The pulmonary vasodilator properties of potassium channel opening drugs

Kir6.1 and SUR2B in Cantú syndrome

Kir6.1 and SUR2 are subunits of ATP-sensitive potassium (KATP) channels expressed in a wide range of tissues. Extensive study has implicated roles of these channel subunits in diverse physiological functions. Together they generate the predominant KATP conductance in vascular smooth muscle and are the target of vasodilatory drugs. Roles for Kir6.1/SUR2 dysfunction in disease have been suggested based on studies of animal models and human genetic discoveries. In recent years, it has become clear that gain-of-function (GoF) mutations in both genes result in Cantú syndrome (CS)-a complex, multisystem disorder. There is currently no targeted therapy for CS, but studies of mouse models of the disease reveal that pharmacological reversibility of cardiovascular and gastrointestinal pathologies can be achieved by administration of the KATP channel inhibitor, glibenclamide. Here we review the function, structure, and physiological and pathological roles of Kir6.1/SUR2B channels, with a focus on CS. Recent studies have led to much improved understanding of the underlying pathologies and the potential for treatment, but important questions remain: Can the study of genetically defined CS reveal new insights into Kir6.1/SUR2 function? Do these reveal new pathophysiological mechanisms that may be important in more common diseases? And is our pharmacological armory adequately stocked?

The Mechanism of High-Output Cardiac Hypertrophy Arising From Potassium Channel Gain-of-Function in Cantú Syndrome

Dramatic cardiomegaly arising from gain-of-function (GoF) mutations in the ATP-sensitive potassium (K_ATP) channels genes, ABCC9 and KCNJ8, is a characteristic feature of Cantú syndrome (CS). How potassium channel over-activity results in cardiac hypertrophy, as well as the long-term consequences of cardiovascular remodeling in CS, is unknown. Using genome-edited mouse models of CS, we therefore sought to dissect the pathophysiological mechanisms linking K_ATP channel GoF to cardiac remodeling. We demonstrate that chronic reduction of systemic vascular resistance in CS is accompanied by elevated renin-angiotensin signaling, which drives cardiac enlargement and blood volume expansion. Cardiac enlargement in CS results in elevation of basal cardiac output, which is preserved in aging. However, the cardiac remodeling includes altered gene expression patterns that are associated with pathological hypertrophy and are accompanied by decreased exercise tolerance, suggestive of reduced cardiac reserve. Our results identify a high-output cardiac hypertrophy phenotype in CS which is etiologically and mechanistically distinct from other myocardial hypertrophies, and which exhibits key features of high-output heart failure (HOHF). We propose that CS is a genetically-defined HOHF disorder and that decreased vascular smooth muscle excitability is a novel mechanism for HOHF pathogenesis.

Ion Channels in Pulmonary Hypertension: A Therapeutic Interest?

Pulmonary arterial hypertension (PAH) is a multifactorial and severe disease without curative therapies. PAH pathobiology involves altered pulmonary arterial tone, endothelial dysfunction, distal pulmonary vessel remodeling, and inflammation, which could all depend on ion channel activities (K⁺, Ca²⁺, Na⁺ and Cl^-). This review focuses on ion channels in the pulmonary vasculature and discusses their pathophysiological contribution to PAH as well as their therapeutic potential in PAH.

Hypoxia Exacerbates Inflammatory Acute Lung Injury via the Toll-Like Receptor 4 Signaling Pathway

