Evidence for specific regional patterns of responses to different vasoconstrictors and vasodilators in sheep isolated pulmonary arteries and veins

Purinergic receptor stimulation induces calcium oscillations and smooth muscle contraction in small pulmonary veins

KEY POINTS

We investigated the excitation-contraction coupling mechanisms in small pulmonary veins (SPVs) in rat precision-cut lung slices. We found that SPVs contract strongly and reversibly in response to extracellular ATP and other vasoconstrictors, including angiotensin-II and endothelin-1. ATP-induced vasoconstriction in SPVs was associated with the stimulation of purinergic P2Y2 receptors in vascular smooth muscle cell, activation of phospholipase C-β and the generation of intracellular Ca²⁺ oscillations mediated by cyclic Ca²⁺ release events via the inositol 1,4,5-trisphosphate receptor. Active constriction of SPVs may play an important role in the development of pulmonary hypertension and pulmonary oedema.

ABSTRACT

The small pulmonary veins (SPVs) may play a role in the development of pulmonary hypertension and pulmonary oedema via active changes in SPV diameter, mediated by vascular smooth muscle cell (VSMC) contraction. However, the excitation-contraction coupling mechanisms during vasoconstrictor stimulation remain poorly understood in these veins. We used rat precision-cut lung slices and phase-contrast and confocal microscopy to investigate dynamic changes in SPV cross-sectional luminal area and intracellular Ca²⁺ signalling in their VSMCs. We found that the SPV (∼150 μm in diameter) contract strongly in response to extracellular ATP and other vasoconstrictors, including angiotensin-II and endothelin-1. ATP-induced SPV contraction was fast, concentration-dependent, completely reversible upon ATP washout, and inhibited by purinergic receptor antagonists suramin and AR-C118925 but not by MRS2179. Immunofluorescence showed purinergic P2Y2 receptors expressed in SPV VSMCs. ATP-induced SPV contraction was inhibited by phospholipase Cβ inhibitor U73122 and accompanied by intracellular Ca²⁺ oscillations in the VSMCs. These Ca²⁺ oscillations and SPV contraction were inhibited by the inositol 1,4,5-trisphosphate receptor inhibitor 2-APB but not by ryanodine. The results of the present study suggest that ATP-induced vasoconstriction in SPVs is associated with the activation of purinergic P2Y2 receptors in VSMCs and the generation of Ca²⁺ oscillations.

Endothelin-1 may play an important role in the Fontan circulation

OBJECTIVES

Our goal was to evaluate whether endothlin-1 (ET-1) plays an important role in the Fontan circulation.

METHODS

Thirteen patients with single-ventricle physiology (Glenn circulation, n = 7; Fontan circulation, n = 6) were evaluated using lung histopathological and immunohistochemical studies and then compared with the normal autopsied controls without congenital heart disease (n = 13). We evaluated the medial thickness of the small pulmonary arteries. For 10 of these patients, quantitative real-time polymerase chain reaction analyses of ET-1, endothelin receptors Type A and Type B, endothelin-converting enzyme-1 and endothelial nitric oxide synthase were performed.

RESULTS

The medial thickness of the small pulmonary arteries in patients with single-ventricle physiology was greater than that of those in the control group (P = 0.0341). Severe medial hypertrophy of the pulmonary arteries was observed in patients who had poor outcomes. Immunohistochemical studies revealed that the marked expression of ET-1 was observed in the endothelium and media of their pulmonary arteries. In these patients, the messenger RNA expression of ET-1 was also increased. Two patients showed high levels of expression of ETAR and ETBR, although these 2 cases maintain good Fontan circulation.

CONCLUSIONS

Medial hypertrophy and the overexpression of ET-1 in the pulmonary arteries were observed in some patients in whom the Fontan circulation failed. Our data suggest that ET-1 may play an important role in maintaining the Fontan circulation.

