Voltage-gated K(+)-channel activity in ovine pulmonary vasculature is developmentally regulated

Redox Regulation of Ion Channels and Receptors in Pulmonary Hypertension

Significance: Pulmonary hypertension (PH) is characterized by elevated vascular resistance due to vasoconstriction and remodeling of the normally low-pressure pulmonary vasculature. Redox stress contributes to the pathophysiology of this disease by altering the regulation and activity of membrane receptors, K+ channels, and intracellular Ca2+ homeostasis. Recent Advances: Antioxidant therapies have had limited success in treating PH, leading to a growing appreciation that reductive stress, in addition to oxidative stress, plays a role in metabolic and cell signaling dysfunction in pulmonary vascular cells. Reactive oxygen species generation from mitochondria and NADPH oxidases has substantial effects on K+ conductance and membrane potential, and both receptor-operated and store-operated Ca2+ entry. Critical Issues: Some specific redox changes resulting from oxidation, S-nitrosylation, and S-glutathionylation are known to modulate membrane receptor and ion channel activity in PH. However, many sites of regulation that have been elucidated in nonpulmonary cell types have not been tested in the pulmonary vasculature, and context-specific molecular mechanisms are lacking. Future Directions: Here, we review what is known about redox regulation of membrane receptors and ion channels in PH. Further investigation of the mechanisms involved is needed to better understand the etiology of PH and develop better targeted treatment strategies.

Reactive oxygen species as mediators of oxygen signaling during fetal-to-neonatal circulatory transition

Reactive oxygen species (ROS) are frequently seen as pathological agents of oxidative stress. However, ROS are not always deleterious and can also act as cell signaling molecules. Vascular oxygen sensing and signaling during fetal-to-neonatal circulatory transition is a remarkable example of the physiological regulatory actions of ROS. The fetal relative hypoxic environment induces hypoxic pulmonary vasoconstriction (HPV) and ductus arteriosus (DA) relaxation favoring the presence of high pulmonary vascular resistance and right-to-left ductal shunt. At birth, the increase in oxygen tension causes relaxation of pulmonary arteries (PAs) and normoxic DA vasoconstriction (NDAV), thus diverting blood flow to the lungs. Although the response to changes in oxygen tension is diametrically opposite, the mechanisms responsible for HPV and NDAV appear to be the result of a similar interaction between triggering and modulating factors that lead to an increase in cytosolic Ca²⁺ concentration and Ca²⁺ sensitization of the contractile apparatus. Growing evidence points to an increase in ROS (mitochondria- and/or NADPH-derived superoxide and/or H₂O₂), leading to inhibition of voltage-gated K⁺ channels, membrane depolarization, and activation of voltage-gated L-type Ca²⁺ channels as critical events in the signaling pathway of both HPV and NDAV. Several groups of investigators have completed this pathway adding other elements such as neutral sphingomyelinase-derived ceramide, the sarcoplasmic/endoplasmic reticulum (through ryanodine and inositol 1,4,5-trisphosphate receptors), Rho kinase-mediated Ca²⁺ sensitization, or transient receptor potential channels. The present review focus on the role of ROS as mediators of the homeostatic oxygen sensing system during fetal and neonatal life not only in the PAs and DA but also in systemic arteries.

Large conductance voltage- and calcium-gated potassium channels (BK) in cerebral artery myocytes of perinatal fetal primates share several major characteristics with the adult phenotype

Large conductance voltage- and calcium-gated channels (BK) control fundamental processes, including smooth muscle contractility and artery diameter. We used a baboon (Papio spp) model of pregnancy that is similar to that of humans to characterize BK channels in the middle cerebral artery and its branches in near-term (165 dGa) primate fetuses and corresponding pregnant mothers. In cell-attached patches (K+pipette = 135 mM) on freshly isolated fetal cerebral artery myocytes, BK currents were identified by large conductance, and voltage- and paxilline-sensitive effects. Their calcium sensitivity was confirmed by a lower Vhalf (transmembrane voltage needed to reach half-maximal current) in inside-out patches at 30 versus 3 μM [Ca2+]free. Immunostaining against the BK channel-forming alpha subunit revealed qualitatively similar levels of BK alpha protein-corresponding fluorescence in fetal and maternal myocytes. Fetal and maternal BK currents recorded at 3 μM [Ca2+]free from excised membrane patches had similar unitary current amplitude, and Vhalf. However, subtle differences between fetal and maternal BK channel phenotypes were detected in macroscopic current activation kinetics. To assess BK function at the organ level, fetal and maternal artery branches were pressurized in vitro at 30 mmHg and probed with the selective BK channel blocker paxilline (1 μM). The degree of paxilline-induced constriction was similar in fetal and maternal arteries, yet the constriction of maternal arteries was achieved sooner. In conclusion, we present a first identification and characterization of fetal cerebral artery BK channels in myocytes from primates. Although differences in BK channels between fetal and maternal arteries exist, the similarities reported herein advance the idea that vascular myocyte BK channels are functional near-term, and thus may serve as pharmacological targets during the perinatal-neonatal period.

