Hypoxia-inducible factors activator, roxadustat, increases pulmonary vascular resistance in rats

Activators of hypoxia inducible factors (HIFs), such as roxadustat, are promising agents for anemia treatment. However, since HIFs are also involved in the regulation of the pulmonary circulation, we hypothesized that roxadustat increases pulmonary vascular resistance and vasoconstrictor reactivity. Using isolated, cell-free solution perfused rat lungs, we found perfusion pressure-flow curves to be shifted to higher pressures by 2 weeks of roxadustat treatment (10 mg/kg every other day), although not as much as by chronic hypoxic exposure. Vasoconstrictor reactivity to angiotensin II and acute hypoxic challenges was not altered by roxadustat. Since roxadustat may inhibit angiotensin-converting enzyme 2 (ACE2), we also tested a purported ACE2 activator, diminazene aceturate (DIZE, 0.1 mM). It produced paradoxical, unexplained pulmonary vasoconstriction. We conclude that the risk of serious pulmonary hypertension is not high when roxadustat is given for 14 days, but monitoring is advisable.

Simvastatin and dehydroepiandrosterone sulfate effects against hypoxic pulmonary hypertension are not additive

Pulmonary hypertension is a group of disorders characterized by elevated mean pulmonary artery pressure (mPAP) and pulmonary vascular resistance. To test our hypothesis that combining two drugs useful in experimental pulmonary hypertension, statins and dehydroepiandrosterone sulfate (DHEA S), is more effective than either agent alone, we induced pulmonary hypertension in adult male rats by exposing them to hypoxia (10%O2) for 3 weeks. We treated them with simvastatin (60 mg/l) and DHEA S (100 mg/l) in drinking water, either alone or in combination. Both simvastatin and DHEA S reduced mPAP (froma mean±s.d. of 34.4±4.4 to 27.6±5.9 and 26.7±4.8 mmHg, respectively), yet their combination was not more effective (26.7±7.9 mmHg). Differences in the degree of oxidative stress (indicated by malondialdehydeplasma concentration),the rate of superoxide production (electron paramagnetic resonance), or blood nitric oxide levels (chemiluminescence) did not explain the lack of additivity of the effect of DHEA S and simvastatin on pulmonary hypertension. We propose that the main mechanism of both drugs on pulmonary hypertension could be their inhibitory effect on 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase, which could explain their lack of additivity.

Alcohol and fetoplacental vasoconstrictor reactivity

Alcohol abuse during pregnancy is a well-known factor in fetal morbidity, including smaller fetal size. We have shown that chronic hypoxia, considered the main pathogenetic factor in intrauterine growth restriction, elevates fetoplacental vascular resistance (and vasoconstrictor reactivity) and thus, presumably, reduces placental blood flow. We thus hypothesized that alcohol may affect the fetus - in addition to other mechanisms - by altering fetoplacental vascular resistance and/or reactivity. Using isolated, double-perfused rat placenta model, we found that maternal alcohol intake in the last third of gestation doubled the vasoconstrictor responses to angiotensin II but did not affect resting vascular resistance. Reactivity to acute hypoxic challenges was unchanged. Chronic maternal alcohol intake in a rat model alters fetoplacental vasculature reactivity; nevertheless, these changes do not appear as serious as other detrimental effects of alcohol on the fetus.

Nitric oxide elevation in polytrauma is driven by oxygen radicals

A common problem in management of polytrauma - a simultaneous injury to more than one organ or organ system, at least one of them lethal without intervention - is a discrepancy between a relatively good initial state and a serious subsequent development. Since nitric oxide (NO) is produced in high quantities during tissue injury, we assumed that serum levels of NO (and its oxidation products, NOx) might serve as a prognostic marker of polytrauma severity. However, we found recently that NOx was increased in polytrauma, but not in the most severe cases. The present study was undertaken to test the hypothesis that serum NOx is reduced in severe polytrauma by concomitant overproduction of reactive oxygen species (ROS). Polytrauma was induced in rats under anesthesia by bilateral fracture of femurs and tibiae plus incision of the right liver lobe through laparotomy. Serum NOx was measured by chemiluminescence after hot acidic reduction. The role of ROS was assessed by treatment with an antioxidant, N-acetyl-L-cysteine (NAC). Experimental polytrauma elevated NOx from 11.0+/-0.7 to 23.8+/-4.5 ppb. This was completely prevented by NAC treatment (9.1+/-2.2 ppb). Serum NOx is elevated in severe polytrauma, and this is not reduced by ROS. On the contrary, ROS are necessary for the NOx elevation, probably because ROS produced by inflammatory cells activated by the polytrauma induce massive NO production.

Chronic hypoxia increases fetoplacental vascular resistance in rat placenta perfused with blood

BACKGROUND

Chronic hypoxia elevates vascular resistance on the fetal side of the placenta. However, when a low-viscosity perfusate is used, the increase in resistance caused by chronic hypoxia is relatively small (12 mmHg). In the present study, we tested the hypothesis that perfusion with more viscous fluid (blood) will reveal more substantial effect of chronic hypoxia.

METHODS

Using an isolated, dually perfused rat placenta perfused at a constant flow rate with homologous blood, perfusion pressure on the fetal side was measured. Then, the relationship between perfusion pressure and flow (P/Q) was determined.

RESULTS

Fetoplacental vascular resistance was increased by chronic hypoxia (10 % O2) during the last third of gestation. This was observed when the placentas were perfused with a low viscosity Krebs solution and more enhanced when perfused with blood. Nevertheless, we found no clear advantage for using blood instead of Krebs solution to study the effects of hypoxia on the fetoplacental vasculature. The elevation of fetoplacental vascular resistance caused by chronic hypoxia was at least partly resistant to acute reoxygenation.

CONCLUSION

Using blood for the perfusion of the isolated rat placenta does not confer any clear methodological advantage over using Krebs solution (Tab. 2, Fig. 2, Ref. 21).

Nitric oxide as an indicator for severity of injury in polytrauma

BACKGROUND

Patients with injuries to multiple organs or organ systems are in a serious risk of shock, multiorgan failure and death. Although there are scoring systems available to assess the extent of polytrauma and guide the prognosis, their usefulness is limited by their considerably subjective nature. As the production of nitric oxide (NO) by many cell types is elevated in tissue injury, we hypothesized that serum concentration of NO (and its oxidation products, NOx) represents a suitable marker of polytrauma correlating with prognosis. We wanted to prove that nitric oxide could serve as an indicator for severity of injury in polytrauma.

METHODS

We measured serum NOx and standard biochemical parameters in 93 patients with various degrees of polytrauma, 15 patients with minor injuries and 20 healthy volunteers.

RESULTS

On admission, serum NOx was higher in patients with moderate polytrauma than both in controls and patients with minor injury, and it was even higher in patients with severe polytrauma. Surprisingly, NOx on admission was normal in the group of patients that required cardiopulmonary resuscitation or died within 48 hours after admission. In the groups, where it was elevated on admission, serum NOx dropped to normal values within 12 hours. Blood lactate levels on admission were elevated in proportion to the severity of subsequent clinical course.

CONCLUSION

Elevated serum NOx and blood lactate in patients with polytrauma are markers of serious clinical course, while normal NOx combined with a very high lactate may signal a fatal prognosis (Fig. 4, Ref. 8).

