K(+) channel inhibition, calcium signaling, and vasomotor tone in canine pulmonary artery smooth muscle

Vasoconstrictor Mechanisms in Chronic Hypoxia-Induced Pulmonary Hypertension: Role of Oxidant Signaling

Elevated resistance of pulmonary circulation after chronic hypoxia exposure leads to pulmonary hypertension. Contributing to this pathological process is enhanced pulmonary vasoconstriction through both calcium-dependent and calcium sensitization mechanisms. Reactive oxygen species (ROS), as a result of increased enzymatic production and/or decreased scavenging, participate in augmentation of pulmonary arterial constriction by potentiating calcium influx as well as activation of myofilament sensitization, therefore mediating the development of pulmonary hypertension. Here, we review the effects of chronic hypoxia on sources of ROS within the pulmonary vasculature including NADPH oxidases, mitochondria, uncoupled endothelial nitric oxide synthase, xanthine oxidase, monoamine oxidases and dysfunctional superoxide dismutases. We also summarize the ROS-induced functional alterations of various Ca2+ and K+ channels involved in regulating Ca2+ influx, and of Rho kinase that is responsible for myofilament Ca2+ sensitivity. A variety of antioxidants have been shown to have beneficial therapeutic effects in animal models of pulmonary hypertension, supporting the role of ROS in the development of pulmonary hypertension. A better understanding of the mechanisms by which ROS enhance vasoconstriction will be useful in evaluating the efficacy of antioxidants for the treatment of pulmonary hypertension.

Hypoxia-dependent reactive oxygen species signaling in the pulmonary circulation: focus on ion channels

SIGNIFICANCE

An acute lack of oxygen in the lung causes hypoxic pulmonary vasoconstriction, which optimizes gas exchange. In contrast, chronic hypoxia triggers a pathological vascular remodeling causing pulmonary hypertension, and ischemia can cause vascular damage culminating in lung edema.

RECENT ADVANCES

Regulation of ion channel expression and gating by cellular redox state is a widely accepted mechanism; however, it remains a matter of debate whether an increase or a decrease in reactive oxygen species (ROS) occurs under hypoxic conditions. Ion channel redox regulation has been described in detail for some ion channels, such as Kv channels or TRPC6. However, in general, information on ion channel redox regulation remains scant.

CRITICAL ISSUES AND FUTURE DIRECTIONS

In addition to the debate of increased versus decreased ROS production during hypoxia, we aim here at describing and deciphering why different oxidants, under different conditions, can cause both activation and inhibition of channel activity. While the upstream pathways affecting channel gating are often well described, we need a better understanding of redox protein modifications to be able to determine the complexity of ion channel redox regulation. Against this background, we summarize the current knowledge on hypoxia-induced ROS-mediated ion channel signaling in the pulmonary circulation.

Synaptic effects of low molecular weight components from Chilean Black Widow spider venom

alpha-Latrotoxin is the principal component of the venom from the euroasiatic Black Widow spider and has been studied for its pharmacological use as a synaptic modulator. Interestingly, smaller molecular weight fractions have been found to be associated with this toxin, but their cellular actions have not been studied in detail. The venom from the Chilean Black Widow spider (Latrodectus mactans) does not produce alpha-latrotoxin, however it does contain several small polypeptides. We have recently demonstrated cellular effects of these peptides at the synaptic level using whole-cell patch clamp techniques. Purified venom from the glands of L. mactans was studied in 12 DIV rat hippocampal neuronal cultures. Venom at a concentration of 10nM was able to decrease neuronal conductance thereby increasing membrane resistance. This effect on the passive properties of the neurons induced a change in action potential kinetics simulating the action of classic potassium channel blockers. These changes produced an increase in spontaneous synaptic activity in rat hippocampal cultures in the presence of the venom in a concentration- and time-dependent manner. These results indicate that venom from Chilean spider L. mactans is capable of increasing cell membrane resistance, prolonging the action potential and generating an increase in synaptic activity demonstrating an interesting pharmacological effect of these low molecular weight fragments.

