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HIGHLIGHTED TOPICS
Pulmonary Circulation and Hypoxia
1Department of Anesthesia and Critical Care and the 2Cardiovascular Research Center, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts
Submitted 2 August 2004 ; accepted in final form 30 September 2004
| ABSTRACT |
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-nitro-L-arginine methyl ester (L-NAME) augmented the increase of LPVR induced by LMBO in wild-type mice (183 ± 67% in L-NAME treated vs. 109 ± 36% in saline treated, P < 0.01) but not in NOS3-deficient mice. Similarly, systemic oxygenation during one-lung ventilation was augmented by L-NAME in wild-type mice but not in NOS3-deficient mice. These findings indicate that NO derived from NOS3 modulates HPV in vivo and that inhibition of NOS3 improves systemic oxygenation during acute unilateral lung hypoxia. hypoxia; shunt; ventilation-perfusion matching
Nitric oxide (NO), synthesized by NO synthases (NOS1, NOS2, and NOS3), plays a pivotal role in regulation of vascular tone. Although all three NOS isoforms are found in the lung (5, 18, 26), the principal isoform expressed in the normal pulmonary vasculature is NOS3 (18, 23). We and others have previously reported that NOS3 contributes to the low pulmonary vascular resistance in mice (7, 20). Studies have suggested an important modulatory role of NO on HPV in a variety of species (4, 10, 23). For instance, inhaled NO reverses human HPV (9), whereas an acute, nonselective inhibition of NOS enhances HPV in isolated, perfused rabbit lungs (23). Using buffer-perfused isolated lungs from genetically modified mice, Fagan et al. (8) observed that NOS3 attenuated HPV to a greater extent than did NOS1 or NOS2. Although these studies support a prominent modulatory role of NOS3 on HPV, there is little information as to whether NO derived from NOS3 affects intrapulmonary blood shunting and systemic oxygenation in intact animals.
In the present study, we hypothesized that reduced endogenous production of NO due to a congenital deficiency of NOS3 or pharmacological inhibition of NOS would augment HPV in a mouse model of acute unilateral lung hypoxia. This model enabled us to examine the in vivo effects of regional hypoxia on pulmonary vascular tone, intrapulmonary shunt, as well as systemic oxygenation. We report that NOS3 deficiency augments HPV and systemic oxygenation during acute unilateral lung hypoxia in mice.
| MATERIALS AND METHODS |
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Experiments were performed in wild-type and NOS3/ mice treated with and without N
-nitro-L-arginine methyl ester (L-NAME), an inhibitor of all three NOS isoforms. Mice that were not treated with L-NAME received the same volume of vehicle (normal saline) as control.
Anesthesia and surgical preparation. Mice were anesthetized by intraperitoneal injection of ketamine (0.1 mg/g body wt) and fentanyl (0.25 µg/g). Muscle relaxation was obtained by intraperitoneal injection of pancuronium (5 µg/g) after tracheostomy. Volume-controlled ventilation was initiated at a respiratory rate of 100 breaths/min with a tidal volume of 11 ml/kg (model 687; Harvard Apparatus, Holliston, MA) at inspired O2 fraction = 1.0 and a positive end-expiratory pressure of 1 cmH2O. Systemic arterial pressure (SAP) was monitored via a PE-10 catheter placed in the right carotid artery. Lactated Ringer solution was infused via the carotid arterial catheter at 0.060.09 ml·h1·g1 with the use of a syringe pump (Kent Scientific, Torrington, CT). L-NAME (0.1 mg/g) or the same volume of saline (0.01 ml/g) was administered intraperitoneally immediately after tracheotomy (30 min before hemodynamic measurements). The dose of L-NAME was selected on the basis of previous studies (12, 20). All physiological signals were displayed and recorded with a digital data acquisition system (DI720; Dataq Instruments, Akron, OH).
Measurement of HPV.
Magnitude of HPV was assessed in 14 saline-treated wild-type mice, 13 saline-treated NOS3/ mice, 12 L-NAME-treated wild-type mice, and 10 L-NAME-treated NOS3/ mice. After thoracotomy, a small-vessel ultrasonic flow probe (0.5VB; Transonic Systems, Ithaca, NY) was placed around the left pulmonary artery to obtain left pulmonary artery blood flow (
lpa). A catheter (a 30-gauge needle connected to PE-10 tubing) was placed by direct puncture of the main pulmonary artery to measure pulmonary arterial pressure (PAP). The inferior vena cava (IVC) was partially occluded with a circumferential 5-0 silk ligature to transiently reduce cardiac output until
lpa was reduced by
50%. The slope, derived by linear regression between mean
lpa and mean PAP during IVC occlusion, represents left lung pulmonary vascular resistance (LPVR) (13, 14, 21). Unilateral alveolar hypoxia was induced by left mainstem bronchus occlusion (LMBO) with a microvascular clip. Transient IVC occlusion was repeated three times before LMBO and 5 min after the microvascular clip was placed (during LMBO). The percentage of increase in LPVR induced by LMBO was calculated as (LPVRduringLMBO LPVRbeforeLMBO)/LPVRbeforeLMBO x 100.
