Endotoxin alters hypoxic pulmonary vasoconstriction in isolated rat lungs

Current Pharmacological Approach to ARDS: The Place of Bosentan

Acute respiratory distress syndrome is characterized by dyspnea at presentation, tachypnea on physical examination, findings of bilateral infiltration in chest radiography, refractory hypoxia, and high mortality. Although the main treatment approach is to address the underlying disease, there are also pharmacological and nonpharmacological options for supportive treatment. There is currently no pharmacological agent with proven efficacy in this syndrome, and many drugs are being studied for this purpose. One of these is the endothelin receptor antagonist bosentan.

Evaluation of the serotonin receptor blocker methiothepin in broilers injected intravenously with lipopolysaccharide and microparticles

There has been considerable interest in the role of serotonin (5-hydroxytryptamine, 5-HT) in the pathogenesis of pulmonary hypertension due to episodes of primary pulmonary hypertension in humans linked to serotoninergic appetite-suppressant drugs. In this study, we investigated the effect of 5-HT on the development of pulmonary hypertension induced by injecting bacterial lipopolysaccharide (LPS; endotoxin) and cellulose microparticles intravenously, using the nonselective 5-HT(1/2)receptor, antagonist methiothepin. In Experiment 1, broilers selected for ascites susceptibility or resistance under conditions of hypobaric hypoxia were treated with methiothepin or saline, followed by injection of LPS, while recording pulmonary arterial pressure (PAP). In Experiment 2 ascites-susceptible broilers were treated with methiothepin or saline, followed by injection of cellulose microparticles, while recording PAP. In Experiment 3, an i.v. microparticle injection dose shown to cause 50% mortality was injected into ascites-susceptible and ascites-resistant broilers after methiothepin or saline treatment. Injecting methiothepin reduced PAP below baseline values in ascites-susceptible and ascites-resistant broilers, suggesting a role for 5-HT in maintaining the basal tone of the pulmonary vasculature in broilers. Injecting microparticles into the wing vein had no affect on the PAP in the broilers treated with methiothepin, suggesting that 5-HT is an important mediator in the pulmonary hypertensive response of broilers to microparticles. Furthermore, injecting an 50% lethal dose of microparticles into ascites-susceptible and ascites-resistant broilers pretreated with methiothepin resulted in reduced mortality. Serotonin appears to play a less prominent role in the pulmonary hypertensive response of broilers to intravenously injected LPS, indicating that other mediators within the innate response to inflammatory stimuli may also be involved. These results are consistent with our hypothesis that pulmonary hypertension syndrome ensues when vasoconstrictors, such as 5-HT, overwhelm the dilatory effects of vasodilators, such as NO, thereby effectively reducing the pulmonary vascular capacity of pulmonary hypertension syndrome-susceptible broilers.

Role of endogenous nitric oxide in endotoxin-induced alteration of hypoxic pulmonary vasoconstriction in mice

Pulmonary vasoconstriction in response to alveolar hypoxia (HPV) is frequently impaired in patients with sepsis or acute respiratory distress syndrome or in animal models of endotoxemia. Pulmonary vasodilation due to overproduction of nitric oxide (NO) by NO synthase 2 (NOS2) may be responsible for this impaired HPV after administration of endotoxin (LPS). We investigated the effects of acute nonspecific (N(G)-nitro-L-arginine methyl ester, L-NAME) and NOS2-specific [L-N6-(1-iminoethyl)lysine, L-NIL] NOS inhibition and congenital deficiency of NOS2 on impaired HPV during endotoxemia. The pulmonary vasoconstrictor response and pulmonary vascular pressure-flow (P-Q) relationship during normoxia and hypoxia were studied in isolated, perfused, and ventilated lungs from LPS-pretreated and untreated wild-type and NOS2-deficient mice with and without L-NAME or L-NIL added to the perfusate. Compared with lungs from untreated mice, lungs from LPS-challenged wild-type mice constricted less in response to hypoxia (69 +/- 17 vs. 3 +/- 7%, respectively, P < 0.001). Perfusion with L-NAME or L-NIL restored this blunted HPV response only in part. In contrast, LPS administration did not impair the vasoconstrictor response to hypoxia in NOS2-deficient mice. Analysis of the pulmonary vascular P-Q relationship suggested that the HPV response may consist of different components that are specifically NOS isoform modulated in untreated and LPS-treated mice. These results demonstrate in a murine model of endotoxemia that NOS2-derived NO production is critical for LPS-mediated development of impaired HPV. Furthermore, impaired HPV during endotoxemia may be at least in part mediated by mechanisms other than simply pulmonary vasodilation by NOS2-derived NO overproduction.