Regional heterogeneity in the reactivity of equine small pulmonary blood vessels

Regional differences in large equine pulmonary artery reactivity exist. It is not known if this heterogeneity extends into small vessels. The hypothesis that there is regional heterogeneity in small pulmonary artery and vein reactivity to sympathomimetics (phenylephrine and isoproterenol) and a parasympathomimetic (methacholine) was tested using wire myography on small vessels from caudodorsal (CD) and cranioventral (CV) lung of 12 horses [9 mares, 3 geldings, 8.67 ± 0.81 (age ± SE) yr, of various breeds that had never raced]. To study relaxation, vessels were precontracted with U46619 (10(-6) M). Methacholine mechanism of action was investigated using L-nitroarginine methylester (L-NAME, 100 μM) and indomethacin (10 μM). Phenylephrine did not contract any vessels. Isoproterenol relaxed CD arteries more than CV arteries (maximum relaxation 28.18% and 48.67%; Log IC50 ± SE -7.975 ± 0.1327 and -8.033 ± 0.1635 for CD and CV, respectively, P < 0.0001), but not veins. Methacholine caused contraction of CD arteries (maximum contraction 245.4%, Log EC50 ± SE -6.475 ± 0.3341), and relaxation of CV arteries (maximum relaxation 40.14%, Log IC50 ± SE -6.791 ± 0.1954) and all veins (maximum relaxation 50.62%, Log IC50 ± SE -6.932 ± 0.1986) in a nonregion-dependent manner. L-NAME (n = 8, P < 0.0001) and indomethacin (n = 7, P < 0.0001) inhibited methacholine-induced relaxation of CV arteries, whereas indomethacin augmented CD artery contraction (n = 8, P < 0.0001). Our data demonstrate significant regional heterogeneity in small blood vessel reactivity when comparing the CD to the CV region of the equine lung.

The role of endothelin-1 in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a rare but debilitating disease, which if left untreated rapidly progresses to right ventricular failure and eventually death. In the quest to understand the pathogenesis of this disease differences in the profile, expression and action of vasoactive substances released by the endothelium have been identified in patients with PAH. Of these, endothelin-1 (ET-1) is of particular interest since it is known to be an extremely powerful vasoconstrictor and also involved in vascular remodelling. Identification of ET-1 as a target for pharmacological intervention has lead to the discovery of a number of compounds that can block the receptors via which ET-1 mediates its effects. This review sets out the evidence in support of a role for ET-1 in the onset and progression of the disease and reviews the data from the various clinical trials of ET-1 receptor antagonists for the treatment of PAH.

Milrinone relaxes pulmonary veins in guinea pigs and humans

Introduction

The phosphodiesterase-III inhibitor milrinone improves ventricular contractility, relaxes pulmonary arteries and reduces right ventricular afterload. Thus, it is used to treat heart failure and pulmonary hypertension (PH). However, its action on pulmonary veins (PVs) is not defined, although particularly PH due to left heart disease primarily affects the pulmonary venous bed. We examined milrinone-induced relaxation in PVs from guinea pigs (GPs) and humans.

Material and Methods

Precision-cut lung slices (PCLS) were prepared from GPs or from patients undergoing lobectomy. Milrinone-induced relaxation was studied by videomicroscopy in naïve PVs and in PVs pre-constricted with the ET_A-receptor agonist BP0104. Baseline luminal area was defined as 100%. Intracellular cAMP was measured by ELISA and milrinone-induced changes of segmental vascular resistances were studied in the GP isolated perfused lung (IPL).

Results

In the IPL (GP), milrinone (10 µM) lowered the postcapillary resistance of pre-constricted vessels. In PCLS (GP), milrinone relaxed naïve and pre-constricted PVs (120%) and this relaxation was attenuated by inhibition of protein kinase G (KT 5823), adenyl cyclase (SQ 22536) and protein kinase A (KT 5720), but not by inhibition of NO-synthesis (L-NAME). In addition, milrinone-induced relaxation was dependent on the activation of K_ATP-, BK_Ca²⁺- and K_v-channels. Human PVs also relaxed to milrinone (121%), however only if pre-constricted.

