxia-induced mitochondrial superoxide release has been suggested as a critical step in the signaling pathway underlying HPV. However, the identity of the primary oxygen sensor and the mechanism of superoxide release in acute hypoxia, as well as its relevance for chronic pulmonary oxygen sensing, remain unresolved.

OBJECTIVES

To investigate the role of the pulmonary-specific isoform 2 of subunit 4 of the mitochondrial complex IV (Cox4i2) and the subsequent mediators superoxide and hydrogen peroxide for pulmonary oxygen sensing and signaling.

METHODS AND RESULTS

Isolated ventilated and perfused lungs from Cox4i2^-/- mice lacked acute HPV. In parallel, pulmonary arterial smooth muscle cells (PASMCs) from Cox4i2^-/- mice showed no hypoxia-induced increase of intracellular calcium. Hypoxia-induced superoxide release which was detected by electron spin resonance spectroscopy in wild-type PASMCs was absent in Cox4i2^-/- PASMCs and was dependent on cysteine residues of Cox4i2. HPV could be inhibited by mitochondrial superoxide inhibitors proving the functional relevance of superoxide release for HPV. Mitochondrial hyperpolarization, which can promote mitochondrial superoxide release, was detected during acute hypoxia in wild-type but not Cox4i2^-/- PASMCs. Downstream signaling determined by patch-clamp measurements showed decreased hypoxia-induced cellular membrane depolarization in Cox4i2^-/- PASMCs compared with wild-type PASMCs, which could be normalized by the application of hydrogen peroxide. In contrast, chronic hypoxia-induced pulmonary hypertension and pulmonary vascular remodeling were not or only slightly affected by Cox4i2 deficiency, respectively.

CONCLUSIONS

Cox4i2 is essential for acute but not chronic pulmonary oxygen sensing by triggering mitochondrial hyperpolarization and release of mitochondrial superoxide which, after conversion to hydrogen peroxide, contributes to cellular membrane depolarization and HPV. These findings provide a new model for oxygen-sensing processes in the lung and possibly also in other organs.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

How does the heart sense changes in oxygen tension: a role for ion channels?

SIGNIFICANCE

Oxygen plays a key role in cellular metabolism and function. Oxygen delivery to cells is crucial, and a lack of oxygen such as that which occurs during myocardial infarction can be lethal. Cells should, therefore, be able to respond to changes in oxygen tension.

RECENT ADVANCES

Since the first studies examining the acute cellular effect of hypoxia on activation of transmitter release from glomus or type I chemoreceptor cells, it is now known that virtually all cells are able to respond to changes in oxygen tension.

CRITICAL ISSUES

Despite advances made in characterizing hypoxic responses, the identity of the "oxygen sensor" remains debated. Recently, more evidence has evolved as to how cardiac myocytes sense acute changes in oxygen. This review will examine the available evidence in support of acute oxygen-sensing mechanisms providing a brief historical perspective and then more detailed insights into the heart and the role of cardiac ion channels in hypoxic responses.

FUTURE DIRECTIONS

A further understanding of these cellular processes should result in interventions that assist in preventing the deleterious effects of acute changes in oxygen tension such as alterations in contractile function and cardiac arrhythmia.

Regulation of ion channels by pyridine nucleotides

Recent research suggests that in addition to their role as soluble electron carriers, pyridine nucleotides [NAD(P)(H)] also regulate ion transport mechanisms. This mode of regulation seems to have been conserved through evolution. Several bacterial ion-transporting proteins or their auxiliary subunits possess nucleotide-binding domains. In eukaryotes, the Kv1 and Kv4 channels interact with pyridine nucleotide-binding β-subunits that belong to the aldo-keto reductase superfamily. Binding of NADP(+) to Kvβ removes N-type inactivation of Kv currents, whereas NADPH stabilizes channel inactivation. Pyridine nucleotides also regulate Slo channels by interacting with their cytosolic regulator of potassium conductance domains that show high sequence homology to the bacterial TrkA family of K(+) transporters. These nucleotides also have been shown to modify the activity of the plasma membrane K(ATP) channels, the cystic fibrosis transmembrane conductance regulator, the transient receptor potential M2 channel, and the intracellular ryanodine receptor calcium release channels. In addition, pyridine nucleotides also modulate the voltage-gated sodium channel by supporting the activity of its ancillary subunit-the glycerol-3-phosphate dehydrogenase-like protein. Moreover, the NADP(+) metabolite, NAADP(+), regulates intracellular calcium homeostasis via the 2-pore channel, ryanodine receptor, or transient receptor potential M2 channels. Regulation of ion channels by pyridine nucleotides may be required for integrating cell ion transport to energetics and for sensing oxygen levels or metabolite availability. This mechanism also may be an important component of hypoxic pulmonary vasoconstriction, memory, and circadian rhythms, and disruption of this regulatory axis may be linked to dysregulation of calcium homeostasis and cardiac arrhythmias.

Function and regulation of large conductance Ca(2+)-activated K+ channel in vascular smooth muscle cells

Large conductance Ca(2+)-activated K(+) (BK(Ca)) channels are abundantly expressed in vascular smooth muscle cells. Activation of BK(Ca) channels leads to hyperpolarization of cell membrane, which in turn counteracts vasoconstriction. Therefore, BK(Ca) channels have an important role in regulation of vascular tone and blood pressure. The activity of BK(Ca) channels is subject to modulation by various factors. Furthermore, the function of BK(Ca) channels are altered in both physiological and pathophysiological conditions, such as pregnancy, hypertension and diabetes, which has dramatic impacts on vascular tone and hemodynamics. Consequently, compounds and genetic manipulation that alter activity and expression of the channel might be of therapeutic interest.