Major histocompatibility (B) complex control of responses against Rous sarcomas

The chicken major histocompatibility (B) complex (MHC) affects disease outcome significantly. One of the best characterized systems of MHC control is the response to the oncogenic retrovirus, Rous sarcoma virus (RSV). Genetic selection altered the tumor growth pattern, either regressively or progressively, with the data suggesting control by one or a few loci. Particular MHC genotypes determine RSV tumor regression or progression indicating the crucial B complex role in Rous sarcoma outcome. Analysis of inbred lines, their crosses, congenic lines, and noninbred populations has revealed the anti-RSV response of many B complex haplotypes. Tumor growth disparity among lines identical at the MHC but differing in their background genes suggested a non-MHC gene contribution to tumor fate. Genetic complementation in tumor growth has also been demonstrated for MHC and non-MHC genes. RSV tumor expansion reflects both tumor cell proliferation and viral replication generating new tumor cells. In addition, the B complex controls tumor growth induced by a subviral DNA construct encoding only the RSV v-src oncogene. Immunity to subsequent tumors and metastasis also exhibit MHC control. Genotypes that regressed either RSV or v-src DNA primary tumors had enhanced protection against subsequent homologous challenge. Regressor B genotypes had lower tumor metastasis compared with progressor types. Together, the data indicate that B complex control of RSV tumor fate is strongly defined by the response to a v-src-determined function. Differential RSV tumor outcomes among various B genotypes may include immune recognition of a tumor-specific antigen or immune system influences on viral replication.

Nω-Nitro-L-Arginine Methyl Ester (L-NAME) Amplifies the Pulmonary Hypertensive Response to Endotoxin in Broilers

The pulmonary hypertensive response to bacterial lipopolysaccharide (LPS, endotoxin) varies widely among individual broilers, leading to the suggestion that innate variability may exist in the proportions or profiles of chemical mediators released during the ensuing inflammatory cascade. LPS induces the expression of nitric oxide synthase (iNOS), which produces the vasodilator nitric oxide (NO) to modulate the responses to concurrently produced vasoconstrictors. In experiment 1, broilers were given the NOS inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME), followed by a supra-maximal dose of LPS while the pulmonary arterial pressure was recorded. In experiment 2 the cardiac output also was recorded before and following the i.v. injection of L-NAME. In both experiments, injection with L-NAME modestly increased the pulmonary arterial pressure when compared with control values, confirming previous reports that tonic/basal NO synthesis is required to promote flow-dependent pulmonary vasodilation in chickens. This response to L-NAME occurred in spite of a tendency for cardiac output and stroke volume to decline and, therefore, can be attributed to pulmonary vasoconstriction (an increase in the pulmonary vascular resistance) rather than an increase in pulmonary blood flow. When L-NAME was used to block NO synthesis induced by LPS, an early peak of pulmonary hypertension was revealed that rarely develops in broilers in the absence of L-NAME, and that has been correlated with the release of platelet activating factor and thromboxane A2 in mammals. The control group responded to LPS with a delayed-onset pulmonary hypertension that was typical in timing, amplitude, and duration of the responses previously observed in broilers and that has been attributed to endothelin-mediated thromboxane A2 synthesis in mammals. This delayed-onset pulmonary hypertensive response to LPS was longer in duration and higher in amplitude in the L-NAME group when compared with the control group. These observations are consistent with the hypothesis that NO modulates the responses to vasoconstrictors released concurrently during the LPS-mediated inflammatory cascade. Inhibition of NOS by L-NAME apparently reduced the modulatory influence of NO and exposed a more dramatic pulmonary hypertensive response to LPS.