Depletion of alveolar macrophages attenuates hypoxic pulmonary hypertension but not hypoxia-induced increase in serum concentration of MCP-1

Exposure to hypoxia, leading to hypoxic pulmonary hypertension (HPH), is associated with activation of alveolar macrophages (AM). However, it remains unclear how AM participate in this process. There are studies which imply that the AM product monocyte chemoattractant protein-1 (MCP-1) plays an important role. Thus we tested: 1. if the selective elimination of AM attenuates HPH in rats, 2. the correlation of MCP-1 plasmatic concentrations with the presence and absence of AM during exposure to hypoxia, 3. the direct influence of hypoxia on MCP-1 production in isolated AM. We found that experimental depletion of AM attenuated the chronic hypoxia-induced increase in mean pulmonary arterial pressure, but did not affect the serum MCP-1 concentrations. Furthermore, the MCP-1 production by AM in vitro was unaffected by hypoxia. Thus we conclude that AM play a significant role in the mechanism of HPH, but MCP-1 release from these cells is most likely not involved in this process. The increase of MCP-1 accompanying the development of HPH probably originates from other sources than AM.

Combined suppression of the intrarenal and circulating vasoconstrictor renin-ACE-ANG II axis and augmentation of the vasodilator ACE2-ANG 1-7-Mas axis attenuates the systemic hypertension in Ren-2 transgenic rats exposed to chronic hypoxia

The aim of the present study was to test the hypothesis that chronic hypoxia would aggravate hypertension in Ren-2 transgenic rats (TGR), a well-defined monogenetic model of hypertension with increased activity of endogenous renin-angiotensin system (RAS). Systolic blood pressure (SBP) in conscious rats and mean arterial pressure (MAP) in anesthetized TGR and normotensive Hannover Sprague-Dawley (HanSD) rats were determined under normoxia that was either continuous or interrupted by two weeks´ hypoxia. Expression, activities and concentrations of individual components of RAS were studied in plasma and kidney of TGR and HanSD rats under normoxic conditions and after exposure to chronic hypoxia. In HanSD rats two weeks´ exposure to chronic hypoxia did not alter SBP and MAP. Surprisingly, in TGR it decreased markedly SBP and MAP; this was associated with substantial reduction in plasma and kidney renin activities and also of angiotensin II (ANG II) levels, without altering angiotensin-converting enzyme (ACE) activities. Simultaneously, in TGR the exposure to hypoxia increased kidney ACE type 2 (ACE2) activity and angiotensin 1-7 (ANG 1-7) concentrations as compared with TGR under continuous normoxia. Based on these results, we propose that suppression of the hypertensiogenic ACE-ANG II axis in the circulation and kidney tissue, combined with augmentation of the intrarenal vasodilator ACE2-ANG 1-7 axis, is the main mechanism responsible for the blood pressure-lowering effects of chronic hypoxia in TGR.

Rho-kinase inhibition attenuates acute hypoxic fetoplacental vasoconstriction in the rat

The vessels on the fetal side of the placenta differ from most other vascular beds except the lungs in that they respond to acute hypoxia by vasoconstriction. An essential role of calcium influx in the mechanism of this hypoxic fetoplacental vasoconstriction (HFPV) has been shown previously. That finding does not, however, exclude the possible involvement of other mechanisms of vascular tone regulation. In this study we tested the hypothesis that Rho-kinase-mediated calcium sensitization is involved in HFPV. We used a model of isolated rat placenta dually perfused (from both the maternal and fetal side) with Krebs salt solution saturated with normoxic and hypoxic gas mixture respectively at constant flow rate. Rho-kinase pathway was inhibited by fasudil (10 microM). We found that fasudil reduced basal normoxic fetoplacental vascular resistance and completely prevented HFPV. This suggests that the activity of Rho-kinase signaling pathway is essential for HFPV.

Regulation of fetoplacental vascular bed by hypoxia

Important fetal and perinatal pathologies, especially intrauterine growth restriction (IUGR), are thought to stem from placental hypoxia-induced vasoconstriction of the fetoplacental vessels, leading to placental hypoperfusion and thus fetal undernutrition. However, the effects of hypoxia on the fetoplacental vessels have been surprisingly little studied. We review here available experimental data on acute hypoxic fetoplacental vasoconstriction (HFPV) and on chronic hypoxic elevation of fetoplacental vascular resistance. The mechanism of HFPV includes hypoxic inhibition of potassium channels in the plasma membrane of fetoplacental vascular smooth muscle and consequent membrane depolarization that activates voltage gated calcium channels. This in turn causes calcium influx and contractile apparatus activation. The mechanism of chronic hypoxic elevation of fetoplacental vascular resistance is virtually unknown except of signs of the involvement of morphological remodeling.

Hypercapnia attenuates hypoxic pulmonary hypertension by inhibiting lung radical injury

Chronic lung hypoxia results in hypoxic pulmonary hypertension. Concomitant chronic hypercapnia partly inhibits the effect of hypoxia on pulmonary vasculature. Adult male rats exposed to 3 weeks hypoxia (Fi(02)=0.1) combined with hypercapnia (Fi(C02)=0.04-0.05) had lower pulmonary arterial blood pressure, increased weight of the right heart ventricle, and less pronounced structural remodeling of the peripheral pulmonary arteries compared with rats exposed only to chronic hypoxia (Fi(02)=0.1). According to our hypothesis, hypoxic pulmonary hypertension is triggered by hypoxic injury to the walls of the peripheral pulmonary arteries. Hypercapnia inhibits release of both oxygen radicals and nitric oxide at the beginning of exposure to the hypoxic environment. The plasma concentration of nitrotyrosine, the marker of peroxynitrite activity, is lower in hypoxic rats exposed to hypercapnia than in those exposed to hypoxia alone. Hypercapnia blunts hypoxia-induced collagenolysis in the walls of prealveolar pulmonary arteries. We conclude that hypercapnia inhibits the development of hypoxic pulmonary hypertension by the inhibition of radical injury to the walls of peripheral pulmonary arteries.

Voltage-gated calcium channels mediate hypoxic vasoconstriction in the human placenta

Unlike all vascular beds with the exception of the pulmonary circulation, fetoplacental vessels respond to acute hypoxia with vasoconstriction. While this hypoxic fetoplacental vasoconstriction (HFPV) is considered essential in the pathogenesis of intrauterine growth retardation, its mechanism is largely unknown. Hypoxia inhibits potassium channels and thus causes depolarization in fetoplacental vascular smooth muscle. We propose that this hypoxia-induced depolarization leads to vasoconstriction by activating voltage-dependent calcium (Ca) channels and Ca influx. We compared HFPV between isolated perfused human cotyledons treated with an inhibitor of L-type channels, nifedipine, and preparations receiving only vehicle. While the solvent (diluted DMSO) had no inhibitory effect on HFPV, the hypoxic responses were completely abolished even by a relatively low dose of nifedipine (1 nM). We conclude that activation of L-type Ca channels is an essential part of HFPV.

Validity reassessment of Trichobilharzia species using Lymnaea stagnalis as the intermediate host

The systematics within the genus Trichobilharzia is complicated. After the description of the type species Trichobilharzia ocellata, the name was routinely used for nearly all European findings of ocellate furcocercariae. T. ocellata was also described from North America and Japan. However, the identity of T. ocellata remains questionable. Comparison of data from the literature showed differences among various T. ocellata isolates and led us to the conclusion that the North American and the Japanese findings are not identical with European T. ocellata. In addition, the description of T. szidati corresponds with the recently reported European T. ocellata isolates. Sequence analysis of the ITS region confirmed that they are identical.

