Impaired physiological responses to chronic hypoxia in mice partially deficient for hypoxia-inducible factor 1alpha.

Chronic hypoxia induces polycythemia, pulmonary hypertension, right ventricular hypertrophy, and weight loss. Hypoxia-inducible factor 1 (HIF-1) activates transcription of genes encoding proteins that mediate adaptive responses to hypoxia, including erythropoietin, vascular endothelial growth factor, and glycolytic enzymes. Expression of the HIF-1alpha subunit increases exponentially as O2 concentration is decreased. Hif1a-/- mouse embryos with complete deficiency of HIF-1alpha due to homozygosity for a null allele at the Hif1a locus die at midgestation, with multiple cardiovascular malformations and mesenchymal cell death. Hif1a+/- heterozygotes develop normally and are indistinguishable from Hif1a+/+ wild-type littermates when maintained under normoxic conditions. In this study, the physiological responses of Hif1a+/- and Hif1a+/+ mice exposed to 10% O2 for one to six weeks were analyzed. Hif1a+/- mice demonstrated significantly delayed development of polycythemia, right ventricular hypertrophy, pulmonary hypertension, and pulmonary vascular remodeling and significantly greater weight loss compared with wild-type littermates. These results indicate that partial HIF-1alpha deficiency has significant effects on multiple systemic responses to chronic hypoxia.

Acute and chronic cardiovascular effects of intermittent hypoxia in C57BL/6J mice.

We investigated the effects of 1) acute hypoxia and 2) 5 wk of chronic intermittent hypoxia (IH) on the systemic and pulmonary circulations of C57BL/6J mice. Mice were chronically instrumented with either femoral artery or right ventricular catheters. In response to acute hypoxia (4 min of 10% O2; n = 6), systemic arterial blood pressure fell (P < 0.005) from 107.7 +/- 2.5 to 84.7 +/- 6.5 mmHg, whereas right ventricular pressure increased (P < 0.005) from 11.7 +/- 0.8 to 14.9 +/- 1.3 mmHg. Another cohort of mice was then exposed to IH for 5 wk (O2 nadir = 5%, 60-s cycles, 12 h/day) and then implanted with catheters. In response to 5 wk of chronic IH, mice (n = 8) increased systemic blood pressure by 7.5 mmHg, left ventricle + septum weight by 32.2 +/- 7.5 x 10(-2) g/100 g body wt (P < 0.015), and right ventricle weight by 19.3 +/- 3.2 x 10(-2) g/100 g body wt (P < 0.001), resulting in a 14% increase in the right ventricle/left ventricle + septum weight (P < 0.005). We conclude that in C57BL/6J mice 1) acute hypoxia causes opposite effects on the pulmonary and systemic circulations, leading to preferential loading of the right heart; and 2) chronic IH in mice results in mild to moderate systemic and pulmonary hypertension, with resultant left- and right-sided ventricular hypertrophy.

Flow-induced responses in cat isolated pulmonary arteries.

Isolated, cannulated, endothelium-intact cat pulmonary arteries, averaging 692 +/- 104 microns in diameter, were set at a transmural pressure of 10 mmHg and monitored with a video system. Intraluminal flow was increased in steps from 0 to 1.6 ml/min by using a syringe pump. An electronic system held pressure constant by changing outflow resistance. Flow-diameter curves were generated in physiological saline solution. At constant transmural pressure, the arteries constricted in response to increased intraluminal flow. Constriction was not affected by removing extracellular Ca2+ but was abolished after treatment with ryanodine to deplete intracellular Ca2+ stores, with the endothelin-1 synthesis inhibitor phosphoramidon, with the endothelin A-receptor antagonist BQ-123, with the protein kinase C inhibitor staurosporine, or with glutaraldehyde to reduce endothelial cell deformability. The results indicate that isolated pulmonary arteries can constrict in response to intraluminal flow and suggest that constriction is mediated by endothelin-1 and depends on intracellular Ca2+ release and protein kinase C activation.

Altered pulmonary vasoreactivity in the chronically hypoxic lung.