Unique aspects of the developing lung circulation: structural development and regulation of vasomotor tone

This review summarizes our current knowledge on lung vasculogenesis and angiogenesis during normal lung development and the regulation of fetal and postnatal pulmonary vascular tone. In comparison to that of the adult, the pulmonary circulation of the fetus and newborn displays many unique characteristics. Moreover, altered development of pulmonary vasculature plays a more prominent role in compromised pulmonary vasoreactivity than in the adult. Clinically, a better understanding of the developmental changes in pulmonary vasculature and vasomotor tone and the mechanisms that are disrupted in disease states can lead to the development of new therapies for lung diseases characterized by impaired alveolar structure and 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.

Prenatal programming of pulmonary hypertension induced by chronic hypoxia or ductal ligation in sheep

Pulmonary hypertension of the newborn is caused by a spectrum of functional and structural abnormalities of the cardiopulmonary circuit. The existence of multiple etiologies and an incomplete understanding of the mechanisms of disease progression have hindered the development of effective therapies. Animal models offer a means of gaining a better understanding of the fundamental basis of the disease. To that effect, a number of experimental animal models are being used to generate pulmonary hypertension in the fetus and newborn. In this review, we compare the mechanisms associated with pulmonary hypertension caused by two such models: in utero ligation of the ductus arteriosus and chronic perinatal hypoxia in sheep fetuses and newborns. In this manner, we make direct comparisons between ductal ligation and chronic hypoxia with respect to the associated mechanisms of disease, since multiple studies have been performed with both models in a single species. We present evidence that the mechanisms associated with pulmonary hypertension are dependent on the type of stress to which the fetus is subjected. Such an analysis allows for a more thorough evaluation of the disease etiology, which can help focus clinical treatments. The final part of the review provides a clinical appraisal of current treatment strategies and lays the foundation for developing individualized therapies that depend on the causative factors.

Antenatal hypoxia and pulmonary vascular function and remodeling

This review provides evidence that antenatal hypoxia, which represents a significant and worldwide problem, causes prenatal programming of the lung. A general overview of lung development is provided along with some background regarding transcriptional and signaling systems of the lung. The review illustrates that antenatal hypoxic stress can induce a continuum of responses depending on the species examined. Fetuses and newborns of certain species and specific human populations are well acclimated to antenatal hypoxia. However, antenatal hypoxia causes pulmonary vascular disease in fetuses and newborns of most mammalian species and humans. Disease can range from mild pulmonary hypertension, to severe vascular remodeling and dangerous elevations in pressure. The timing, length, and magnitude of the intrauterine hypoxic stress are important to disease development, however there is also a genetic-environmental relationship that is not yet completely understood. Determining the origins of pulmonary vascular remodeling and pulmonary hypertension and their associated effects is a challenging task, but is necessary in order to develop targeted therapies for pulmonary hypertension in the newborn due to antenatal hypoxia that can both treat the symptoms and curtail or reverse disease progression.

Hypoxic pulmonary hypertension of the newborn

Hypoxic pulmonary hypertension of the newborn is characterized by elevated pulmonary vascular resistance and pressure due to vascular remodeling and increased vessel tension secondary to chronic hypoxia during the fetal and newborn period. In comparison to the adult, the pulmonary vasculature of the fetus and the newborn undergoes tremendous developmental changes that increase susceptibility to a hypoxic insult. Substantial evidence indicates that chronic hypoxia alters the production and responsiveness of various vasoactive agents such as endothelium-derived nitric oxide, endothelin-1, prostanoids, platelet-activating factor, and reactive oxygen species, resulting in sustained vasoconstriction and vascular remodeling. These changes occur in most cell types within the vascular wall, particularly endothelial and smooth muscle cells. At the cellular level, suppressed nitric oxide-cGMP signaling and augmented RhoA-Rho kinase signaling appear to be critical to the development of hypoxic pulmonary hypertension of the newborn.

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

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

Hypoxic Pulmonary Vasoconstriction

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

Hypoxia-inducible factor-1α regulates KCNMB1 expression in human pulmonary artery smooth muscle cells

Previously, we observed that hypoxia increases the expression of the β1-subunit (KCNMB1) of the calcium-sensitive potassium channel (BK(Ca)). Herein, we elucidate the mechanism whereby hypoxia increases KCNMB1 expression in human pulmonary artery smooth muscle cells (hPASMC). In response to hypoxia, the expression of both the transcription factor hypoxia-inducible factor 1-α (HIF-1α) and KCNMB1 are increased. Knockdown of HIF-1α using a shRNA plasmid blocked the hypoxic induction of KCNMB1 expression. Chromatin immunoprecipitation (ChIP) demonstrated HIF-1α binding to three discrete regions of the human KCNMB1 promoter known to contain hypoxia response elements (HREs). A KCNMB1 promoter reporter assay combined with site-directed mutagenesis identified two adjacent HREs located between -3,540 bp and -3,311 bp that are essential for the hypoxic induction of KCNMB1 promoter activity. Furthermore, additional ChIP assays demonstrated recruitment of the HIF-1α transcriptional coactivator, p300, to this same promoter region. Treatment of hPASMC with the histone deacetylase inhibitor, trichostatin, prolonged the increase in KCNMB1 observed with hypoxia, suggesting that alterations in chromatin remodeling function to limit the hypoxic induction of KCNMB1. Finally, KCNMB1 knockdown potentiated the hypoxia-induced increase in cytosolic calcium in hPASMC, highlighting the contribution of the β1-subunit in modulating vascular SMC tone in response to acute hypoxia. In conclusion, HIF-1α increases KCNMB1 expression in response to hypoxia in hPASMC by binding to two HREs located at -3,540 to -3,311 of the KCNMB1 promoter. We speculate that selective modulation of KCNMB1 expression may serve as a novel therapeutic approach to address diseases characterized by an increase in vascular tone.