Hypoxic Pulmonary Vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Hypoxic pulmonary vasoconstriction: mechanisms of oxygen-sensing

PURPOSE OF REVIEW

Hypoxic pulmonary vasoconstriction (HPV) is driven by the intrinsic response to hypoxia of pulmonary arterial smooth muscle and endothelial cells. These are representatives of a group of specialized O2-sensing cells, defined by their acute sensitivity to relatively small changes in pO2, which have evolved to modulate respiratory and circulatory function in order to maintain O2 supply within physiological limits. The aim of this article is to discuss recent investigations into the mechanism(s) of hypoxia-response coupling and, in light of these, provide a critical assessment of current working hypotheses.

RECENT FINDINGS

Upon exposure to hypoxia state-of-the-art technologies have now confirmed that mitochondrial oxidative phosphorylation is inhibited in all O2-sensing cells, including pulmonary arterial smooth muscle cells. Thereafter, evidence has been presented to indicate a role as principal effector for the 'gasotransmitters' carbon monoxide and hydrogen sulphide, reactive oxygen species or, in marked contrast, reduced cellular redox couples. Considering recent evidence in favour and against these proposals we suggest that an alternative mechanism may be key, namely the activation of adenosine monophosphate-activated protein kinase consequent to inhibition of mitochondrial oxidative phosphorylation.

SUMMARY

HPV supports ventilation-perfusion matching in the lung by diverting blood flow away from oxygen-deprived areas towards regions rich in O2. However, in diseases such as emphysema and cystic fibrosis, widespread HPV leads to hypoxic pulmonary hypertension and ultimately right heart failure. Determining the precise mechanism(s) that underpins hypoxia-response coupling will therefore advance understanding of the fundamental processes contributing to related pathophysiology and provide for improved therapeutics.

Modulation of K(ATP) currents in rat ventricular myocytes by hypoxia and a redox reaction

AIM

The present study investigated the possible regulatory mechanisms of redox agents and hypoxia on the K(ATP) current (I(KATP)) in acutely isolated rat ventricular myocytes.

METHODS

Single-channel and whole-cell patch-clamp techniques were used to record the K(ATP) current (I(KATP)) in acutely isolated rat ventricular myocytes.

RESULTS

Oxidized glutathione (GSSG, 1 mmol/L) increased the I(KATP), while reduced glutathione (GSH, 1 mmol/L) could reverse the increased I(KATP) during normoxia. To further corroborate the effect of the redox agent on the K(ATP) channel, we employed the redox couple DTT (1 mmol/L)/H2O2 (0.3, 0.6, and 1 mmol/L) and repeated the previous processes, which produced results similar to the previous redox couple GSH/GSSG during normoxia. H2O2 increased the I(KATP) in a concentration dependent manner, which was reversed by DTT (1 mmol/L). In addition, our results have shown that 15 min of hypoxia increased the I(KATP), while GSH (1 mmol/L) could reverse the increased I(KATP). Furthermore, in order to study the signaling pathways of the I(KATP) augmented by hypoxia and the redox agent, we applied a protein kinase C(PKC) inhibitor bisindolylmaleimide VI (BIM), a protein kinase G(PKG) inhibitor KT5823, a protein kinase A (PKA) inhibitor H-89, and Ca2+/calmodulin-dependent protein kinase II (CaMKII) inhibitors KN-62 and KN-93. The results indicated that BIM, KT5823, KN-62, and KN-93, but not H-89, inhibited the I(KATP) augmented by hypoxia and GSSG; in addition, these results suggest that the effects of both GSSG and hypoxia on K(ATP) channels involve the activation of the PKC, PKG, and CaMK II pathways, but not the PKA pathway.

CONCLUSION

The present study provides electrophysiological evidence that hypoxia and the oxidizing reaction are closely related to the modulation of I(KATP).

Cardiac Na+ current regulation by pyridine nucleotides

RATIONALE

Mutations in glycerol-3-phosphate dehydrogenase 1-like (GPD1-L) protein reduce cardiac Na+ current (I(Na)) and cause Brugada Syndrome (BrS). GPD1-L has >80% amino acid homology with glycerol-3-phosphate dehydrogenase, which is involved in NAD-dependent energy metabolism.

OBJECTIVE

Therefore, we tested whether NAD(H) could regulate human cardiac sodium channels (Na(v)1.5).

METHODS AND RESULTS

HEK293 cells stably expressing Na(v)1.5 and rat neonatal cardiomyocytes were used. The influence of NADH/NAD+ on arrhythmic risk was evaluated in wild-type or SCN5A(+/-) mouse heart. A280V GPD1-L caused a 2.48+/-0.17-fold increase in intracellular NADH level (P<0.001). NADH application or cotransfection with A280V GPD1-L resulted in decreased I(Na) (0.48+/-0.09 or 0.19+/-0.04 of control group, respectively; P<0.01), which was reversed by NAD+, chelerythrine, or superoxide dismutase. NAD+ antagonism of the Na+ channel downregulation by A280V GPD1-L or NADH was prevented by a protein kinase (PK)A inhibitor, PKAI(6-22). The effects of NADH and NAD+ were mimicked by a phorbol ester and forskolin, respectively. Increasing intracellular NADH was associated with an increased risk of ventricular tachycardia in wild-type mouse hearts. Extracellular application of NAD+ to SCN5A(+/-) mouse hearts ameliorated the risk of ventricular tachycardia.