Discussion

Milrinone relaxes PVs from GPs and humans. In GPs, milrinone-induced relaxation is based on K_ATP-, BK_Ca²⁺- and K_v-channel-activation and on cAMP/PKA/PKG. The relaxant properties of milrinone on PVs lead to reduced postcapillary resistance and hydrostatic pressures. Hence they alleviate pulmonary edema and suggest beneficial effects of milrinone in PH due to left heart disease.

Characterization of vasomotor responses in different vascular territories of C57BL/6J mice

The mouse is a quickly reproducing, inexpensive animal and often used for transgenic approaches. Due to its small size, only the aorta is frequently taken to assess vascular function. However, atherosclerosis is a generalized disease and becomes symptomatic when the perfusion of specific organs is impaired. We have therefore compared the thoracic and abdominal aorta with carotid, femoral, mesenteric, renal and coronary arteries to see whether aortic vasomotion can indeed serve as a surrogate for other, organ-specific vascular territories. Arterial segments of male C57BL/6J mice were dissected and mounted on a myograph for isometric force measurement. Vasoconstriction was determined in response to depolarization by potassium chloride (KCl), which was not different with or without an α-adrenoceptor antagonist. Vascular responses were determined in response to receptor activation by the neurotransmitter norepinephrine (±inhibition of nitric oxide synthase; ±α- and β-adrenoceptor antagonists) and the platelet-derived mediator serotonin (±inhibition of nitric oxide synthesis; ±5-hydroxytryptamine receptor antagonist). Endothelium-dependent and -independent vasodilation was determined in response to carbachol and nitroprusside after norepinephrine-induced pre-constriction (±β-adrenoceptor antagonist). Vasoconstriction in response to KCl, norepinephrine and serotonin differed in magnitude between thoracic and abdominal aorta and between aorta and the other arterial segments. Endothelium-dependent and -independent vasodilation differed also in magnitude between the arterial segments. Thus, the murine aorta is not a general surrogate to assess vascular function of organ-specific vascular territories.

Long-term effects of prenatal hypoxia on endothelium-dependent relaxation responses in pulmonary arteries of adult sheep

Chronic hypoxia during the course of pregnancy is a common insult to the fetus. However, its long-term effect on the pulmonary vasculature in adulthood has not been described. In this study, the vasorelaxation responses of conduit pulmonary arteries in adult female sheep that were chronically hypoxic as fetuses and raised postnatally at sea level were investigated. Vessel tension studies revealed that endothelium-dependent relaxation responses were attenuated in pulmonary arteries from adult sheep that experienced prenatal hypoxia. Endothelial nitric oxide synthase (eNOS) protein expression was unchanged, but eNOS activity was significantly decreased in pulmonary arteries from prenatally hypoxic sheep. Protein expression of eNOS partners, caveolin-1, calmodulin, and heat shock protein 90 (Hsp90) did not change following prenatal hypoxia. However, the association between eNOS and caveolin-1, its inhibitory binding partner, was significantly increased, whereas association between eNOS and its stimulatory partners calmodulin and Hsp90 was greatly decreased. Furthermore, phosphorylation of Ser(1177) in eNOS decreased, whereas phosphorylation of Thr(495) increased, in the prenatally hypoxic pulmonary arteries, events that are related to eNOS activity. These data demonstrate that prenatal hypoxia results in persistent abnormalities in endothelium-dependent relaxation responses of pulmonary arteries in adult sheep due to decreased eNOS activity resulting from altered posttranslational regulation.

Regional heterogeneity of substance P-induced endothelium-dependent contraction, relaxation, and -independent contraction in rabbit pulmonary arteries

AIMS

The present study examined whether substance P (SP)-induced endothelium-dependent TXA(2)-mediated contraction (EDC), nitric oxide (NO)-mediated relaxation (EDR), and endothelium-independent contraction (EIC) are different between the rabbit proximal and distal intrapulmonary arteries.

MAIN METHODS

The helically cut strips of isolated proximal and distal arteries were fixed vertically between hooks in organ bath, and changes in isometric tension were measured.