Prolonged exposure to alveolar hypoxia induces physiological changes in the pulmonary vasculature that result in the development of pulmonary hypertension. A hallmark of hypoxic pulmonary hypertension is an increase in vasomotor tone. In vivo, pulmonary arterial smooth muscle cell contraction is influenced by vasoconstrictor and vasodilator factors secreted from the endothelium, lung parenchyma and in the circulation. During chronic hypoxia, production of vasoconstrictors such as endothelin-1 and angiotensin II is enhanced locally in the lung, while synthesis of vasodilators may be reduced. Altered reactivity to these vasoactive agonists is another physiological consequence of chronic exposure to hypoxia. Enhanced contraction in response to endothelin-1 and angiotensin II, as well as depressed vasodilation in response to endothelium-derived vasodilators, has been documented in models of hypoxic pulmonary hypertension. Chronic hypoxia may also have direct effects on pulmonary vascular smooth muscle cells, modulating receptor population, ion channel activity or signal transduction pathways. Following prolonged hypoxic exposure, pulmonary vascular smooth muscle exhibits alterations in K+ current, membrane depolarization, elevation in resting cytosolic calcium and changes in signal transduction pathways. These changes in the electrophysiological parameters of pulmonary vascular smooth muscle cells are likely associated with an increase in basal tone. Thus, hypoxia-induced modifications in pulmonary arterial myocyte function, changes in synthesis of vasoactive factors and altered vasoresponsiveness to these agents may shift the environment in the lung to one of contraction instead of relaxation, resulting in increased pulmonary vascular resistance and elevated pulmonary arterial pressure.

Mobilization of intracellular Ca(2+) by endothelin-1 in rat intrapulmonary arterial smooth muscle cells.

Endothelin-1 (ET-1) increases intracellular Ca(2+) concentration ([Ca(2+)](i)) in pulmonary arterial smooth muscle cells (PASMCs); however, the mechanisms for Ca(2+) mobilization are not clear. We determined the contributions of extracellular influx and intracellular release to the ET-1-induced Ca(2+) response using Indo 1 fluorescence and electrophysiological techniques. Application of ET-1 (10(-10) to 10(-8) M) to transiently (24-48 h) cultured rat PASMCs caused concentration-dependent increases in [Ca(2+)](i). At 10(-8) M, ET-1 caused a large, transient increase in [Ca(2+)](i) (>1 microM) followed by a sustained elevation in [Ca(2+)](i) (<200 nM). The ET-1-induced increase in [Ca(2+)](i) was attenuated (<80%) by extracellular Ca(2+) removal; by verapamil, a voltage-gated Ca(2+)-channel antagonist; and by ryanodine, an inhibitor of Ca(2+) release from caffeine-sensitive stores. Depleting intracellular stores with thapsigargin abolished the peak in [Ca(2+)](i), but the sustained phase was unaffected. Simultaneously measuring membrane potential and [Ca(2+)](i) indicated that depolarization preceded the rise in [Ca(2+)](i). These results suggest that ET-1 initiates depolarization in PASMCs, leading to Ca(2+) influx through voltage-gated Ca(2+) channels and Ca(2+) release from ryanodine- and inositol 1,4,5-trisphosphate-sensitive stores.

Chronic hypoxia alters effects of endothelin and angiotensin on K+ currents in pulmonary arterial myocytes.

We tested the hypothesis that chronic hypoxia alters the regulation of K+ channels in intrapulmonary arterial smooth muscle cells (PASMCs). Charybdotoxin-insensitive, 4-aminopyridine-sensitive voltage-gated K+ (K(V,CI)) and Ca2+-activated K+ (KCa) currents were measured in freshly isolated PASMCs from rats exposed to 21 or 10% O2 for 17-21 days. In chronically hypoxic PASMCs, K(V, CI) current was reduced and KCa current was enhanced. 4-Aminopyridine (10 mM) depolarized both normoxic and chronically hypoxic PASMCs, whereas charybdotoxin (100 nM) had no effect in either group. The inhibitory effect of endothelin (ET)-1 (10(-7) M) on K(V,CI) current was significantly reduced in PASMCs from chronically hypoxic rats, whereas inhibition by angiotensin (ANG) II (10(-7) M) was enhanced. Neither ET-1 nor ANG II altered K(Ca) current in normoxic PASMCs; however, both stimulated K(Ca) current at positive potentials in chronically hypoxic PASMCs. These results suggest that although modulation of K(V,CI) and KCa channels by ET-1 and ANG II is altered by chronic hypoxia, the role of these channels in the regulation of resting membrane potential was not changed.

