Role of basal extracellular Ca2+ entry during 5-HT-induced vasoconstriction of canine pulmonary arteries

Long-Term Hypoxia Negatively Influences Ca2+ Signaling in Basilar Arterial Myocytes of Fetal and Adult Sheep

Cerebral arterial vasoreactivity is vital to the regulation of cerebral blood flow. Depolarization of arterial myocytes elicits whole-cell Ca²⁺ oscillations as well as subcellular Ca²⁺ sparks due to activation of ryanodine receptors on the sarcoplasmic reticulum. Previous evidence illustrates that contraction of cerebral arteries from sheep and underlying Ca²⁺ signaling pathways are modified by age and that long-term hypoxia (LTH) causes aberrations in Ca²⁺ signaling pathways and downstream effectors impacting vasoregulation. We hypothesize that age and LTH affect the influence of membrane depolarization on whole-cell intracellular Ca²⁺ oscillations and sub-cellular Ca²⁺ spark activity in cerebral arteries. To test this hypothesis, we examined Ca²⁺ oscillatory and spark activities using confocal fluorescence imaging techniques of Fluo-4 loaded basilar arterial myocytes of low- and high-altitude term fetal (∼145 days of gestation) and adult sheep, where high-altitude pregnant and non-pregnant sheep were placed at 3,801 m for >100 days. Ca²⁺ oscillations and sparks were recorded using an in situ preparation evaluated in the absence or presence of 30 mM K⁺ (30K) to depolarize myocytes. Myocytes from adult animals tended to have a lower basal rate of whole-cell Ca²⁺ oscillatory activity and 30K increased the activity within cells. LTH decreased the ability of myocytes to respond to depolarization independent of age. These observations illustrate that both altitude and age play a role in affecting whole-cell and localized Ca²⁺ signaling, which are important to arterial vasoreactivity and cerebral blood flow.

Novel Experimental Therapies for Treatment of Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a progressive and devastating disease characterized by pulmonary artery vasoconstriction and vascular remodeling leading to vascular rarefaction with elevation of pulmonary arterial pressures and pulmonary vascular resistance. Often PAH will cause death from right heart failure. Current PAH-targeted therapies improve functional capacity, pulmonary hemodynamics and reduce hospitalization. Nevertheless, today PAH still remains incurable and is often refractory to medical therapy, underscoring the need for further research. Over the last three decades, PAH has evolved from a disease of unknown pathogenesis devoid of effective therapy to a condition whose cellular, genetic and molecular underpinnings are unfolding. This article provides an update on current knowledge and summarizes the progression in recent advances in pharmacological therapy in PAH.

Comparison of signalling mechanisms underlying UTP-evoked vasoconstriction of rat pulmonary and tail arteries

The endogenous nucleotide, UTP, acts at smooth muscle P2Y receptors to constrict rat pulmonary and tail arteries, but the underlying signalling pathways are poorly understood. The aim was to characterise the contribution of Ca²⁺ release and influx, rho kinase and protein kinase C to these contractions. Isometric tension was recorded from endothelium-denuded rat intralobar pulmonary and tail artery rings mounted on a wire myograph. Contractions were evoked by UTP and peak amplitude measured. Thapsigargin (1 µM), but not ryanodine (10 µM), significantly depressed contractions in both by 30-40% (P < 0.05). Nifedipine (1 µM) significantly reduced contractions in tail artery by ~60% (P < 0.01). Y27632 (10 µM), a rho kinase inhibitor and GF109203X (10 µM), a protein kinase C inhibitor, each significantly reduced pulmonary vasoconstriction by ~20%, and tail artery contractions by ~80% and ~40%, respectively (P < 0.01). In pulmonary artery, Y27632, GF109203X and thapsigargin, acted in an additive manner, but nifedipine less so. Adding all four together abolished the UTP response. In tail artery, Y27632 plus thapsigargin or GF109203X or nifedipine abolished contractions. Thapsigargin, GF109203X and nifedipine, coapplied pair-wise, acted additively and applying all three together abolished UTP-evoked contractions. So, Ca²⁺ release from the sarcoplasmic reticulum and influx through Ca_v1.2 channels, but not ryanodine receptors, play significant roles in UTP-evoked vasoconstriction of rat pulmonary and tail arteries. Rho kinase and protein kinase C are also involved, but more so in tail artery. Thus UTP activates multiple signalling mechanisms that lead to vasoconstriction, but their relative importance differs in pulmonary compared with systemic arteries.