KEY FINDINGS

SP-induced EDC was greater in the distal than proximal arteries, and EDR was greater in the proximal than distal arteries. However, under the complete blockade of NK(2) receptors and NO production, SP (10(-9)-3x10(-7) M)-induced EDC did not differ between proximal and distal arteries. Under the complete blockade of NK(2) receptors and TXA(2) production, SP (3x10(-10)-3x10(-8) M)-induced EDR was greater in the proximal than distal arteries. Neither contraction induced by U-46619, a TXA(2) agonist, nor relaxation by sodium nitroprusside, an NO donor, was different between both portions of the arteries. Both ionomycin (10(-8) M)- and l-arginine (1 mM)-induced EDRs were also significantly greater in the proximal than distal arteries. Under the blockade of NK(1) receptors and NO and TXA(2) production, SP (10(-7) M)-induced EIC was greater in the distal than proximal arteries. In summary, the capacity for NO production is higher in the proximal than distal arteries, resulting in SP-induced higher EDR and lower EDC in the proximal arteries.

SIGNIFICANCE

These regional differences in responses to SP may play important roles in maintaining the homogenous distribution of blood flow in the lung.

Contractile and relaxant properties of rat-isolated pulmonary veins related to localization and histology

The aim of this study was to investigate the in vitro vasomotor properties of rat extra-and intralobar pulmonary veins (PVs) related to their localization and to assess the modulatory role of endothelium on these properties. Segments from PVs were mounted in small vessel myograph and stretched at various diameters (D(10), D(20), D(30)) corresponding to intraluminal pressures of 10, 20 or 30 mmHg. At D(10) or D(20), contractile responses to phenylephrine, U46619 and angiotensin II of distal intralobar part of PVs were smaller compared with those of proximal extralobar part, but no longer different when distal part was stretched at D(30). When submitted to an NO donor, sodium nitroprusside, distal part of PV relaxed more strongly when stretched at D(30) compared with D(10). Acetylcholine and bradykinin were devoid of relaxing effect on distal parts stretched at D(10), but in contrast to acetylcholine, bradykinin slightly relaxed preparations stretched at D(30). Isoprenaline strongly relaxed PVs ( approximately 80% of initial precontraction), with the distal part exhibiting a higher sensitivity to the agonist compared with the proximal part. This relaxation was also observed with salbutamol and suppressed with ICI 118551, which is in favour of the involvement of beta(2)-adrenoceptors in this effect. Preincubation of the preparations with N(G)-nitro-l-arginine methyl ester (10(-4) m) and indomethacin (10(-5) m) did not modify the contractile responses to U46619, nor the relaxing response to isoprenaline, which support that endothelium does not appear to play a significant modulatory role in these responses. Histological and electron microscopical examinations of proximal and distal sections of the same vein show that the layers of smooth muscle cells and cardiomyocytes were thicker in the proximal compared with the distal part. This study shows that, because of morphological heterogeneity of the PVs, the site of dissection and the initial condition of tension can play a significant role upon the sensitivity and the magnitude of the responses to both contractile and relaxing agonists.

Propofol and thiopental attenuate adenosine triphosphate-sensitive potassium channel relaxation in pulmonary veins