Responses to pulsatile flow in piglet isolated cerebral arteries.

Because cerebrovascular hemorrhage in newborns is often associated with fluctuations in cerebral blood flow, this study was designed to investigate the effects of pulsatile flow in isolated cerebral arteries from neonatal piglets. Arteries mounted on cannulas were bathed in and perfused with a physiologic saline solution. An electronic system produced pulsations, the amplitude and frequency of which were independently controlled. At constant mean transmural pressure (20 mm Hg), increasing flow in steps from 0 to 1.6 mL/min under steady flow conditions caused a biphasic response, constriction at low flow, and dilation at high flow. Under pulsatile flow conditions (pulse amplitude 16-24 mm Hg; 2 Hz), the arteries dilated upon flow initiation and continued to dilate as mean flow increased. Dilation to pulsatile flow did not depend on the level of mean flow because switching from steady to pulsatile flow at each flow step also caused dilation. Arteries dilated further upon increasing either pulse amplitude (12-28 mm Hg; 2 Hz) or frequency (16-24 mm Hg; 4 Hz). Inhibiting nitric oxide synthesis with Nomega-nitro-L-arginine or perfusing with glutaraldehyde to decrease endothelial cell deformability significantly reduced dilations to pulsatile flow and to increased amplitude and frequency. These data suggest that the arterial response to flow is highly dependent on the mode of flow. Dilation induced by initiating pulsatile flow or increasing either pulse amplitude or frequency appears to be mediated by augmented nitric oxide release as result of shear stress-induced deformation of the endothelial cells.

Inhibition of voltage-gated K+ current in rat intrapulmonary arterial myocytes by endothelin-1.

Although endothelin (ET)-1 is an important regulator of pulmonary vascular tone, little is known about the mechanisms by which ET-1 causes contraction in this tissue. Using the whole cell patch-clamp technique in rat intrapulmonary arterial smooth muscle cells, we found that ET-1 and the voltage-dependent K+ (Kv)-channel antagonist 4-aminopyridine, but not the Ca(2+)-activated K(+)-channel antagonist charybdotoxin (ChTX), caused membrane depolarization. In the presence of 100 nM ChTX, ET-1 (10(-10) to 10(-7) M) caused a concentration-dependent inhibition of K+ current (56.2 +/- 3.8% at 10(-7) M) and increased the rate of current inactivation. These effects of ET-1 on K+ current were markedly reduced by inhibitors of protein kinase C (staurosporine and GF 109203X) and phospholipase C (U-73122) or under Ca(2+)-free conditions and were mimicked by activators of protein kinase C (phorbol 12-myristate 13-actetate and 1,2-dioctanoyl-sn-glycerol). These data suggest that ET-1 modulated pulmonary vascular reactivity by depolarizing pulmonary arterial smooth muscle, due in part to the inhibition of Kv current that occurred via activation of the phospholipase C-protein kinase C signal transduction pathway.

Capacitative calcium entry and TRPC channel proteins are expressed in rat distal pulmonary arterial smooth muscle.

Mammalian homologs of transient receptor potential (TRP) genes in Drosophila encode TRPC proteins, which make up cation channels that play several putative roles, including Ca2+ entry triggered by depletion of Ca2+ stores in endoplasmic reticulum (ER). This capacitative calcium entry (CCE) is thought to replenish Ca2+ stores and contribute to signaling in many tissues, including smooth muscle cells from main pulmonary artery (PASMCs); however, the roles of CCE and TRPC proteins in PASMCs from distal pulmonary arteries, which are thought to be the major site of pulmonary vasoreactivity, remain uncertain. As an initial test of the possibility that TRPC channels contribute to CCE and Ca2+ signaling in distal PASMCs, we measured [Ca2+]i by fura-2 fluorescence in primary cultures of myocytes isolated from rat intrapulmonary arteries (>4th generation). In cells perfused with Ca2+-free media containing cyclopiazonic acid (10 microM) and nifedipine (5 microM) to deplete ER Ca2+ stores and block voltage-dependent Ca2+ channels, restoration of extracellular Ca2+ (2.5 mM) caused marked increases in [Ca2+]i whereas MnCl2 (200 microM) quenched fura-2 fluorescence, indicating CCE. SKF-96365, LaCl3, and NiCl2, blocked CCE at concentrations that did not alter Ca2+ responses to 60 mM KCl (IC50 6.3, 40.4, and 191 microM, respectively). RT-PCR and Western blotting performed on RNA and protein isolated from distal intrapulmonary arteries and PASMCs revealed mRNA and protein expression for TRPC1, -4, and -6, but not TRPC2, -3, -5, or -7. Our results suggest that CCE through TRPC-encoded Ca2+ channels could contribute to Ca2+ signaling in myocytes from distal intrapulmonary arteries.