Effects of a Selective Nitric Oxide Synthase Inhibitor on Endotoxin-Induced Alteration in Hypoxic Pulmonary Vasoconstriction in Sheep

It has been suggested that overproduction of nitric oxide (NO) by nitric oxide synthase (NOS) contributes to blunted hypoxic pulmonary vasoconstriction (HPV) during endotoxemia. We investigated the effect of a selective inducible NOS (iNOS) inhibitor, ONO-1714, on the loss of HPV during endotoxemia in awake sheep to clarify the role of iNOS. We prepared 11 intubated, awake sheep with hemodynamic monitoring. Hypoxic challenges (FiO2; 12%) were performed before, and 5, 24, 48, 72 hours after endotoxin (1 microg/kg) infusion for 15 minutes. Pulmonary artery (Ppa) and left atrial pressure (Pla) were continuously measured and cardiac output (CO) was measured by the thermodilution method. Pulmonary vascular resistance (PVR) was calculated by (Ppa - Pla)/CO. The percent change in PVR (%PVR) before (pre-PVR) and after (post-PVR) hypoxia was calculated as (post-PVR - pre-PVR)/pre-HPV x 100. ONO-1714 (0.1 mg/kg, n=5, Exp 1) or normal saline (n=6, Exp 2) was administered 5 hours before hypoxic challenge every day. ONO-1714 did not affect the baseline pulmonary hemodynamics before endotoxin administration. % PVR before and after hypoxic exposure was significantly decreased 5 hours after endotoxin administration and gradually improved to baseline at 72 hours. Treatment with iNOS inhibition significantly restored % HPV (24.7+/-5.5% in Exp1 versus -3.1+/-3.6% in Exp 2, 5 hours, 25.3+/-2.5% in Exp 1 versus 7.7+/-2.2% in Exp 2, 24 hours). It is suggested that inducible nitric oxide is related to pulmonary vascular hyporesponsiveness to hypoxia during endotoxemia in sheep.

Pulmonary hypertensive response to endotoxin in cellulose-primed and unprimed broiler chickens

Previous studies indicate that individual broilers vary widely in their pulmonary vascular responsiveness to i.v. injections of endotoxin. This individual variability may reflect differences acquired during previous respiratory challenges or genetic variability that may be associated with susceptibility to pulmonary hypertension syndrome (ascites). In the present study, we compared the endotoxin responses of 4- to 5- wk-old control broilers (unprimed) and broilers in which the pulmonary vasculature had been immunologically challenged 48 h previously by an i.v. injection of cellulose micro-particles (primed). The injected cellulose micro-particles are carried in the venous blood to the lungs, where they become trapped in the pulmonary vasculature and initiate acute focal inflammatory responses within the surrounding lung parenchyma. Physiological variables (respiratory rate, heart rate, pulmonary and systemic arterial pressures) were evaluated prior to and following the i.v. administration of 1 mg of Salmonella typhimurium endotoxin. Prior to endotoxin injection, the respiratory rate was higher in primed than in unprimed broilers; however, the heart rate, pulmonary arterial pressure, and systemic arterial pressure did not differ between groups. Broilers in both groups exhibited similar ranges of individual variability in their endotoxin responses. The overall time of onset, magnitude, and duration of the pulmonary hypertensive responses were similar for both groups. Accordingly, the initiation of a preexisting inflammatory response within the lung parenchyma did not alter the timing, amplitude, or variability of the subsequent pulmonary hypertensive response to endotoxin in broilers.

Alveolar mechanics alter hypoxic pulmonary vasoconstriction

OBJECTIVES

Hypoxic pulmonary vasoconstriction is the primary physiologic mechanism that maintains a proper ventilation/perfusion match, but it fails in diffuse lung injuries such as acute respiratory distress syndrome. Acute respiratory distress syndrome is associated with pulmonary surfactant loss that alters alveolar mechanics (i.e., dynamic change in alveolar size and shape during ventilation), converting normal stable alveoli into unstable alveoli. We hypothesized that alveolar instability stents open pulmonary microvessels and is the mechanism of hypoxic pulmonary vasoconstriction failure associated with acute respiratory distress syndrome.