CONCLUSIONS

Our results show that Na(v)1.5 is regulated by pyridine nucleotides, suggesting a link between metabolism and I(Na). This effect required protein kinase C activation and was mediated by oxidative stress. NAD+ could prevent this effect by activating PKA. Mutations of GPD1-L may downregulate Na(v)1.5 by altering the oxidized to reduced NAD(H) balance.

Cellular localization of mitochondria contributes to Kv channel-mediated regulation of cellular excitability in pulmonary but not mesenteric circulation

Mitochondria are proposed to be a major oxygen sensor in hypoxic pulmonary vasoconstriction (HPV), a unique response of the pulmonary circulation to low oxygen tension. Mitochondrial factors including reactive oxygen species, cytochrome c, ATP, and magnesium are potent modulators of voltage-gated K(+) (K(v)) channels in the plasmalemmal membrane of pulmonary arterial (PA) smooth muscle cells (PASMCs). Mitochondria have also been found close to the plasmalemmal membrane in rabbit main PA smooth muscle sections. Therefore, we hypothesized that differences in mitochondria localization in rat PASMCs and systemic mesenteric arterial smooth muscle cells (MASMCs) may contribute to the divergent oxygen sensitivity in the two different circulations. Cellular localization of mitochondria was compared with immunofluorescent labeling, and differences in functional coupling between mitochondria and K(v) channels was evaluated with the patch-clamp technique and specific mitochondrial inhibitors antimycin A (acting at complex III of the mitochondrial electron transport chain) and oligomycin A (which inhibits the ATP synthase). It was found that mitochondria were located significantly closer to the plasmalemmal membrane in PASMCs compared with MASMCs. Consistent with these findings, the effects of the mitochondrial inhibitors on K(v) current (I(Kv)) were significantly more potent in PASMCs than in MASMCs. The cytoskeletal disruptor cytochalasin B (10 microM) also altered mitochondrial distribution in PASMCs and significantly attenuated the effect of antimycin A on the voltage-dependent parameters of I(Kv). These findings suggest a greater structural and functional coupling between mitochondria and K(v) channels specifically in PASMCs, which could contribute to the regulation of PA excitability in HPV.

Oxidant and redox signaling in vascular oxygen sensing: implications for systemic and pulmonary hypertension

It has been well known for >100 years that systemic blood vessels dilate in response to decreases in oxygen tension (hypoxia; low PO2), and this response appears to be critical to supply blood to the stressed organ. Conversely, pulmonary vessels constrict to a decrease in alveolar PO2 to maintain a balance in the ventilation-to-perfusion ratio. Currently, although little question exists that the PO2 affects vascular reactivity and vascular smooth muscle cells (VSMCs) act as oxygen sensors, the molecular mechanisms involved in modulating the vascular reactivity are still not clearly understood. Many laboratories, including ours, have suggested that the intracellular calcium concentration ([Ca2+]i), which regulates vasomotor function, is controlled by free radicals and redox signaling, including NAD(P)H and glutathione (GSH) redox. In this review article, therefore, we discuss the implications of redox and oxidant alterations seen in pulmonary and systemic hypertension, and how key targets that control [Ca2+]i, such as ion channels, Ca2+ release from internal stores and uptake by the sarcoplasmic reticulum, and the Ca2+ sensitivity to the myofilaments, are regulated by changes in intracellular redox and oxidants associated with vascular PO2sensing in physiologic or pathophysiologic conditions.

Redox reaction modulates transient and persistent sodium current during hypoxia in guinea pig ventricular myocytes

Whole-cell and cell-attached patch clamp techniques were applied on isolated guinea pig ventricular myocytes to study the possible regulatory mechanisms of redox agent on persistent and transient sodium current related to hypoxia. The results showed that hypoxia for 15 min increased persistent sodium current (I (Na.P)) and decreased transient sodium current (I (Na.T)) at the same time, while 1 mmol/l of reduced glutathione (GSH) could reverse the increased I (Na.P) and the decreased I (Na.T) simultaneously. Both persistent and transient sodium channel activities could be reversed concurrently again by application of 1 mmol/l oxidized glutathione (GSSG). Hypoxia for 15 min decreased the action potential amplitude (APA) and shortened action potential duration at 90% repolarization (APD(90)) of ventricular papillary cells simultaneously, while 1 mmol/GSH could reverse the decreased APA and the shortened APD(90) at the same time; 1 mmol/l GSSG strengthened the decrease of APA induced by hypoxia and attenuated the decurtation of APD(90) induced by hypoxia compared with pure hypoxia. The correlation between I (Na.P) and I (Na.T) and the effects of GSH and GSSG on them suggested that during hypoxia, redox regulation played a tremendous part in sodium channel activity and that I (Na.P) and I (Na.T) might be charged by the same channel with different gating modes in guinea pig ventricular myocytes. Judging from their alterations during hypoxia and exposure to GSH and GSSG, we speculated that an interconversion might exist between I (Na.P) and I (Na.T). That was when one of them was increased, the other was decreased, and vice versa.