Flow-induced responses in piglet isolated cerebral arteries.

Although cerebral hemorrhage is a widely occurring neurologic disorder thought to be caused by fluctuating blood flow, the response to flow in the neonatal cerebrovasculature has not been characterized. In the present study, we examined the effect of changing flow on middle cerebral artery diameter and pathways by which flow modulates cerebrovascular tone. Arteries from 2-14-d-old piglets were mounted on cannulas and bathed in and perfused with physiologic saline solution. An electronic system controlled pressure and a syringe pump provided constant flow. The transmural pressure was held constant at 20 mm Hg, and changes in vessel diameter were measured as flow was increased in steps from 0 to 1.60 mL/min (flow/diameter curves). Increasing flow at constant pressure resulted in constriction at flows from 0.077 to 0.152 mL/min and dilation at flows from 0.212 to 1.60 mL/min. The flow/diameter curves were repeated in arteries bathed in Na(+)-reduced or Ca(2+)-free physiologic saline solution; denervated with 6-hydroxydopamine; or treated with indomethacin, N-nitro-L-arginine methyl ester, N omega-nitro-L-arginine (NLA), and L-arginine), ryanodine, or glutaraldehyde. In Na(+)-reduced and in Ca(2+)-free physiologic saline solution, flow constriction was eliminated. Neither indomethacin nor 6-hydroxydopamine affected the biphasic response. N-Nitro-L-arginineL, NLA, and ryanodine blocked dilation, whereas L-arginine restored dilation in NLA-treated arteries. These data suggest that neither prostaglandins nor adrenergic nerve endings participate in flow-induced responses in piglet cerebral arteries. Elimination of flow-constriction by Na+ reduction or Ca2+ removal is consistent with findings in other artery types. The elimination of dilation by N-nitro-L-arginine methyl ester, NLA, and ryanodine suggests that dilation is mediated by nitric oxide and intracellular Ca2+. Whereas the contractile and dilatory responses to agonists remained intact after glutaraldehyde perfusion, both flow-induced constriction and dilation were eliminated, indicating that both types of flow responses result from endothelial cell deformation.

L-type Ca(2+) channels, resting [Ca(2+)](i), and ET-1-induced responses in chronically hypoxic pulmonary myocytes.

In the lung, chronic hypoxia (CH) causes pulmonary arterial smooth muscle cell (PASMC) depolarization, elevated endothelin-1 (ET-1), and vasoconstriction. We determined whether, during CH, depolarization-driven activation of L-type Ca(2+) channels contributes to 1) maintenance of resting intracellular Ca(2+) concentration ([Ca(2+)](i)), 2) increased [Ca(2+)](i) in response to ET-1 (10(-8) M), and 3) ET-1-induced contraction. Using indo 1 microfluorescence, we determined that resting [Ca(2+)](i) in PASMCs from intrapulmonary arteries of rats exposed to 10% O(2) for 21 days was 293.9 +/- 25.2 nM (vs. 153.6 +/- 28.7 nM in normoxia). Resting [Ca(2+)](i) was decreased after extracellular Ca(2+) removal but not with nifedipine (10(-6) M), an L-type Ca(2+) channel antagonist. After CH, the ET-1-induced increase in [Ca(2+)](i) was reduced and was abolished after extracellular Ca(2+) removal or nifedipine. Removal of extracellular Ca(2+) reduced ET-1-induced tension; however, nifedipine had only a slight effect. These data indicate that maintenance of resting [Ca(2+)](i) in PASMCs from chronically hypoxic rats does not require activation of L-type Ca(2+) channels and suggest that ET-1-induced contraction occurs by a mechanism primarily independent of changes in [Ca(2+)](i).