Measurements of intrapulmonary shunt fraction. Intrapulmonary shunt fraction (QS/QT) was determined in four additional groups of mice (n = 6 in each group): NOS3/ and wild-type mice that had one-lung ventilation with LMBO and NOS3/ and wild-type mice that had two-lung ventilation without LMBO. Blood samples were simultaneously withdrawn from the carotid and pulmonary arteries and analyzed for pH, PO2, and PCO2 by a blood-gas analyzer (Rapidlab 840; Chiron Diagnostics, Medfield, MA). Oxygen saturation and the concentration of total hemoglobin were measured with a hemoximeter (OSM3; Radiometer, Copenhagen, Denmark). Barometric pressure was noted daily to calculate alveolar PO2. Oxygen content of arterial blood, pulmonary end-capillary blood, and mixed venous blood was calculated assuming a hemoglobin oxygen binding capacity of 1.31 ml/g and pulmonary end-capillary blood oxygen saturation of 1.0. QS/QT was calculated with a standard equation (17).
Measurements of left atrial pressure and systemic oxygenation. In another subset of mice (9 saline-treated wild-type mice, 9 saline-treated NOS3/ mice, 5 L-NAME-treated wild-type mice, and 5 L-NAME-treated NOS3/ mice), a catheter (a 30-gauge needle connected to PE-10 tubing) was placed in the left atrium to measure left atrial pressure (LAP) before and 5 min after the left mainstem bronchus was occluded. To allow insertion of the left atrial catheter, the flow probe on the left pulmonary artery was omitted in these experiments. Arterial blood sample was obtained during LMBO by direct withdrawal from left ventricle with a 27-gauge needle for blood-gas analysis. To further assess the impact of NOS3 deficiency on systemic oxygenation during LMBO in four wild-type and four NOS3/ mice, a flexible polarographic Clark-type oxygen micro probe (0.5 mm OD; LICOX CC1.R, GMS, Kiel-Mielkendorf, Germany) was advanced into the abdominal aorta via the left femoral artery. PaO2 was measured in real time and recorded continuously. The PaO2 electrodes were calibrated before and after each experiment in air at ambient pressure according to the manufacturer's instructions.
Statistical analysis. All data are expressed as means ± SD. For results of HPV and QS/QT measurements, a two-way ANOVA with a post hoc Student-Newman-Keuls test was used for evaluating differences between genotypes and treatments. Measurements made with the intra-aortic PaO2 probe were compared by unpaired t-test in wild-type and NOS3/ mice. Measurements within the same experimental group before and during LMBO were compared by a paired t-test. If the normality test failed, Mann-Whitney rank sum test was applied. A SigmaStat 3.0 software package (Systat Software, Richmond, CA) was used for statistical analysis. A value of P < 0.05 was considered statistically significant.
| RESULTS |
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lpa, and increased LPVR compared with baseline values in both wild-type and NOS3/ mice (Table 1, P < 0.001 for all comparisons by paired t-test). Although LMBO more than doubled LPVR in both wild-type and NOS3/ mice, NOS3/ mice had a higher LPVR during LMBO than did wild-type mice (287 ± 69 vs. 205 ± 81 mmHg·ml1·min·g body wt, P < 0.05). Similarly, the percent increase in LPVR induced by LMBO was greater in NOS3/ mice than in wild-type mice (151 ± 39% vs. 109 ± 36%, P < 0.05, Fig. 1), suggesting that HPV was augmented in NOS3/ mice.
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Effect of L-NAME on the magnitude of HPV.
Before thoracotomy, L-NAME-treated wild-type mice had a higher SAP than did saline-treated wild-type mice (134 ± 19 vs. 92 ± 20 mmHg, P < 0.001). On the other hand, L-NAME treatment did not change SAP in NOS3/ mice. Administration of L-NAME increased PAP, decreased
lpa, and increased LPVR in both wild-type and NOS3/ mice before and during LMBO compared with saline-treated mice of corresponding genotypes (Table 1). Administration of L-NAME augmented the percent increase in LPVR induced by LMBO in wild-type mice (P < 0.01, Fig. 1) but not in NOS3/ mice.