Long-term high-altitude hypoxia influences pulmonary arterial L-type calcium channel-mediated Ca2+ signals and contraction in fetal and adult sheep

Long-term hypoxia (LTH) has a profound effect on pulmonary arterial vasoconstriction in the fetus and adult. Dysregulation in Ca²⁺ signaling is important during the development of LTH-induced pulmonary hypertension. In the present study, we tested the hypothesis that L-type Ca²⁺ channels (Ca_L), which are voltage dependent and found in smooth, skeletal, and cardiac muscle, are important in the adaptation of pulmonary arterial contractions in postnatal maturation and in response to LTH. Pulmonary arteries were isolated from fetal or adult sheep maintained at low or high altitude (3,801 m) for >100 days. The effects were measured using an L-type Ca²⁺ channel opener FPL 64176 (FPL) in the presence or absence of an inhibitor, Nifedipine (NIF) on arterial contractions, intracellular Ca²⁺ oscillations, and ryanodine receptor-driven Ca²⁺ sparks. FPL induced pulmonary arterial contractions in all groups were sensitive to NIF. However, when compared with 125 mM K⁺, FPL contractions were greater in fetuses than in adults. FPL reduced Ca²⁺ oscillations in myocytes of adult but not fetal arteries, independently of altitude. The FPL effects on Ca²⁺ oscillations were reversed by NIF in myocytes of hypoxic but not normoxic adults. FPL failed to enhance Ca²⁺ spark frequency and had little impact on spatiotemporal firing characteristics. These data suggest that Ca_L-dependent contractions are largely uncoupled from intracellular Ca²⁺ oscillations and the development of Ca²⁺ sparks. This raises questions regarding the coupling of pulmonary arterial contractility to membrane depolarization, attendant Ca_L facilitation, and the related associations with the activation of Ca²⁺ oscillations and Ca²⁺ sparks.

S-nitrosothiols dilate the mesenteric artery more potently than the femoral artery by a cGMP and L-type calcium channel-dependent mechanism

S-nitrosothiols (SNOs) are metabolites of NO with potent vasodilatory activity. Our previous studies in sheep indicated that intra-arterially infused SNOs dilate the mesenteric vasculature more than the femoral vasculature. We hypothesized that the mesenteric artery is more responsive to SNO-mediated vasodilation, and investigated various steps along the NO/cGMP pathway to determine the mechanism for this difference. In anesthetized adult sheep, we monitored the conductance of mesenteric and femoral arteries during infusion of S-nitroso-l-cysteine (L-cysNO), and found mesenteric vascular conductance increased (137 ± 3%) significantly more than femoral conductance (26 ± 25%). Similar results were found in wire myography studies of isolated sheep mesenteric and femoral arteries. Vasodilation by SNOs was attenuated in both vessel types by the presence of ODQ (sGC inhibitor), and both YC-1 (sGC agonist) and 8-Br-cGMP (cGMP analog) mediated more potent relaxation in mesenteric arteries than femoral arteries. The vasodilatory difference between mesenteric and femoral arteries was eliminated by antagonists of either protein kinase G or L-type Ca(2+) channels. Western immunoblots showed a larger L-type Ca(2+)/sGC abundance ratio in mesenteric arteries than in femoral arteries. Fetal sheep mesenteric arteries were more responsive to SNOs than adult mesenteric arteries, and had a greater L-Ca(2+)/sGC ratio (p = 0.047 and r = -0.906 for correlation between Emax and L-Ca(2+)/sGC). These results suggest that mesenteric arteries, especially those in fetus, are more responsive to SNO-mediated vasodilation than femoral arteries due to a greater role of the L-type calcium channel in the NO/cGMP pathway.

Local and systemic vasodilatory effects of low molecular weight S-nitrosothiols

S-nitrosothiols (SNOs) such as S-nitroso-L-cysteine (L-cysNO) are endogenous compounds with potent vasodilatory activity. During circulation in the blood, the NO moiety can be exchanged among various thiol-containing compounds by S-transnitrosylation, resulting in SNOs with differing capacities to enter the cell (membrane permeability). To determine whether the vasodilating potency of SNOs is dependent upon membrane permeability, membrane-permeable L-cysNO and impermeable S-nitroso-D-cysteine (D-cysNO) and S-nitroso-glutathione (GSNO) were infused into one femoral artery of anesthetized adult sheep while measuring bilateral femoral and systemic vascular conductances. L-cysNO induced vasodilation in the infused hind limb, whereas D-cysNO and GSNO did not. L-cysNO also increased intracellular NO in isolated arterial smooth muscle cells, whereas GSNO did not. The infused SNOs remained predominantly in a low molecular weight form during first-passage through the hind limb vasculature, but were converted into high molecular weight SNOs upon systemic recirculation. At systemic concentrations of ~0.6 μmol/L, all three SNOs reduced mean arterial blood pressure by ~50%, with pronounced vasodilation in the mesenteric bed. Pharmacokinetics of L-cysNO and GSNO were measured in vitro and in vivo and correlated with their hemodynamic effects, membrane permeability, and S-transnitrosylation. These results suggest local vasodilation by SNOs in the hind limb requires membrane permeation, whereas systemic vasodilation does not. The systemic hemodynamic effects of SNOs occur after equilibration of the NO moiety amongst the plasma thiols via S-transnitrosylation.