DESIGN

Prospective, randomized, controlled study.

SETTING

University research laboratory.

SUBJECTS

Ten adult pigs.

INTERVENTIONS

Anesthetized ventilated pigs were prepared surgically for hemodynamic monitoring and were subjected to a right thoracotomy. An in vivo microscope was attached to the right lung, and the microvascular response to hypoxia (F(IO(2)), 15%) was measured in a lung with normal stable alveoli and in a lung with unstable alveoli caused by surfactant deactivation (Tween lavage).

MEASUREMENTS AND MAIN RESULTS

Alveolar instability, defined as the difference between alveolar area at peak inspiration and end expiration and assessed as a percentage change (I-E Delta%), was significantly increased after Tween (23.9 +/- 3.0, I-E Delta%) compared with baseline (2.4 +/- 1.0, I-E Delta%). Alveolar instability was associated with the following microvascular changes: a) increased vasoconstriction (Tween, 14.9 +/- 1.0%) in response to hypoxia compared with baseline (10.8 +/- 1.2%, p <.05); and b) increased mean vascular diameter (Tween, 41.2 +/- 1.5 microm) compared with the mean diameter at baseline (24.6 +/- 1.0 microm, p <.05).

CONCLUSION

Unstable alveoli stent open pulmonary vessels, which may explain the failure of hypoxic pulmonary vasoconstriction in acute respiratory distress syndrome.

Hypoxic pulmonary vasoconstriction increases during endotoxemia in the perfused rat lung

BACKGROUND

Several investigations have studied hypoxic pulmonary vasoconstriction (HPV) during endotoxemia, as in this situation there is an increase in the activation of the inducible nitric oxide synthases, producing a greater liberation of nitric oxide (NO) in the pulmonary vessels. However, these studies yielded conflicting or at times contradictory results, since reference has been made to both enhancement and inhibition of HPV. Our objective was to determine the effect of hypoxia on the isolated blood-perfused lung of endotoxemic rats, and to give at least a partial explanation of its production mechanism.

METHODS

Pulmonary arterial pressure (PAP) was measured in a blood-perfused lung preparation from Wistar rats in normoxia (O2, 20%; CO2, 5%; N, 75%) and hypoxia (O2, 2%; CO2, 5%; N, 93%). There were three experimental protocols. We studied the effect of hypoxia in a control group (CG) and an endotoxemic group (EG). Second, we studied the effect of hypoxia in endotoxemic rats pretreated with indomethacin (E+IG). Third, we assessed the effect of two inhibitors of NO synthesis: N-methyl-l-arginine (NMLA) and methylene blue (MB) on two subgroups of groups CG (CGnmla and CGmb) and EG (EGnmla and EGmb). With the exception of the CG, all specimens were pretreated with a 20-mg/kg intraperitoneal injection of Escherichia coli lipopolysaccharide.

RESULTS

DeltaPAP elicited by hypoxia in the EG group (15.90 +/- 4.75 mm Hg) was 2.30 times higher than in the CG (6.89 +/- 1.96 mm Hg). In the E+IG group, hypoxia produced a DeltaPAP of 15.20 +/- 3.56 mm Hg, similar to that in the EG. The addition of MB in the EGmb subgroup increased base PAP during normoxia from 19.1 +/- 1.23 mm Hg to 32.2 +/- 6.1 mm Hg (p < 0.05).

CONCLUSION

In an isolated-perfused rat model, E. coli lipopolysaccharide (20 mg/kg) significantly increased HPV. This response is maintained over time. Inhibition of NO release by hypoxia may be responsible for the enhanced HPV after endotoxin.