Hypoxic constriction of porcine distal pulmonary arteries: endothelium and endothelin dependence.

To determine the role of endothelium in hypoxic pulmonary vasoconstriction (HPV), we measured vasomotor responses to hypoxia in isolated seventh-generation porcine pulmonary arteries < 300 microm in diameter with (E+) and without endothelium. In E+ pulmonary arteries, hypoxia decreased the vascular intraluminal diameter measured at a constant transmural pressure. These constrictions were complete in 30-40 min; maximum at PO(2) of 2 mm Hg; half-maximal at PO(2) of 40 mm Hg; blocked by exposure to Ca(2+)-free conditions, nifedipine, or ryanodine; and absent in E+ bronchial arteries of similar size. Hypoxic constrictions were unaltered by indomethacin, enhanced by indomethacin plus N(G)-nitro-L-arginine methyl ester, abolished by BQ-123 or endothelial denudation, and restored in endothelium-denuded pulmonary arteries pretreated with 10(-10) M endothelin-1 (ET-1). Given previous demonstrations that hypoxia caused contractions in isolated pulmonary arterial myocytes and that ET-1 receptor antagonists inhibited HPV in intact animals, our results suggest that full in vivo expression of HPV requires basal release of ET-1 from the endothelium to facilitate mechanisms of hypoxic reactivity in pulmonary arterial smooth muscle.

Partial HIF-1alpha deficiency impairs pulmonary arterial myocyte electrophysiological responses to hypoxia.

Chronic hypoxia depolarizes and reduces K+ current in pulmonary arterial smooth muscle cells (PASMCs). Our laboratory previously demonstrated that hypoxia-inducible factor-1 (HIF-1) contributed to the development of hypoxic pulmonary hypertension. In this study, electrophysiological parameters were measured in PASMCs isolated from intrapulmonary arteries of mice with one null allele at the Hif1a locus encoding HIF-1alpha [Hif1a(+/-)] and from their wild-type [Hif1a(+/+)] littermates after 3 wk in air or 10% O2. Hematocrit and right ventricular wall and left ventricle plus septum weights were measured. Capacitance, K+ current, and membrane potential were measured with whole cell patch clamp. Similar to our laboratory's previous results, hypoxia-induced right ventricular hypertrophy and polycythemia were blunted in Hif1a(+/-) mice. Hypoxia increased PASMC capacitance in Hif1a(+/+) mice but not in Hif1a(+/-) mice. Chronic hypoxia depolarized and reduced K+ current density in PASMCs from Hif1a(+/+) mice. In PASMCs from hypoxic Hif1a(+/-) mice, no reduction in K+ current density was observed, and depolarization was significantly blunted. Thus partial deficiency of HIF-1alpha is sufficient to impair hypoxia-induced depolarization, reduction of K+ current density, and PASMC hypertrophy.

Effects of hypoxia in porcine pulmonary arterial myocytes: roles of K(V) channel and endothelin-1.

Effects of acute hypoxia on intracellular Ca(2+) concentration ([Ca(2+)](i)) and cell length were recorded simultaneously in proximal and distal pulmonary (PASMCs) and femoral (FASMCs) arterial smooth muscle cells. Reducing PO(2) from normoxia to severe hypoxia (PO(2) < 10 mmHg) caused small but significant decreases in length and a reversible increase in [Ca(2+)](i) in distal PASMCs and a small decrease in length in proximal PASMCs but had no effect in FASMCs, even though all three cell types contracted significantly to vasoactive agonists. Inhibition of voltage-dependent K(+) (K(V)) channel with 4-aminopyridine produced a greater increase in [Ca(2+)](i) in distal than in proximal PASMCs. In distal PASMCs, severe hypoxia caused a slight inhibition of K(V) currents; however, it elicited further contraction in the presence of 4-aminopyridine. Endothelin-1 (10(-10) M), which itself did not alter cell length or [Ca(2+)](i), significantly potentiated the hypoxic contraction. These results suggest that hypoxia only has small direct effects on porcine PASMCs. These effects cannot be fully explained by inhibition of K(V) channels and were greatly enhanced via synergistic interactions with the endothelium-derived factor endothelin-1.

Hypoxic constriction and reactive oxygen species in porcine distal pulmonary arteries.