Ion channels and transporters as therapeutic targets in the pulmonary circulation

Pulmonary circulation is a low pressure, low resistance, high flow system. The low resting vascular tone is maintained by the concerted action of ion channels, exchangers and pumps. Under physiological as well as pathophysiological conditions, they are targets of locally secreted or circulating vasodilators and/or vasoconstrictors, leading to changes in expression or to posttranslational modifications. Both structural changes in the pulmonary arteries and a sustained increase in pulmonary vascular tone result in pulmonary vascular remodeling contributing to morbidity and mortality in pediatric and adult patients. There is increasing evidence demonstrating the pivotal role of ion channels such as K(+) and Cl(-) or transient receptor potential channels in different cell types which are thought to play a key role in vasoconstrictive remodeling. This review focuses on ion channels, exchangers and pumps in the pulmonary circulation and summarizes their putative pathophysiological as well as therapeutic role in pulmonary vascular remodeling. A better understanding of the mechanisms of their actions may allow for the development of new options for attenuating acute and chronic pulmonary vasoconstriction and remodeling treating the devastating disease pulmonary hypertension.

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.

Cerebral artery signal transduction mechanisms: developmental changes in dynamics and Ca2+ sensitivity

As compared to the adult, the developing fetus and newborn infant are at much greater risk for dysregulation of cerebral blood flow (CBF), with complications such as intraventricular and germinal matrix hemorrhage with resultant neurologic sequelae. To minimize this dysregulation and its consequences presents a major challenge. Although in many respects the fundamental signal transduction mechanisms that regulate relaxation and contraction pathways, and thus cerebrovascular tone and CBF in the immature organism are similar to those of the adult, the individual elements, pathways, and roles differ greatly. Here, we review aspects of these maturational changes of relaxation/contraction mechanisms in terms of both electro-mechanical and pharmaco-mechanical coupling, their biochemical pathways and signaling networks. In contrast to the adult cerebrovasculature, in addition to attenuated structure with differences in multiple cytoskeletal elements, developing cerebrovasculature of fetus and newborn differs in many respects, such as a strikingly increased sensitivity to [Ca(2+)]i and requirement for extracellular Ca(2+) for contraction. In essence, the immature cerebrovasculature demonstrates both "hyper-relaxation" and "hypo-contraction". A challenge is to unravel the manner in which these mechanisms are integrated, particularly in terms of both Ca(2+)-dependent and Ca(2+)-independent pathways to increase Ca(2+) sensitivity. Gaining an appreciation of these significant age-related differences in signal mechanisms also will be critical to understanding more completely the vulnerability of the developing cerebral vasculature to hypoxia and other stresses. Of vital importance, a more complete understanding of these mechanisms promises hope for improved strategies for therapeutic intervention and clinical management of intensive care of the premature newborn.

Effect of chronic perinatal hypoxia on the role of rho-kinase in pulmonary artery contraction in newborn lambs

Exposure to chronic hypoxia during gestation predisposes infants to neonatal pulmonary hypertension, but the underlying mechanisms remain unclear. Here, we test the hypothesis that moderate continuous hypoxia during gestation causes changes in the rho-kinase pathway that persist in the newborn period, altering vessel tone and responsiveness. Lambs kept at 3,801 m above sea level during gestation and the first 2 wk of life were compared with those with gestation at low altitude. In vitro studies of isolated pulmonary arterial rings found a more forceful contraction in response to KCl and 5-HT in high-altitude compared with low-altitude lambs. There was no difference between the effects of blockers of various pathways of extracellular Ca(2+) entry in low- and high-altitude arteries. In contrast, inhibition of rho-kinase resulted in significantly greater attenuation of 5-HT constriction in high-altitude compared with low-altitude arteries. High-altitude lambs had higher baseline pulmonary artery pressures and greater elevations in pulmonary artery pressure during 15 min of acute hypoxia compared with low-altitude lambs. Despite evidence for an increased role for rho-kinase in high-altitude arteries, in vivo studies found no significant difference between the effects of rho-kinase inhibition on hypoxic pulmonary vasoconstriction in intact high-altitude and low-altitude lambs. We conclude that chronic hypoxia in utero results in increased vasopressor response to both acute hypoxia and serotonin, but that rho-kinase is involved only in the increased response to serotonin.

Maternal high-altitude hypoxia and suppression of ryanodine receptor-mediated Ca2+ sparks in fetal sheep pulmonary arterial myocytes

Ca(2+) sparks are fundamental Ca(2+) signaling events arising from ryanodine receptor (RyR) activation, events that relate to contractile and dilatory events in the pulmonary vasculature. Recent studies demonstrate that long-term hypoxia (LTH) can affect pulmonary arterial reactivity in fetal, newborn, and adult animals. Because RyRs are important to pulmonary vascular reactivity and reactivity changes with ontogeny and LTH we tested the hypothesis that RyR-generated Ca(2+) signals are more active before birth and that LTH suppresses these responses. We examined these hypotheses by performing confocal imaging of myocytes in living arteries and by performing wire myography studies. Pulmonary arteries (PA) were isolated from fetal, newborn, or adult sheep that lived at low altitude or from those that were acclimatized to 3,801 m for > 100 days. Confocal imaging demonstrated preservation of the distance between the sarcoplasmic reticulum, nucleus, and plasma membrane in PA myocytes. Maturation increased global Ca(2+) waves and Ca(2+) spark activity, with sparks becoming larger, wider, and slower. LTH preferentially depressed Ca(2+) spark activity in immature pulmonary arterial myocytes, and these sparks were smaller, wider, and slower. LTH also suppressed caffeine-elicited contraction in fetal PA but augmented contraction in the newborn and adult. The influence of both ontogeny and LTH on RyR-dependent cell excitability shed new light on the therapeutic potential of these channels for the treatment of pulmonary vascular disease in newborns as well as adults.

