Spontaneous transient outward currents and delayed rectifier K+ current: effects of hypoxia

A novel KCND3 mutation associated with early-onset lone atrial fibrillation

Atrial fibrillation (AF) is the most common arrhythmia in the clinic. While previous studies have identified AF-associated mutations in several genes, the genetic basis for AF remains unclear. Here, we identified a novel T361S missense mutation in potassium voltage-gated channel, shal-related subfamily, member 3 (KCND3) from a Chinese Han family ancestor with lone AF. The wild-type (WT) or mutant T361S of K_v4.3 protein (encoded by KCND3) were co-expressed with the auxiliary subunit K⁺ channel-Interacting Protein (KChIP2) in HEK293 cells, and transient outward potassium current (I_to) were recorded using patch-clamp methods, and the surface or total protein levels of K_v4.3 were analyzed by western blot. I_to density, measured at 60 mV, for T361S was significantly higher than that for WT. Both the steady-state activation and inactivation curves showed a remarkable hyperpolarizing shift in T361S. Moreover, recovery from inactivation after a 500-ms depolarizing pulse was significantly delayed for T361S compared with that for WT. Mechanistically, the gain of function of I_to elicited by T361S was associated with the increased expression of cell surface and total cell protein of K_v4.3. The computer stimulation revealed that the T361S mutation shortened the action potential duration through an increased I_toin Human Atrial Model. In conclusion, we identified a novel T361S mutation in KCND3 associated with AF in the Chinese Han family. The T361S mutant result in the changes in channel kinetics as well as the up-regulation of K_v4.3 protein, which may be a critical driver for lone AF as observed in the patient.

Constitutive calcium entry and cancer: updated views and insights

Tight control of basal cytosolic Ca²⁺ concentration is essential for cell survival and to fine-tune Ca²⁺-dependent cell functions. A way to control this basal cytosolic Ca²⁺ concentration is to regulate membrane Ca²⁺ channels including store-operated Ca²⁺ channels and secondary messenger-operated channels linked to G-protein-coupled or tyrosine kinase receptor activation. Orai, with or without its reticular STIM partner and Transient Receptor Potential (TRP) proteins, were considered to be the main Ca²⁺ channels involved. It is well accepted that, in response to cell stimulation, opening of these Ca²⁺ channels contributes to Ca²⁺ entry and the transient increase in cytosolic Ca²⁺ concentration involved in intracellular signaling. However, in various experimental conditions, Ca²⁺ entry and/or Ca²⁺ currents can be recorded at rest, without application of any experimental stimulation. This led to the proposition that some plasma membrane Ca²⁺ channels are already open/activated in basal condition, contributing therefore to constitutive Ca²⁺ entry. This article focuses on direct and indirect observations supporting constitutive activity of channels belonging to the Orai and TRP families and on the mechanisms underlying their basal/constitutive activities.

Membrane potential and Ca2+ concentration dependence on pressure and vasoactive agents in arterial smooth muscle: A model

A mathematical model incorporating junctional and stretch-activated microdomains and 37 protein components describes the myogenic response in arterial smooth muscle cells.

Arterial smooth muscle (SM) cells respond autonomously to changes in intravascular pressure, adjusting tension to maintain vessel diameter. The values of membrane potential (V_m) and sarcoplasmic Ca²⁺ concentration (Ca_in) within minutes of a change in pressure are the results of two opposing pathways, both of which use Ca²⁺ as a signal. This works because the two Ca²⁺-signaling pathways are confined to distinct microdomains in which the Ca²⁺ concentrations needed to activate key channels are transiently higher than Ca_in. A mathematical model of an isolated arterial SM cell is presented that incorporates the two types of microdomains. The first type consists of junctions between cisternae of the peripheral sarcoplasmic reticulum (SR), containing ryanodine receptors (RyRs), and the sarcolemma, containing voltage- and Ca²⁺-activated K⁺ (BK) channels. These junctional microdomains promote hyperpolarization, reduced Ca_in, and relaxation. The second type is postulated to form around stretch-activated nonspecific cation channels and neighboring Ca²⁺-activated Cl⁻ channels, and promotes the opposite (depolarization, increased Ca_in, and contraction). The model includes three additional compartments: the sarcoplasm, the central SR lumen, and the peripheral SR lumen. It incorporates 37 protein components. In addition to pressure, the model accommodates inputs of α- and β-adrenergic agonists, ATP, 11,12-epoxyeicosatrienoic acid, and nitric oxide (NO). The parameters of the equations were adjusted to obtain a close fit to reported V_m and Ca_in as functions of pressure, which have been determined in cerebral arteries. The simulations were insensitive to ±10% changes in most of the parameters. The model also simulated the effects of inhibiting RyR, BK, or voltage-activated Ca²⁺ channels on V_m and Ca_in. Deletion of BK β1 subunits is known to increase arterial–SM tension. In the model, deletion of β1 raised Ca_in at all pressures, and these increases were reversed by NO.