Acute lung injury (ALI) is characterized by non-cardiogenic diffuse alveolar damage and often leads to a lethal consequence, particularly when hypoxia coexists. The treatment of ALI remains a challenge: pulmonary inflammation and hypoxia both contribute to its onset and progression and no effective prevention approach is available. Here, we aimed to investigate the underlying mechanism of hypoxia interaction with inflammation in ALI and to evaluate hypoxia-inducible factor 1 alpha (HIF-1α)—the crucial modulator in hypoxia—as a potential therapeutic target against ALI. First, we developed a novel ALI rat model induced by a combined low-dose of lipopolysaccharides (LPS) with acute hypoxia. Second, we used gene microarray analysis to evaluate the inflammatory profiles of bronchi alveolar lavage fluid cells of ALI rats. Third, we employed an alveolar macrophage cell line, NR8383 as an in vitro system together with a toll-like receptor 4 (TLR4) antagonist TAK-242, to verify our in vivo findings from ALI animals. Finally, we tested the therapeutic effects of HIF-1α augmentation against inflammation and hypoxia in ALI. We demonstrated that (i) LPS upregulated inflammatory genes, tumor necrosis factor alpha (TNF-α), interleukin-1 beta (IL-1β), and interleukin-6 (IL-6), in the alveolar macrophages of ALI rats, which were further enhanced when ALI combined with hypoxia; (ii) hypoxia exposure could further enhance the upregulation of alveolar macrophageal TLR4 that was noticed in LPS-induced inflammatory ALI, conversely, TLR4 antagonist TAK-242 could suppress the macrophageal expression of TLR4 and inflammatory cytokines, including TNF-α, IL-1β, and IL-6, suggesting that the TLR4 signaling pathway as a central link between inflammation and hypoxia in ALI; (iii) manipulation of HIF-1α in vitro could suppress TLR4 expression induced by combined LPS and hypoxia, via suppressing promoter activity of the TLR4 gene; (iv) preconditioning augmentation of HIF-1α in vivo by HIF hydroxylase inhibitor, DMOG excreted protection against inflammatory, and hypoxic processes in ALI. Together, we see that hypoxia can exacerbate inflammation in ALI via the activation of the TLR4 signaling pathway in alveolar macrophages and predispose impairment of the alveolar-capillary barrier in the development of ALI. Targeting HIF-1α can suppress TLR4 expression and macrophageal inflammation, suggesting the potential therapeutic and preventative value of HIF-1α/TLR4 crosstalk pathway in ALI.

Potassium Channel Subfamily K Member 3 (KCNK3) Contributes to the Development of Pulmonary Arterial Hypertension

BACKGROUND

Mutations in the KCNK3 gene have been identified in some patients suffering from heritable pulmonary arterial hypertension (PAH). KCNK3 encodes an outward rectifier K(+) channel, and each identified mutation leads to a loss of function. However, the pathophysiological role of potassium channel subfamily K member 3 (KCNK3) in PAH is unclear. We hypothesized that loss of function of KCNK3 is a hallmark of idiopathic and heritable PAH and contributes to dysfunction of pulmonary artery smooth muscle cells and pulmonary artery endothelial cells, leading to pulmonary artery remodeling: consequently, restoring KCNK3 function could alleviate experimental pulmonary hypertension (PH).

METHODS AND RESULTS

We demonstrated that KCNK3 expression and function were reduced in human PAH and in monocrotaline-induced PH in rats. Using a patch-clamp technique in freshly isolated (not cultured) pulmonary artery smooth muscle cells and pulmonary artery endothelial cells, we found that KCNK3 current decreased progressively during the development of monocrotaline-induced PH and correlated with plasma-membrane depolarization. We demonstrated that KCNK3 modulated pulmonary arterial tone. Long-term inhibition of KCNK3 in rats induced distal neomuscularization and early hemodynamic signs of PH, which were related to exaggerated proliferation of pulmonary artery endothelial cells, pulmonary artery smooth muscle cell, adventitial fibroblasts, and pulmonary and systemic inflammation. Lastly, in vivo pharmacological activation of KCNK3 significantly reversed monocrotaline-induced PH in rats.

CONCLUSIONS

In PAH and experimental PH, KCNK3 expression and activity are strongly reduced in pulmonary artery smooth muscle cells and endothelial cells. KCNK3 inhibition promoted increased proliferation, vasoconstriction, and inflammation. In vivo pharmacological activation of KCNK3 alleviated monocrotaline-induced PH, thus demonstrating that loss of KCNK3 is a key event in PAH pathogenesis and thus could be therapeutically targeted.