Regulation of the Pulmonary Circulation in the Fetus and Newborn

During the development of the pulmonary vasculature in the fetus, many structural and functional changes occur to prepare the lung for the transition to air breathing. The development of the pulmonary circulation is genetically controlled by an array of mitogenic factors in a temporo-spatial order. With advancing gestation, pulmonary vessels acquire increased vasoreactivity. The fetal pulmonary vasculature is exposed to a low oxygen tension environment that promotes high intrinsic myogenic tone and high vasocontractility. At birth, a dramatic reduction in pulmonary arterial pressure and resistance occurs with an increase in oxygen tension and blood flow. The striking hemodynamic differences in the pulmonary circulation of the fetus and newborn are regulated by various factors and vasoactive agents. Among them, nitric oxide, endothelin-1, and prostaglandin I2 are mainly derived from endothelial cells and exert their effects via cGMP, cAMP, and Rho kinase signaling pathways. Alterations in these signaling pathways may lead to vascular remodeling, high vasocontractility, and persistent pulmonary hypertension of the newborn.

Endothelium-dependent control of vascular tone during early postnatal and juvenile growth

Endothelial dysfunction can develop at an early age in children with risk factors for cardiovascular disease. A clear understanding of the nature of this dysfunction and how it can worsen over time requires detailed information on the normal growth-related changes in endothelial function on which the pathological changes are superimposed. This review summarizes our current understanding of these normal changes, as derived from studies in four different mammalian species. Although the endothelium plays an important role in controlling vascular tone from birth onward, the vasoactive molecules that mediate this control often change during postnatal or juvenile growth. The specifics of this transition to an adult endothelial cell phenotype can vary depending on the vascular bed. During growth, the contribution of nitric oxide to endothelium-dependent dilation generally increases in the lung, cerebral cortex, and skeletal muscle, but decreases in the intestine. Endothelial capacity for release of other vasoactive factors (e.g., cyclooxygenase products, hydrogen peroxide, carbon monoxide) can also increase or decrease during growth. Although these changes have been well documented, there is less information on their underlying cellular or molecular events. Further research is required to clarify these mechanisms, and to evaluate the functional significance of such shifts in endothelial phenotype.

Impaired voltage gated potassium channel responses in a fetal lamb model of persistent pulmonary hypertension of the newborn

We investigated the hypothesis that oxidative stress in persistent pulmonary hypertension of the newborn (PPHN) impairs voltage gated potassium (Kv) channel function. We induced PPHN in fetal lambs by prenatal ligation of ductus arteriosus; controls had sham ligation. We studied changes in the tone of pulmonary artery (PA) rings and Kv channel current of freshly isolated PA smooth muscle cells (PASMC) using standard techniques. 4-Aminopyridine (4-AP), a Kv channel antagonist, induced dose-dependent constriction of control PA rings; this response was attenuated in PPHN pulmonary arteries. Exogenous superoxide and peroxynitrite inhibited the response to 4-AP in control rings. Tiron, a superoxide scavenger, improved the response to 4-AP in PPHN rings. 4-AP inhibited the NOS-independent relaxation response to ATP in control PA rings. Relaxation response to ATP was blunted in PPHN rings and was improved by NOS antagonist, N-nitro-L-arginine methyl ester (L-NAME). 4-AP attenuated this response in L-NAME-treated PPHN rings. Exogenous superoxide suppressed 4-AP sensitive Kv current in control PASMC. Kv channel current was attenuated in cells from PPHN lambs and was restored by tiron. Oxidative stress impairs Kv channel function in PPHN. Superoxide scavengers may improve pulmonary vasodilation in PPHN in part by restoring Kv channel function.

Developmental changes in the expression of voltage-gated potassium channels in the ductus arteriosus of the fetal rat

Oxygen-sensitive, voltage-gated potassium channels (Kv) may contribute to the determination of the membrane potential in smooth muscle cells of the ductus arteriosus (DA), and thus to regulation of contractile tone in response to oxygen. Developmental changes in Kv during gestation may be related to closure of the DA after birth. This study investigated developmental changes in the expression of Kv in the DA and compared it with that of the pulmonary artery (PA) and the aorta (Ao). The DA, PA, and Ao were isolated from fetal rats at days 19 and 21 of gestation (term: 21.5 days). The expression of Kv1.2, Kv1.5, Kv2.1, and Kv3.1, putative oxygen-sensitive Kv channels that open in response to oxygen, was evaluated at both the mRNA and protein levels, using quantitative real-time polymerase chain reaction and immunohistochemistry. In the Kv family studied, Kv1.5 mRNA was expressed most abundantly in the DA, PA, and Ao in both day-19 and day-21 fetuses. Although the expression levels of Kv1.2, Kv1.5, Kv2.1, and Kv3.1 did not change much with development in the PA and Ao, in the DA they decreased with development. The decrease in the expression of Kv channels may enhance DA closure after birth by eliminating the opening of Kv channels when oxygen increases.