Pulmonary veins (PV) make a significant contribution to total pulmonary vascular resistance. We investigated the cellular mechanisms by which the intravenous anesthetics propofol and thiopental alter adenosine triphosphate-sensitive potassium (KATP+) channel relaxation in canine PV. The effects of KATP+ channel inhibition (glybenclamide), cyclooxygenase inhibition (indomethacin), nitric oxide synthase inhibition (L-NAME), and L-type voltage-gated Ca2+ channel inhibition (nifedipine) on vasorelaxation responses to levcromakalim (KATP+ channel activator) alone and in combination with the anesthetics were assessed. The maximal relaxation response to levcromakalim was attenuated by removing the endothelium and by L-NAME, but not by indomethacin. Propofol (10(-5), 3x10(-5), and 10(-4) M) and thiopental (10(-4) and 3x10(-4) M) each attenuated levcromakalim relaxation in endothelium-intact (E+) rings, whereas propofol (3x10(-5) and 10(-4) M) and thiopental (3x10(-4) M) attenuated levcromakalim relaxation in endothelium-denuded (E-) rings. In E+ rings, the anesthesia-induced attenuation of levcromakalim relaxation was decreased after pretreatment with L-NAME but not with indomethacin. In E-strips, propofol (10(-4) M) and thiopental (3x10(-4) M) inhibited decreases in tension and intracellular Ca2+ concentration ([Ca2+]i) in response to levcromakalim, and these changes were abolished by nifedipine. These findings indicate that propofol and thiopental attenuate the endothelium-dependent component of KATP+ channel-induced PV vasorelaxation via an inhibitory effect on the nitric oxide pathway. Both anesthetics also attenuate the PV smooth muscle component of KATP+ channel-induced relaxation by reducing the levcromakalim-induced decrease in [Ca2+]i via an inhibitory effect on L-type voltage-gated Ca2+ channels.

Role of NO and prostaglandins in acute hypoxic vasoconstriction in sheep pulmonary veins

The aim of this study was to investigate the effect of hypoxia on and the role of nitric oxide (NO) and cyclooxgenase inhibition in hypoxia-induced vasoconstriction in sheep isolated pulmonary veins. We used the potent pulmonary vasoconstrictor U46619, a thromboxane analog, as a precontractile agent. Our results showed that hypoxia caused a vasoconstriction both under resting tone and in U46619 (10(-6) mol/l) precontracted pulmonary veins. In the presence of the nonselective NO synthase inhibitior Nomega-nitro-L-arginine methyl ester (L-NAME; 3 x 10(-5) mol/l), the hypoxic pulmonary vasoconstriction (HPV) was significantly increased in veins under resting force. However, there was a decrease in HPV in pulmonary veins precontracted with U46619 in the presence of L-NAME. Moreover, L-NAME markedly augmented the U46619-induced pulmonary contractions under normoxic conditions. Cyclooxygenase inhibition with indomethacin (10(-5) mol/l) significantly reduced the HPV both under resting tone and in precontracted veins. Indomethacin also significantly decreased the U46619-induced pulmonary contractions prior to the induction of hypoxia. Our findings suggest that NO and prostaglandins can act as a modulators of the hypoxic vasoconstriction in isolated pulmonary veins.

Role of NO and EDHF-mediated endothelial function in the porcine pulmonary circulation: Comparison between pulmonary artery and vein

OBJECTIVE

To compare electrophysiological measurement of nitric oxide (NO) release and endothelium-derived hyperpolarizing factor (EDHF)-mediated endothelial function in porcine pulmonary arteries and veins.

METHODS

Isolated pulmonary interlobular arteries (PA) and veins (PV) were obtained from a local slaughterhouse. By using a NO-specific electrode and a conventional intracellular microelectrode, the amount of NO released from endothelial cells and hyperpolarization of smooth muscle cells were investigated. The bradykinin (BK)-induced relaxation in the precontraction by U(46619) was examined in the absence or presence of N(G)-nitro-l-arginine (l-NNA), indomethacin (INDO) plus oxyhemoglobin (HbO).

RESULTS

The basal release of NO was 7.0+/-1.2 nmol/L in PA (n=8) and 5.5+/-1.6 nmol/L in PV (n=8, p<0.01). BK-induced release of NO was 160.4+/-10.3 nmol/L in PA (n=8) and 103.0+/-14.7 nmol/L in PV (n=8, p<0.001) with longer releasing duration in PA than in PV (14.3+/-1.3 vs. 12.1+/-0.8 min, p<0.01). BK evoked an endothelium-dependent hyperpolarization and relaxation that were reduced by l-NNA, INDO, and HbO (hyperpolarization: 12.8+/-1.3 vs. 8.0+/-1.4 mV in PA, n=6, p<0.001 and 8.3+/-1.4 vs. 3.0+/-0.8 mV in PV, n=6, p<0.001; relaxation: 92.8+/-3.1% vs. 19.6+/-11.1% in PA n=8, p<0.001 and 70.3+/-7.9% vs. 6.0+/-6.8% in PV, n=8, p<0.001). Both hyperpolarization (8.0+/-1.4 vs. 3.0+/-0.8 mV, p<0.001) and relaxation (19.6+/-11.1% vs. 6.0+/-6.8%, p<0.01) were greater in PA than in PV.