Dehydroepiandrosterone sulphate reduces chronic hypoxic pulmonary hypertension in rats

Pathogenesis of pulmonary hypertension includes vascular smooth muscle cell membrane depolarisation and consequent calcium influx. Usually, calcium-gated potassium channels are activated under such conditions and repolarise the membrane. However, in pulmonary hypertension they are downregulated. The authors hypothesised that pharmacological augmentation of these channels would reduce pulmonary hypertension. Dehydroepiandrosterone sulphate (DHEA-S, 0.1 mg x mL(-1)), a recently characterised activator of calcium-gated potassium channels, was given to rats in drinking water. Pulmonary arterial blood pressure, increased by 4 weeks of hypoxia (from 15 +/- 0.2 to 29.4 +/- 2.5 mmHg), was selectively attenuated in rats treated with DHEA-S for the whole duration of the hypoxic exposure (23.9 +/- 0.9 mmHg) and in rats given DHEA-S only after pulmonary hypertension had fully developed (last 2 weeks of hypoxia; 24.4 +/- 1.4 mmHg). Pulmonary vascular remodelling and right ventricular hypertrophy associated with pulmonary hypertension were also reduced by DHEA-S. Cardiac index and systemic arterial blood pressure did not differ among the groups. The authors conclude that treatment with an activator of calcium-gated potassium channels, dehydroepiandrosterone sulphate, known to be well tolerated by humans, reduces hypoxic pulmonary hypertension in rats.

Comparison of european Trichobilharzia species based on ITS1 and ITS2 sequences

Schistosomes are parasites of considerable medical and veterinary importance and, therefore, all aspects of their biology have been intensively studied. In contrast, our knowledge of species of the largest genus, Trichobilharzia, is insufficient. Because morphological characterization of Trichobilharzia species provides a limited number of criteria for species determination, molecular data are required. In the present paper, we sequenced internal transcribed spacers ITS1 and ITS2, and 5.8S ribosomal RNA (rRNA) genes of 3 European Trichobilharzia species (T. regenti, T. szidati and T. franki). We showed that ITS1 and ITS2 sequences can be used in species identification. Repetitive elements were found in ITS1 of all 3 Trichobilharzia species; their number and length varied depending on the species. Phylogenetic analysis showed that the visceral T. franki is more related to the nasal T. regenti, than to the visceral T. szidati. The newly designed primer, which is specific for T. regenti, might be used as a tool for diagnosis of this potential pathogen.

Hyperoxia and recovery from hypoxia alter collagen in peripheral pulmonary arteries similarly

Chronic hypoxia causes pulmonary hypertension, the mechanism of which includes altered collagen metabolism in the pulmonary vascular wall. This chronic hypoxic pulmonary hypertension is gradually reversible upon reoxygenation. The return to air after the adjustment to chronic hypoxia resembles in some aspects a hyperoxic stimulus and we hypothesize that the changes of extracellular matrix proteins in peripheral pulmonary arteries may be similar. Therefore, we studied the exposure to moderate chronic hyperoxia (FiO2 = 0.35, 3 weeks) in rats and compared its effects on the rat pulmonary vasculature to the effects of recovery (3 weeks) from chronic hypoxia (FiO2 = 0.1, 3 weeks). Chronically hypoxic rats had pulmonary hypertension (Pap = 26 +/- 3 mm Hg, controls 16 +/- 1 mm Hg) and right ventricular hypertrophy. Pulmonary arterial blood pressure and right ventricle weight normalized after 3 weeks of recovery in air (Pap = 19 +/- 1 mm Hg). The rats exposed to moderate chronic hyperoxia also did not have pulmonary hypertension (Pap = 18 +/- 1 mm Hg, controls 17 +/- 1 mm Hg). Collagenous proteins isolated from the peripheral pulmonary arteries (100-300 microm) were studied using polyacrylamide gel electrophoresis. A dominant low molecular weight peptide (approx. 76 kD) was found in hypoxic rats. The proportion of this peptide decreases significantly in the course of recovery in air. In addition, another larger peptide doublet was found in rats recovering from chronic hypoxia. It was localized in polyacrylamide gels close to the zone of alpha2 chain of collagen type I. It was bound to anticollagen type I antibodies. An identically localized peptide was found in rats exposed to moderate chronic hyperoxia. The apparent molecular weight of this collagen fraction suggests that it is a product of collagen type I cleavage by a rodent-type interstitial collagenase (MMP-13). We conclude that chronic moderate hyperoxia and recovery from chronic hypoxia have a similar effect on collagenous proteins of the peripheral pulmonary arterial wall.

Fenfluramine-induced pulmonary vasoconstriction: role of serotonin receptors and potassium channels

The anorexic agent fenfluramine considerably increases the risk of primary pulmonary hypertension. The mechanism of this effect is unknown. The appetite-reducing action of fenfluramine is mediated by its interaction with the metabolism of serotonin [5-hydroxytryptamine (5-HT)] in the brain. We tested the hypothesis that the pulmonary vasoconstrictive action of fenfluramine is at least in part mediated by 5-HT receptor activation. In addition, we sought to determine whether pharmacological reduction of voltage-gated potassium (K(V)) channel activity would potentiate the pulmonary vascular reactivity to fenfluramine. Using isolated rat lungs perfused with Krebs-albumin solution, we compared the inhibitory effect of ritanserin, an antagonist of 5-HT(2) receptors, on fenfluramine- and 5-HT-induced vasoconstriction. Both 5-HT (10(-5) mol/l) and fenfluramine (5 x 10(-4) mol/l) caused significant increases in perfusion pressure. Ritanserin at a dose (10(-7) mol/l) sufficient to inhibit >80% of the response to 5-HT reduced the response to fenfluramine by approximately 50%. A higher ritanserin dose (10(-5) mol/l) completely abolished the responses to 5-HT but had no more inhibitory effect on the responses to fenfluramine. A pharmacological blockade of K(V) channels by 4-aminopyridine (3 x 10(-3) mol/l) markedly potentiated the pulmonary vasoconstrictor response to fenfluramine but was without effect on the reactivity to 5-HT. These data indicate that the pulmonary vasoconstrictor response to fenfluramine is partly mediated by 5-HT receptors. Furthermore, the pulmonary vasoconstrictor potency of fenfluramine is elevated when the K(V)-channel activity is low. This finding suggests that preexisting K(V)-channel insufficiency may predispose some patients to the development of pulmonary hypertension during fenfluramine treatment.

Construction and bootstrap analysis of DNA fingerprinting-based phylogenetic trees with the freeware program FreeTree: application to trichomonad parasites

The Win95/98/NT program FreeTree for computation of distance matrices and construction of phylogenetic or phenetic trees on the basis of random amplified polymorphic DNA (RAPD), RFLP and allozyme data is presented. In contrast to other similar software, the program FreeTree (available at http://www.natur.cuni.cz/~flegr/programs/freetree or http://ijs.sgmjournals.org/content/vol51/issue3/) can also assess the robustness of the tree topology by bootstrap, jackknife or operational taxonomic unit-jackknife analysis. Moreover, the program can be also used for the analysis of data obtained in several independent experiments performed with non-identical subsets of taxa. The function of the program was demonstrated by an analysis of RAPD data from 42 strains of 10 species of trichomonads. On the phylogenetic tree constructed using FreeTree, the high bootstrap values and short terminal branches for the Tritrichomonas foetus/suis 14-strain branch suggested relatively recent and probably clonal radiation of this species. At the same time, the relatively lower bootstrap values and long terminal branches for the Trichomonas vaginalis 20-strain branch suggested more ancient radiation of this species and the possible existence of genetic recombination (sexual reproduction) in this human pathogen. The low bootstrap values and the star-like topology of the whole Trichomonadidae tree confirm that the RAPD method is not suitable for phylogenetic analysis of protozoa at the level of higher taxa. It is proposed that the repeated bootstrap analysis should be an obligatory part of any RAPD study. It makes it possible to assess the reliability of the tree obtained and to adjust the amount of collected data (the number of random primers) to the amount of phylogenetic signals in the RAPD data of the taxon analysed. The FreeTree program makes such analysis possible.