Tanshinone IIA modulates pulmonary vascular response to agonist and hypoxia primarily via inhibiting Ca2+ influx and release in normal and hypoxic pulmonary hypertension rats

The present study was designed to investigate the vascular effects and underlying mechanisms of tanshinone IIA on isolated rat pulmonary artery. Isometric tension was recorded in the arteries from normal and hypoxic pulmonary hypertension rats under normoxia or hypoxia condition. The results showed that tanshinone IIA exerted a biphasic effect on rat pulmonary artery. The constriction was attenuated by endothelium-denudation but was enhanced by inhibition of nitric oxide synthase. Pretreatment with tetraethylammonium (Ca2+-activated K+ channel inhibitor) upward shifted the concentration-response curve without affecting the maximum dilatation. Pretreatment with zinc protoporphyrin IX (heme oxygenase-1 inhibitor), 4-aminopyridine (KV channel inhibitor), glibenclamide (KATP channel inhibitor) or BaCl2 (inwardly rectifying K+ channel inhibitor) did not affect the vasoreactivity. Meanwhile, tanshinone IIA almost abolished vasoconstriction induced by extracellular Ca2+. Under hypoxia condition, tanshinone IIA eliminated acute hypoxia-induced initial contraction, potentiated following vasorelaxation, attenuated and reversed sustained contraction to relaxation in pulmonary artery from normal rats, and reversed phenylephrine-induced sustained constriction to sustained relaxation in remodeled pulmonary artery from hypoxic pulmonary hypertension rats. We concluded that the mild constrictive effect induced by tanshinone IIA was affected by integrity of endothelium and production of nitric oxide, while the potent dilative effect was endothelium-independent and produced primarily by inhibiting extracellular Ca2+ influx and partially by inhibiting intracellular Ca2+ release, as well as activating Ca2+-activated K+ channels. The modulation of tanshinone IIA on pulmonary vasoreactivity under both acute and chronic hypoxia condition may provide a new insight for curing hypoxic pulmonary hypertension.

KCNQ potassium channels: new targets for pulmonary vasodilator drugs?

Smooth muscle cells regulate the diameter of pulmonary arteries and the resistance to blood flow in the pulmonary circulation. These cells are normally relaxed to maintain low intrinsic vessel tone, but are contracted in pulmonary arterial hypertension (PAH). Potassium channels in the smooth muscle cell help to maintain low tone by polarising the membrane and preventing Ca(2+) influx through voltage-operated Ca(2+) channels. There is a loss of K(+) channel activity in PAH, so drugs that open K(+) channels are predicted to have a beneficial effect, provided their action can be restricted to the pulmonary circulation. Here we review the myriad of K(+) channels that are expressed in pulmonary arteries and suggest the roles that each might play in regulating pulmonary artery tone. We conclude that members of the KCNQ family of K(+) channels, the most recent K(+) channels to be discovered in pulmonary artery, may be a useful therapeutic target for the treatment of PAH. KCNQ channels appear to be preferentially expressed in pulmonary arteries and drugs that modulate their activity have potent effects on pulmonary artery tone.

Electrophysiological modelling of pulmonary artery smooth muscle cells in the rabbits--special consideration to the generation of hypoxic pulmonary vasoconstriction

In vascular smooth muscle cells, it has been suggested that membrane potential is an important component that initiates contraction. We developed a mathematical model to elucidate the quantitative contributions of major ion currents [a voltage-gated L-type Ca2+ current (ICaL), a voltage-sensitive K+ current (IKV), a Ca2+-activated K+ current (IKCa) and a nonselective cation current (INSC)] to membrane potential. In order to typify the diverse nature of pulmonary artery smooth muscle cells (PASMCs), we introduced parameters that are not fixed (variable parameters). The population of cells with different parameters was constructed and the cells that have the electrophysiological properties of PASMCs were selected. The contributions of each membrane current were investigated by sensitivity analysis and modification of the current parameters. Consequently, IKV and INSC were found to be the most important currents that affect the membrane potential. The occurrence of depolarisation in hypoxic pulmonary vasoconstriction (HPV) was also examined. In hypoxia, IKV and IKCa were reduced, but the consequent depolarisation in simulation was not enough to initiate contractions. If we add an increase of INSC (2.5-fold), the calculated membrane potential was enough to induce contraction. From the results, we conclude that the balance of various ion channel activities determines the resting membrane potential of PASMCs and our model was successful in explaining the depolarisation in HPV. Therefore, this model can be a powerful tool to investigate the various electrical properties of PASMCs in both normal and pathological conditions.

