Inhaled carbon monoxide does not cause pulmonary vasodilation in the late-gestation fetal lamb

Enhanced ventricular pump function and decreased reservoir backflow sustain rise in pulmonary blood flow after reduction of lung liquid volume in fetal lambs

Although a reduction in lung liquid volume increases fetal pulmonary blood flow, the changes in central flow patterns that sustain this increased pulmonary perfusion are unknown. To address this issue, eight anesthetized late-gestation fetal sheep were instrumented with pulmonary trunk (PT), ductus arteriosus (DA), and left pulmonary artery (PA) micromanometer catheters and transit-time flow probes, with blood flow profile and wave intensity analyses performed at baseline and after withdrawal of lung liquid via an endotracheal tube. Reducing lung liquid volume by 19 ± 6 ml/kg (mean ± SD) augmented right ventricular power by 34% (P < 0.001), with distribution of an accompanying increase in mean PT blood flow (245 ± 63 ml/min, P < 0.001) to the lungs (169 ± 91 ml/min, P = 0.001) and across the DA (77 ± 92 ml/min, P = 0.04). However, although PT and DA flow increments were confined to systole and were related to an increased magnitude of flow-increasing, forward-running compression waves, the rise in PA flow spanned both systole (108 ± 66 ml/min) and diastole (61 ± 32 ml/min). Flow profile analysis showed that the step-up in PA diastolic flow was associated with diminished PA diastolic backflow and accompanied by a lesser degree of diastolic right-to-left DA shunting. These data suggest that an increased pulmonary blood flow after reduction of lung liquid volume is associated with substantial changes in PT-DA-PA interactions and underpinned by two main factors: 1) enhanced right ventricular pump function that increases PA systolic inflow and 2) decreased PA diastolic backflow that arises from a fundamental change in PA reservoir function, thereby resulting in greater passage of systolic inflow through the lungs.

The heme oxygenase-carbon monoxide system in the regulation of cardiorespiratory function at high altitude

Pulmonary arterial hypertension is one of the most serious pathologies that can affect the 140 million people living at altitudes over 2500 m. The primary emphasis of this review is pulmonary artery hypertension in mammals (sheep and llamas) at high altitude, with specific focus on the heme oxygenase and carbon monoxide (HO-CO) system. We highlight the fact that the neonatal llama has neither pulmonary artery hypertension nor pulmonary vascular remodeling in the Andean altiplano. These neonates have an enhanced HO-CO system function, increasing the HO-1 protein expression and CO production by the pulmonary vessels, when compared to llamas raised at low altitude, or neonatal sheep raised at high altitude. The neonatal sheep has high altitude pulmonary artery hypertension in spite of enhancement of the NO system, with high eNOS protein expression and NO production by the lung. The gasotransmitters NO and CO are important in the regulation of the pulmonary vascular function at high altitudes in both high altitude acclimatized species, such as the sheep, and high altitude adapted species, such as the llama.

Effects of environmental tobacco smoke on the pulmonary circulation in the ovine fetus

OBJECTIVE

Epidemiologic studies have highlighted an association between maternal smoking and persistent pulmonary hypertension of the newborn infant. However, the impact of exposure to tobacco smoke on the perinatal pulmonary circulation is currently unknown. The purpose of this study was to assess the pulmonary vascular effects of environmental tobacco smoke in the fetus.

STUDY DESIGN

We performed surgery on 16 fetal lambs and placed catheters in the main pulmonary artery, aorta, and left atrium to measure pressures. An ultrasonic blood flow transducer was placed around the left pulmonary artery to measure blood flow. The ewes were exposed to tobacco smoke for 2 hours. In another set of experiments, the pulmonary vascular response to increased fetal O(2) tension has been assessed after 2 hours of tobacco smoke inhalation or not (control group).

RESULTS

Exposure to tobacco smoke decreased pulmonary blood flow by 30% and elevated pulmonary vascular resistance by 40%. The vasodilator response to increased O(2) tension was blunted after smoke inhalation compared with control animals. Smoke inhalation was associated with a decrease in fetal PaO(2) and SaO(2).

CONCLUSION

Exposure to environmental tobacco smoke in pregnant ewes impairs both basal tone and vascular reactivity of the fetal lung.