Perinatal history of hypoxia leads to lower vascular pressures and hyporeactivity to angiotensin II in isolated lungs of adult rats

The most dramatic changes in pulmonary circulation occur at the time of birth. We hypothesized that some of the effects of perinatal hypoxia on pulmonary vessels are permanent. We studied the consequences of perinatal exposure to hypoxia (12 % O2 one week before and one week after birth) in isolated lungs of adult male rats (approximately 12 weeks old) perfused with homologous blood. Perfusion pressure-flow relationship was tilted towards lower pressures in the perinatally hypoxic as compared to the control, perinatally normoxic rats. A non-linear, distensible vessel model analysis revealed that this was due to increased vascular distensibility in perinatally hypoxic rats (4.1 +/- 0.6 %/mm Hg vs. 2.3 +/- 0.4 %/mm Hg in controls, P = 0.03). Vascular occlusion techniques showed that lungs of the perinatally hypoxic rats had lower pressures at both the pre-capillary and post-capillary level. To assess its role, basal vascular tone was eliminated by a high dose of sodium nitroprusside (20 microM). This reduced perfusion pressures only in the lungs of rats born in hypoxia, indicating that perinatal hypoxia leads to a permanent increase in the basal tone of the pulmonary vessels. Pulmonary vasoconstrictor reactivity to angiotensin II (0.1-0.5 microg) was reduced in rats with the history of perinatal-hypoxia. These data show that perinatal hypoxia has permanent effects on the pulmonary circulation that may be beneficial and perhaps serve to offset the previously described adverse consequences.

Role of nitric oxide in the pathogenesis of chronic pulmonary hypertension

Chronic pulmonary hypertension is a serious complication of a number of chronic lung and heart diseases. In addition to vasoconstriction, its pathogenesis includes injury to the peripheral pulmonary arteries leading to their structural remodeling. Increased pulmonary vascular synthesis of an endogenous vasodilator, nitric oxide (NO), opposes excessive increases of intravascular pressure during acute pulmonary vasoconstriction and chronic pulmonary hypertension, although evidence for reduced NO activity in pulmonary hypertension has also been presented. NO can modulate the degree of vascular injury and subsequent fibroproduction, which both underlie the development of chronic pulmonary hypertension. On one hand, NO can interrupt vascular wall injury by oxygen radicals produced in increased amounts in pulmonary hypertension. NO can also inhibit pulmonary vascular smooth muscle and fibroblast proliferative response to the injury. On the other hand, NO may combine with oxygen radicals to yield peroxynitrite and other related, highly reactive compounds. The oxidants formed in this manner may exert cytotoxic and collagenolytic effects and, therefore, promote the process of reparative vascular remodeling. The balance between the protective and adverse effects of NO is determined by the relative amounts of NO and reactive oxygen species. We speculate that this balance may be shifted toward more severe injury especially during exacerbations of chronic diseases associated with pulmonary hypertension. Targeting these adverse effects of NO-derived radicals on vascular structure represents a potential novel therapeutic approach to pulmonary hypertension in chronic lung diseases.

[Anorectics and pulmonary hypertension]

Anorectics (appetite-suppressant drugs) are frequently requested by patients. Their usage, however, can have serious, life-threatening side effects, such as pulmonary hypertension and valve defects. The association of anorexigen use with pulmonary hypertension was first detected at the end of the sixties. Back than, the incidence of pulmonary hypertension, diagnosed as primary, increased soon after an anorexigen, aminorex was introduced. After aminorex was recalled several years latter, the incidence of the disease returned to the usual low levels. A recent epidemiological study proved that a newer anorexigen, fenfluramine (or its stereoisomer, dexfenfluramine) considerably increases the risk of pulmonary hypertension. Currently, it is unclear how the anorectics contribute to the development of pulmonary hypertension. One possibility may be the increase in plasma serotonin concentration. Serotonin is a pulmonary vasoconstrictor in many species. However, even if this mechanism plays any role in humans, it cannot completely explain the influence of anorectics on the pulmonary circulation. The anorectics cause membrane depolarization of the pulmonary vascular smooth muscle cells by inhibiting potassium channel activity. The depolarization activates voltage-operated calcium channels, thus increasing intracellular calcium ion concentration, which is the well-known stimulus for vasoconstriction. The increase in vascular tension can be especially significant when there is a deficiency in mechanisms acting against vasoconstriction, such as endothelial production of nitric oxide (NO). Such pre-existing defects may be the reason why only a fraction of patients using anorectics actually develop pulmonary hypertension.

[Nitric oxide and regulation of pulmonary vessels]

One of the most important differences between the pulmonary and systemic circulation is a considerably lower blood pressure and hemodynamic resistance in the pulmonary circulation. After the discovery of the endogenous vasodilatator, nitric oxide (NO), it had been assumed that pulmonary vasculature is characterized by a high tonic NO production, which might contribute to the low basal tone of these vessels. Incapacity to maintain this high NO production would then lead to the development of pulmonary hypertension. However, numerous studies summarized in this review gradually proved that, in reality, this simple scheme does not apply. Healthy pulmonary vessels usually produce only relatively small amounts of NO. In pulmonary hypertension, NO synthesis increases. The rise in pulmonary artery pressure is partly blunted by this protective mechanism, at least until the pulmonary endothelium (a major source of NO) is damaged by the permanent and severe elevation of the intravascular pressure.

Aerosol delivery of diethylenetriamine/nitric oxide, a nitric oxide adduct, causes selective pulmonary vasodilation in perinatal lambs

Postnatal adaptation of the pulmonary circulation is mediated partly by endothelium-derived nitric oxide (NO). Recent studies have demonstrated that inhaled NO causes selective and sustained vasodilation in infants with persistent pulmonary hypertension of the newborn. Because the short half-life of NO limits its clinical application, we hypothesized that aerosol delivery of an NO-adduct, diethylenetriamine (DETANO), can cause sustained and selective pulmonary vasodilation. To test the acute effects of DETANO, we studied the pulmonary vascular response of late-gestation fetal lambs (n = 8; age = 138 days; term = 147) to aerosolized DETANO in the presence of an endothelium-derived NO inhibitor, nitro-L-arginine. To determine whether DETANO has a sustained effect, fetal lambs were ventilated with FiO2 0.10 before and 15 minutes after they were treated with aerosolized DETANO. Fetal lambs were acutely prepared. Nitro-L-arginine (1 mg/min x 30 minutes) was infused into the left pulmonary artery before ventilation with FiO2 1.00 for 30 minutes, followed by continued ventilation with FiO2 0.10 for 10 minutes. This represented the control period. Ventilation was continued with FiO2 1.00, and aerosolized DETANO was given in doses of 0.1, 0.4, and 1.0 mg. Fifteen minutes after the last dose of DETANO was administered, animals were ventilated with FiO2 0.10. In the control period, during ventilation with FiO2 0.10, left pulmonary artery flow was 122+/-33 mL/min and decreased to 104+/-22 mL/min. Aerosol delivery of DETANO increased left pulmonary artery flow to 176+/-26 mL/min (P<.05) and had no effect on aortic pressure or heart rate. After DETANO was administered, ventilation with FiO2 0.10 did not cause any change in left pulmonary artery flow. We conclude that DETANO can cause selective fetal pulmonary vasodilation. Aerosol delivery of DETANO may increase the clinical applications of NO.