Depolarization-dependent contraction increase after birth and preservation following long-term hypoxia in sheep pulmonary arteries

Membrane depolarization is critical to pulmonary arterial (PA) contraction. Both L-type Ca²⁺ channels (Ca_L) and Rho-kinase are important signaling components of this process and mitochondrial and non-mitochondrial generated superoxides can be part of the signaling process. Maturation and long-term hypoxia (LTH) each can modify depolarization-dependent contraction and the role of superoxides. By the use of wire myography, we tested the hypothesis that maturation and LTH increase pulmonary arterial reactivity to high-K⁺-induced membrane depolarization through enhancements in the importance of Ca_Land Rho-kinase-dependent pathways. The data show that maturation, but not LTH, increases contraction to 125 mM KCl (high-K⁺) without altering the EC₅₀. High-K⁺-dependent contraction was inhibited to a similar extent in fetal and adult PA by multiple Ca_L blockers, including 10 μM diltiazem, 10 μM verapamil, and 10 μM nifedipine. Postnatal maturation increased the role for 10 μM nifedipine-sensitive Ca_L, and decreased that for 10 μM Y-27632-sensitive Rho-kinase. In all groups, the combination of nifedipine and Y-27632 effectively inhibited high-K⁺ contraction. Tempol (3 mM) but not 100 μM apocynin slightly reduced contraction in arteries from fetal hypoxic and adult normoxic and hypoxic sheep, indicating a limited role for non-mitochondrial derived superoxide to high-K⁺-induced contraction. Western immunoblot for alpha smooth muscle actin indicated small increases in relative abundance in the adult. The data suggest that while Ca_L therapies more effectively vasodilate PA in adults and rho-kinase therapies are more effective in newborns, combination therapies would provide greater efficacy in both young and mature patients regardless of normoxic or hypoxic conditions.

Inositol trisphosphate receptors in smooth muscle cells

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are a family of tetrameric intracellular calcium (Ca(2+)) release channels that are located on the sarcoplasmic reticulum (SR) membrane of virtually all mammalian cell types, including smooth muscle cells (SMC). Here, we have reviewed literature investigating IP(3)R expression, cellular localization, tissue distribution, activity regulation, communication with ion channels and organelles, generation of Ca(2+) signals, modulation of physiological functions, and alterations in pathologies in SMCs. Three IP(3)R isoforms have been identified, with relative expression and cellular localization of each contributing to signaling differences in diverse SMC types. Several endogenous ligands, kinases, proteins, and other modulators control SMC IP(3)R channel activity. SMC IP(3)Rs communicate with nearby ryanodine-sensitive Ca(2+) channels and mitochondria to influence SR Ca(2+) release and reactive oxygen species generation. IP(3)R-mediated Ca(2+) release can stimulate plasma membrane-localized channels, including transient receptor potential (TRP) channels and store-operated Ca(2+) channels. SMC IP(3)Rs also signal to other proteins via SR Ca(2+) release-independent mechanisms through physical coupling to TRP channels and local communication with large-conductance Ca(2+)-activated potassium channels. IP(3)R-mediated Ca(2+) release generates a wide variety of intracellular Ca(2+) signals, which vary with respect to frequency, amplitude, spatial, and temporal properties. IP(3)R signaling controls multiple SMC functions, including contraction, gene expression, migration, and proliferation. IP(3)R expression and cellular signaling are altered in several SMC diseases, notably asthma, atherosclerosis, diabetes, and hypertension. In summary, IP(3)R-mediated pathways control diverse SMC physiological functions, with pathological alterations in IP(3)R signaling contributing to disease.

Preservation of serotonin-mediated contractility in adult sheep pulmonary arteries following long-term high-altitude hypoxia

Long-term hypoxia (LTH) can increase serotonin (5-HT) signaling as well as extracellular calcium entry in adult rodent pulmonary arteries (PA), and 5-HT is associated with pulmonary hypertension. Because LTH, 5-HT, and calcium entry are related, we tested the hypothesis that LTH increases 5-HT-mediated PA contractility and associated calcium influx through L-type Ca2+ channels, nonselective cation channels (NSCC), and reverse-mode sodium-Ca2+ exchange. We performed wire myography and confocal calcium imaging on pulmonary arteries from adult ewes that lived near sea level or were maintained at high-altitude (3801 m) for ∼110 days. LTH did not increase the arterial medial wall thickness, nor did it affect the potency or efficacy for 5-HT-induced PA contraction. Ketanserin (100 nM), a 5-HT2A antagonist, shifted the 5-HT potency to a far greater extent than 1 μM GR-55562, a 5-HT1B/D inhibitor. These influences were unaffected by LTH. The rank order for reducing 5-HT-induced PA contraction in normoxic animals was extracellular calcium removal≈10 mM Ni2+≈10 μM verapamil≈10 μM nifedipine with 50 μM SKF 96365>30 μM KB-R7943≈100 μM flufenamic acid≈10 μM nifedipine≈100 μM Gd3+> 100 μM La3+>500 μM Ni2+≈10 μM diltiazem≈50 μM 2-APB≈100 μM LOE 908. Contraction was not reduced by 100 μM spermine or 30 μM SN-6. LTH increased the effects of KB-R7943 and mitigated those of nifedipine but did not affect calcium responses in imaging studies. Overall, in adult sheep, arterial structure and 5-HT2A and 5HT1B/D functions are preserved following LTH while the role of NSCC-related calcium-dependent contraction is increased. These elements indicate preservation of PA contractility in LTH with minimal functional changes.