Relaxation of human placental arteries and veins by ATP-sensitive potassium channel openers

BACKGROUND

Adenosine triphosphate (ATP)-sensitive potassium channels (K(ATP)) are important modulators of vascular tone. Preliminary data from our laboratory suggests that K(ATP) channels are expressed in the fetoplacental vasculature where addition of pinacidil, a specific K(ATP) opener, promotes relaxation. We aimed to assess the effects of KRN2391 and KRN4884 on the fetoplacental vasculature, which are putative K(ATP) channel openers.

MATERIALS AND METHODS

Functional activity of K(ATP) channels was assessed in chorionic plate arteries and veins using wire myography. Cromakalim-, KRN2391- and KRN4884-induced relaxations were assessed in the presence and absence of agonist-induced pretone. Cromakalim, an established K(ATP) channel opener, acted as control.

RESULTS

KRN2391 evoked significantly greater relaxation of chorionic plate arteries and veins than either KRN4884 or cromakalim. KRN2391-induced relaxation of precontracted arteries and veins was reduced in the presence of inhibitors of the nitric oxide pathway (L-NNA or LY83583). With KRN4884, there was no contribution of nitric oxide to the induced relaxation.

CONCLUSIONS

We conclude that K(ATP) channels play an important role in controlling placental vascular tone. KRN2391 induces relaxation of human placental blood vessels by activation of K(ATP) channels and via activation of nitric oxide-dependent pathways.

Hypoxic pulmonary hypertension (HPH) and iptakalim, a novel ATP-sensitive potassium channel opener targeting smaller arteries in hypertension

Hypoxic pulmonary hypertension (HPH) is a serious and potentially devastating chronic disorder of the pulmonary circulation. Attempts to use drugs in the therapy of hypoxic pulmonary hypertension indicated the importance of prevention or reduction of vasoconstriction as well as of the reversal of remodeling within the cardiovascular system. Iptakalim (2,3-dimethyl-N-(1-methylethyl)-2-butylamine), a novel ATP-sensitive potassium channel opener, has the desired effects on hypoxic pulmonary arteries. Iptakalim decreases the elevated mean pressure in pulmonary arteries, and attenuates remodeling in the right ventricle, pulmonary arteries and airways. Moreover, iptakalim has selective antihypertensive effects: it significantly lowers arterial pressure in hypertensive animals, but has little if any effect in normotensive animals. In HPH iptakalim has selective effects on smaller arteries. Long-term iptakalim therapy decreases expression of sulfonylurea receptor 2 and of mRNA of inwardly rectifying potassium channel in smaller arteries of spontaneously hypertensive rats. Iptakalim inhibits the effects of endothelin-1, reduces the intracellular calcium concentration and inhibits the cell cycle in smooth muscle cells of pulmonary arteries. There is no evidence for the development of tolerance to the long-lasting antihypertensive action of iptakalim. At therapeutic doses iptakalim has no effects on the central nervous, respiratory, digestive, or endocrine systems. It has a broad therapeutic range, so that it can be safely used in the therapy of HPH.

Glibenclamide Inhibits Agonist-induced Vasoconstriction of Placental Chorionic Plate Arteries

BACKGROUND

Preliminary data suggest that K(ATP) channels may be expressed in placental arteries and veins [Wareing M, Turner C, Greenwood SL, Baker PN, Fyfe GK. Expression of mRNA encoding K+ channels in chorionic plate arteries and veins. J Soc Gynecol Investig 2004;11:353A]. However, no data exist on glibenclamide's effects in placental chorionic plate arteries.

AIM

To assess the effect of glibenclamide on placental chorionic plate arterial vasoconstriction.