Molecular identification of the role of voltage-gated K+ channels, Kv1.5 and Kv2.1, in hypoxic pulmonary vasoconstriction and control of resting membrane potential in rat pulmonary artery myocytes

Hypoxia initiates pulmonary vasoconstriction (HPV) by inhibiting one or more voltage-gated potassium channels (Kv) in the pulmonary artery smooth muscle cells (PASMCs) of resistance arteries. The resulting membrane depolarization increases opening of voltage-gated calcium channels, raising cytosolic Ca2+ and initiating HPV. There are presently nine families of Kv channels known and pharmacological inhibitors lack the specificity to distinguish those involved in control of resting membrane potential (Em) or HPV. However, the Kv channels involved in Em and HPV have characteristic electrophysiological and pharmacological properties which suggest their molecular identity. They are slowly inactivating, delayed rectifier currents, inhibited by 4-aminopyridine (4-AP) but insensitive to charybdotoxin. Candidate Kv channels with these traits (Kv1.5 and Kv2.1) were studied. Antibodies were used to immunolocalize and functionally characterize the contribution of Kv1. 5 and Kv2.1 to PASMC electrophysiology and vascular tone. Immunoblotting confirmed the presence of Kv1.1, 1.2, 1.3, 1.5, 1.6, and 2.1, but not Kv1.4, in PASMCs. Intracellular administration of anti-Kv2.1 inhibited whole cell K+ current (IK) and depolarized Em. Anti-Kv2.1 also elevated resting tension and diminished 4-AP-induced vasoconstriction in membrane-permeabilized pulmonary artery rings. Anti-Kv1.5 inhibited IK and selectively reduced the rise in [Ca2+]i and constriction caused by hypoxia and 4-AP. However, anti-Kv1.5 neither caused depolarization nor elevated basal pulmonary artery tone. This study demonstrates that antibodies can be used to dissect the whole cell K+ currents in mammalian cells. We conclude that Kv2. 1 is an important determinant of resting Em in PASMCs from resistance arteries. Both Kv2.1 and Kv1.5 contribute to the initiation of HPV.

Anorexic agents aminorex, fenfluramine, and dexfenfluramine inhibit potassium current in rat pulmonary vascular smooth muscle and cause pulmonary vasoconstriction

BACKGROUND

The appetite suppressant aminorex fumarate is thought to have caused an epidemic of pulmonary hypertension in Europe in the 1960s. More recently, pulmonary hypertension has been described in some patients taking other amphetamine-like, anorexic agents: fenfluramine and its d-isomer, dexfenfluramine. No mechanism has been demonstrated that might account for the association between anorexic drugs and pulmonary hypertension.

METHODS AND RESULTS

Using the whole-cell, patch-clamp technique, we found that aminorex, fenfluramine, and dexfenfluramine inhibit potassium current in smooth muscle cells taken from the small resistance pulmonary arteries of the rat lung. Dexfenfluramine causes reversible membrane depolarization in these cells. These actions are similar to those of hypoxia, which initiates pulmonary vasoconstriction by inhibiting a potassium current in pulmonary vascular smooth muscle. In the isolated, perfused rat lung, aminorex, fenfluramine, and dexfenfluramine induce a dose-related increase in perfusion pressure. When the production of endogenous NO is inhibited by N-nitro-L-arginine methyl ester, the pressor response to dexfenfluramine is greatly enhanced.

CONCLUSIONS

These observations indicate that anorexic agents, like hypoxia, can inhibit potassium current, cause membrane depolarization, and stimulate pulmonary vasoconstriction. They suggest one mechanism that could be responsible for initiating pulmonary hypertension in susceptible individuals. It is possible that susceptibility is the result of the reduced production of an endogenous vasodilator, such as NO, but this remains speculative.

Origins of breath nitric oxide in humans

STUDY OBJECTIVES

Nitric oxide (NO) exists in the human breath, but little is known about its site of origin or enzyme source. The aims of this study were to locate the main site of NO release into human breath and to decide whether the inducible isoform of NO synthase (iNOS) and nasal bacteria contribute to breath NO.

DESIGN

Using a chemiluminescence assay, NO levels were measured in air exhaled from the nose, mouth, trachea, and distal airway. The susceptibility of breath NO to treatment with a topical corticosteroid (to inhibit iNOS; intranasal beclomethasone dipropionate for 2 weeks) and with antibiotics (systemic amoxicillin plus clavulanic acid and intranasal bacitracin zinc, 5 to 10 days) was also tested.

PARTICIPANTS

Twenty-one healthy subjects, 9 intubated patients, and 7 patients undergoing bronchoscopy. All subjects were nonsmokers free of pneumonia, rhinitis, and bronchitis.

MEASUREMENTS AND RESULTS

Breath NO levels, collected in the gas sampling bags, were greater (p < 0.05) in the nose (25 +/- 2 parts per billion [ppb]) than in the mouth (6 +/- 1 ppb), trachea (3 +/- 1 ppb), or distal airway (1 +/- 2 ppb). Similar results were obtained when NO was sampled directly by cannula from nose or mouth during resting breathing. Nasal breath NO signal increased sharply during 30 s of breath-holding. Beclomethasone, but not antibiotics, decreased nasal NO levels without changing oral breath NO.

CONCLUSIONS

Most NO in normal human breath derives locally from the nose where it can reach high levels during breath-holding. NO is synthesized, at least in part, by a steroid-inhibitable, nonbacterial, NO synthase, presumably iNOS.

The somatostatin analog angiopeptin does not reduce chronic hypoxic pulmonary hypertension in rats

Angiopeptin is an analog of somatostatin-14, which has been found to inhibit cellular proliferation in several models of systemic vascular injury. As proliferation plays a major role in pulmonary hypertension, we examined the hypothesis that angiopeptin would inhibit the development of chronic hypoxic pulmonary hypertension in the rat. Angiopeptin was infused intravenously (90-100 microg/kg/day) by minipumps in 10 rats during a 3-week exposure to hypobaric hypoxia and in six normoxic rats. Normal saline was infused in six hypoxic control rats and in seven normoxic control rats. Angiopeptin produced no significant difference in mean pulmonary arterial pressure and resistance, right ventricular weight, or medial thickness of small pulmonary vessels. Vasoconstrictor responses of isolated lungs to acute hypoxia were not affected by angiopeptin. We conclude that angiopeptin, at the high intravenous dose used, does not significantly reduce the development of chronic hypoxic pulmonary hypertension in rats.