LY294002 inhibits glucocorticoid-induced COX-2 gene expression in cardiomyocytes through a phosphatidylinositol 3 kinase-independent mechanism

Glucocorticoids induce COX-2 expression in rat cardiomyocytes. While investigating whether phosphatidylinositol 3 kinase (PI3K) plays a role in corticosterone (CT)-induced COX-2, we found that LY294002 (LY29) but not wortmannin (WM) attenuates CT from inducing COX-2 gene expression. Expression of a dominant-negative mutant of p85 subunit of PI3K failed to inhibit CT from inducing COX-2 expression. CT did not activate PI3K/AKT signaling pathway whereas LY29 and WM decreased the activity of PI3K. LY303511 (LY30), a structural analogue and a negative control for PI3K inhibitory activity of LY29, also suppressed COX-2 induction. These data suggest PI3K-independent mechanisms in regulating CT-induced COX-2 expression. LY29 and LY30 do not inhibit glucocorticoid receptor transactivity. Both compounds have been reported to inhibit Casein Kinase 2 activity and modulate potassium and calcium levels independent of PI3K, while LY29 has been reported to inhibit mammalian Target of Rapamycin (mTOR), and DNA-dependent Protein Kinase (DNA-PK). Inhibitor of Casein Kinase 2 (CK2), mTOR or DNA-PK failed to prevent CT from inducing COX-2 expression. Tetraethylammonium (TEA), a potassium channel blocker, and nimodipine, a calcium channel blocker, both attenuated CT from inducing COX-2 gene expression. CT was found to increase intracellular Ca(2+) concentration, which can be inhibited by LY29, TEA or nimodipine. These data suggest a possible role of calcium instead of PI3K in CT-induced COX-2 expression in cardiomyocytes.

Store-operated calcium entry in vascular smooth muscle

In non-excitable cells, activation of G-protein-coupled phospholipase C (PLC)-linked receptors causes the release of Ca(2+) from intracellular stores, which is followed by transmembrane Ca(2+) entry. This Ca(2+) entry underlies a small and sustained phase of the cellular [Ca(2+)](i) increases and is important for several cellular functions including gene expression, secretion and cell proliferation. This form of transmembrane Ca(2+) entry is supported by agonist-activated Ca(2+)-permeable ion channels that are activated by store depletion and is referred to as store-operated Ca(2+) entry (SOCE) and represents a major pathway for agonist-induced Ca(2+) entry. In excitable cells such as smooth muscle cells, Ca(2+) entry mechanisms responsible for sustained cellular activation are normally considered to be mediated via either voltage-operated or receptor-operated Ca(2+) channels. Although SOCE occurs following agonist activation of smooth muscle, this was thought to be more important in replenishing Ca(2+) stores rather than acting as a source of activator Ca(2+) for the contractile process. This review summarizes our current knowledge of SOCE as a regulator of vascular smooth muscle tone and discusses its possible role in the cardiovascular function and disease. We propose a possible hypothesis for its activation and suggest that SOCE may represent a novel target for pharmacological therapeutic intervention.

High altitude pulmonary hypertension: role of K+ and Ca2+ channels

Global alveolar hypoxia, as experienced at high-altitude living, has a serious impact on vascular physiology, particularly on the pulmonary vasculature. The effects of sustained hypoxia on pulmonary arteries include sustained vasoconstriction and enhanced medial hypertrophy. As the major component of the vascular media, pulmonary artery smooth muscle cells (PASMC) are the main effectors of the physiological response(s) induced during or following hypoxic exposure. Endothelial cells, on the other hand, can sense humoral and hemodynamic changes incurred by hypoxia, triggering their production of vasoactive and mitogenic factors that then alter PASMC function and growth. Transmembrane ion flux through channels in the plasma membrane not only modulates excitation- contraction coupling in PASMC, but also regulates cell volume, apoptosis, and proliferation. In this review, we examine the roles of K+ and Ca2+ channels in the pulmonary vasoconstriction and vascular remodeling observed during chronic hypoxia-induced pulmonary hypertension.

Pharmacological evidence for a key role of voltage-gated K+ channels in the function of rat aortic smooth muscle cells