Mitochondrial inhibition prior to oxygen-withdrawal facilitates the occurrence of hypoxia-induced spreading depression in rat hippocampal slices

Oxygen withdrawal blocks mitochondrial respiration. In rat hippocampal slices, this triggers a massive depolarization of CA1 neurons and a negative shift of the extracellular DC potential, the characteristic sign of hypoxia-induced spreading depression (HSD). To unveil the contribution of mitochondria to the sensing of hypoxia and the ignition of HSD, we modified mitochondrial function. Mitochondrial uncoupling by carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP, 1 microM) prior to hypoxia hastened the onset and shortened the duration of HSD. Blocking mitochondrial ATP synthesis by oligomycin (10 microg/ml) was without effect. Inhibition of mitochondrial respiration by rotenone (20 microM), diphenyleneiodonium (25 microM), or antimycin A (20 microM) also hastened HSD onset and shortened HSD duration. 3-nitropropionic acid (1 mM) increased HSD duration. Cyanide (100 microM) hastened HSD onset and increased HSD duration. At higher concentrations, cyanide (1 mM), azide (2 mM), and FCCP (10 microM) triggered SD episodes on their own. Compared with control HSD, the spatial extent of the intrinsic optical signals of cyanide- and azide-induced SDs was more pronounced. Monitoring NADH (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide) autofluorescence and mitochondrial membrane potential verified the mitochondrial targeting by the drugs used. Except 1 mM cyanide, no treatment reduced cellular ATP levels severely and no correlation was found between ATP, NADH, or FAD levels and the time to HSD onset. Therefore ATP depletion or a cytosolic reducing shift due to NADH/FADH2 accumulation cannot serve as a general explanation for the hastening of HSD onset on mitochondrial inhibition. Additional redox couples (glutathione) or events downstream of the mitochondrial depolarization need to be considered.

Oxidant and redox signaling in vascular oxygen sensing mechanisms: basic concepts, current controversies, and potential importance of cytosolic NADPH

Vascular smooth muscle (VSM) derived from pulmonary arteries generally contract to hypoxia, whereas VSM from systemic arteries usually relax, indicating the presence of basic oxygen-sensing mechanisms in VSM that are adapted to the environment from which they are derived. This review considers how fundamental processes associated with the generation of reactive oxygen species (ROS) by oxidase enzymes, the metabolic control of cytosolic NADH, NADPH and glutathione redox systems, and mitochondrial function interact with signaling systems regulating vascular force in a manner that is potentially adapted to be involved in Po2 sensing. Evidence for opposing hypotheses of hypoxia, either decreasing or increasing mitochondrial ROS, is considered together with the Po2 dependence of ROS production by Nox oxidases as sensors potentially contributing to hypoxic pulmonary vasoconstriction. Processes through which ROS and NAD(P)H redox changes potentially control interactive signaling systems, including soluble guanylate cyclase, potassium channels, and intracellular calcium are discussed together with the data supporting their regulation by redox in responses to hypoxia. Evidence for hypothesized potential differences between systemic and pulmonary arteries originating from properties of mitochondrial ROS generation and the redox sensitivity of potassium channels is compared with a new hypothesis in which differences in the control of cytosolic NADPH redox by the pentose phosphate pathway results in increased NADPH and Nox oxidase-derived ROS in pulmonary arteries, whereas lower levels of glucose-6-phosphate dehydrogenase in coronary arteries may permit hypoxia to activate a vasodilator mechanism controlled by oxidation of cytosolic NADPH.

Nitric oxide synthase inhibition affects sarcoplasmic reticulum Ca2+ release in skeletal muscle fibres from mouse

Nitric oxide (NO) generated by skeletal muscle is believed to regulate force production but how this is achieved remains poorly understood. In the present work we tested the effects of NO synthase (NOs) inhibitors on membrane current and intracellular calcium in isolated skeletal muscle fibres from mouse, under voltage-clamp conditions. Resting [Ca(2+)] and [Ca(2+)] transients evoked by large depolarizations exhibited similar properties in control fibres and in fibres loaded with tenth millimolar levels of the NOs inhibitor N-nitro-L-arginine (L-NNA). Yet the voltage dependence of calcium release was found to be shifted by approximately 15 mV towards negative values in the presence of L-NNA. This effect could be reproduced by the other NOs inhibitor S-methyl-L-thiocitrulline (L-SMT). Separate experiments showed that the voltage dependence of charge movement and of the slow calcium current were unaffected by the presence of L-NNA, ruling out an effect on the voltage sensor. A negative shift in the voltage dependence of calcium release with no concurrent alteration in the properties of charge movement was also observed in fibres exposed to the oxidant H(2)O(2) (1 mM). Conversely the reducing agent dithiothreitol (10 mM) had no obvious effect on Ca(2+) release. Overall, the results indicate that physiological levels of NO exert a tonic inhibitory control on the activation of the calcium release channels. Changes in the voltage dependence of Ca(2+) release activation may be a ubiquitous physiological consequence of redox-related modifications of the ryanodine receptor.