Intravenous Endotoxin Triggers Pulmonary Vasoconstriction and Pulmonary Hypertension in Broiler Chickens

Bacterial endotoxins stimulate endothelin-mediated, thromboxane-dependent increases in pulmonary vascular resistance in mammals, and thromboxane has been shown to cause an immediate but transient pulmonary vasoconstriction in broiler chickens. In the present study, i.v. injections of 1 mg endotoxin into anesthetized male broilers caused a pulmonary vasoconstrictive response that was delayed in onset by 15 min and that elevated the pulmonary arterial pressure by 10 mm Hg within 25 min postinjection. Thereafter, pulmonary hemodynamic variables gradually (> or = 15 min) returned toward pre-injection levels, and supplemental injections of 4 mg endotoxin during this recovery period failed to reinitiate pulmonary hypertension. In contrast, injecting the thromboxane A2 mimetic U44069 during the endotoxin recovery period triggered pulmonary vasoconstriction and pulmonary hypertension similar in magnitude to the responses triggered by U44069 before endotoxin had been administered. The time course and magnitude of the pulmonary hemodynamic responses to endotoxin were highly variable among individual broilers, whereas the individual responses to U44069 were more consistent. Unanesthetized broilers resembled anesthetized broilers in the time course, magnitude, and variability of their pulmonary hemodynamic responses to endotoxin. Overall, these observations are consistent with the hypothesis that endotoxin initiates a biochemical cascade, culminating in the delayed onset of pulmonary vasoconstriction and pulmonary hypertension within 20 min postinjection. Subsequently, the pulmonary vasculature remains responsive to large bolus injections of exogenous thromboxane mimetic; however depletion of endogenous vasoconstrictive components of the endotoxin-mediated cascade, a compensatory increase in endogenous vasodilators, or the induction of a transient cellular tolerance to endotoxin prevented fourfold higher doses of endotoxin from reversing the return toward a normal pulmonary vascular tone. Individual differences among broilers in their susceptibility to pulmonary hypertension syndrome (ascites) may be related to innate or acquired variability in their pulmonary vascular responsiveness to vasoactive mediators.

Cyclooxygenase inhibitors attenuate bradykinin-induced vasoconstriction in septic isolated rat lungs

Cyclooxygenase (COX) products play an important role in modulating sepsis and subsequent endothelial injury. We hypothesized that COX inhibitors may attenuate endothelial dysfunction during sepsis, as measured by receptor-mediated bradykinin (BK)-induced vasoconstriction and/or receptor-independent hypoxic pulmonary vasoconstriction (HPV). Rats were administered intraperitoneally a nonselective COX inhibitor (indomethacin, 5 or 10 mg/kg) or a selective COX-2 inhibitor (NS-398, 4 or 8 mg/kg) 1 h before lipopolysaccharide (LPS, 15 mg/kg), or saline (control). Three hours later, the rats were anesthetized, the lungs were isolated, and pulmonary vasoreactivity was assessed with BK (0.3, 1.0, and 3.0 microg) and HPV (3% O(2)). Perfusion pressure was monitored as an index of vasoconstriction. To investigate what receptor-subtype is mediating BK responses, the BK(1)-receptor antagonist des-Arg(9)-[Leu(8)]-BK, the BK(2)-receptor antagonist HOE-140, or the thromboxane A(2)-receptor antagonist SQ 29548 (all at 1 microM) were added to the perfusate. BK-induced vasoconstriction was significantly increased in LPS lungs (1.4-5.2 mm Hg) compared with control (0.1-1.1 mm Hg). In LPS lungs, indomethacin 10 mg/kg significantly decreased BK vasoconstriction by 78% +/- 9%, whereas 5 mg/kg did not. NS-398, 4 mg/kg, significantly attenuated BK vasoconstriction at 0.3 microg (71% +/- 7%) and 1.0 microg (56% +/- 12%), whereas 8 mg/kg attenuated 0.3 microg BK (57% +/- 14%), compared with LPS lungs. HPV was increased in LPS lungs (21.5 +/- 2 mm Hg) compared with control lungs (9.8 +/- 0.6 mm Hg). Indomethacin 5 mg/kg increased HPV in LPS lungs; otherwise, HPV was not altered by COX inhibition. BK-induced vasoconstriction was prevented by BK(2), but not BK(1) or thromboxane A(2)-receptor antagonism. This study suggests that nonselective COX inhibition, and possibly inhibition of the inducible isoform COX-2, may attenuate sepsis-induced, receptor-mediated vasoconstriction in rats.