Voltage-gated K+ channels at an early stage of chronic hypoxia-induced pulmonary hypertension in newborn piglets

Our purpose was to determine whether smooth muscle cell membrane properties are altered in small pulmonary arteries (SPA) of piglets at an early stage of pulmonary hypertension. Piglets were raised in either room air (control) or hypoxia for 3 days. A microelectrode technique was used to measure smooth muscle cell membrane potential ( Em) in cannulated, pressurized SPA (100- to 300-μm diameter). SPA responses to the voltage-gated K+ (KV) channel antagonist 4-aminopyridine (4-AP) and the KV1 family channel antagonist correolide were measured. Other SPA were used to assess amounts of KV1.2, KV1.5, and KV2.1 (immunoblot technique). Em was more positive in SPA of chronically hypoxic piglets than in SPA of comparable-age control piglets. The magnitude of constriction elicited by either 4-AP or correolide was diminished in SPA from hypoxic piglets. Abundances of KV1.2 were reduced, whereas abundances of both KV1.5 and KV2.1 were unaltered, in SPA from hypoxic piglets. At least partly because of reduced amounts of KV1.2, smooth muscle cell membrane properties are altered such that Em is depolarized and KV channel family function is impaired in SPA of piglets at an early stage of chronic hypoxia-induced pulmonary hypertension.

Segmental heterogeneity in the mechanism of sodium nitroprusside-induced relaxation in ovine pulmonary artery

Segmental heterogeneity in relaxation response to nitric oxide (NO) was examined using NO donor sodium nitroprusside (SNP) in second- (medium) and fourth-generation (small) ovine isolated intralobar pulmonary arteries. In vessels precontracted with serotonin, NO donors SNP and S-nitroso-N-acetylpenicillamine (SNAP) were more potent in relaxing medium, in comparison to the small, arteries. Soluble guanylyl cyclase (sGC) inhibitor [1,2,4]oxadiazolo-[4,3-a]quinoxaline-1-one (ODQ 3 microM) caused a profound inhibition of SNP relaxation in small as compared with medium-sized arteries. However, both basal and SNP (10 microM)-stimulated intracellular cyclic guanosine monophosphate (cGMP) content was identical in these 2 arterial segments. The Na,K-ATPase inhibitor ouabain (1 microM) had a marked inhibitory effect on SNP-mediated relaxation in both segments. There was no segmental difference in SNP (10 microM)-stimulated plasma membrane Na,K-ATPase activity and ouabain-sensitive Rb-uptake. 4-AP (1 mM), a relatively selective inhibitor of Kv channels, decreased the potency of SNP relaxation by about 10-fold in the medium-sized vessels. On the other hand, 4-AP was without effect on the vasodilator potency of SNP in small vessels. Interestingly, in the presence of 4-AP, SNP was equipotent in dilating both medium (pD2 = 5.80 +/- 0.07; Emax = 84 +/- 1.6%, n = 7) and small (pD2 = 5.74 +/- 0.15; Emax = 83 +/- 2.5%, n = 7) pulmonary arteries. In conclusion, the results of the present study suggest that Kv channels determine the segmental heterogeneity of NO-mediated relaxation in ovine pulmonary artery.

Oxygen tension modulates the expression of pulmonary vascular BKCa channel alpha- and beta-subunits

At birth, the lung environment changes from low to relatively high O(2) tension. Pulmonary blood flow increases and pulmonary artery pressure decreases. Recent data suggest that pulmonary vascular calcium-sensitive K(+) channel (BK(Ca)) activation mediates perinatal pulmonary vasodilation. Although BK(Ca) channel expression is developmentally regulated, the molecular mechanisms responsible for BK(Ca) expression remain unknown. We tested the hypothesis that the low-O(2) tension environment of the normal fetus modulates BK(Ca) channel expression. We analyzed BK(Ca) expression under conditions of hypoxia and normoxia both in vitro and in vivo. BK(Ca) alpha-subunit mRNA expression increased twofold in ovine pulmonary artery smooth muscle cell (PASMC) primary cultures maintained in hypoxia. In vivo, BK(Ca) expression was similarly affected by hypoxia. When adult Sprague-Dawley rats were placed in hypobaric hypoxic chambers for 3 wk, hypoxic animals showed an increase of threefold in the expression of BK(Ca) alpha- and more than twofold in the expression of BK(Ca) beta(1)-subunit mRNA. Immunochemical staining was consistent with the genetic data. To assess transcriptional activation of the beta-subunit of the BK(Ca), both BK(Ca) beta(1)- and beta(2)-subunit luciferase (K(Ca) beta:luc(+)) reporter genes were constructed. Hypoxia increased PASMC K(Ca) beta(1):luc(+) reporter expression by threefold and K(Ca) beta(2):luc(+) expression by 35%. Fetal PASMC treated with the hypoxia-inducible factor-1 mimetic deferoxamine showed a 63 and 41% increase in BK(Ca) channel alpha- and beta(1)-subunit expression, respectively. Together, these results suggest that oxygen tension modulates BK(Ca) channel subunit mRNA expression, and the regulation is, at least in part, at the transcriptional level.