Acute hypoxia differentially regulates K+ channels. Implications with respect to cardiac arrhythmia

The first ion channels demonstrated to be sensitive to changes in oxygen tension were K(+) channels in glomus cells of the carotid body. Since then a number of hypoxia-sensitive ion channels have been identified. However, not all K(+) channels respond to hypoxia alike. This has raised some debate about how cells detect changes in oxygen tension. Because ion channels respond rapidly to hypoxia it has been proposed that the channel is itself an oxygen sensor. However, channel function can also be modified by thiol reducing and oxidizing agents, implicating reactive oxygen species as signals in hypoxic events. Cardiac ion channels can also be modified by hypoxia and redox agents. The rapid and slow components of the delayed rectifier K(+) channel are differentially regulated by hypoxia and beta-adrenergic receptor stimulation. Mutations in the genes that encode the subunits for the channel are associated with Long QT syndrome and sudden cardiac death. The implications with respect to effects of hypoxia on the channel and triggering of cardiac arrhythmia will be discussed.

O2 sensing in the human ductus arteriosus: redox-sensitive K+ channels are regulated by mitochondria-derived hydrogen peroxide

The ductus arteriosus (DA) is a fetal artery that allows blood ejected from the right ventricle to bypass the pulmonary circulation in utero. At birth, functional closure of the DA is initiated by an O2-induced, vasoconstrictor mechanism which, though modulated by endothelial-derived endothelin and prostaglandins, is intrinsic to the smooth muscle cell (DASMC) [Michelakis et al., Circ. Res. 91 (2002); pp. 478-486]. As pO2 increases, a mitochondrial O2-sensor (electron transport chain complexes I or III) is activated, which generates a diffusible redox mediator (H2O2). H2O2 inhibits voltage-gated K+ channels (Kv) in DASMC. The resulting membrane depolarization activates L-type Ca2+ channels, thereby promoting vasoconstriction. Conversely, inhibiting mitochondrial ETC complexes I or III mimics hypoxia, depolarizing mitochondria, and decreasing H2O2 levels. The resulting increase in K+ current hyperpolarizes the DASMC and relaxes the DA. We have developed two models for study of the DA's O2-sensor pathway, both characterized by decreased O2-constriction and Kv expression: (i) preterm rabbit DA, (ii) ionically-remodeled, human term DA. The O2-sensitive channels Kv1.5 and Kv2.1 are important to DA O2-constriction and overexpression of either channel enhances DA constriction in these models. Understanding this O2-sensing pathway offers therapeutic targets to modulate the tone and patency of human DA in vivo, thereby addressing a common form of congenital heart disease in preterm infants.

Metabolic regulation of potassium channels

Potassium (K+) channels exist in all three domains of organisms: eubacteria, archaebacteria, and eukaryotes. In higher animals, these membrane proteins participate in a multitude of critical physiological processes, including food and fluid intake, locomotion, stress response, and cognitive functions. Metabolic regulatory factors such as O2, CO2/pH, redox equivalents, glucose/ATP/ADP, hormones, eicosanoids, cell volume, and electrolytes regulate a diverse group of K+ channels to maintain homeostasis.

Pentose phosphate pathway coordinates multiple redox-controlled relaxing mechanisms in bovine coronary arteries

Pentose phosphate pathway (PPP) inhibitors, 6-aminonicotinamide (6-AN) and epiandrosterone (Epi), were employed to examine whether changes in NADP(H) redox regulates contractile force in endothelium-removed bovine coronary arteries (BCAs). 6-AN (0.01-5 mM) or Epi (1-500 microM) elicited dose-dependent relaxation in BCAs contracted with 30 mM KCl, 0.1 microM U-44619, and endothelin-1 but not with phorbol 12,13-dibutyrate, a protein kinase C activator that causes Ca2+-independent contraction. Relaxation to PPP inhibition was associated with oxidation of NADPH and glutathione (GSH). Relaxation to 6-AN was not mediated by H2O2, because it was not altered by hypoxia or the peroxide scavenger ebselen (100 microM). The thiol reductant DTT (3 mM) attenuated the relaxation to 6-AN and Epi by 30-40%. Inhibition of glycolysis or mitochondrial electron transport did not elicit relaxation in BCAs contracted with 30 mM KCl, suggesting these pathways may not be involved in relaxation elicited by PPP inhibition. High doses of K+ channel blockers [e.g., TEA (10 mM) and 4-aminopyridine (10 mM)] only partially inhibited the relaxation to 6-AN. On the basis of changes in the fura-2 fluorescence ratio, 6-AN and Epi appeared to markedly reduce intracellular Ca2+. Thus PPP inhibition oxidizes NADPH and GSH and appears to activate a novel coordination of redox-controlled relaxing mechanisms in BCAs mediated primarily through decreasing intracellular Ca2+.

Dehydroepiandrosterone (DHEA) prevents and reverses chronic hypoxic pulmonary hypertension

Pulmonary artery (PA) hypertension was studied in a chronic hypoxic-pulmonary hypertension model (7-21 days) in the rat. Increase in PA pressure (measured by catheterism), cardiac right ventricle hypertrophy (determined by echocardiography), and PA remodeling (evaluated by histology) were almost entirely prevented after oral dehydroepiandrosterone (DHEA) administration (30 mg/kg every alternate day). Furthermore, in hypertensive rats, oral administration, or intravascular injection (into the jugular vein) of DHEA rapidly decreased PA hypertension. In PA smooth muscle cells, DHEA reduced the level of intracellular calcium (measured by microspectrofluorimetry). The effect of DHEA appears to involve a large conductance Ca2+-activated potassium channel (BKCa)-dependent stimulatory mechanism, at both function and expression levels (isometric contraction and Western blot), via a redox-dependent pathway. Voltage-gated potassium (Kv) channels also may be involved because the antagonist 4-amino-pyridine blocked part of the DHEA effect. The possible pathophysiological and therapeutic significance of the results is discussed.