CONCLUSIONS

Both NO and EDHF play an important role in regulation of porcine pulmonary arterial and venous tones. The more significant role of NO and EDHF is revealed in pulmonary arteries than in veins.

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.

Cellular mechanisms of thromboxane A2-mediated contraction in pulmonary veins

Our objectives were to identify the relative contributions of [Ca2+]i and myofilament Ca2+ sensitivity in the pulmonary venous smooth muscle (PVSM) contractile response to the thromboxane A2 mimetic U-46619 and to assess the roles of PKC, tyrosine kinases (TK), and Rho-kinase (ROK) in that response. We tested the hypothesis that U-46619-induced contraction in PVSM is mediated by both increases in [Ca2+]i and myofilament Ca2+ sensitivity and that the PKC, TK, and ROK signaling pathways are involved. Isometric tension was measured in isolated endothelium-denuded (E-) canine pulmonary venous (PV) rings. In addition, [Ca2+]i and tension were simultaneously measured in fura-2-loaded E- PVSM strips. U-46619 (0.1 nM-1 microM) caused dose-dependent (P < 0.001) contraction in PV rings. U-46619 contraction was attenuated by inhibitors of L-type voltage-operated Ca2+ channels (nifedipine, P < 0.001), inositol 1,4,5-trisphosphate-mediated Ca2+ release (2-aminoethoxydiphenylborate, P < 0.001), PKC (bisindolylmaleimide I, P < 0.001), TK (tyrphostin A-47, P = 0.014), and ROK (Y-27632, P = 0.008). In PV strips, U-46619 contraction was associated with increases in [Ca2+]i and myofilament Ca2+ sensitivity. Both Ca2+ influx and release mediated the early transient increase in [Ca2+]i, whereas the late sustained increase in [Ca2+]i only involved Ca2+ influx. Inhibition of both PKC and ROK (P = 0.006 and P = 0.002, respectively), but not TK, attenuated the U-46619-induced increase in myofilament Ca2+ sensitivity. These results suggest that U-46619 contraction is mediated by Ca2+ influx, Ca2+ release, and increased myofilament Ca2+ sensitivity. The PKC, TK, and ROK signaling pathways are involved in U-46619 contraction.

Role of veins in regulation of pulmonary circulation

Pulmonary veins have been seen primarily as conduit vessels; however, over the past two decades, a large amount of evidence has accumulated to indicate that pulmonary veins can exhibit substantial vasoactivity. In this review, the role of veins in regulation of the pulmonary circulation, particularly during the perinatal period and under certain pathophysiological conditions, is discussed. In the fetus, pulmonary veins contribute a significant fraction to total pulmonary vascular resistance. At birth, the veins as well as the arteries relax in response to endothelium-derived nitric oxide and dilator prostaglandins, thereby assisting in the fall in pulmonary vascular resistance. These effects are oxygen dependent and modulated by cGMP-dependent protein kinase. Under chronic hypoxic conditions, pulmonary veins undergo remodeling and demonstrate substantial constriction and hypertrophy. In a number of species, including the human, pulmonary veins are also the primary sites of action of certain vasoconstrictors such as endothelin and thromboxane. In various pathological conditions, there is an increased synthesis of these vasoactive agents that may lead to pulmonary venous constriction, increased microvascular pressures for fluid filtration, and formation of pulmonary edema. In conclusion, the significant role of veins in regulation of the pulmonary circulation needs to be appreciated to better prevent, diagnose, and treat lung disease.