Increased myocardial matrix metalloproteinases in hypoxic newborn pigs during resuscitation: effects of oxygen and carbon dioxide

BACKGROUND

Perinatal asphyxia is associated with cardiac dysfunction, and it is important to prevent further tissue injury during resuscitation. There is increasing evidence that myocardial matrix metalloproteinases (MMPs) are involved in myocardial hypoxaemia-reoxygenation injury.

OBJECTIVE

To assess MMPs and antioxidant capacity in newborn pigs after global ischaemia and subsequent resuscitation with ambient air or 100% O(2) at different PaCO(2)-levels.

METHODS

Newborn pigs (12-36 h of age) were resuscitated for 30 min by ventilation with 21% or 100% O(2) at different PaCO(2) levels after a hypoxic insult, and thereafter observed for 150 min. In myocardial tissue extracts, MMPs were analyzed by gelatin zymography and broad matrix-degrading capacity (total MMP). Total endogenous antioxidant capacity in myocardial tissue extracts was measured by the oxygen radical absorbance capacity (ORAC) assay.

RESULTS

Matrix metalloproteinase-2 more than doubled from baseline values (P < 0.001), and was higher in piglets resuscitated with 100% O(2) than with ambient air (P = 0.012). The ORAC value was considerably decreased (P < 0.001). In piglets with elevated PaCO(2), total MMP-activity in the right ventricle was more increased than in the left ventricle (P = 0.008). In the left ventricle, total MMPactivity was higher in the piglets with low PaCO(2) than in the piglets with elevated PaCO(2) (P = 0.013).

CONCLUSION

In hypoxaemia-reoxygenation injury the MMP-2 level was highly increased and was most elevated in the piglets resuscitated with 100% O(2). Antioxidant capacity was considerably decreased. Assessed by total MMP-activity, elevated PaCO(2) during resuscitation might protect the left ventricle, and probably increase right ventricle injury of the myocardium.

Chronic intrauterine pulmonary hypertension compromises fetal pulmonary artery smooth muscle cell O2 sensing

To test the hypothesis that chronic intrauterine pulmonary hypertension (PHTN) compromises pulmonary artery (PA) smooth muscle cell (SMC) O2 sensing, fluorescence microscopy was used to study the effect of an acute increase in Po2 on the cytosolic Ca2+ concentration ([Ca2+]i) of chronically hypoxic subconfluent monolayers of PA SMC in primary culture. PA SMCs were derived from fetal lambs with PHTN due to intrauterine ligation of the ductus arteriosus. Acute normoxia decreased [Ca2+]i in control but not PHTN PA SMC. In control PA SMC, [Ca2+]i increased after Ca2+-sensitive (KCa) and voltage-sensitive (Kv) K+ channel blockade and decreased after diltiazem treatment. In PHTN PA SMC, KCa blockade had no effect, whereas Kv blockade and diltiazem increased [Ca2+]i. Inhibition of sarcoplasmic reticulum Ca2+ ATPase activity caused a greater increase in [Ca2+]i in controls compared with PHTN PA SMC. Conversely, ryanodine caused a greater increase of [Ca2+]i in PHTN compared with control PA SMC. KCa channel mRNA is decreased and Kv channel mRNA is unchanged in PHTN PA SMC compared with controls. We conclude that PHTN compromises PA SMC O2 sensing, alters intracellular Ca2+ homeostasis, and changes the predominant ion channel that determines basal [Ca2+]i from KCa to Kv.

Impaired voltage-gated K+ channel expression in brain during experimental cancer cachexia

Cancer-induced cachexia affects most advanced cancer patients. It is characterized by anorexia, profound metabolic dysfunctions, and severe neurological disorders. Here we show that voltage-gated potassium channel (Kv) expression is impaired in the brain of tumor-bearing animals. Expression of both delayed rectifier (Kv1.1, Kv1.2, Kv1.3, Kv1.5, Kv1.6, Kv2.1, Kv3.1, Kv4.2) and A-type potassium channels (Kv1.4, Kv3.3, Kv3.4) was greatly down-regulated in brain from animals bearing a Yoshida AH-130 ascites hepatoma. The possible compensatory mechanisms (Kv1.4/Kv4.2), expression of redundant genes (Kv3.1/Kv3.3) and heteromultimeric channel formation (Kv2.1/Kv9.3) were also affected. The high circulating levels of TNFalpha and the reduced expression of the anti-apoptotic protein Bcl-XL found in the brain of tumor-bearing animals indicate that this response could be mediated by an increase in brain cell death due to apoptosis. The results suggest that brain function is impaired during cancer cachexia, and may account for the cancer-induced anorectic response and other neurological alterations.