Redox signaling in oxygen sensing by vessels

In response to the increase in oxygen tension at birth, the resistance pulmonary arteries dilate, while the ductus arteriosus constricts. Although modulated by the endothelium, these opposite responses are intrinsic to the vascular smooth muscle. While still controversial, it seems likely that during normoxia the production of reactive oxygen species (ROS) increases and the smooth muscle cell cytoplasm is more oxidized in both pulmonary arteries and ductus, compared to hypoxia. However, the effect of changes in the endogenous redox status or the addition of a redox agent, oxidizing or reducing, is exactly opposite in the two vessels. A reducing agent, dithiothreitol, like hypoxia, in the pulmonary artery will inhibit potassium current, cause depolarization, increase cytosolic calcium and lead to contraction. Responses to dithiothreitol in the ductus are opposite and removal of endogenous H(2)O(2) by intracellular catalase in the ductus increases potassium current. Oxygen sensing in both vessels is probably mediated by redox effects on both calcium influx and calcium release from the sarcoplasmic reticulum (SR).

Inhibitors of pentose phosphate pathway cause vasodilation: involvement of voltage-gated potassium channels

Cytosolic reducing cofactors, such as NADPH and NADH, are thought to regulate vascular smooth muscle ion channel activity and vascular tone. In this study, the effects of pentose phosphate pathway (PPP) inhibitors, 6-aminonicotinamide (6-AN), epiandrosterone (EPI), and dehydroepiandrosterone (DHEA), on vascular tone were studied in isolated perfused lungs and pulmonary artery (PA) and aortic rings from rats. In addition, effects of 6-AN on voltage-gated K(+) (K(v)) current in PA smooth muscle cells (SMCs) were also examined. Pretreatment of lungs with 6-AN and EPI reduced the pressor response to acute hypoxia and decreased tissue NADPH levels. 6-AN, EPI, and DHEA relaxed isolated PA and aortic rings precontracted with 30 mM KCl in a dose-dependent manner. The PPP inhibitor-induced PA relaxations were reduced in PA rings precontracted with 80 mM KCl but not by pretreatment with nitro-L-arginine or endothelial removal. Pretreatment of PA rings with tetraethylammonium chloride or 4-aminopyridine caused rightward shifts of concentration-relaxation curves for 6-AN, EPI, and DHEA. In contrast, glybenclamide, charybdotoxin, or apamin did not inhibit the relaxant effects of 6-AN, EPI, and DHEA. 6-AN caused an increase in K(v) current in PASMC. These results indicate that reduction of NADPH by the PPP inhibitors causes vasodilation at least partly through opening of K(v) channels.

Molecular basis of hypoxia-induced pulmonary vasoconstriction: role of voltage-gated K+ channels

The hypoxia-induced membrane depolarization and subsequent constriction of small resistance pulmonary arteries occurs, in part, via inhibition of vascular smooth muscle cell voltage-gated K+ (KV) channels open at the resting membrane potential. Pulmonary arterial smooth muscle cell KV channel expression, antibody-based dissection of the pulmonary arterial smooth muscle cell K+ current, and the O2 sensitivity of cloned KV channels expressed in heterologous expression systems have all been examined to identify the molecular components of the pulmonary arterial O2-sensitive KV current. Likely components include Kv2.1/Kv9.3 and Kv1.2/Kv1.5 heteromeric channels and the Kv3.1b alpha-subunit. Although the mechanism of KV channel inhibition by hypoxia is unknown, it appears that KV alpha-subunits do not sense O2 directly. Rather, they are most likely inhibited through interaction with an unidentified O2 sensor and/or beta-subunit. This review summarizes the role of KV channels in hypoxic pulmonary vasoconstriction, the recent progress toward the identification of KV channel subunits involved in this response, and the possible mechanisms of KV channel regulation by hypoxia.

Hydrogen peroxide--an intracellular signal in the pulmonary circulation: involvement in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) is a regulatory feature of the pulmonary circulation that ensures consistent matching of perfusion to ventilation in the normal lung. However, under pathophysiological conditions, HPV contributes to the elevated pulmonary arterial pressure inherent to numerous disease states. Consequently, control of HPV is an avenue of potential therapy for such conditions. This review discusses the role of hydrogen peroxide (H(2)O(2)) as an intracellular signal in the pulmonary circulation, concentrating on the potential involvement of H(2)O(2) in HPV and in the control of pulmonary arterial tone. Sites of hypoxic pulmonary arterial H(2)O(2) production include the mitochondrial electron transport chain, a microsomal electron transport chain containing an NADH oxidoreductase and alternatively, a membrane-bound NADPH oxidase. Each of these sources of H(2)O(2) and the effect of hypoxia on the production of reactive oxygen species are considered. The review also discusses the variance in vascular reactivity of H(2)O(2), which is described to elicit both pulmonary arterial vasoconstriction and dilatation at varying concentrations. The redox capabilities of H(2)O(2) are also considered. The relevance of all of these actions of H(2)O(2) are also assessed as potential pharmacological targets for the future development of therapy for lung diseases that are characterised by some degree of HPV and in the pathogenesis of pulmonary diseases in which reactive oxygen species are implicated.