Effect of NOS3 deficiency on intrapulmonary shunt. As depicted in Fig. 2, when both lungs were ventilated, there was no difference in QS/QT between NOS3/ mice and wild-type mice (9.4 ± 3.0 vs. 8.6 ± 1.7%). One-lung ventilation increased QS/QT in both wild-type and NOS3/ mice, compared with mice of respective genotype that had two-lung ventilation (P < 0.001 for both). However, one-lung ventilation with LMBO induced a larger increase in QS/QT in wild-type than in NOS3/ mice (21.7 ± 2.4 vs. 17.1 ± 3.6%, P < 0.05).
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| DISCUSSION |
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Although NOS3/ mice had higher SAP than did wild-type mice when measured with intact chest, thoracotomy and surgical instrumentation decreased SAP in both genotypes and abolished the difference in SAP (see Table 1). It is likely that surgical bleeding and mechanical interference of cardiovascular function by instruments (e.g., flow probe on left pulmonary artery and pulmonary artery catheter) decreased cardiac output after thoracotomy. It is also possible that open chest caused left ventricular dysfunction, thereby decreasing SAP and obscuring the hemodynamic differences between the two genotypes (11, 16). On the other hand, it is notable that NOS3 deficiency significantly augmented HPV despite the obscuring effects of thoracotomy on hemodynamic parameters in this model. It is conceivable that NOS3 deficiency may have an even greater impact on the magnitude of HPV in closed-chest animal.
In the present study, inhibition of all three isoforms of NOS by L-NAME enhanced HPV and systemic oxygenation in wild-type mice, confirming previous results obtained in isolated perfused rabbits lungs (23). L-NAME has also been shown to enhance systemic oxygenation in septic patients (2) and improve ventilation-perfusion distribution matching in anesthetized, mechanically ventilated dogs (3). These results indicate an important negative regulation of HPV by NO. Interestingly, L-NAME did not augment HPV or systemic oxygenation in NOS3/ mice in the present study. The latter results are in agreement with a recent report by Fagan et al. (8), who found that NG-nitro-L-arginine, another nonselective NOS inhibitor, doubled HPV in perfused lungs isolated from wild-type mice but not from NOS3/ mice. Together, these findings suggest that the ability of nonselective NOS inhibitors to augment HPV in response to unilateral alveolar hypoxia is predominantly dependent on inhibition of NOS3.
It is of note that the LMBO-induced increase of LPVR tended to be greater and systemic oxygenation was significantly better during LMBO in L-NAME-treated wild-type mice than in L-NAME-treated NOS3/ mice. It is possible that the vasoconstrictor effects of L-NAME were downregulated in NOS3/ mice due to chronic hypertension. Alternatively, life-long deficiency of NOS3 may lead to upregulation of an L-NAME-insensitive vasodilator, which serves to inhibit HPV.
Using the unilateral alveolar hypoxia model, our group (13) previously reported that mutation or inhibition of cytosolic phospholipase A2 attenuated HPV, and vasoconstrictors such as indomethacin and L-NAME restored HPV. The present study demonstrated that mutation or inhibition of an enzyme, NOS3, responsible for synthesis of a vasodilator, NO, augmented HPV and improved systemic oxygenation. These findings suggest that HPV is critically dependent on the underlying tone of the pulmonary vasculature. Moreover, these finding provide further support for the hypothesis that genetic mechanisms contribute to the regulation of HPV. It has recently become increasingly appreciated that polymorphisms in the NOS3 gene are associated with a variety of cardiovascular diseases, including essential hypertension, smoking-associated coronary heart disease, and carotid atherosclerosis (15, 22, 24). Droma et al. (6) reported that two polymorphisms in the NOS3 gene were associated with high-altitude pulmonary edema, a disorder characterized by exaggerated pulmonary hypertension. Taken together, these findings suggest that genetic variations in the NOS3 gene may contribute to differences among individuals in the manner in which the pulmonary vasculature responds to hypoxia.
In conclusion, we demonstrated that NOS3 deficiency augments HPV as well as reduces intrapulmonary shunt and improves systemic oxygenation during one-lung ventilation in mice. Inhibition of all three isoforms of NOS by L-NAME also enhances HPV in wild-type mice but exerts little or no effects in NOS3-deficient mice. These results support the hypothesis that NO derived from NOS3 is an important modulator of HPV in vivo, and inhibition of NOS improves oxygenation during regional lung hypoxia.
| GRANTS |
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| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
| REFERENCES |
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