Chronic infusion of nitric oxide in experimental pulmonary hypertension: pulmonary pressure-flow analysis

Inhaled-nitric oxide (NO) is a selective pulmonary vasodilator in short-term clinical studies. Use of NO inhalation in chronic pulmonary hypertension is complicated by potential problems with ambulatory NO delivery. We hypothesized that long-term infusion of NO solution into the central venous circulation, which did not suffer from this drawback, might reduce chronic pulmonary hypertension. Saturated NO solution was infused in chronically hypoxic rats by implantable minipumps at a rate which was effective in reducing acute hypoxic vasoconstriction in isolated, Krebs' albumin perfused lungs (2.5 microL.h-1). Pulmonary haemo dynamics and the pressure-flow relationship were studied after 4 weeks of infusion. NO was still present in the minipumps at the end of the infusion period. Despite causing methaemoglobinaemia, NO infusion did not significantly attenuate pulmonary arterial pressure, pulmonary vascular resistance, right ventricular hypertrophy, or the parameters of the pulmonary vascular pressure-flow relationship. Pressure-flow curves, analysed with the nonlinear, distensible vessel model, indicated increased near-zero pressure resistance (Ro) in chronic hypoxia. The tendency of chronic NO infusion to reduce Ro did not reach statistical significance. Long-term infusion of nitric oxide solution is technically feasible but does not effectively reverse chronic pulmonary hypertension. The failure of infused NO to reduce pulmonary hypertension is explained by the fact that the inactivation of NO by haemoglobin is much faster than anticipated.

Differential distribution of electrophysiologically distinct myocytes in conduit and resistance arteries determines their response to nitric oxide and hypoxia

The cellular mechanisms that determine differences in reactivity of arteries of varying size and origin are unknown. We evaluated the hypothesis that there is diversity in the distribution of K+ channels between vascular smooth muscle (VSM) cells within a single segment of the pulmonary arteries (PAs) and that there are differences in the prevalence of these cell types between conduit and resistance arteries, which contribute to segmental differences in the vascular response to NO and hypoxia. Three types of VSM cells can be identified in rat PAs on the basis of their whole-cell electrophysiological properties- current density and the pharmacological dissection of whole-cell K+ current(I(K))-and morphology. Cells are referred to as "K(Ca), K(Dr), or mixed," acknowledging the type of K+ channel that dominates the IK: the Ca2+-sensitive (K(Ca)) channel, delayed rectifier (K(Dr)) channel, or a mixture of both. The three cell types were identified by light and electron microscopy. K(Ca) cells are large and elongated, and they have low current density and currents that are inhibited by tetraethylammonium (5 mmol/L) or charybdotoxin (100 nmol/L). K(Dr) cells are smaller, with a perinuclear bulge, but have high current density and currents that are inhibited by 4-aminopyridine (5 mmol/L). Conduit arteries contain significant numbers of K(Ca) cells, whereas resistance arteries have a majority of K(Dr) cells and few K(Ca) cells. NO rapidly and reversibly increases I(K) and hyperpolarizes K(Ca) cells because of an increase in open probability of a 170-pS K(Ca) channel. Hypoxia depolarizes K(Dr) cells by rapidly and reversibly inhibiting one or more of the tonically active K(Dr) channels (including a 37-pS channel) that control resting membrane potential. The effects of both hypoxia and NO on K+ channels are evident at negative membrane potentials, supporting their physiological relevance. The functional correlate of this electrophysiological diversity is that K(Dr)-enriched resistance vessels constrict to hypoxia, whereas conduit arteries have a biphasic response predominated by relaxation. Although effective in both segments, NO relaxes conduit more than resistance rings, in both cases by a cGMP-dependent mechanism. We conclude that regional electrophysiological diversity among smooth muscle cells is a major determinant of segmental differences in vascular reactivity.

Nebulized nitric oxide/nucleophile adduct reduces chronic pulmonary hypertension

OBJECTIVE

Inhaled nitric oxide (NO) is a selective pulmonary vasodilator, but its use has been restricted almost exclusively to the intensive care setting due to the complexity of its delivery. NO/nucleophile adducts, such as diethylenetriamine/NO (DETA/NO), spontaneously release NO in aqueous solutions. We hypothesized that a nebulized DETA/NO (half-time of NO release > 20 h) would stay in the lower airways and continuously supply sufficient NO to achieve sustained vasodilation in chronic pulmonary hypertension.

METHODS

Chronic pulmonary hypertension was induced in rats by a monocrotaline injection. Nineteen days later, nebulizations of DETA/NO were given on 4 consecutive days (5 and 50 mu mol; 10 min/day). One day after the last nebulization, pulmonary and systemic arterial pressure and cardiac output were measured after thoracotomy. The lungs were isolated and perfused to study the pressure-flow relationship. The effect of DETA/NO nebulization on acute vasoconstrictor reactivity was studied in additional isolated lungs.

RESULTS

Total pulmonary, but not systemic, vascular resistance was significantly reduced by both DETA/NO doses, suggesting that DETA/NO, like NO, causes preferential dilation of the pulmonary circulation. The pulmonary perfusion pressure-flow curves were shifted downwards by DETA/NO treatment, indicating improved resistive properties of the pulmonary vasculature. DETA/NO nebulization into isolated lungs increased exhaled NO levels and progressively reduced vasoconstrictor responses to angiotensin II and acute hypoxia. These effects were not reversed by perfusate exchange. In intact rats, carotid artery pressure and plasma NO2- + NO3- levels did not change during and after DETA/NO nebulization.

CONCLUSION

DETA/NO nebulization offers a possibility of once a day, ambulatory delivery of NO and is a potential treatment for chronic pulmonary hypertension, although further studies are needed to establish safety and selectivity.

Dithionite increases radical formation and decreases vasoconstriction in the lung. Evidence that dithionite does not mimic alveolar hypoxia

Dithionite is a powerful reducing agent used to deoxygenate hemoglobin and create anaerobic conditions in vitro. Recently, dithionite has been used as a convenient means of creating "hypoxia" in experiments studying the O2 sensor in the pulmonary circulation and carotid body. We evaluated the hypothesis that hypoxia created by hypoxic ventilation and that created by dithionite have different effects on the pulmonary circulation. In vitro, dithionite (10(-5) to 10(-3) mol/L), added to oxygenated Krebs' solution, rapidly created superoxide anion in a dose-dependent manner. Dithionite consumed O2 in parallel with the generation of superoxide radical, with both processes peaking within seconds. Anoxia was sustained only if resupply of O2 was prevented. In isolated rat lungs (whether perfused with autologous blood or Krebs' solution), hypoxic ventilation alone lowered perfusate PO2 from approximately 140 to 40 mm Hg and decreased lung levels of activated oxygen species (AOS), measured by luminol-enhanced chemiluminescence, before the onset of hypoxic pulmonary vasoconstriction. Constrictor responses to angiotensin II and KCl were not impaired by intermittent hypoxic challenges, and lung weight did not increase. In contrast, dithionite impaired constrictor responses of the Krebs' solution-perfused lungs to all vasoconstrictors tested and increased lung weight. When given as a bolus (5 x 10(-3) mol/L) into the pulmonary artery during normoxic ventilation, dithionite caused no vasoconstriction and only briefly lowered PO2 (because of constant resupply of O2 from the alveoli). When superimposed on hypoxic ventilation, dithionite further lowered PO2 from approximately 40 to approximately 0 mm Hg and caused additional constriction. Unlike hypoxic ventilation, dithionite increased AOS production. Antioxidant enzymes diminished dithionite-induced radical production and diminished the loss of vascular reactivity and lung edema. In conclusion, unlike hypoxic ventilation, dithionite causes edema and loss of vascular reactivity in the lung by generating superoxide anion and hydrogen peroxide. Hypoxia elicited by dithionite is not equivalent to authentic hypoxia because of the obligatory associated generation of AOS. Dithionite usage should not be substituted for authentic hypoxia in studies of O2 sensing.