Long-term exposure to high-altitude chronic hypoxia during gestation induces neonatal pulmonary hypertension at sea level

We determined whether postnatal pulmonary hypertension induced by 70% of pregnancy at high altitude (HA) persists once the offspring return to sea level and investigated pulmonary vascular mechanisms operating under these circumstances. Pregnant ewes were divided into two groups: conception, pregnancy, and delivery at low altitude (580 m, LLL) and conception at low altitude, pregnancy at HA (3,600 m) from 30% of gestation until delivery, and return to lowland (LHL). Pulmonary arterial pressure (PAP) was measured in vivo. Vascular reactivity and morphometry were assessed in small pulmonary arteries (SPA). Protein expression of vascular mediators was determined. LHL lambs had higher basal PAP and a greater increment in PAP after N^G-nitro-l-arginine methyl ester (20.9 ± 1.1 vs. 13.7 ± 0.5 mmHg; 39.9 ± 5.0 vs. 18.3 ± 1.3 mmHg, respectively). SPA from LHL had a greater maximal contraction to K⁺ (1.34 ± 0.05 vs. 1.16 ± 0.05 N/m), higher sensitivity to endothelin-1 and nitroprusside, and persistence of dilatation following blockade of soluble guanylate cyclase. The heart ratio of the right ventricle-to-left ventricle plus septum was higher in the LHL relative to LLL. The muscle area of SPA (29.3 ± 2.9 vs. 21.1 ± 1.7%) and the protein expression of endothelial nitric oxide synthase (1.7 ± 0.1 vs. 1.1 ± 0.2), phosphodiesterase (1.4 ± 0.1 vs. 0.7 ± 0.1), and Ca²⁺-activated K⁺ channel (0.76 ± 0.16 vs. 0.30 ± 0.01) were greater in LHL compared with LLL lambs. In contrast, LHL had decreased heme oxygenase-1 expression (0.82 ± 0.26 vs. 2.22 ± 0.44) and carbon monoxide production (all P < 0.05). Postnatal pulmonary hypertension induced by 70% of pregnancy at HA promotes cardiopulmonary remodeling that persists at sea level.

[Carbon monoxide--poison or potential therapeutic?]

Carbon monoxide (CO) is much more than just a toxic gas. Carbon monoxide is produced endogenously by the enzyme heme oxygenase and has important functions under physiological and pathophysiological conditions. Recent studies suggested antioxidative, anti-inflammatory, antiproliferative, anti-apoptotic, and vasodilating characteristics. Regarding clinically-relevant diseases in anesthesiology and critical care medicine, such as adult respiratory distress syndrome (ARDS), sepsis, or during organ transplantation, cytoprotective properties have been demonstrated by low-dose CO in experimental models. In view of a potential CO application in future human studies, this review discusses what is known to date about CO as it relates to functional, protective and toxic aspects.

Soluble guanylate cyclase activator reverses acute pulmonary hypertension and augments the pulmonary vasodilator response to inhaled nitric oxide in awake lambs

BACKGROUND

Inhaled nitric oxide (NO) is a potent and selective pulmonary vasodilator, which induces cGMP synthesis by activating soluble guanylate cyclase (sGC) in ventilated lung regions. Carbon monoxide (CO) has also been proposed to influence smooth muscle tone via activation of sGC. We examined whether direct stimulation of sGC by BAY 41-2272 would produce pulmonary vasodilation and augment the pulmonary responses to inhaled NO or CO.

METHODS AND RESULTS

In awake, instrumented lambs, the thromboxane analogue U-46619 was intravenously administered to increase mean pulmonary arterial pressure to 35 mm Hg. Intravenous infusion of BAY 41-2272 (0.03, 0.1, and 0.3 mg x kg(-1) x h(-1)) reduced mean pulmonary arterial pressure and pulmonary vascular resistance and increased transpulmonary cGMP release in a dose-dependent manner. Larger doses of BAY 41-2272 also produced systemic vasodilation and elevated the cardiac index. N(omega)-nitro-l-arginine methyl ester abolished the systemic but not the pulmonary vasodilator effects of BAY 41-2272. Furthermore, infusing BAY 41-2272 at 0.1 mg x kg(-1) x h(-1) potentiated and prolonged the pulmonary vasodilation induced by inhaled NO (2, 10, and 20 ppm). In contrast, inhaled CO (50, 250, and 500 ppm) had no effect on U-46619-induced pulmonary vasoconstriction before or during administration of BAY 41-2272.