Distinct activity of BK channel β1-subunit in cerebral and pulmonary artery smooth muscle cells

This study was designed to test a hypothesis that the functional activity of big-conductance, Ca(2+)-activated K(+) (BK) channels is different in cerebral and pulmonary artery smooth muscle cells (CASMCs and PASMCs). Using patch-clamp recordings, we found that the activity of whole cell and single BK channels were significantly higher in CASMCs than in PASMCs. The voltage and Ca(2+) sensitivity of BK channels were greater in CASMCs than in PASMCs. Targeted gene knockout of β(1)-subunits significantly reduced BK currents in CASMCs but had no effect in PASMCs. Western blotting experiments revealed that BK channel α-subunit protein expression level was comparable in CASMCs and PASMCs; however, β(1)-subunit protein expression level was higher in CASMCs than in PASMCs. Inhibition of BK channels by the specific blocker iberiotoxin enhanced norepinephrine-induced increase in intracellular calcium concentration in CASMCs but not in PASMCs. Systemic artery blood pressure was elevated in β(1)(-/-) mice. In contrast, pulmonary artery blood pressure was normal in β(1)(-/-) mice. These findings provide the first evidence that the activity of BK channels is higher in cerebral than in PASMCs. This heterogeneity is primarily determined by the differential β(1)-subunit function and contributes to diverse cellular responses in these two distinct types of cells.

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.

Hypoxia induces intracellular Ca2+ release by causing reactive oxygen species-mediated dissociation of FK506-binding protein 12.6 from ryanodine receptor 2 in pulmonary artery myocytes

Here we attempted to test a novel hypothesis that hypoxia may induce Ca(2+) release through reactive oxygen species (ROS)-mediated dissociation of FK506-binding protein 12.6 (FKBP12.6) from ryanodine receptors (RyRs) on the sarcoplasmic reticulum (SR) in pulmonary artery smooth muscle cells (PASMCs). The results reveal that hypoxic exposure significantly decreased the amount of FKBP12.6 on the SR of PAs and increased FKBP12.6 in the cytosol. The colocalization of FKBP12.6 with RyRs was decreased in intact PASMCs. Pharmacological and genetic inhibition of intracellular ROS generation prevented hypoxia from decreasing FKBP12.6 on the SR and increasing FKBP12.6 in the cytosol. Exogenous ROS (H(2)O(2)) reduced FKBP12.6 on the SR and augmented FKBP12.6 in the cytosol. Oxidized FKBP12.6 was absent on the SR from PAs pretreated with and without hypoxia, but it was present with a higher amount in the cytosol from PAs pretreated with than without hypoxia. Hypoxia and H(2)O(2) diminished the association of FKBP12.6 from type 2 RyRs (RyR2). The activity of RyRs was increased in PAs pretreated with hypoxia or H(2)O(2). FKBP12.6 removal enhanced, whereas RyR2 gene deletion blocked the hypoxic increase in [Ca(2+)](i) in PASMCs. Collectively, we conclude that hypoxia may induce Ca(2+) release by causing ROS-mediated dissociation of FKBP12.6 from RyR2 in PASMCs.

Role of ROS signaling in differential hypoxic Ca2+ and contractile responses in pulmonary and systemic vascular smooth muscle cells

Hypoxia causes a large increase in [Ca2+]i and attendant contraction in pulmonary artery smooth muscle cells (PASMCs), but not in systemic artery SMCs. The different responses meet the respective functional needs in these two distinct vascular myocytes; however, the underlying molecular mechanisms are not well known. We and other investigators have provided extensive evidence to reveal that voltage-dependent K+ (KV) channels, canonical transient receptor potential (TRPC) channels, ryanodine receptor Ca2+ release channels (RyRs), cyclic adenosine diphosphate-ribose, FK506 binding protein 12.6, protein kinase C, NADPH oxidase and reactive oxygen species (ROS) are the essential effectors and signaling intermediates in the hypoxic increase in [Ca2+]i in PASMCs and HPV, but they may not primarily underlie the diverse cellular responses in pulmonary and systemic vascular myocytes. Hypoxia significantly increases mitochondrial ROS generation in PASMCs, which can induce intracellular Ca2+ release by opening RyRs, and may also cause extracellular Ca2+ influx by inhibiting KV channels and activating TRPC channels, leading to a large increase in [Ca2+]i in PASMCs and HPV. In contrast, hypoxia has no or a minor effect on mitochondrial ROS generation in systemic SMCs, thereby causing no change or a negligible increase in [Ca2+]i and contraction. Further preliminary work indicates that Rieske iron-sulfur protein in the mitochondrial complex III may perhaps serve as a key initial molecular determinant for the hypoxic increase in [Ca2+]i in PASMCs and HPV, suggesting its potential important role in different cellular changes to respond to hypoxic stimulation in pulmonary and systemic artery myocytes. All these findings have greatly improved our understanding of the molecular processes for the differential hypoxic Ca2+ and contractile responses in vascular SMCs from distinct pulmonary and systemic circulation systems.