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.

Long-term maternal hypoxia: the role of extracellular Ca2+ entry during serotonin-mediated contractility in fetal ovine pulmonary arteries

Antenatal maternal long-term hypoxia (LTH) can alter serotonin (5-HT) and calcium (Ca(2+)) signaling in fetal pulmonary arteries (PAs) and is associated with persistent pulmonary hypertension of the newborn (PPHN). In humans, the antenatal maternal hypoxia can be secondary to smoking, anemia, and chronic obstructive pulmonary disorders. However, the mechanisms of antenatal maternal hypoxia-related PPHN are unresolved. Because both LTH and 5-HT are associated with PPHN, we tested the hypothesis that antenatal maternal LTH can increase 5-HT-mediated PA contraction and associated extracellular Ca(2+) influx through L-type Ca(2+) channels (Ca(L)), nonselective cation channels (NSCCs), and reverse-mode sodium-calcium exchanger (NCX) in the near-term fetus. We performed wire myography and confocal-Ca(2+) imaging approaches on fetal lamb PA (∼ 140 days of gestation) from normoxic ewes or those acclimatized to high-altitude LTH (3801 m) for ∼110 days. Long-term hypoxia reduced the potency but not the efficacy of 5-HT-induced PA contraction. Ketanserin (100 nmol/L), a 5-HT(2A) antagonist, shifted 5-HT potency irrespective of LTH, while GR-55562 (1 µmol/L), a 5-HT(1B/D) inhibitor, antagonized 5-HT-induced contraction in normoxic fetuses only. Various inhibitors for Ca(L), NSCC, and reverse-mode NCX were used in contraction studies. Contraction was reliant on extracellular Ca(2+) regardless of maternal hypoxia, NSCC was more important to contraction than Ca(L), and reverse-mode NCX had little or no role in contraction. Long-term hypoxia also attenuated the effects of 2-APB and flufenamic acid and reduced Ca(2+) responses observed by imaging studies. Overall, LTH reduced 5HT(1B/D) function and increased NSCC-related Ca(2+)-dependent contraction in ovine fetuses, which may compromise pulmonary vascular function in the newborn.

Inhaled nitrite reverses hemolysis-induced pulmonary vasoconstriction in newborn lambs without blood participation

BACKGROUND

Nitrite can be converted to nitric oxide (NO) by a number of different biochemical pathways. In newborn lambs, an aerosol of inhaled nitrite has been found to reduce pulmonary blood pressure, possibly acting via conversion to NO by reaction with intraerythrocytic deoxyhemoglobin. If so, the vasodilating effects of nitrite would be attenuated by free hemoglobin in plasma that would rapidly scavenge NO.

METHODS AND RESULTS

Pulmonary vascular pressures and resistances to flow were measured in anesthetized newborn lambs. Plasma hemoglobin concentrations were then elevated, resulting in marked pulmonary hypertension. This effect was attenuated if infused hemoglobin was first oxidized to methemoglobin, which does not scavenge NO. These results further implicate NO as a tonic pulmonary vasodilator. Next, while free hemoglobin continued to be infused, the lambs were given inhaled NO gas (20 ppm), inhaled sodium nitrite aerosol (0.87 mol/L), or an intravascular nitrite infusion (3 mg/h bolus, 5 mg · kg⁻¹ · h⁻¹ infusion). Inhaled NO and inhaled nitrite aerosol both resulted in pulmonary vasodilation. Intravascular infusion of nitrite, however, did not. Increases in exhaled NO gas were observed in lambs while breathing the nitrite aerosol (≈ 20 ppb NO) but not during intravascular infusion of nitrite.

CONCLUSIONS

We conclude that the pulmonary vasodilating effect of inhaled nitrite results from its conversion to NO in airway and parenchymal lung tissue and is not dependent on reactions with deoxyhemoglobin in the pulmonary circulation. Inhaled nitrite aerosol remains a promising candidate to reduce pulmonary hypertension in clinical application.