IMPLICATIONS

This study demonstrated that, in an isolated rat lung model, nonselective inhibition of the cyclooxygenase pathway, and possibly selective inhibition of the inducible cyclooxygenase-2 isoform, may attenuate sepsis-induced endothelial dysfunction.

Pulmonary vasoregulation by endothelin in conscious dogs after left lung transplantation

We tested the hypothesis that regulation of the pulmonary circulation by endogenous endothelin (ET) during normoxia and hypoxia was altered in conscious dogs 1 mo after left lung autotransplantation (LLA). Sham-operated control and post-LLA dogs were chronically instrumented to measure the left pulmonary vascular pressure-flow (LP-Q) relationship. LP-Q plots were generated on separate days during normoxia and hypoxia (arterial PO(2) approximately 50 Torr) in the intact condition, after selective ET(A)-receptor inhibition (BQ-485), and after combined ET(A+B)-receptor inhibition (bosentan). Although LLA resulted in a chronic increase in pulmonary vascular resistance, the ET-receptor antagonists had no effect on the LP-Q relationship during normoxia in either group. The magnitude of hypoxic pulmonary vasoconstriction (HPV) was flow dependent in both groups, and the HPV response was potentiated post-LLA compared with control. ET(A)-receptor inhibition attenuated the HPV response to the same extent in both groups. ET(A+B)-receptor inhibition attenuated the HPV response to a greater extent than did ET(A)-receptor inhibition alone, and this effect was greater post-LLA compared with control. Plasma ET-1 concentration only increased during hypoxia in the LLA group. These results indicate that ET does not regulate the baseline LP-Q relationship in either group. Both ET(A)- and ET(B)-receptor activation mediate a component of HPV in conscious dogs, and the vasoconstrictor influence of ET(B)-receptor activation is enhanced post-LLA.

Nitric oxide differentially attenuates microvessel response to hypoxia and hypercapnia in injured lungs

The issue of whether the acinar microvessel response to alveolar hypoxia and hypercapnia is impaired in injured lungs has not been vigorously addressed, despite the importance of knowing whether it is or not when treating patients with serious lung injury in terms of permissive hypercapnia. Applying a real-time laser confocal luminescence microscope, we studied hypoxia- and hypercapnia-induced changes in the diameter of the intra-acinar arterioles, venules, and capillaries of isolated rat lungs harvested from animals exposed for 48 h to 21% O(2) (group N) or 90% O(2) (group H). Measurements were made with and without inhibition of nitric oxide (NO) synthase (NOS) by N(omega)-nitro-L-arginine methyl ester or of cyclooxygenase (COX) by indomethacin at different basal vascular tones evoked by thromboxane A(2) (TXA(2)) analog. Hypoxia in the absence of TXA(2) contracted arterioles in group N but not in group H. Attenuated hypoxia-induced arteriole constriction was restored almost fully by inhibiting NOS and partially by inhibiting COX. Hypercapnia induced venule dilation in group N, but did not dilate venules in group H, irrespective of TXA(2). NOS inhibition in hypercapnia unexpectedly enhanced venule and arteriole dilation in group H. These responses no longer occurred when NOS and COX were inhibited simultaneously. In conclusion, microvessel reactions to hypoxia and hypercapnia are abnormal in hyperoxia-injured acini, in which NO directly attenuates hypoxia-induced arteriole constriction, whereas COX inhibited by excessive NO impedes hypercapnia-induced microvessel dilation.