ROS-dependent signaling mechanisms for hypoxic Ca(2+) responses in pulmonary artery myocytes

Hypoxic exposure causes pulmonary vasoconstriction, which serves as a critical physiologic process that ensures regional alveolar ventilation and pulmonary perfusion in the lungs, but may become an essential pathologic factor leading to pulmonary hypertension. Although the molecular mechanisms underlying hypoxic pulmonary vasoconstriction and associated pulmonary hypertension are uncertain, increasing evidence indicates that hypoxia can result in a significant increase in intracellular reactive oxygen species concentration ([ROS](i)) through the mitochondrial electron-transport chain in pulmonary artery smooth muscle cells (PASMCs). The increased mitochondrial ROS subsequently activate protein kinase C-epsilon (PKCepsilon) and NADPH oxidase (Nox), providing positive mechanisms that further increase [ROS](i). ROS may directly cause extracellular Ca(2+) influx by inhibiting voltage-dependent K(+) (K(V)) channels and opening of store-operated Ca(2+) (SOC) channels, as well as intracellular Ca(2+) release by activating ryanodine receptors (RyRs), leading to an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) and associated contraction. In concert with ROS, PKCepsilon may also affect K(V) channels, SOC channels, and RyRs, contributing to hypoxic Ca(2+) and contractile responses in PASMCs.

Heterogeneous gene expression and functional activity of ryanodine receptors in resistance and conduit pulmonary as well as mesenteric artery smooth muscle cells

BACKGROUND

Hypoxia causes heterogeneous contractile responses in resistance and conduit pulmonary as well as systemic (mesenteric) artery smooth muscle cells (RPASMCs, CPASMCs and MASMCs), but the underlying mechanisms are largely unknown. In this study, we aimed to investigate whether the gene expression and functional activity of ryanodine receptors (RyRs) would be different in these 3 cell types.

METHODS

RyR mRNA expression, Ca(2+) sparks and [Ca(2+)](i) were measured by real-time quantitative RT-PCR, laser scanning confocal microscopy and wide-field fluorescence microscopy, respectively.

RESULTS

All 3 RyR subtype (RyR1, RyR2 and RyR3) mRNAs are expressed in RPASMCs, CPASMCs and MASMCs, but their expression levels are different. Spontaneous Ca(2+) sparks (functional events of RyRs) show distinct frequency, amplitude, duration, size and kinetics in these 3 cell types. Similarly, activation of RyRs by caffeine, 4-chloro-m-cresol or high K(+) induces differential Ca(2+) release. Moreover, hypoxia-induced increase in [Ca(2+)](i) is largest in MASMCs relative to CPSAMCs and smallest in RPASMCs.

CONCLUSION

This study provides comprehensive evidence that RyRs are heterogeneous in gene expression and functional activity in RPASMCs, CPASMCs and MASMCs, which may contribute to the diversity of excitation-contraction coupling and hypoxic Ca(2+) responses in different vascular smooth muscle cells.

Hypoxia reduces KCa channel activity by inducing Ca2+ spark uncoupling in cerebral artery smooth muscle cells