METHODS

Arteries were dissected from placental chorionic plate biopsies obtained at term from uncomplicated pregnancies (N=63). Arteries were mounted onto a wire myograph in HCO3- -buffered physiological salt solution (PSS) at 37 degrees C (5% O2/5% CO2 bubbling) and normalised at 0.9 of L5.1 kPa. Constriction viability was assessed with 120 mmol l(-1) potassium solution (KPSS). Dose-response curves were produced with the thromboxane-mimetics U46619 and U44069 (10(-10)-2 x 10(-6)M), arginine vasopressin (10(-10)-5 x 10(-8)M) and endothelin-1 (10(-11)-3 x 10(-7)M) in the presence or absence of 50 micromol l(-1) glibenclamide. The effect of glibenclamide on arginine vasopressin- and U46619-induced constriction was also assessed in the presence of the cyclo-oxygenase inhibitor indomethacin (10 micromol l(-1)).

RESULTS

Pre-incubation with 50 micromol l(-1) glibenclamide significantly right-shifted dose-response curves to all vasoconstrictive agonists tested (repeated measures ANOVA). Indomethacin did not modify the inhibitory effect of glibenclamide.

CONCLUSION

Glibenclamide's effects on agonist-induced constrictions are unlikely to be via an inhibition of ATP-sensitive K+ channels, and with U46619- and U44069-induced constrictions, glibenclamide may be acting as a competitive antagonist of thromboxane receptors.

Vasorelaxing effect of U50,488H in pulmonary artery and underlying mechanism in rats

AIM

To investigate the relaxation effect and underlying mechanism of U50,488H (a selective kappa-opioid receptor agonist) in pulmonary artery in the rat.

METHODS

Isolated pulmonary artery ring was perfused and the tension of the vessel was measured.

RESULTS

U50,488H relaxed the pulmonary artery ring in a dose-dependent manner and the effect was abolished by nor-binaltorphimine, a selective kappa-opioid receptor antagonist. The relaxation effect of U50,488H in pulmonary artery was partially endothelium-dependent and was significantly attenuated in the presence of L-NAME. The relaxation effect of U50,488H was significantly attenuated by K(V) channel blocker 4-AP (4-aminopyridine), but not by glibenclamide (ATP-sensitive K+ channel blocker) nor TEA (tetraethylamonium, Ca2+-activated K+ channel blocker). Further study also showed that endothelium denudation and 4-AP have an additive inhibitory effect on pulmonary artery relaxation caused by U50,488H.

CONCLUSION

Kappa-opioid receptor activation by U50,488H relaxes pulmonary artery via two separate pathways: one is endothelium-derived nitric oxide, the other is K(V) channel in pulmonary artery smooth muscle.

Two-pore domain K channel, TASK-1, in pulmonary artery smooth muscle cells

Pulmonary vascular tone is strongly influenced by the resting membrane potential of smooth muscle cells, depolarization promoting Ca2+ influx, and contraction. The resting potential is determined largely by the activity of K+-selective ion channels, the molecular nature of which has been debated for some time. In this study, we provide strong evidence that the two-pore domain K+ channel, TASK-1, mediates a noninactivating, background K+ current (IKN), which sets the resting membrane potential in rabbit pulmonary artery smooth muscle cells (PASMCs). TASK-1 mRNA was found to be present in PASMCs, and the membranes of PASMCs contained TASK-1 protein. Both IKN and the resting potential were found to be exquisitely sensitive to extracellular pH, acidosis inhibiting the current and causing depolarization. Moreover, IKN and the resting potential were enhanced by halothane (1 mmol/L), inhibited by Zn2+ (100 to 200 micromol/L) and anandamide (10 micromol/L), but insensitive to cytoplasmic Ca2+. These properties are all diagnostic of TASK-1 channels and add to previously identified features of IKN that are shared with TASK-1, such as inhibition by hypoxia, low sensitivity to 4-aminopyridine and quinine and insensitivity to tetraethylammonium ions. It is therefore concluded that TASK-1 channels are major contributors to the resting potential in pulmonary artery smooth muscle. They are likely to play an important role in mediating pulmonary vascular responses to changes in extracellular pH, and they could be responsible for the modulatory effects of pH on hypoxic pulmonary vasoconstriction.