Hypoxia potentiates nitric oxide synthesis and transiently increases cytosolic calcium levels in pulmonary artery endothelial cells

There is indirect, contradictory evidence both for increased and reduced synthesis of the endothelium-derived vasodilator, nitric oxide, in the pulmonary circulation during acute hypoxia. Therefore, we decided to directly measure the effect of acute hypoxia on nitric oxide production by cultured pulmonary endothelium. Because increases in the intracellular free calcium concentration are known to initiate nitric oxide synthesis, we also studied cytosolic calcium levels. We measured the accumulation of the stable nitric oxide metabolite, nitrite, in the fluid used to superfuse the cultured bovine pulmonary artery endothelial cells at an oxygen tension (PO2) of either 20.3 (normoxia) or 4.9 kPa (hypoxia) (152 or 37 mmHg). Intracellular calcium levels were measured with dual-excitation microfluorimetry after loading the cells with the fluorescent calcium indicator, fura 2. Basal NO synthesis, measured as nitrite accumulation over 10 min, was significantly higher under hypoxic than normoxic conditions (8.3 +/- 2.2 versus 4.6 +/- 0.8 nM). Hypoxia transiently increased cytosolic calcium concentration (from 113 +/- 10 to 231 +/- 45 nM). Ryanodine and thapsigargin (which deplete intracellular calcium stores), but not the removal of extracellular calcium, inhibited the hypoxic increase in cytosolic calcium, indicating that it resulted primarily from release of intracellular calcium. Bradykinin-elicited NO synthesis was potentiated by hypoxia. Bradykinin-induced increase in cytosolic calcium was not inhibited by hypoxia. We conclude that hypoxia acutely increases cytosolic calcium levels and basal and bradykinin-stimulated nitric oxide synthesis in pulmonary artery endothelium.

Hypoxia potentiates nitric oxide synthesis and transiently increases cytosolic calcium levels in pulmonary artery endothelial cells

There is indirect, contradictory evidence both for increased and reduced synthesis of the endothelium-derived vasodilator, nitric oxide, in the pulmonary circulation during acute hypoxia. Therefore, we decided to directly measure the effect of acute hypoxia on nitric oxide production by cultured pulmonary endothelium. Because increases in the intracellular free calcium concentration are known to initiate nitric oxide synthesis, we also studied cytosolic calcium levels. We measured the accumulation of the stable nitric oxide metabolite, nitrite, in the fluid used to superfuse the cultured bovine pulmonary artery endothelial cells at an oxygen tension (PO2) of either 20.3 (normoxia) or 4.9 kPa (hypoxia) (152 or 37 mmHg). Intracellular calcium levels were measured with dual-excitation microfluorimetry after loading the cells with the fluorescent calcium indicator, fura 2. Basal NO synthesis, measured as nitrite accumulation over 10 min, was significantly higher under hypoxic than normoxic conditions (8.3 +/- 2.2 versus 4.6 +/- 0.8 nM). Hypoxia transiently increased cytosolic calcium concentration (from 113 +/- 10 to 231 +/- 45 nM). Ryanodine and thapsigargin (which deplete intracellular calcium stores), but not the removal of extracellular calcium, inhibited the hypoxic increase in cytosolic calcium, indicating that it resulted primarily from release of intracellular calcium. Bradykinin-elicited NO synthesis was potentiated by hypoxia. Bradykinin-induced increase in cytosolic calcium was not inhibited by hypoxia. We conclude that hypoxia acutely increases cytosolic calcium levels and basal and bradykinin-stimulated nitric oxide synthesis in pulmonary artery endothelium.

Long-term effects of prenatal indomethacin administration on the pulmonary circulation in rats

Mechanical properties of the adult pulmonary vasculature are affected by perinatal experience of hypoxic pulmonary hypertension. In the present study, we followed the long-term effects of perinatal pulmonary hypertension induced by means other than hypoxia in rats. Daily injections of indomethacin (1 mg.kg-1 body weight (BW)) were given to the parturient rats. Their newborn pups had significantly increased number of muscularized peripheral pulmonary vessels. Pulmonary hypertension, however, did not persist to adulthood (mean pulmonary arterial pressure (Ppa) was 17.2 +/- 1.3 torr in the experimental group and 16.4 +/- 0.8 torr in controls). Pulmonary hypertension induced in adult rats by exposure to chronic hypoxia or by acute hypoxic challenges was similar in indomethacin-treated and control rats. Normoxic perfusion pressure/flow (P/Q) plots in isolated lungs were less steep in indomethacin-treated than in control rats. Acute hypoxia increased the slope of P/Q plots in indomethacin treated rats but not in controls. The described changes in the pulmonary vasculature induced by indomethacin are similar to those found previously in adult rats born in hypoxia. We conclude that perinatal pulmonary hypertension permanently modifies the pulmonary vasculature.

Nitric oxide and cGMP cause vasorelaxation by activation of a charybdotoxin-sensitive K channel by cGMP-dependent protein kinase

Nitric oxide (NO)-induced relaxation is associated with increased levels of cGMP in vascular smooth muscle cells. However, the mechanism by which cGMP causes relaxation is unknown. This study tested the hypothesis that activation of Ca-sensitive K (KCa) channels, mediated by a cGMP-dependent protein kinase, is responsible for the relaxation occurring in response to cGMP. In rat pulmonary artery rings, cGMP-dependent, but not cGMP-independent, relaxation was inhibited by tetraethylammonium, a classical K-channel blocker, and charybdotoxin, an inhibitor of KCa channels. Increasing extracellular K concentration also inhibited cGMP-dependent relaxation, without reducing vascular smooth muscle cGMP levels. In whole-cell patch-clamp experiments, NO and cGMP increased whole-cell K current by activating KCa channels. This effect was mimicked by intracellular administration of (Sp)-guanosine cyclic 3',5'-phosphorothioate, a preferential cGMP-dependent protein kinase activator. Okadaic acid, a phosphatase inhibitor, enhanced whole-cell K current, consistent with an important role for channel phosphorylation in the activation of NO-responsive KCa channels. Thus NO and cGMP relax vascular smooth muscle by a cGMP-dependent protein kinase-dependent activation of K channels. This suggests that the final common pathway shared by NO and the nitrovasodilators is cGMP-dependent K-channel activation.

Increased endothelium-derived NO in hypertensive pulmonary circulation of chronically hypoxic rats

The hypothesis that the endothelium-derived relaxing factor/nitric oxide (EDNO) activity is elevated in chronic hypoxic pulmonary hypertension (CH-PHT) was tested using isolated Krebs-albumin-perfused rat lungs. Concentration of the EDNO decomposition products (NOx) in the lungs' effluent was measured by a modified chemiluminescence assay. The functional significance of basal EDNO production was studied by measuring the vasoconstrictor response to an EDNO synthesis inhibitor, N omega-nitro-L-arginine methyl ester (L-NAME). Reactivity to the endothelium-dependent vasodilator substance P and to exogenous NO was also studied. More NOx was found in effluent from CH-PHT (22.3 +/- 9.8 nM) than control (0.4 +/- 3.9 nM) lungs. The L-NAME-induced vasoconstriction was greater in CH-PHT than in control rats. The sensitivity, but not the maximal vasodilation, to exogenous NO was elevated in CH-PHT. The substance P-induced vasodilation was potentiated in CH-PHT compared with control rats and blocked by L-NAME in both groups. We conclude that basal and agonist-stimulated pulmonary EDNO activity is enhanced in this model of CH-PHT. The EDNO synthesis may play a counterregulatory role in CH-PHT.