Arterial smooth muscle cell large-conductance Ca(2+)-activated potassium (K(Ca)) channels have been implicated in modulating hypoxic dilation of systemic arteries, although this is controversial. K(Ca) channel activity in arterial smooth muscle cells is controlled by localized intracellular Ca(2+) transients, termed Ca(2+) sparks, but hypoxic regulation of Ca(2+) sparks and K(Ca) channel activation by Ca(2+) sparks has not been investigated. We report here that in voltage-clamped (-40 mV) cerebral artery smooth muscle cells, a reduction in dissolved O(2) partial pressure from 150 to 15 mmHg reversibly decreased Ca(2+) spark-induced transient K(Ca) current frequency and amplitude to 61% and 76% of control, respectively. In contrast, hypoxia did not alter Ca(2+) spark frequency, amplitude, global intracellular Ca(2+) concentration, or sarcoplasmic reticulum Ca(2+) load. Hypoxia reduced transient K(Ca) current frequency by decreasing the percentage of Ca(2+) sparks that activated a transient K(Ca) current from 89% to 63%. Hypoxia reduced transient K(Ca) current amplitude by attenuating the amplitude relationship between Ca(2+) sparks that remained coupled and the evoked transient K(Ca) currents. Consistent with these data, in inside-out patches at -40 mV hypoxia reduced K(Ca) channel apparent Ca(2+) sensitivity and increased the K(d) for Ca(2+) from approximately 17 to 32 microM, but did not alter single-channel amplitude. In summary, data indicate that hypoxia reduces K(Ca) channel apparent Ca(2+) sensitivity via a mechanism that is independent of cytosolic signaling messengers, and this leads to uncoupling of K(Ca) channels from Ca(2+) sparks. Transient K(Ca) current inhibition due to uncoupling would oppose hypoxic cerebrovascular dilation.

Chronic carbon monoxide exposure of hypoxic rats increases in vitro sensitivity of pulmonary artery smooth muscle

Exogenous carbon monoxide (CO) induces pulmonary vasodilation by acting directly on pulmonary artery (PA) smooth muscle cells. We investigated the contribution of K+ channels and soluble guanylyl cyclase to the regulation of PA tone by acute CO in chronic hypoxic rats (3 weeks at 0.5 atm (1 atm = 101.325 kPa); hypoxic) and in chronic hypoxic rats exposed to exogenous CO (3 weeks at 0.5 atm + 50 ppm CO; hypoxic-CO). Acute CO induced relaxation in PA rings from all animals. However, the amplitude of CO relaxation was significantly decreased in hypoxic rings and increased in hypoxic-CO rings. This different effect occurred with a decrease and an increase of pD2, respectively, in hypoxic and hypoxic-CO rings. We showed a positive relation between the percentage of inhibition of CO relaxation by a blocker of K+ channels and the increase of CO sensitivity. Thus, we showed for the first time that chronic hypoxia decreases acute CO sensitivity, which in contrast, increases in the presence of chronic CO. The present study provides initial evidence of a link between increased K(+)-channel activity and CO sensitivity.

Metabolic inhibition with cyanide induces calcium release in pulmonary artery myocytes and Xenopus oocytes

We examined the effects of metabolic inhibition on intracellular Ca(2+) release in single pulmonary arterial smooth muscle cells (PASMCs). Severe metabolic inhibition with cyanide (CN, 10 mM) increased intracellular calcium concentration ([Ca(2+)](i)) and activated Ca(2+)-activated Cl(-) currents [I(Cl(Ca))] in PASMCs, responses that were greatly inhibited by BAPTA-AM or caffeine. Mild metabolic inhibition with CN (1 mM) increased spontaneous transient inward currents and Ca(2+) sparks in PASMCs. In Xenopus oocytes, CN also induced Ca(2+) release and activated I(Cl(Ca)), and these responses were inhibited by thapsigargin and cyclopiazonic acid to deplete sarcoplasmic reticulum (SR) Ca(2+), whereas neither heparin nor anti-inositol 1,4,5-trisphosphate receptor (IP(3)R) antibodies affected CN responses. In both PASMCs and oocytes, CN-evoked Ca(2+) release was inhibited by carbonyl cyanide m-chlorophenylhydrazone (CCCP) and oligomycin or CCCP and thapsigargin. Whereas hypoxic stimuli resulted in Ca(2+) release in pulmonary but not mesenteric artery myocytes, CN induced release in both cell types. We conclude that metabolic inhibition with CN increases [Ca(2+)](i) in both pulmonary and systemic artery myocytes by stimulating Ca(2+) release from the SR and mitochondria.

Chronic carbon monoxide enhanced IbTx-sensitive currents in rat resistance pulmonary artery smooth muscle cells

Exogenous carbon monoxide (CO) can induce pulmonary vasodilation by acting directly on pulmonary artery (PA) smooth muscle cells. We investigated the contribution of K+ channels to the regulation of resistance PA resting membrane potential on control (PAC) rats and rats exposed to CO for 3 wk at 530 parts/million, labeled as PACO rats. Whole cell patch-clamp experiments revealed that the resting membrane potential of PACO cells was more negative than that of PAC cells. This was associated with a decrease of membrane resistance in PACO cells. Additional analysis showed that outward current density in PACO cells was higher (50% at +60 mV) than in PAC cells. This was linked to an increase of iberiotoxin (IbTx)-sensitive current. Chronic CO hyperpolarized membrane of pressurized PA from -46.9 +/- 1.2 to -56.4 +/- 2.6 mV. Additionally, IbTx significantly depolarized membrane of smooth muscle cells from PACO arteries but not from PAC arteries. The present study provides initial evidence of an increase of Ca2+-activated K+ current in smooth muscle cells from PA of rats exposed to chronic CO.