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.

KRN2391: dual action on rat pulmonary artery and no loss of potency in pulmonary hypertension

1. The pulmonary vasorelaxant properties of KRN2391 (N-cyano-N'-(2-nitroxyethyl)-3-pyridinecarboximidamide) were examined in isolated ring preparations of main (MPA) and intralobar (IPA) pulmonary artery from control and pulmonary hypertensive rats (exposure to hypoxia, 10% oxygen, for 1 week). 2. On both MPA and IPA, pulmonary vasorelaxant responses were inhibited by methylene blue (10 micromol/L) or glibenclamide (1 or 10 micromol/L). Thus, KRN2391 has the properties of both a nitric oxide (NO) donor and a K(ATP) channel opener on rat pulmonary arteries. 3. KRN2391 was more potent and gave a greater maximum relaxation on MPA (-log EC(50) 6.47; maximum 92% reversal of induced contraction) than on IPA (-log EC(50) 6.09; maximum 58% reversal of induced contraction). Comparable differences between MPA and IPA were seen for SIN-1 (NO donor) and levcromakalim (K(ATP) channel opener). 4. KRN2391 was equipotent in MPA from control and pulmonary hypertensive rats but, when glibenclamide (10 micromol/L) was present, KRN2391 was six-fold less potent in preparations from pulmonary hypertensive than control rats. An eight-fold reduction in potency was seen for SIN-1 (no glibenclamide) in arteries from pulmonary hypertensive rats, confirming previous findings with other NO donors. 5. It is concluded that the dual mechanism of action of KRN2391 accounts for the finding that this drug is equally potent in pulmonary arteries from pulmonary hypertensive and control rats. In the context of pulmonary hypertension, this property of the drug could give it an advantage over drugs that act solely as NO donors because these decline in potency, at least in animal models of this disease.

Hypoxic pulmonary vasoconstriction

Hypoxic vasoconstriction is unique to pulmonary circulation. The pulmonary response is part of a self-regulatory mechanism by which pulmonary capillary blood flow is automatically adjusted to alveolar ventilation for maintaining the optimal balance of ventilation and perfusion. In pathological conditions, hypoxic pulmonary vasoconstriction may occur as an acute episode or as a sustained response with pulmonary hypertension and vascular remodeling. Vasoactive substances produced from the endothelial cells (prostanoids, nitric oxide, or endothelin) or other mediators such as 5 hydroxytryptamine have been examined as possible mediators of hypoxic vasoconstriction. These appear more likely to be modulators than mediators of the vasoconstrictor response to hypoxia. Recent hypotheses have emerged indicating that O2 levels per se can regulate ion channel activity. The modulation of both K+ and Ca2+ channels differs according to the conduit or resistance pulmonary vessel type, tending to extend the former and contract the latter, thereby opposing the ventilation to perfusion mismatching. In the absence of drugs that act selectively on pulmonary circulation, inhaled therapy is an alternative in the treatment of pulmonary hypertension. According to its short half-life and to its potential cytotoxicity, nitric oxide is only of value in the management of patients with acute respiratory disease. Aerosolized prostacyclin and iloprost result in a sustained efficacy of the inhaled vasodilator regimen in patients with severe pulmonary hypertension and offer a new strategy for treatment of this disease. At the moment, therapy aimed at reversing the structural remodeling and matrix deposition in pulmonary arteries remains experimental. New drugs such as potassium channel openers or endothelin receptor antagonists warrant further investigations as possible therapeutic candidates in the treatment of pulmonary hypertension.