Chronic hypoxic pulmonary hypertension. Is thrombin involved?

Thrombin contracts vascular smooth muscle and stimulates its proliferation. Using a specific thrombin inhibitor, hirudin, we studied whether thrombin contributes to the pulmonary vasoconstriction and vascular proliferation that occurs in pulmonary hypertension. Hirudin was infused intravenously (0.2 mg/h/kg) by minipumps in nine rats during a 3-wk exposure to hypobaric hypoxia (HH). Vehicle (normal saline) was infused in eight hypoxic control (HC) and seven normoxic control (NC) rats. Sufficient hirudin delivery was confirmed by a failure of undiluted plasma from HH, but not from NC and HC, to clot in response to thrombin. When the plasma samples were diluted 1:10, the thrombin time was significantly prolonged in HH when compared with that in both NC and HC. Although hirudin slightly reduced mean pulmonary arterial pressure in open-chest rats, there was no significant difference between the hypoxic groups in total pulmonary resistance, right ventricle weight, morphologic remodeling of lung vessels, or the perfusion pressure-flow relationship in isolated lungs. Vasoconstrictor responses of isolated lungs to angiotensin II and acute hypoxic challenges were not affected by hirudin treatment. We conclude that hirudin, in a dose sufficient to reduce thrombin's catalytic effect on fibrinogen, does not significantly prevent the development of chronic hypoxic pulmonary hypertension.

Chronic EDRF inhibition and hypoxia: effects on pulmonary circulation and systemic blood pressure

It has been suggested that chronic hypoxic pulmonary hypertension results from chronic hypoxic inhibition of endothelium-derived relaxing factor (EDRF) synthesis. We tested this hypothesis by studying whether chronic EDRF inhibition by N omega-nitro-L-arginine methyl ester (L-NAME) would induce pulmonary hypertension similar to that found in chronic hypoxia. L-NAME (1.85 mM) was given for 3 wk in drinking water to rats living in normoxia or hypoxia. Unlike chronic hypoxia, chronic L-NAME treatment did not increase pulmonary arterial pressure. Cardiac output was reduced and mean systemic arterial pressure was increased by chronic L-NAME treatment. The vascular pressure-flow relationship in isolated lungs was shifted toward higher pressures by chronic hypoxia and, to a lesser degree, by L-NAME intake. In isolated lungs, vasoconstriction in response to angiotensin II and acute hypoxia and vasodilation in response to sodium nitroprusside were increased by chronic L-NAME treatment in normoxia and chronic hypoxia. Chronic hypoxia, but not L-NAME, induced hypertensive pulmonary vascular remodeling. Chronic supplementation with the EDRF precursor L-arginine did not have any significant effect on chronic hypoxic pulmonary hypertension. We conclude that the chronic EDRF deficiency state, induced by L-NAME, does not mimic chronic hypoxic pulmonary hypertension in our model. In addition, EDRF proved to be less important for basal tone regulation in the pulmonary than in the systemic circulation.

NG-monomethyl-L-arginine causes nitric oxide synthesis in isolated arterial rings: trouble in paradise

Arginine analogs are commonly used as inhibitors of the synthesis of endothelium-derived relaxing factor, nitric oxide. However, their effect on nitric oxide levels is rarely measured. Using a chemiluminescence assay for nitric oxide, we found that NG-monomethyl-L-arginine enhanced, rather than reduced, nitric oxide synthesis in pulmonary arterial and aortic rings. NG-monomethyl-L-arginine inhibited relaxation to the endothelium-dependent vasodilator A23187 in aortic but not pulmonary arterial rings. In contrast, N omega-nitro-L-arginine did not stimulate nitric oxide synthesis and it inhibited relaxation to A23187 in all rings. We conclude that NG-monomethyl-L-arginine is a partial agonist for nitric oxide synthesis.

Almitrine in low dose potentiates vasoconstrictor responses of isolated rat lungs to moderate hypoxia

To test whether the effect of almitrine on hypoxic pulmonary vasoconstriction was dose-dependent, two series of experiments were performed on isolated rat lungs perfused with constant flow of blood. In the first series, the effects of different doses of almitrine on perfusion pressure were measured. Baseline perfusion pressure was not changed by solvent or by 0.25 micrograms.ml-1 almitrine, but it was increased by 0.5 and 2.0 micrograms.ml-1 almitrine. The increase in perfusion pressure in response to 10 min ventilation with hypoxic gas mixture (5% O2) was significantly (p less than 0.05) higher after 0.25 micrograms.ml-1 almitrine (12.0 +/- 0.8 torr) than before addition of the drug (5.43 +/- 1.8 torr). Responses to hypoxia were insignificant after higher doses (0.5 and 2.0 micrograms.ml-1) of almitrine. In the second series of experiments the responses to varying degrees of hypoxia were measured after administration of one dose of almitrine (0.25 micrograms.ml-1). Almitrine, compared to solvent alone, significantly altered the shape of the dose-response curve to hypoxia. Increases in perfusion pressure in response to moderate degrees of hypoxia were potentiated (10% O2: 8.7 +/- 1.8 torr after almitrine, 2.1 +/- 0.6 torr after solvent, p less than 0.05), whereas responses to severe hypoxia (3% O2) were not changed by almitrine. Reactivity to angiotensin II was decreased by 0.25 micrograms.ml-1 almitrine. We conclude that almitrine in low but not in high dose augments pulmonary vasoconstriction induced by mild degrees of hypoxia.

Almitrine in low dose potentiates vasoconstrictor responses of isolated rat lungs to moderate hypoxia

To test whether the effect of almitrine on hypoxic pulmonary vasoconstriction was dose-dependent, two series of experiments were performed on isolated rat lungs perfused with constant flow of blood. In the first series, the effects of different doses of almitrine on perfusion pressure were measured. Baseline perfusion pressure was not changed by solvent or by 0.25 micrograms.ml-1 almitrine, but it was increased by 0.5 and 2.0 micrograms.ml-1 almitrine. The increase in perfusion pressure in response to 10 min ventilation with hypoxic gas mixture (5% O2) was significantly (p less than 0.05) higher after 0.25 micrograms.ml-1 almitrine (12.0 +/- 0.8 torr) than before addition of the drug (5.43 +/- 1.8 torr). Responses to hypoxia were insignificant after higher doses (0.5 and 2.0 micrograms.ml-1) of almitrine. In the second series of experiments the responses to varying degrees of hypoxia were measured after administration of one dose of almitrine (0.25 micrograms.ml-1). Almitrine, compared to solvent alone, significantly altered the shape of the dose-response curve to hypoxia. Increases in perfusion pressure in response to moderate degrees of hypoxia were potentiated (10% O2: 8.7 +/- 1.8 torr after almitrine, 2.1 +/- 0.6 torr after solvent, p less than 0.05), whereas responses to severe hypoxia (3% O2) were not changed by almitrine. Reactivity to angiotensin II was decreased by 0.25 micrograms.ml-1 almitrine. We conclude that almitrine in low but not in high dose augments pulmonary vasoconstriction induced by mild degrees of hypoxia.