To determine whether reactive oxygen species (ROS) play an essential role in hypoxic pulmonary vasoconstriction (HPV) and the cellular locus of ROS production and action during HPV, we measured internal diameter (ID) at constant transmural pressure, lucigenin-derived chemiluminescence (LDCL), and electron paramagnetic resonance (EPR) spin adduct spectra in small distal porcine pulmonary arteries, and dichlorofluorescein (DCF) fluorescence in myocytes isolated from these arteries. Hypoxia (4% O2) decreased ID, increased DCF fluorescence, tended to increase LDCL, and in some preparations produced EPR spectra consistent with hydroxyl and alkyl radicals. Superoxide dismutase (SOD, 150 U/ml) or SOD + catalase (CAT, 200 U/ml) did not alter ID during normoxia but reduced or abolished the constriction induced by hypoxia. SOD also blocked HPV in endothelium-denuded arteries after restoration of the response by exposure to 10-10 M endothelin-1. Confocal fluorescence microscopy demonstrated that labeled SOD and CAT entered pulmonary arterial myocytes. SOD, SOD + CAT, and CAT blocked the increase in DCF fluorescence induced by hypoxia, but SOD + CAT and CAT also caused a stable increase in fluorescence during normoxia, suggesting that CAT diminished efflux of DCF from cells or oxidized the dye directly. We conclude that HPV required increased concentrations of ROS produced by and acting on pulmonary arterial smooth muscle rather than endothelium.

Temporal, spatial, and oxygen-regulated expression of hypoxia-inducible factor-1 in the lung.

Hypoxia-inducible factor (HIF)-1 is a basic helix-loop-helix transcription factor that transactivates genes encoding proteins that participate in homeostatic responses to hypoxia. Several of these downstream gene products, such as erythropoietin, vascular endothelial growth factor, heme oxygenase-1, and inducible nitric oxide synthase, may contribute to the pathogenesis of pulmonary hypertension. Previous studies demonstrated increased HIF-1 mRNA levels in rats and mice subjected to hypoxia. In this study, we have demonstrated spatial, temporal, and O2-dependent expression of HIF-1 protein. Immunoblot analysis revealed hypoxic induction of HIF-1 in all cultured pulmonary cell types assayed, including those derived from pulmonary arterial endothelium and smooth muscle, bronchial epithelium, alveolar macrophages, alveolar epithelium, and microvascular endothelium. In contrast to all other cell types, pulmonary arterial smooth muscle cells expressed HIF-1 under nonhypoxic conditions. Immunohistochemistry and immunoblot analysis of ferret lungs demonstrated pulmonary expression of HIF-1 in vivo. HIF-1 protein expression was induced maximally when lungs were ventilated with 0 or 1% O2 for 4 h. On reoxygenation, HIF-1 was rapidly degraded, with a half-life of <1 min. These findings demonstrate that HIF-1 expression is tightly coupled to O2 concentration in vivo and are consistent with the involvement of HIF-1 in the physiological and pathophysiological responses to hypoxia in the lung.

Inhibition of voltage-gated K(+) currents by endothelin-1 in human pulmonary arterial myocytes.

Recent studies demonstrate that endothelin-1 (ET-1) constricts human pulmonary arteries (PA). In this study, we examined possible mechanisms by which ET-1 might constrict human PA. In smooth muscle cells freshly isolated from these arteries, whole cell patch-clamp techniques were used to examine voltage-gated K(+) (K(V)) currents. K(V) currents were isolated by addition of 100 nM charybdotoxin and were identified by current characteristics and inhibition by 4-aminopyridine (10 mM). ET-1 (10(-8) M) caused significant inhibition of K(V) current. Staurosporine (1 nM), a protein kinase C (PKC) inhibitor, abolished the effect of ET-1. Rings of human intrapulmonary arteries (0.8-2 mm OD) were suspended in tissue baths for isometric tension recording. ET-1-induced contraction was maximal at 10(-8) M, equal to that induced by K(V) channel inhibition with 4-aminopyridine, and attenuated by PKC inhibitors. These data suggest that ET-1 constricts human PA, possibly because of myocyte depolarization via PKC-dependent inhibition of K(V). Our results are consistent with data we reported previously in the rat, suggesting similar mechanisms may be operative in both species.