Pulmonary vascular K+ channel expression and vasoreactivity in a model of congenital heart disease

K+ channels play an important role in mediating pulmonary vasodilation caused by increased oxygen tension, nitric oxide, alkalosis, and shear stress. To test the hypothesis that lung K+ channel gene expression may be altered by chronic increases in pulmonary blood flow, we measured gene and protein expression of calcium-sensitive (K Ca ) and voltage-gated (Kv2.1) K+ channels, and a pH-sensitive K+ channel (TASK), in distal lung from fetal lambs in which an aortopulmonary shunt was placed at 139 days gestation. Under baseline conditions, animals with an aortopulmonary shunt showed elevated pulmonary artery pressure and pulmonary blood flow compared with twin controls. Hypoxia caused a greater increase in pulmonary vascular tone in shunt animals compared with controls. Alkalosis caused pulmonary vasodilation in control but not shunt animals. To determine lung K+ channel mRNA levels, we performed quantitative RT-PCR. In comparison with control animals, lung K Ca channel mRNA content was increased in shunt animals, whereas TASK mRNA levels were decreased. There was no difference in Kv2.1 mRNA levels. Channel protein expression was consistent with these findings. We conclude that, in the presence of elevated pulmonary blood flow, K Ca channel expression is increased and TASK is decreased.

Different Kv2.1/Kv9.3 heteromer expression during brain and lung post-natal development in the rat

The Kv2.1/Kv9.3 heteromer generates an O2 sensitive potassium channel and induces a slow deactivation that has important consequences for brain and lung physiology. We examined the developmental regulation of Kv2.1 and Kv9.3 mRNAs in brain and lung. Both genes followed parallel expression patterns in brain, increasing progressively through post-natal life. In lung, however, the expression of the two genes followed opposite trends: Kv2.1 transcripts decreased, while Kv9.3 mRNA increased. The Kv9.3/Kv2.1 ratio shows that while in brain the expression of both genes followed a similar pattern, the relative abundance of Kv9.3 increased steadily through post-natal life in lung. Furthermore, there is selective regulation of gene expression during the suckling-weaning transition. Our results suggest that different Kv9.3/Kv2.1 ratios could have physiological implications in both organs during post-natal development, and that diet composition and selective tissue-specific insulin regulation modulate the expression of Kv2.1 and Kv9.3.

Inhibition of inwardly rectifying K(+) channels by cGMP in pulmonary vascular endothelial cells

Endothelial barrier dysfunction is typically triggered by increased intracellular Ca(2+) concentration. Membrane-permeable analogs of guanosine 3',5'-cyclic monophosphate (cGMP) prevent disruption of endothelial cell integrity. Because membrane potential (E(m)), which influences the electrochemical gradient for Ca(2+) influx, is regulated by K(+) channels, we investigated the effect of 8-bromo-cGMP on E(m) and inwardly rectifying K(+) (K(IR)) currents in bovine pulmonary artery and microvascular endothelial cells (BPAEC and BMVEC), using whole cell patch-clamp techniques. Both cell types exhibited inward currents at potentials negative to -50 mV that were abolished by application of 10 microM Ba(2+), consistent with K(IR) current. Ba(2+) also depolarized both cell types. 8-Bromo-cGMP (10(-3) M) depolarized BPAEC and BMVEC and inhibited K(IR) current. Pretreatment with Rp-8-cPCT-cGMPS or KT-5823, protein kinase G (PKG) antagonists, did not prevent current inhibition by 8-bromo-cGMP. These data suggest that 8-bromo-cGMP induces depolarization in BPAEC and BMVEC due, in part, to PKG-independent inhibition of K(IR) current. The depolarization could be a protective mechanism that prevents endothelial cell barrier dysfunction by reducing the driving force for Ca(2+) entry.

Maturation alters the contribution of potassium channels to resting and 5HT-induced tone in small cerebral arteries of the sheep

To address the hypothesis that maturation alters the contribution of K-channels to resting and agonist-induced tone in small cerebral arteries, second branch middle cerebral arteries (approximately 200 microm) were taken from term fetal (139-141 days gestation) and adult sheep, denuded of endothelium, and mounted in myographs. After determination of length-tension relations, the arteries were stretched to 55, 100, and 145% of optimum length. At each level of stretch, contractile responses to 5 mM 4-aminopyridine (4-AP, voltage-sensitive K-channel blocker), 100 nM iberiotoxin (calcium-sensitive K-channel blocker), 10 microM glibenclamide (ATP-sensitive K-channel blocker), or 10 microM Ba(2+) (inward rectifier K-channel blocker) were recorded. In separate experiments, concentration--response relations were determined for 5-HT in the presence and absence of each of the four K-channel blockers at the same concentrations. Both 4-AP and iberiotoxin produced stretch-dependent contractions of greater magnitude in adult (37% for 4-AP and 43% for iberiotoxin at 100% optimum) than in fetal (5% for 4-AP and 7% for iberiotoxin at 100% optimum) arteries. 4-AP also enhanced the pD(2) for 5-HT in adult (from 7.15 to 7.49), but not in fetal, arteries. Conversely, glibenclamide attenuated the pD(2) for 5-HT in fetal (from 7.02 to 6.71), but not in adult, arteries. Iberiotoxin enhanced the pD(2) for 5-HT in both fetal (from 7.05 to 7.51) and adult (from 7.15 to 7.75) arteries. In addition, iberiotoxin enhanced maximum responses to 5-HT (from 59 to 82%) in adult but not fetal arteries. Finally, 4-AP enhanced the maximum responses to 5-HT in both fetal (from 67 to 85%) and adult (from 59 to 79%) arteries. These results indicate that maturation modulates the contribution of K(V), K(Ca), and K(ATP), but not K(IR) channels to basal and/or 5HT-induced cerebrovascular tone, and demonstrate that K(V) and K(Ca) channels are coupled to stretch-sensitive receptors, and that K(V) and K(Ca) limit contractile responses to 5-HT. To the extent that changes in pD(2) values reflect changes in agonist--ligand interactions, the data also suggest that K(V), K(Ca), and K(ATP) channels may possibly influence ligand--receptor binding for 5-HT.