Oxygen-sensing potassium currents in pulmonary artery

The pulmonary vasculature is sensitive to the relative components of the respiratory gases and will vasoconstrict in response to decreased oxygen (O2) levels. This hypoxic pulmonary vasoconstriction (HPV) controls pulmonary blood flow in the fetus and serves to maximize ventilation perfusion matching in the adult lung. The exact mechanism of HPV is not fully understood but it appears to involve direct effects on both the endothelium and smooth muscle cells within the vessel wall. There is growing evidence to suggest that hypoxia mediates vasoconstriction, at least in part through the inhibition of outward potassium (K+) current in smooth muscle. A number of K+ currents present in the pulmonary vasculature have been shown to be sensitive to O2, with hypoxia acting to inhibit these currents in the majority of cases. Differences in the expression of these O2-sensitive K+ channels may explain regional and generic variations observed in the HPV response. The mechanism by which these K+ channels sense changes in O2 levels may involve changes in the cellular redox state, oxidative phosphorylation or a direct effect on the channel protein itself.

Recognition and management of pulmonary hypertension

Pulmonary hypertension (mean pulmonary arterial pressure > 20mm Hg at rest or > 30mm Hg during exercise) occurs (i) as primary pulmonary hypertension (no known underlying cause), (ii) as persistent pulmonary hypertension of the newborn or (iii) secondary to a variety of lung and cardiovascular diseases. In the last 10 to 15 years there have been significant advances in the medical management of this debilitating and life-threatening disorder. The main drugs in current use are anticoagulants (warfarin, heparin) and vasodilators, especially oral calcium antagonists, intravenous prostacyclin (prostaglandin I2; epoprostenol) and inhaled nitric oxide. Calcium antagonists, (e.g. nifedipine, diltiazem) are used chiefly in primary pulmonary hypertension. They are effective in patients who give a pulmonary vasodilator response to an acute challenge with a short acting vasodilator (e.g. prostacyclin, nitric oxide or adenosine), and are used in doses greater than are usual in the treatment of other cardiovascular disorders. Prostacyclin, given by continuous intravenous infusion, is effective in patients even if they do not respond to an acute vasodilator challenge. The long term benefit in these patients is thought to reflect the antiproliferative effects of the drug and/or its ability to inhibit platelet aggregation. It is used either as long term therapy or as a bridge to transplantation. Inhaled nitric oxide, which is used mainly in persistent pulmonary hypertension of the newborn, has the particular benefit of being pulmonary selective, due to its route of administration and rapid inactivation. Anticoagulants have a specific role in the treatment of pulmonary thromboembolic pulmonary hypertension and are also used routinely in patients with primary pulmonary hypertension. Nondrug treatments for pulmonary hypertension include (i) supplemental oxygen (> or = 15 h/day), which is the primary therapy in patients with pulmonary hypertension secondary to chronic obstructive pulmonary disease and (ii) heart-lung or lung transplantation, which nowadays is regarded as a last resort. Different types of pulmonary hypertension require different treatment strategies. Future advances in the treatment of pulmonary hypertension may come from the use of drug combinations, the development of new drugs, such as endothelin antagonists, nitric oxide donors and potassium channel openers, or the application of gene therapy.

Vasorelaxant effects of SCA40 (a phosphodiesterase III inhibitor) in pulmonary vascular preparations in rats

1. The novel phosphodiesterase (PDE) inhibitor SCA40 (6-bromo-8(methylamino)imidazo[1,2-a]pyrazine-2-carbonitrile) was examined for its vasorelaxant activity on isolated pulmonary vascular preparations from rats. 2. SCA40 relaxed ring preparations of main and intralobar pulmonary artery precontracted submaximally with either phenylephrine or U46619 (thromboxane-mimetic). Based on negative log EC50 values, SCA40 was six-to 14-fold more potent than the PDE III inhibitor milrinone or the non-selective PDE inhibitor 3-isobutyl-1-methyl xanthine (IBMX). The potency of SCA40 corresponded to its reported potency as a PDE III inhibitor. 3. In isolated perfused lungs, SCA40 reversed the vasoconstriction induced by alveolar hypoxia. It was 49-fold more potent than IBMX. 4. In main pulmonary artery the vasorelaxation induced by SCA40 was not blocked by the large-conductance calcium-activated potassium channel (BKCa) inhibitors iberiotoxin (50 and 100 nmol/L) or charybdotoxin (100 and 300 nmol/L). This was in contrast to data on guinea-pig trachea, where responses to SCA40 were significantly inhibited by charybdotoxin (100 nmol/L). 5. It is concluded that opening of BKCa channels does not contribute to the pulmonary vasorelaxant effects of SCA40 in main pulmonary artery and it is likely that responses reflect the PDE III inhibitory properties of the drug. 6. It is postulated that SCA40 may be useful as a pulmonary vasodilator in disorders such as pulmonary hypertension.