Thromboxane mimics the pulmonary but not systemic vascular responses to bolus HCl injections in broiler chickens

Bolus i.v. injections of 1.2 N HCl elicit a rapid but transient pulmonary vasoconstriction in broiler chickens. In mammals, the pulmonary vasoconstrictive response to bolus acid injection depends on increased synthesis of thromboxane A2; however, the vascular responsiveness of domestic fowl to thromboxane previously had not been evaluated. In the present study, we tested the hypothesis that, if HCl triggers pulmonary vasoconstriction by stimulating thromboxane A2 synthesis in broilers, then bolus i.v. injections of the potent thromboxane A2 mimetic U44069 (9,11-dideoxy-9alpha,11alpha-epoxy-methanoprostaglandin++ + F2alpha; 1 micromol/mL; 0.5 mL injected volume) should trigger hemodynamic responses similar to those elicited by HCl (1.2 N; 1.5 mL injected volume). Both HCl and the thromboxane mimetic elicited twofold or greater increases in pulmonary vascular resistance, which in turn increased pulmonary arterial pressure by 50% despite concurrent reductions in cardiac output. The reductions in cardiac output were associated with reductions in stroke volume but not heart rate. The thromboxane mimetic also increased the total peripheral resistance, which minimized the reduction in mean systemic arterial pressure associated with the decrease in cardiac output. In contrast, HCl injections did not increase total peripheral resistance; consequently, the reduction in cardiac output caused the mean systemic arterial pressure to decrease by 30 mm Hg. Mannitol (2.5%; 1.5 mL) was injected i.v. as a volume control, and had no influence on any of the variables. This study provides the first direct evidence that thromboxane is a potent pulmonary vasoconstrictor in broilers, and provides support for the hypothesis that thromboxane mediates the pulmonary vasoconstrictive response to bolus i.v. injections of HCl. The differential response of the systemic vasculature to the thromboxane mimetic and HCl may indicate that cardiopulmonary responses to HCl injections are not mediated solely via thromboxane production. Alternatively, a direct dilatory effect of elevated hydrogen ion concentrations on the systemic vasculature may have counteracted any tendency for simultaneously evolved endogenous thromboxane to elicit systemic vasoconstriction.

Hypoxic contraction of small pulmonary arteries from normal and endotoxemic rats: fundamental role of NO

The present study was aimed at examining the role of nitric oxide (NO) in the hypoxic contraction of isolated small pulmonary arteries (SPA) in the rat. Animals were treated with either saline (sham experiments) or Escherichia coli lipolysaccharide [LPS, to obtain expression of the inducible NO synthase (iNOS) in the lung] and killed 4 h later. SPA (300- to 600-micrometer outer diameter) were mounted as rings in organ chambers for the recording of isometric tension, precontracted with PGF2alpha, and exposed to either severe (bath PO2 8 +/- 3 mmHg) or milder (21 +/- 3 mmHg) hypoxia. In SPA from sham-treated rats, contractions elicited by severe hypoxia were completely suppressed by either endothelium removal or preincubation with an NOS inhibitor [NG-nitro-L-arginine methyl ester (L-NAME), 10(-3) M]. In SPA from LPS-treated rats, contractions elicited by severe hypoxia occurred irrespective of the presence or absence of endothelium and were largely suppressed by L-NAME. The milder hypoxia elicited no increase in vascular tone. These results indicate an essential role of NO in the hypoxic contractions of precontracted rat SPA. The endothelium independence of HPV in arteries from LPS-treated animals appears related to the extraendothelial expression of iNOS. The severe degree of hypoxia required to elicit any contraction is consistent with a mechanism of reduced NO production caused by a limited availability of O2 as a substrate for NOS.

Impaired hypoxic vasoconstriction in intraacinar microvasculature in hyperoxia-exposed rat lungs