NADPH oxidase: a universal oxygen sensor?

NADPH oxidase is classically regarded as a key enzyme of neutrophils, where it is involved in the pathogenic production of reactive oxygen species. However, NADPH oxidase-like enzymes have recently been identified in non-neutrophil cells, supporting a separate role for NADPH-oxidase derived oxygen species in oxygen sensitive processes. This article reviews the current literature surrounding the potential role of NADPH oxidase in the oxygen sensing processes which underlie hypoxic pulmonary vasoconstriction, systemic vascular smooth muscle proliferation, carotid and airways chemoreceptor activation, erythropoietin gene expression, and oxytropic responses of plant cells.

Effect of ceramide on KCa channel activity and vascular tone in coronary arteries

A sphingomyelin metabolite, ceramide, serves as a second messenger in a variety of mammalian cells. Little is known regarding the production and actions of this novel intracellular signaling lipid molecule in the vasculature. The present study was designed to test the hypothesis that a ceramide-mediated signaling pathway is present in coronary arterial smooth muscle and that ceramide serves as an inhibitor of the large-conductance Ca2+-activated potassium (KCa) channels and mediates vasoconstriction in coronary circulation. We found that C2-ceramide produced a concentration-dependent decrease in KCa channel activity in vascular smooth muscle cells from small bovine coronary arteries. The average channel activity of the KCa channels in cell-attached patches decreased from 0.046+/-0.01 to 0. 008+/-0.001 at a C2-ceramide concentration of 10 micromol/L. In inside-out patches, C2-ceramide (1 micromol/L) reduced the average channel activity of the KCa channels from 0.06+/-0.007 to 0.016+/-0. 004. Dithiothreitol, an inhibitor of acidic sphingomyelinase (1 mmol/L), increased the average channel activity of the KCa channels in cell-attached patches from 0.05+/-0.02 of control to 0.26+/-0.04, a 5-fold increase that was reversed by addition of 1 micromol/L ceramide. Glutathione, an inhibitor of neutral sphingomyelinase, was without effect. C2-ceramide significantly reduced the diameter of isolated perfused small coronary arteries in a concentration-dependent manner. Addition of 1 micromol/L C2-ceramide decreased average arterial diameter by 28%. When 14C-sphingomyelin was incubated with coronary arterial homogenates at pH 7.4 and pH 5. 0, 14C-choline phosphate and ceramide were produced. The conversion rates of 14C-sphingomyelin into 14C-choline phosphate and ceramide were 65.1+/-1.0 fmol/min per milligram protein at pH 7.4 and 114. 6+/-8.3 fmol/min per milligram protein at pH 5.0. We conclude that both acidic and neutral sphingomyelinases are present in the bovine coronary arteries and that ceramide inactivates the KCa channel in arterial smooth muscle cells and hence exerts a tonic vasoconstrictor action in coronary microcirculation.

Cysteine and glutathione in catabolic conditions and immunological dysfunction

The conspicuous increase in the plasma cysteine disulphide/thiol ratio in elderly persons and cancer patients indicates a shift of the plasma redox state. The most important redox buffers in skeletal muscle tissue and blood plasma, i.e. glutathione and albumin, respectively, are significantly decreased in different models of cachexia. Treatment with N-acetyl cysteine, i.e. a thiol-containing antioxidant, was found to increase the plasma albumin level and to ameliorate the loss of body cell mass in cancer patients and healthy individuals. The treatment of HIV infection with N-acetyl cysteine, in contrast, serves mainly as a tool to ameliorate the physiological and immunological consequences of the virus-induced cysteine deficiency.

Contribution of Ca2+-activated K+ channels and non-selective cation channels to membrane potential of pulmonary arterial smooth muscle cells of the rabbit

1. Using the perforated patch-clamp or whole-cell clamp technique, we investigated the contribution of Ca2+-activated K+ current (IK(Ca)) and non-selective cation currents (INSC) to the membrane potential in small pulmonary arterial smooth muscle cells of the rabbit. 2. The resting membrane potential (Vm) was -39.2 +/- 0.9 mV (n = 72). It did not stay at a constant level, but hyperpolarized irregularly, showing spontaneous transient hyperpolarizations (STHPs). The mean frequency and amplitude of the STHPs was 5.6 +/- 1. 1 Hz and -7.7 +/- 0.7 mV (n = 12), respectively. In the voltage-clamp mode, spontaneous transient outward currents (STOCs) were recorded with similar frequency and irregularity. 3. Intracellular application of BAPTA or extracellular application of TEA or charybdotoxin suppressed both the STHPs and STOCs. The depletion of intracellular Ca2+ stores by caffeine or ryanodine, and the removal of extracellular Ca2+ also abolished STHPs and STOCs. 4. Replacement of extracellular Na+ with NMDG+ caused hyperpolarization Vm of without affecting STHPs. Removal of extracellular Ca2+ induced a marked depolarization of Vm along with the disappearance of STHPs. 5. The ionic nature of the background inward current was identified. The permeability ratio of K+ : Cs+ : Na+ : Li+ was 1.7 : 1.3 : 1 : 0. 9, indicating that it is a non-selective cation current (INSC). The reversal potential of this current in control conditions was calculated to be -13.9 mV. The current was blocked by millimolar concentrations of extracellular Ca2+ and Mg2+. 6. From these results, it was concluded that (i) hyperpolarizing currents are mainly contributed by Ca2+-activated K+ (KCa) channels, and thus STOCs result in transient membrane hyperpolarization, and (ii) depolarizing currents are carried through NSC channels.