Cholesterol depletion alters coronary artery myocyte Ca(2+) signalling in a stimulus-specific manner

Although there is evidence that caveolae and cholesterol play an important role in myocyte signalling processes, details of the mechanisms involved remain sparse. In this paper we have studied for the first time the clinically relevant intact coronary artery and measured in situ Ca(2+) signals in individual myocytes using confocal microscopy. We have examined the effect of the cholesterol-depleting agents, methyl-cyclodextrin (MCD) and cholesterol oxidase, on high K(+), caffeine and agonist-induced Ca(2+) signals. We find that cholesterol depletion produces a stimulus-specific alteration in Ca(2+) responses; with 5-HT (10microM) and endothelin-1 (10nM) responses being selectively decreased, the phenylephrine response (100microM) increased and the responses to high K(+) (60mM) and caffeine (10mM) unaffected. Agonist-induced Ca(2+) signals were restored when cholesterol was replenished using cholesterol-saturated MCD. In additional experiments, enzymatically isolated myocytes were patch clamped. We found that cholesterol depletion caused a selective modification of ion channel function, with whole cell inward Ca(2+) current being unaltered, whereas outward K(+) current was increased, due to BK(Ca) channel activation. There was also a significant decrease in cell capacitance. These data are discussed in terms of the involvement of caveolae in receptor localisation, Ca(2+) entry pathways and SR Ca(2+) release, and the role of these in agonist signalling.

Caffeine inhibits InsP3 responses and capacitative calcium entry in canine pulmonary arterial smooth muscle cells

Caffeine is a well described and characterized ryanodine receptor (RyR) activator. Previous evidence from independent research studies also indicate caffeine inhibits InsP3 receptor functionality, which is important to activation of capacitative Ca2+ entry (CCE) in some cell types. In addition, RyR activation elicits excitatory-coupled Ca2+ entry (ECCE) in skeletal muscle myotubes. Recent studies by our group show that canine pulmonary arterial smooth muscle cells (PASMCs) have functional InsP3 receptors as well as RyRs, and that CCE is dependent on InsP3 receptor activity. The potential for caffeine to activate ECCE as well as inhibit InsP3 receptor function and CCE was examined using fura-2 fluorescent imaging in canine PASMCs. The data show caffeine causes transient as well as sustained cytosolic Ca2+ increases, though this is not due to CCE or ECCE activity as evidenced by a lack of an increase in Mn2+ quench of fura-2. The experiments also show caffeine reversibly inhibits 5-HT elicited-InsP3 mediated Ca2+ responses with an IC50 of 6.87x10(-4) M and 10 mM caffeine fully inhibits CCE. These studies provide the first evidence that caffeine is an inhibitor of InsP3 generated Ca2+ signals and CCE in PASMCs.

Maturation of intracellular calcium homeostasis in sheep pulmonary arterial smooth muscle cells

Cytosolic Ca(2+) signaling dynamics are important to pulmonary arterial reactivity, and alterations are implicated in pulmonary vascular disorders. Yet, adaptations in cellular Ca(2+) homeostasis and receptor-mediated Ca(2+) signaling with maturation from fetal to adult life in pulmonary arterial smooth muscle cells (PASMCs) are not known. The present study tested the hypothesis that cytosolic Ca(2+) homeostasis and receptor-generated Ca(2+) signaling adapt with maturation in sheep PASMCs. Digitalized fluorescence microscopy was performed using isolated PASMCs from fetal and adult sheep that were loaded with the Ca(2+) indicator fura 2. The results show that basal cytosolic and sarcoplasmic reticulum Ca(2+) levels are attained before birth. Similarly, Ca(2+) efflux pathways from the cytosol and basal as well as capacitative Ca(2+) entry (CCE) are also developed before birth. However, receptor-mediated Ca(2+) signaling adapts with maturation. Prominently, serotonin stimulation elicited Ca(2+) elevations in very few fetal compared with adult PASMCs; in contrast, phenylephrine elevated Ca(2+) in a similar percentage of fetal and adult PASMCs. Serotonin and phenylephrine elicited Ca(2+) increases of a similar magnitude in reactive cells of fetus and adult, supporting the assertion that inositol trisphosphate signaling is intact. Caffeine and ATP elevated Ca(2+) in equivalent numbers of fetal and adult PASMCs. However, the caffeine-induced cytosolic Ca(2+) increase was significantly greater in fetal PASMCs, whereas the ATP-elicited increase was greater in adult cells. Overall, the results of this study demonstrate selective adaptations in receptor-mediated Ca(2+) signaling, but not in cellular Ca(2+) homeostasis.

Influence of heat stress on the reactivity of isolated chicken carotid artery to vasoactive agents