Evaluation of bosentan, pinacidil and nitroprusside on human pulmonary arteries: comparison with rat pulmonary arteries

Bosentan (endothelin ETA/ETB antagonist), pinacidil (potassium channel opener) and nitroprusside (nitric oxide donor) were examined on isolated ring preparations of human intralobar pulmonary artery (3rd-5th generation, internal radius > or = 1 mm), rat main pulmonary artery (1st generation; internal radius > or = 1 mm) and rat intralobar pulmonary artery (3rd generation; internal radius 0.1-0.3 mm). The potency of endothelin-1 was the same in all three artery types. In human intralobar artery and rat main pulmonary artery, bosentan (3 and 10 microM) shifted the endothelin-1 concentration response curve to a higher concentration range (endothelin-1 concentration ratios, in human intralobar and rat main pulmonary artery, respectively: 3 microM bosentan, 4.5 and 8.1; 10 microM bosentan, 13.5 and 19.5), but caused no significant block of endothelin-1 in rat intralobar artery. The latter finding may be due to the reported presence of ETB receptors in rat intralobar arteries and the higher potency of bosentan on ETA than on ETB receptors. In contrast, the potencies of nitroprusside and pinacidil (relaxation of submaximal contractions to the thromboxane-mimetic, U46619) agreed on human and rat intralobar arteries but were 6 to 16-fold lower than on rat main pulmonary artery. We conclude that data obtained on pulmonary arteries from rats can be useful in predicting the effects of vasoactive drugs in human pulmonary arteries but selection of the most appropriate rat artery for study will depend on the drug group under investigation. For potassium channel openers and nitric oxide donors, good agreement between human and rat data will be found when using pulmonary arteries from the same anatomical location even though they differ markedly in size. In contrast, for endothelin antagonists, agreement is more likely to be found in arteries of comparable size, despite their different anatomical locations.

Potassium channels in the vasculature

Many important cellular functions are regulated by vascular potassium channels, including the resting membrane potential. Recent evidence suggests that the function of these channels is altered in pathophysiological disorders of the cardiovascular system. Using molecular cloning techniques, considerable effort has been made over the past 5 years to elucidate the structure of various types of potassium channels. Several different potassium channel clones have been identified from neuronal and cardiac tissues, although only a few have so far been identified in smooth muscle.

In vitro hypoxia on rat pulmonary artery: effects on contractions to spasmogens and role of KATP channels

The effect of in vitro hypoxia for 1 h on concentration-response curves to vasoconstrictor spasmogens was examined in rat isolated pulmonary arteries. Hypoxia, like levcromakalim (KATP channel opener), did not affect contractions to endothelin-1 but attenuated contractions to U46619 ((1,5,5,)-hydroxy-11 alpha, 9 alpha-epoxymethano) prosta 5Z, 13E-dienoic acid; thromboxane-mimetic), angiotensin II, noradrenaline and 5-hydroxytryptamine. The attenuation was prevented by glybenclamide. In pre-contracted arteries, subsequent exposure to hypoxia caused a response consisting of four phases (transient relaxation due to endothelium-derived nitric oxide; transient contraction; slow relaxation; sustained contraction). Glybenclamide, if added before hypoxia, did not eliminate either of the relaxant phases but, if added during the sustained contractile phase, caused further contraction. We conclude that exposure of pulmonary arteries to prolonged hypoxia causes KATP channels to open, as in systemic arteries; this diminishes contractions to some, but not all, vasoconstrictor spasmogens. The data suggest that endothelin-1, unlike other vasoconstrictors, would remain a highly effective pulmonary vasoconstrictor under severe hypoxic conditions.