Involvement of tyrosine kinase pathway in acute hypoxic vasoconstriction in sheep isolated pulmonary vein

Tyrosine kinase pathway has been shown to be involved in the effects of hypoxia in pulmonary arteries, but its role in pulmonary vein is not known. The aims of this study were to determine the effect of hypoxia in sheep isolated pulmonary veins and to identify the role of tyrosine kinase pathway in hypoxic response. Genistein and tyrphostin were used as selective tyrosine kinase inhibitors, and sodium orthovanadate was administered for tyrosine kinase activation. Hypoxia (95% N(2) to 5% CO(2)) caused a vasoconstriction either under resting tone or in U46619-precontracted pulmonary veins. Genistein and tyrphostin inhibited hypoxia-induced vasoconstriction both under resting tone and in precontracted veins, while sodium orthovanadate increased these hypoxic contractions. Our findings suggest that tyrosine kinase pathway is involved in hypoxic pulmonary vasoconstriction in sheep isolated pulmonary vein rings.

Prostacyclin release and receptor activation: differential control of human pulmonary venous and arterial tone

1. In human pulmonary vascular preparations, precontracted arteries were more sensitive to the relaxant effect of acetylcholine (ACh) than veins (pD(2) values: 7.25+/-0.08 (n=23) and 5.92+/-0.09 (n=25), respectively). Therefore, the role of prostacyclin (PGI(2)) was explored to examine whether this mediator may be responsible for the difference in relaxation. 2. In the presence of the cyclooxygenase (COX) inhibitor, indomethacin (INDO), the ACh relaxations were reduced in arteries but not in veins. On the contrary, an inhibitor (l-NOARG) of the nitric oxide synthase blocked preferentially the relaxation in veins. 3. A greater release of 6-keto-PGF(1alpha), the stable metabolite of PGI(2), was observed in arterial preparations than in venous preparations when stimulated with either ACh or arachidonic acid (AA). 4. Exogenous PGI(2) produced a reduced relaxant effect in the precontracted vein when compared with the artery. In the presence of the EP(1)-receptor antagonist AH6809, the PGI(2) relaxation of veins was similar to arteries. 5. In veins, AA (0.1 mm) produced a biphasic response, namely, a contraction peak (0.4-0.5 g) followed by a relaxation. These contractions in venous preparations were abolished either in the absence of endothelium or in the presence of INDO or an EP(1)-receptor antagonist (AH6809, SC19220). In the arterial preparations AA induced only relaxations. 6. In both vascular preparations, COX-1 but not the COX-2 protein was detected in microsomal preparations derived from homogenized tissues or freshly isolated endothelial cells. 7. The differential vasorelaxations induced by ACh may be explained, in part, by a more pronounced production and release of PGI(2) in human pulmonary arteries than in the veins. In addition, while PGI(2) induced relaxation by activation of IP-receptors in both types of vessels, a PGI(2) constrictor effect was responsible for masking the relaxation in the veins by activation of the EP(1)-receptor.

Discrepant distribution of big endothelin (ET)-1 and ET receptors in the pulmonary artery

Since pulmonary vasculature is complex in terms of regional difference in structure and function, it is important to understand the site of endothelin (ET) synthesis and the distribution of the ET system along the axial pathways of pulmonary artery. The expression of big ET-1, ET converting enzyme (ECE) and ET(A) receptors were examined in rat pulmonary arteries under normal and hypoxic conditions using an immunohistochemical method and Northern blot analysis. In normal conditions, big ET-1 was expressed in the intima and media of pulmonary arteries with a predominant distribution in the distal segments and a preferential localization in the media, while ETA receptors were dominantly expressed in the proximal segments. ECE was constitutively expressed in the intima and media. Following exposure to hypoxia, messenger ribonucleic acid (mRNA) expression of ET-1 and ET(A) receptors were up-regulated by two-fold and immunoreactivities for big ET-1, ECE, and ET(A) receptors significantly increased by two to five-fold in the distal segments. Smooth muscle cells are an important source of endothelin-1 in the pulmonary artery. The distribution of big endothelin-1 and endothelin A receptors in pulmonary arteries was discrepant in normal conditions while their expression concomitantly increased in the distal segments in hypoxic conditions. This heterogeneity may play an important role in the regulation of pulmonary vascular tone.