Cerebral ischaemia is considered to be an important cause of central nervous system dysfunction in heat stress. We hypothesized that heat stress would alter the reactivity of isolated carotid artery to vasoactive agents. Carotid arteries were isolated from broiler chickens maintained either at 23-24 degrees C with 55-65% humidity (control conditions) or exposed to 40 +/- 1 degrees C with 35% humidity for 4 h (heat stress). Contractions were elicited with vasoconstrictors such as 5-HT, phenylephrine, guanfacine and CaCl(2) (K(+)-depolarized) in endothelium-denuded arterial rings. Heat stress significantly increased the potency of 5-HT, but had no effect on the sensitivity of the vessel to phenylephrine or guanfacine. In contrast, it markedly decreased the potency and efficacy of CaCl(2). Vasodilator responses to ACh (endothelium-intact) and sodium nitroprusside (endothelium-denuded), however, were unaffected. Although cyclopiazonic acid (10 microm) significantly decreased 5-HT responses in both the conditions, the agonist was still more potent in heat stress. Extracellular Ca(2)(+) removal had no effect on contractions caused by 5-HT in control conditions, but it significantly decreased the agonist potency in heat stress. Interestingly, nifedipine (1 microm) markedly inhibited 5-HT-induced contractions both in control conditions and in heat stress, implying an inhibitory effect on both Ca(2)(+) influx and release. Thus, nifedipine had a markedly greater inhibitory effect on 5-HT-induced contractions in heat stress compared with control conditions. The results suggest that heat stress increased the vasoconstrictor responses to 5-HT by a mechanism that involved extracellular Ca(2)(+) influx through nifedipine-sensitive L-type calcium channels.

Inhibition of ryanodine receptors by 4-(2-aminopropyl)-3,5-dichloro-N,N-dimethylaniline (FLA 365) in canine pulmonary arterial smooth muscle cells

Ryanodine is a selective ryanodine receptor (RyR) blocker, with binding dependent on RyR opening. In whole-cell studies, ryanodine binding can lock the RyR in an open-conductance state, short-circuiting the sarcoplasmic reticulum, which restricts studies of inositol-1,4,5-trisphosphate receptor (InsP3R) activity. Other RyR blockers have nonselective effects that also limit their utility. 4-(2-aminopropyl)-3,5-dichloro-N,N-dimethylaniline (FLA 365) blocks RyR-elicited Ca2+ increases in skeletal and cardiac muscle; yet, its actions on smooth muscle are unknown. Canine pulmonary arterial smooth muscle cells (PASMCs) express both RyRs and InsP3Rs; thus, we tested the ability of FLA 365 to block RyR- and serotonin-mediated InsP3R-elicited Ca2+ release by imaging fura-2-loaded PASMCs. Acute exposure to 10 mM caffeine, a selective RyR activator, induced Ca2+ increases that were reversibly reduced by FLA 365, with an estimated IC50 of approximately 1 to 1.5 microM, and inhibited by 10 microM ryanodine or 10 microM cyclopiazonic acid. FLA 365 also blocked L-type Ca2+ channel activity, with 10 microM reducing Ba2+ current amplitude in patch voltage-clamp studies to 54 +/- 6% of control and 100 microM FLA 365 reducing membrane current to 21 +/- 6%. InsP3R-mediated Ca2+ responses elicited by 10 microM 5-hydroxytryptamine (serotonin) in canine PASMCs and 100 microM carbachol in human embryonic kidney (HEK)-293 cells were not reduced by 2 microM FLA 365, but they were reduced by 20 microM FLA 365 to 76 +/- 9% of control in canine PASMCs and 52 +/- 1% in HEK-293 cells. Thus, FLA 365 preferentially blocks RyRs with limited inhibition of L-type Ca2+ channels or InsP3R in canine PASMCs.

Role of InsP3 and ryanodine receptors in the activation of capacitative Ca2+ entry by store depletion or hypoxia in canine pulmonary arterial smooth muscle cells

BACKGROUND AND PURPOSE

Experiments were performed to determine if capacitative Ca(2+) entry (CCE) in canine pulmonary arterial smooth muscle cells (PASMCs) is dependent on InsP(3) receptors or ryanodine receptors as induction of CCE is dependent on simultaneous depletion of the functionally separate InsP(3)- and ryanodine-sensitive sarcoplasmic reticulum (SR) Ca(2+) stores in these cells.

EXPERIMENTAL APPROACH

Myocytes were isolated from canine pulmonary arteries using enzymatic procedures and were used within 8 h of preparation. Measurements of cytosolic Ca(2+) were made by imaging fura-2 loaded individual myocytes that were perfused with physiological buffered saline solution with or without Ca(2+).

KEY RESULTS

Treating myocytes with 10 microM cyclopiazonic acid (CPA), removing extracellular Ca(2+), and briefly applying 10 mM caffeine and 10 microM 5-hydroxytryptamine (5-HT) depleted SR Ca(2+) stores. Extracellular Ca(2+) reintroduction caused cytosolic [Ca(2+)] to elevate above baseline signifying CCE. The InsP(3) receptor inhibitors 2-aminobiphenylborate (50-75 microM; 2-APB) and xestospongin-C (20 microM; XeC) abolished CCE. Yet, CCE was unaffected by 10 microM or 300 microM ryanodine or 10 microM dantrolene, which modify ryanodine receptor activity. Higher dantrolene concentrations (50 microM), however, can inhibit both ryanodine receptors and InsP(3) receptors, did reduce CCE. In contrast, CCE activated by hypoxia was unaffected by XeC (20 microM).

CONCLUSIONS AND IMPLICATIONS

The results provide evidence that CCE activated by depletion of both InsP(3) and ryanodine SR Ca(2+) stores in canine PASMCs is dependent on functional InsP(3) receptors, whereas the activation of CCE by hypoxia appears to be independent of functional InsP(3) receptors.