Reduced glutathione inhibits beta-NAD+-activated non-selective cation currents in the CRI-G1 rat insulin-secreting cell line

1. Whole-cell voltage-clamp recordings were used to study the characteristics of a non-selective cation current, activated by intracellular beta-NAD+, present in CRI-G1 insulin-secreting cells. The monovalent cations Na+, K+ and Cs+ were equally permeant through this channel. 2. The magnitude of the beta-NAD+ current was dependent on the concentration of both beta-NAD+ and Ca2+ in the cell. The properties of the beta-NAD+-activated macroscopic current are similar to those of the beta-NAD+-activated non-selective cation channel (NSNAD) examined at the single channel level in this cell line. 3. The presence of intracellular reduced glutathione (GSH) inhibited the beta-NAD+-activated macroscopic current and the activity of the NSNAD channel in inside-out patch recordings. 4. The inhibition of beta-NAD+-activated currents by GSH is mimicked by its analogue ophthalmic acid but not by another thiol reducing agent dithiothreitol, indicating the presence of a specific GSH binding site present on the NSNAD channel or associated protein.

Glutathione modulates ryanodine receptor from skeletal muscle sarcoplasmic reticulum. Evidence for redox regulation of the Ca2+ release mechanism

In this report, we demonstrate the ability of the cellular thiol glutathione to modulate the ryanodine receptor from skeletal muscle sarcoplasmic reticulum. Reduced glutathione (GSH) inhibited Ca2+-stimulated [3H]ryanodine binding to the sarcoplasmic reticulum and inhibited the single-channel gating activity of the reconstituted Ca2+ release channel. The effects of GSH on both the [3H]ryanodine binding and single-channel measurements were dose-dependent, exhibiting an IC50 of approximately 2.4 mM in binding experiments. Scatchard analysis demonstrated that GSH decreased the binding affinity of ryanodine for its receptor (increased Kd) and lowered the maximal binding occupancy (Bmax). In addition, GSH did not modify the Ca2+ dependence of [3H]ryanodine binding. In single-channel experiments, GSH (5-10 mM), added to the cis side of the bilayer lipid membrane, lowered the open probability (Po) of a Ca2+ (50 microM)-stimulated Ca2+ channel without modifying the single-channel conductance. Subsequent perfusion of the cis chamber with an identical buffer, containing 50 microM Ca2+ without GSH, re-established Ca2+-stimulated channel gating. GSH did not inhibit channel activity when added to the trans side of the bilayer lipid membrane. Similar to GSH, the thiol-reducing agents dithiothreitol and beta-mercaptoethanol also inhibited high affinity [3H]ryanodine binding to sarcoplasmic reticulum membranes. In contrast to GSH, glutathione disulfide (GSSG) was a potent stimulator of high affinity [3H]ryanodine binding and it also stimulated the activity of the reconstituted single Ca2+ release channel. These results provide direct evidence that glutathione interacts with reactive thiols associated with the Ca2+ release channel/ryanodine receptor complex, which are located on the cytoplasmic face of the SR, and support previous observations (Liu, G, Abramson, J. J., Zable, A. C., and Pessah, I. N. (1994) Mol. Pharmacol. 45, 189-200) that reactive thiols may be involved in the gating of the Ca2+ release channel.

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

1. The electrical properties of chemoreceptor cells from neonatal rat and adult rabbit carotid bodies (CBs) are strikingly different. These differences have been suggested to be developmental and/or species related. To distinguish between the two possibilities, the whole-cell configuration of the patch-clamp technique was used to characterize the ionic currents present in isolated chemoreceptor cells from adult rat CBs. Since hypoxia-induced inhibition of O2-sensitive K+ currents is considered a crucial step in O2 chemoreception, the effect of hypoxia on the adult rat chemoreceptor cell currents was also studied. 2. Outward currents were carried mainly by K+, and two different components could be distinguished: a Ca(2+)-dependent K+ current (IK(Ca)) sensitive to Cd2+ and charybdotoxin (CTX), and a Ca(2+)-insensitive, voltage-dependent K+ current (IK(V)). IK(V) showed a slow voltage-dependent activation (time constant (tau) of 87.4 ms at -20 mV and 8.8 ms at +60 mV) and a very slow inactivation, described by the sum of two exponentials (tau 1 = 684 +/- 150 ms and tau 2 = 4.96 +/- 0.76 s at + 30 mV), that was almost voltage insensitive. The kinetic and pharmacological properties of IK(V) are typical of a delayed rectifier K+ channel. 3. Voltage-dependent Ca2+ currents (ICa) were present in nineteen of twenty-seven cells. TTX-sensitive Na+ currents were also observed in about 10% of the cells. 4. Low PO2 (< 10 mmHg) reduced the whole outward current amplitude by 22.17 +/- 1.96% (n = 27) at +20 mV. This effect was absent in the presence of Cd2+. Since low PO2 did not affect ICa, we conclude that hypoxia selectively blocks IK(Ca). 5. The properties of the currents recorded in adult rat chemoreceptor cells, including the specific inhibition of IK(Ca) by hypoxia, are similar to those reported in neonatal rat CB cells, implying that the differences between rat and rabbit chemoreceptor cells are species related.