Modulation of pulmonary vasomotor tone in the fetus and neonate

The high pulmonary vascular resistance (PVR) of atelectatic, hypoxic, fetal lungs limits intrauterine pulmonary blood flow (PBF) to less than 10% of combined right and left ventricular output. At birth, PVR decreases precipitously to accommodate the entire cardiac output. The present review focuses on the role of endothelium-derived nitric oxide (NO), prostacyclin, and vascular smooth muscle potassium channels in mediating the decrease in PVR that occurs at birth, and in maintaining reduced pulmonary vasomotor tone during the neonatal period. The contribution of vasodilator and vasoconstrictor modulator activity to the pathophysiology of neonatal pulmonary hypertension is also addressed.

Oxygen-induced fetal pulmonary vasodilation is mediated by intracellular calcium activation of K(Ca) channels

O(2) sensing in fetal pulmonary artery smooth muscle is critically important in the successful transition to air breathing at birth. However, the mechanism by which the fetal pulmonary vasculature senses and responds to an acute increase in O(2) tension is not known. Isolated fetal pulmonary artery smooth muscle cells were kept in primary culture for 5-14 days in a hypoxic environment (20-30 mmHg). These cells showed a 25.1 +/- 1.7% decrease in intracellular calcium in response to an acute increase in O(2) tension. Low concentrations of caffeine (0.5 mM) and diltiazem also decreased intracellular calcium. The decrease in intracellular calcium concentration in response to increasing O(2) was inhibited by iberiotoxin and ryanodine. Freshly isolated fetal pulmonary artery smooth muscle cells exhibited "spontaneous transient outward currents," indicative of intracellular calcium spark activation of calcium-sensitive potassium channels. The frequency of spontaneous transient outward currents increased when O(2) tension was increased to normoxic levels. Increasing fetal pulmonary O(2) tension in acutely instrumented fetal sheep increased fetal pulmonary blood flow. Ryanodine attenuated O(2)-induced pulmonary vasodilation. This study demonstrates that fetal pulmonary vascular smooth muscle cells are capable of responding to an acute increase in O(2) tension and that this O(2) response is mediated by intracellular calcium activation of calcium-sensitive potassium channels.

Abnormal balance in the angiopoietin-tie2 system in human brain arteriovenous malformations

Brain arteriovenous malformations (BAVMs) are congenital vascular lesions that often present as cerebral hemorrhage in young adults. The variable nature of the clinical course, especially with respect to spontaneous hemorrhage, recurrence, growth, and regression, suggests that BAVMs are lesions with active angiogenesis and vascular remodeling. We examined mRNA and protein expression of angiopoietin 1 (Ang1) and Ang2 by semiquantitative reverse transcriptase-polymerase chain reaction, in situ hybridization, and Western blot in BAVMs and control brains obtained from temporal lobectomy for medically intractable seizures. Although Ang1 mRNA levels were similar in BAVMs and controls, Ang1 protein levels were 30% lower in BAVMs than in controls. Ang2 mRNA levels were 40% higher and Ang2 protein levels were 8-fold higher in BAVMs than in controls. In situ hybridization showed that the Ang2 mRNA was localized to the perivascular area in BAVMs. This abnormal balance in the Ang-Tie2 system may, in part, explain the aberrant vascular phenotype in BAVMs.

Ca(2+)-activated K(+) channels modulate basal and E(2)beta-induced rises in uterine blood flow in ovine pregnancy