The role of voltage-dependent (I(K(v))) and large conductance Ca(2+)-activated (BK(Ca)) K(+) currents in the function of the rat aorta was investigated using specific BK(Ca) and K(V) channel inhibitors in single rat aortic myocytes (RAMs) with patch-clamp technique and in endothelium-denuded aortic rings with isometric tension measurements. The whole-cell K(+) currents were recorded in RAMs dialysed with 200 and 444 nm Ca(2+) and in perforated-patch configuration. Electrophysiological analysis demonstrated that I(K(v)) appeared at >/=-40 mV, while BK(Ca) (isolated using 1 microm paxilline) were seen positive to -20 mV in all conditions. Voltage-dependent characteristics, but not maximal conductance, of I(K(v)) was significantly altered in increased [Ca(2+)](i). Correolide (1 microm) (a K(V)1 channel blocker) did not inhibit the I(K(v)), whereas millimolar concentration of TEA (IC(50)=3.1+/-0.6 mm, n=5) and 4-aminopyridine (4-AP, IC(50)=5.9+/-1.9 mm, n=7) suppressed I(K(v)). These results and immunocytochemical analysis suggest the K(V)2.1 channel to be a molecular correlate for I(K(v)). In nonstimulated aortic rings 1-5 mm TEA and 4-AP (inhibitors of I(K(v))), but not paxilline (1 microm), caused contraction. The frequency of contractile responses to TEA and 4-AP was increased in the presence of 10 mm KCl, which itself did not significantly affect the aortic basal tone. Phenylephrine (15-40 nm) induced sustained tension with superimposed slow oscillatory contractions (termed OWs). OWs were blocked by diltiazem, ryanodine and cyclopiazonic acid, suggesting the involvement of L-type Ca(2+) channels and ryanodine-sensitive Ca(2+) stores in this process. TEA and 4-AP, but not IbTX, paxilline or correolide, increased the duration and amplitude of OWs, indicating that I(K(v)) is involved in the control of oscillatory activity. In conclusion, our findings suggest that the K(V)2.1-mediated I(K(v)), and not BK(Ca), plays an important role in the regulation of the excitability and contractility of rat aorta.

Graded response of K+ current, membrane potential, and [Ca2+]i to hypoxia in pulmonary arterial smooth muscle

Many studies indicate that hypoxic inhibition of some K+ channels in the membrane of the pulmonary arterial smooth muscle cells (PASMCs) plays a part in initiating hypoxic pulmonary vasoconstriction. The sensitivity of the K+ current (I(k)), resting membrane potential (E(m)), and intracellular Ca2+ concentration ([Ca2+]i) of PASMCs to different levels of hypoxia in these cells has not been explored fully. Reducing PO2 levels gradually inhibited steady-state I(k) of rat resistance PASMCs and depolarized the cell membrane. The block of I(k) by hypoxia was voltage dependent in that low O2 tensions (3 and 0% O2) inhibited I(k) more at 0 and -20 mV than at 50 mV. As expected, the hypoxia-sensitive I(k) was also 4-aminopyridine sensitive. Fura 2-loaded PASMCs showed a graded increase in [Ca2+]i as PO2 levels declined. This increase was reduced markedly by nifedipine and removal of extracellular Ca2+. We conclude that, as in the carotid body type I cells, PC-12 pheochromocytoma cells, and cortical neurons, increasing severity of hypoxia causes a proportional decrease in I(k) and E(m) and an increase of [Ca2+]i.

Sugar reception in the blowfly: a possible Ca(++) involvement

The present study investigates the effects of W-7 (a calmodulin antagonist involved in the Ca(++) cascade) on the response of the 'sugar' and 'water' cells of labellar chemosensilla in the blowfly Protophormia terraenovae to stimulation with sucrose or fructose. In order to ascertain whether Ca(++) conductance is involved, the effects of EGTA, one of the most used Ca(++) chelating agent, and of SK&F-96365, an inhibitor of receptor mediated calcium influx, were also studied. Our electrophysiological data indicate that W-7 addition strongly depresses the 'sugar' chemoreceptor response to both sugars and in the case of sucrose stimulation also influences adaptation rate. The Ca(++) chelator has no significant effects on the response of the 'sugar' cell following stimulation with sucrose, but lowers fructose stimulating effectiveness. In the presence of SK&F-96365 both sucrose and fructose responses are inhibited. A possible transduction mechanism for sugar reception is discussed.

Potassium channels underlying the resting potential of pulmonary artery smooth muscle cells

1. The molecular identity of the K channels giving rise to the negative membrane potential of pulmonary artery smooth muscle cells has yet to be determined. 2. To date, most studies have focused on voltage-gated, delayed rectifier channels and their roles in mediating hypoxia-induced membrane depolarization. There is, however, strong evidence that an outwardly rectifying K+ conductance distinct from the classical delayed rectifier is involved. 3. Growing evidence that TASK-like channels can sense hypoxia and are present in pulmonary artery smooth muscle cells suggests that they may be responsible for the resting K+ conductance and resting potential. 4. The present review considers the evidence that particular K channels maintain the resting membrane potential of pulmonary artery smooth muscle cells and mediate the depolarizing response to hypoxia.

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.