Ca(2+) release from ryanodine-sensitive store contributes to mechanism of hypoxic vasoconstriction in rat lungs

Studies of thapsigargin, cyclopiazonic acid, and ryanodine in isolated pulmonary arteries and smooth muscle cells suggest that release of Ca(2+) from inositol 1,4,5-trisphosphate (IP(3))- and/or ryanodine-sensitive sarcoplasmic reticulum Ca(2+) stores is a component of the mechanism of acute hypoxic pulmonary vasoconstriction (HPV). However, the actions of these agents on HPV in perfused lungs have not been reported. Thus we tested effects of thapsigargin and cyclopiazonic acid, inhibitors of sarcoplasmic reticulum Ca(2+)-ATPase, and of ryanodine, an agent that either locks the ryanodine receptor open or blocks it, on HPV in salt solution-perfused rat lungs. After inhibition of cyclooxygenase and nitric oxide synthase, thapsigargin (10 nM) and cyclopiazonic acid (5 microM) augmented the vasoconstriction to 0% but not to 3% inspired O(2). Relatively high concentrations of ryanodine (100 and 300 microM) blunted HPV in nitric oxide synthase-inhibited lungs. The results indicate that release of Ca(2+) from the ryanodine-sensitive, but not the IP(3)-sensitive, store, contributes to the mechanism of HPV in perfused rat lungs and that Ca(2+)-ATPase-dependent Ca(2+) buffering moderates the response to severe hypoxia.

Hypoxic release of calcium from the sarcoplasmic reticulum of pulmonary artery smooth muscle

The hypoxic constriction of isolated pulmonary vessels is composed of an initial transient phase (phase 1) followed by a slowly developing increase in tone (phase 2). We investigated the roles of the endothelium and of intracellular Ca2+ stores in both preconstricted and unpreconstricted intrapulmonary rabbit arteries when challenged with hypoxia (PO2 16-21 Torr). Removing the endothelium did not affect phase 1, but phase 2 appeared as a steady plateau. Removing extracellular Ca2+ had essentially the same effect as removing the endothelium. Depletion of sarcoplasmic reticulum Ca2+ stores with caffeine and ryanodine abolished the hypoxic response. Omitting preconstriction reduced the amplitude of the hypoxic response but did not qualitatively affect any of the above responses. We conclude that hypoxia releases intracellular Ca2+ from ryanodine-sensitive stores by a mechanism intrinsic to pulmonary vascular smooth muscle without the need for Ca2+ influx across the plasmalemma or an endothelial factor. Our results also suggest that extracellular Ca2+ is required for the release of an endothelium-derived vasoconstrictor.

Hypoxia inhibits the Na(+)/Ca(2+) exchanger in pulmonary artery smooth muscle cells

The cellular mechanisms underlying hypoxic pulmonary vasoconstriction are not fully understood. We examined the effect of hypoxia on Ca(2+) efflux from the cytosol in single Fura-2-loaded pulmonary artery myocytes. During mild hypoxia (pO(2)=50-60 Torr), peak [Ca(2+)](i) was increased and the rate of Ca(2+) removal from the cytosol was markedly slowed after stimuli that elevated [Ca(2+)](i). Removal of extracellular Na(+) potentiated the peak [Ca(2+)](i) rise and slowed the Ca(2+) decay rate in cells recorded under normoxic conditions; it did not further slow the Ca(2+) decay rate or potentiate the [Ca(2+)](i) increase in hypoxic cells. An Na(+)/Ca(2+) exchange current was recorded in isolated pulmonary artery myocytes. Switching from Li(+) to Na(+) (130 mM) revealed an inward current with reversal potential consistent with the Na(+)/Ca(2+) exchange current in cells in which [Ca(2+)](i) was clamped at 1 microM similar currents, although smaller, were observed with normal resting [Ca(2+)](i) using the perforated patch clamp technique. The Na(+)/Ca(2+) exchange current was markedly inhibited in myocytes exposed to mild hypoxia. RT-PCR revealed the expression of specific alternatively spliced RNAs of NCX1 in rat pulmonary arteries. These findings provide an enhanced understanding of the molecular mechanisms underlying hypoxic sensing in pulmonary arteries.