A constitutively active nonselective cation conductance underlies resting Ca2+ influx and secretion in bovine adrenal chromaffin cells

We have combined fluorimetric measurements of the intracellular free Ca(2+) concentration ([Ca(2+)](i)) with the patch clamp technique, to investigate resting Ca(2+) entry in bovine adrenal chromaffin cells. Perfusion with nominally Ca(2+)-free medium resulted in a rapid, reversible decrease in [Ca(2+)](i), indicating a resting Ca(2+) permeability across the plasma membrane. Simultaneous whole-cell voltage-clamp showed a resting inward current that increased when extracellular Ca(2+) (Ca(2+)(o)) was lowered. This current had a reversal potential of around 0 mV and was carried by monovalent or divalent cations. In Na(+)-free extracellular medium there was a reduction in current amplitude upon removal of Ca(2+)(o), indicating the current can carry Ca(2+). The current was constitutively active and not enhanced by agents that promote Ca(2+)-store depletion such as thapsigargin. Extracellular La(3+) abolished the resting current, reduced resting [Ca(2+)](i) and inhibited basal secretion. Abolishment of resting Ca(2+) influx depleted the inositol 1,4,5-trisphosphate-sensitive Ca(2+) store without affecting the caffeine-sensitive Ca(2+) store. The results indicate the presence of a constitutively active nonselective cation conductance, permeable to both monovalent and divalent cations, that can regulate [Ca(2+)](i), the repletion state of the intracellular Ca(2+) store and the secretory response in resting cells.

Effects of Ca2+ channel blockers on store-operated Ca2+ channel currents of Kupffer cells after hepatic ischemia/reperfusion injury in rats

AIM

To study the effects of hepatic ischemia/reperfusion (I/R) injury on store-operated calcium channel (SOC) currents (I(SOC)) in freshly isolated rat Kupffer cells, and the effects of Ca(2+) channel blockers, 2-aminoethoxydiphenyl borate (2-APB), SK and F96365, econazole and miconazole, on I(SOC) in isolated rat Kupffer cells after hepatic I/R injury.

METHODS

The model of rat hepatic I/R injury was established. Whole-cell patch-clamp techniques were performed to investigate the effects of 2-APB, SK and F96365, econazole and miconazole on I(SOC) in isolated rat Kupffer cells after hepatic I /R injury.

RESULTS

I/R injury significantly increased I(SOC) from -80.4 +/- 25.2pA to -159.5 +/- 34.5pA ((b)P < 0.01, n = 30). 2-APB (20, 40, 60, 80, 100 micromol/L), SK and F96365 (5, 10, 20, 40, 50 micromol/L), econazole (0.1, 0.3, 1, 3, 10 micromol/L) and miconazole (0.1, 0.3, 1, 3, 10 micromol/L) inhibited I(SOC) in a concentration-dependent manner with IC50 of 37.41 micromol/L (n = 8), 5.89 micromol/L (n = 11), 0.21 micromol/L (n = 13), and 0.28 micromol/L (n = 10). The peak value of I(SOC) in the I-V relationship was decreased by the blockers in different concentrations, but the reverse potential of I(SOC) was not transformed.

CONCLUSION

SOC is the main channel for the influx of Ca(2+) during hepatic I/R injuries. Calcium channel blockers, 2-APB, SK and F96365, econazole and miconazole, have obviously protective effects on I/R injury, probably by inhibiting I(SOC) in Kupffer cells and preventing the activation of Kupffer cells.

Effects of aging on Ca2+ signaling in murine mesenteric arterial smooth muscle cells

Pathophysiological changes in arterial smooth muscle structure and function occur with aging and there are a number of reports illustrating reductions in vascular responsiveness with aging. While much is known about arterial remodeling and functional adaptations with aging, very little is known about the biophysical adaptations in individual arterial myocytes. Cytosolic Ca2+ signaling, involving activation of L-type Ca2+ channels on the plasma membrane as well as InsP3 and ryanodine receptors on the sarcoplasmic reticulum, is integral to vascular tone and reactivity. Thus, we tested the hypothesis that aging results in reductions in the functional expression of L-type channels and temporal aspects of ryanodine receptor and InsP3 receptor Ca2+ signaling, in mesenteric arterial smooth muscle cells isolated from 6 and 30 months old C57Bl/6 mice. Comparisons of L-type current activity were made using dialyzed, whole-cell voltage-clamp techniques and Ba2+ as charge carrier. Ca2+ signaling was measured using fura-2 fluorescence microscopy techniques. Cell morphological changes were also investigated using electrophysiological and immunocytochemical approaches. The amplitudes of L-type Ca2+ currents were increased in older mice, but this was associated with membrane surface area increases of approximately 50%, due to increases in cell length not cell width. Consequently, L-type Ca2+ current densities were preserved with age, indicating functional channel expression was unchanged. In contrast, aging was associated with decrements in Ca2+ signaling in response to either ryanodine receptor stimulation by caffeine or InsP3 receptor activation with phenylephrine. These changes with aging may be related to the previously reported depression in myogenic reactivity.