Role of NO pathway, calcium and potassium channels in the peripheral pulmonary vascular tone in dogs

Because hypoxic pulmonary vasoconstriction occurs mainly in the small pulmonary arteries, the authors investigated the effects of drugs acting on the nitric oxide (NO) pathway and the calcium and potassium channels in the peripheral pulmonary circulation, without interference with the overall pulmonary or systemic circulation. Mixed venous blood was infused in wedged areas to study the pressure/flow relationship and to compute peripheral pulmonary vascular resistance (PPVR). The authors studied the effects of Nomega-nitro-L-arginine methyl ester (L-NAME), an NO synthase inhibitor, sodium nitroprusside (SNP, an NO donor), the calcium channel blockers verapamil, nifedipine and nicardipine, and the potassium channel opener levcromakalim, during normoxia and acute mild normocapnic hypoxia. In the peripheral pulmonary circulation, L-NAME caused an increase in PPVR during normoxia (+95%; p<0.001) and hypoxia (+60%; p<0.01). Following the increase by L-NAME, SNP decreased PPVR during normoxia (-24%; p<0.05) and hypoxia (-23%; p<0.05). Verapamil, nifedipine and nicardipine did not modify PPVR during normoxia but during hypoxia they decreased PPVR (-28%, nonsignificant; -27%, p<0.01 and -33%, p<0.05, respectively). Levcromakalim did not modify PPVR during normoxia or hypoxia. In conclusion, the nitric oxide pathway and voltage-dependent calcium channels, and not adenosine triphosphate sensitive potassium channels, play an important role in the control of peripheral pulmonary circulation in dogs.

The role of nitric oxide in the responses of the ovine digital artery to vasoactive agents and modification of these responses by endotoxin and cytokines

1. Laminitis, an important cause of lameness in domestic ungulates, occurs as a result of altered digital perfusion. Endotoxin and cytokines may mediate the vascular derangements observed through alterations in nitric oxide production. In this study, the vascular responses of the isolated ovine digital artery were examined and the influence of endotoxin and cytokines investigated. 2. Neither removal of the endothelium nor incubation with N(G)-nitro-L-arginine methyl ester (L-NAME, 300 microM) altered the response to phenylephrine (PE, 1 nM to 300 microM). Indomethacin (10 microM) decreased PE log EC(50) from -6.22+/-0.08 to -6.55+/-0.07. Acetylcholine (1 nM to 1 mM) and bradykinin (BK, 100 pM to 3 microM) induced endothelium-dependent relaxation. Bradykinin-induced relaxation was reduced by L-NAME, E(max) falling from -61.7+/-7.4 to -34.0+/-2.1%. Addition of indomethacin further reduced BK E(max) to -9.6+/-2.8%. Sodium nitroprusside (1 nM to 300 microM) produced endothelium-independent relaxation that was unaffected by L-NAME or indomethacin. 3. Following a 6 h incubation with endotoxin (3 microml(-1)), arterial responses to PE and BK did not differ from polymyxin B-treated controls (10 microg ml(-1)). Arteries incubated for 6 h with interferon-gamma (IFN-gamma, 10 ng ml(-1)) and tumour necrosis factor-alpha (TNF-alpha, 5 ng ml(-1)) exhibited greater relaxation to BK (E(max)-50.0+/-5.1%) than polymyxin B-treated controls (E(max)-33.1+/-4.0%), but did not differ in their response to PE. 4. Prolonged incubation (16 h) with endotoxin (3 microg ml(-1)) did not alter the response to PE, however incubation with IFN-gamma (10 ng ml(-1)), TNF-alpha (5 ng ml(-1)) and interleukin-1beta (20 ng ml(-1)) for 16 h increased PE log EC(50) from -6.44+/-0.09 to -6. 10+/-0.11. 5. Nitric oxide is an important mediator of endothelium-dependent relaxation in ovine digital arteries but does not modulate PE-induced vasoconstriction. Incubation with cytokines decreased the sensitivity of digital arteries to PE.