To assess the effects of exposure of the lung to hyperoxic conditions on reactivity of pulmonary microcirculation to hypoxic stimulation, we measured hypoxia-elicited overall pulmonary pressor changes (HPV) and microvascular diameter changes in intraacinar arterioles, venules, and capillaries in isolated perfused rat lungs exposed to a hyperoxic environment (90% O2). To estimate the importance of vasoactive prostaglandins and nitric oxide (NO) for HPV modification, we examined the roles of constitutive and inducible forms of cyclooxygenase (COX-1 and COX-2) and those of NO synthase (eNOS and iNOS). Indomethacin was used for inhibiting both COX-1 and COX-2, while NS-398 was used as a selective inhibitor of COX-2. Both eNOS and iNOS were suppressed by L-NAME, whereas iNOS alone was inhibited by aminoguanidine. Microvascular diameter was measured with a real-time confocal laser scanning luminescence microscope. We found that (1) exposure to hyperoxia caused overall HPV and arteriolar constriction to be attenuated; (2) the blunted HPV was restored by L-NAME but not by aminoguanidine, indomethacin, or NS-398; and (3) arteriolar constriction was improved by either L-NAME, aminoguanidine, or indomethacin but only slightly by NS-398. In conclusion, attenuation of overall HPV in hyperoxia-exposed lungs is explicable mainly by excessive NO generated via eNOS, while impaired arteriolar constriction is caused by NO yielded by eNOS and iNOS as well as by vasodilating prostaglandin(s) produced by COX-1.

Response of intra-acinar pulmonary microvessels to hypoxia, hypercapnic acidosis, and isocapnic acidosis

To elucidate the differential reactivity of pulmonary microvessels in the acini to hypoxia, excessive CO2, and increased H+, we investigated changes in the diameter of precapillary arterioles, postcapillary venules, and capillaries in isolated rat lungs on exposure to normocapnic hypoxia (2% O2), normoxic hypercapnia (15% CO2), and isocapnic acidosis (0.01 mol/L HCl). Microvascular diameters were precisely examined using a real-time confocal laser scanning luminescence microscope coupled to a high-sensitivity camera with an image intensifier. Measurements were made under conditions with and without indomethacin or N(omega)-nitro-L-arginine methyl ester to assess the importance of vasoactive substances produced by cyclooxygenase (COX) or NO synthase (NOS) as it relates to the reactivity of pulmonary microvessels to physiological stimuli. We found that acute hypoxia contracted precapillary arterioles that had diameters of 20 to 30 microm but did not constrict postcapillary venules of similar size. COX- and NOS-related vasoactive substances did not modulate hypoxia-elicited arteriolar constriction. Hypercapnia induced a distinct venular dilatation closely associated with vasodilators produced by COX but not by NOS. Arterioles were appreciably constricted in isocapnic acidosis when NOS, but not COX, was suppressed, whereas venules showed no constrictive response even when both enzymes were inhibited. Capillaries were neither constricted nor dilated under any experimental conditions. These findings suggest that reactivity to hypoxia, CO2, and H+ is not qualitatively similar among intra-acinar microvessels, in which COX- and NOS-associated vasoactive substances function differently.

Nitric oxide synthase inhibition restores hypoxic pulmonary vasoconstriction in sepsis

Hypoxic pulmonary vasoconstriction (HPV) is inhibited by inhaled nitric oxide (NO) in healthy animals and is blunted in endotoxemia. We investigated whether the loss of HPV during sepsis could be reversed by NO synthase (NOS) inhibition. Hypoxic challenges were induced in intubated, awake sheep breathing 100% nitrogen to the left lung and 100% oxygen to the right lung. HPV was assessed as the decrease in left pulmonary blood flow during hypoxia, measured with an ultrasonic flow probe around the left pulmonary artery. Group I (n = 5) received carrier solutions and Groups II (n = 6) and III (n = 8) received an infusion containing Pseudomonas aeruginosa. After 24 h, Group III also received an infusion of 6.6 mg.kg.h-1 N omega-monomethyl-L-arginine (L-NMMA). After 24 h of sepsis, HPV decreased from 60 +/- 9% in Group II and 56 +/- 4% in Group III to 27 +/- 2% and 26 +/- 4%, respectively. Group I showed no change in HPV. During infusion of L-NMMA, HPV increased to 38 +/- 4%. Pulmonary shunt during hypoxia increased in Group III to 161 +/- 10% of its baseline value, and decreased to 121 +/- 11% during infusion of L-NMMA. We conclude that L-NMMA improves but does not restore HPV, indicating that other vasodilatory mediators besides NO also influence HPV in sepsis.