Uterine blood flow (UBF) increases >30-fold during ovine pregnancy. During the last trimester, this reflects vasodilation, which may be due to placentally derived estrogens. In nonpregnant ewes, estradiol-17 beta (E(2)beta) increases UBF >10-fold by activating nitric oxide synthase and large conductance calcium-dependent potassium channels (BK(Ca)). To determine whether BK(Ca) channels modulate basal and E(2)beta-induced increases in UBF, studies were performed in near-term pregnant ewes with uterine artery flow probes and catheters for intra-arterial infusions of tetraethylammonium (TEA), a selective BK(Ca) channel antagonist at <1 mM, in the absence or presence of E(2)beta (1 microg/kg iv). Uterine arteries were collected to measure BK(Ca) channel mRNA. TEA (0.15 mM) decreased basal UBF (P < 0.0001) 40 +/- 8% and 55 +/- 7% (n = 11) at 60 and 90 min, respectively, and increased resistance 175 +/- 48% without affecting (P > 0.1) mean arterial pressure (MAP), heart rate, or contralateral UBF. Systemic E(2)beta increased UBF 30 +/- 6% and heart rate 13 +/- 1% (P < or = 0.0001, n = 13) without altering MAP. Local TEA (0.15 mM) inhibited E(2)beta-induced increases in UBF without affecting increases in heart rate (10 +/- 4%; P = 0.006). BK(Ca) channel mRNA was present in uterine artery myocytes from pregnant and nonpregnant ewes. Exponential increases in ovine UBF in late pregnancy may reflect BK(Ca) channel activation, which may be mediated by placentally derived estrogens.

Pulmonary vascular response to normoxia and K(Ca) channel activity is developmentally regulated

To address developmental regulation of pulmonary vascular O(2) sensing, we tested the hypotheses that 1) fetal but not adult pulmonary artery smooth muscle cells (PASMCs) can directly sense an acute increase in O(2), 2) Ca2+-sensitive K(+) (K(Ca)) channel activity decreases with maturation, and 3) PASMC K(Ca) channel expression decreases with maturation. We used fluorescence microscopy to confirm that fetal but not adult PASMCs are able to sense an acute increase in O(2) tension. Acute normoxia induced a 22 +/- 2% decrease in cytosolic Ca2+ concentration ([Ca2+](i)) in fetal PASMCs and no change in ([Ca2+](i)) in adult PASMCs (P < 0.01). The effects of K(+) channel antagonists were studied on fetal and adult PASMC ([Ca2+](i)). Iberiotoxin (10(-9) M) caused PASMC ([Ca2+](i)) to increase by 694 +/- 22% in the fetus and caused no change in adult PASMCs. K(Ca) channel expression and mRNA levels in distal pulmonary arteries from fetal and adult sheep were examined. Both K(Ca) channel protein and mRNA expression in the distal pulmonary vasculature decreased with maturation. We conclude that maturation-dependent changes in PASMC O(2) sensing render the fetal PASMCs uniquely sensitive to an acute increase in O(2) tension at a biologically critical time point.

Multiple Ca(2+)-dependent modulators mediate alkalosis-induced vasodilation in newborn piglet lungs

We previously found that alkalosis-induced vasodilation was mediated by endothelium-derived nitric oxide (EDNO) in newborn piglet pulmonary artery and vein rings precontracted with the thromboxane mimetic U-46619. In contrast, prostacyclin or K(+) channel activation contributed to the response in other preparations. This study was undertaken to determine whether EDNO alone also mediates alkalosis-induced pulmonary vasodilation in piglet lungs vasoconstricted with hypoxia and, if not, to identify the mediator(s) involved. Responses to alkalosis were measured during hypoxia under control conditions after blocking nitric oxide synthase (N(omega)-nitro-L-arginine), cyclooxygenase (meclofenamate), or both endothelium-derived modulators (Dual); after blocking voltage-dependent (4-aminopyridine), ATP- dependent (glibenclamide), or Ca(2+)-dependent K(+) (K(Ca); tetraethylammonium) K(+) channels; and after blocking both endothelium-derived modulators and K(Ca) channels (Triple). Vasodilator responses measured after 20 min of alkalosis were blunted in Dual and tetraethylammonium lungs and abolished in Triple lungs. Thus alkalosis-induced vasodilation in hypoxic lungs appeared to be mediated by three Ca(2+)-dependent modulators: EDNO, prostacyclin, and K(Ca) channels. In addition, a transient, unidentified modulator contributed to the nadir of the vasodilator response measured at 10 min of alkalosis. Future studies are needed to identify factors that contribute to the discordance between isolated vessels and whole lungs.

Fetal rabbit pulmonary artery smooth muscle cell response to ryanodine is developmentally regulated

To study developmental changes in intracellular calcium handling in pulmonary artery smooth muscle cells (PASMCs), cells were isolated from distal and proximal pulmonary arteries from rabbits at different developmental stages: juvenile (4-6 wk old), newborn (<48 h), and full-term fetal. Isolated PASMCs were studied using the calcium-sensitive dye fura 2. Cells from each age group responded to caffeine with an increase in calcium; however, ryanodine (50 microM) only increased calcium in fetal distal PASMCs. The ryanodine-induced increase was due to influx of extracellular calcium because it was blocked by removal of extracellular calcium or by diltiazem. The calcium-sensitive potassium (K(Ca)) channel blocker iberiotoxin produced a transient increase in calcium in the fetal distal PASMCs, which could be inhibited by prior application of ryanodine. Conversely, the ryanodine response was inhibited if iberiotoxin was given first. With the use of electrophysiology and confocal microscopy, fetal PASMCs were shown to exhibit spontaneous transient outward currents and calcium sparks, respectively. These observations suggest that ryanodine-sensitive release of calcium from the sarcoplasmic reticulum and K(Ca) channels act together to control intracellular calcium only in fetal distal PASMCs.