CONCLUSIONS

In lambs with acute pulmonary hypertension, BAY 41-2272 is a potent pulmonary vasodilator that augments and prolongs the pulmonary vasodilator response to inhaled NO. Direct pharmacological stimulation of sGC, either alone or in combination with inhaled NO, may provide a novel approach for the treatment of pulmonary hypertension.

Heme oxygenase does not contribute to control of basal vascular tone in isolated blood-perfused rat lung

BACKGROUND

Vasoconstriction in pulmonary ischemia-reperfusion injury may involve dysfunction of the physiologic vasodilation of pulmonary arteries. Little is known of the relative importance of heme oxygenase (HO)/carbon monoxide (CO)-dependent vs nitric oxide synthase (NOS)/nitric oxide (NO)-dependent vasodilation of the pulmonary vasculature. We evaluated the significance of HO function on basal pulmonary vascular resistance (PVR) and compared it with the function of NOS.

METHODS

Using an isolated blood-perfusion model, lungs of Lewis rats were assigned to 3 groups (n = 6/group). After stabilization, either an inhibitor of HO (tin-protoporphyrin-9 [SnPP-9]) or an inhibitor of NOS (NG-nitro-L-arginine methylester [L-NAME]) was added to the perfusate (50 micromol/liter and 1 mmol/liter as the final concentration, respectively). Lungs receiving saline served as controls. Gas exchange, hemodynamic and respiratory functions and the levels of cyclic 3',5'-guanosine monophosphate (cGMP) in the perfusate were measured.

RESULTS

Inhibition of NOS by L-NAME resulted in a significant (p < 0.01) increase in PVR (DeltaPVR: 0.110 +/- 0.012 cm H(2)O/ml. min) within 5 minutes. In contrast, PVR was minimally affected by SnPP-9 (DeltaPVR: 0.005 +/- 0.005 cm H(2)O/ml. min), which was comparable to control lungs (DeltaPVR: 0.012 +/- 0.005 cm H(2)O/ml. min). The level of cGMP in the perfusate 5 minutes after drug application was markedly, but not significantly, lower in the L-NAME group (1.67 +/- 0.74 nmol/liter) when compared with controls (2.69 +/- 0.89 nmol/liter) and SnPP-9-treated lungs (2.65 +/- 0.66 nmol/liter).

CONCLUSIONS

NOS but not HO contributes to the control of basal vascular tone in the rat lung.

Circulatory effects and kinetics following acute administration of carbon monoxide in a porcine model

Carbon monoxide is produced in the endothelial cells and has possible vasodilator activity through three different pathways. The aim of this study was to demonstrate circulatory effects after administration of saturated carbon monoxide blood and to describe the pharmacokinetics of carbon monoxide. Six pigs were anesthetized and 150 ml blood was removed. This blood was bubbled with carbon monoxide until the carboxyhemoglobin (COHb) levels were 90-99%. A specific amount of this blood was then injected back to the animal. At predetermined times; arterial and mixed venous blood was drawn and analyzed for carbon monoxide. Systemic and pulmonary vascular resistance index (SVRi and PVRi) were measured and exhaled air was sampled and measured for carbon monoxide. Blood samples were gathered over 300 minutes along with measurements of invasive pressures, heart rate, cardiac output, oxygen saturation (SpO2), Hb, temperature and blood gases. We conclude that this type of exposure to carbon monoxide appears to have little or no effect on general vasomotor tone and, after correcting for basal levels of carbon monoxide, elimination occurs through the lungs as predicted by a single compartment model. The half-life of carbon monoxide was determined to be 60.5 minutes (SEM 4.7).

Low-dose inhaled carbon monoxide reduces pulmonary vascular resistance during acute hypoxemia in adult sheep

Carbon monoxide (CO) is produced by the action of the heme oxygenase (HO) complex through the oxidation of heme. CO, like nitric oxide (NO), is a molecular gas that among other actions stimulates guanylyl cyclase and increases cGMP levels in smooth muscle cells, regulating the vascular tone. Acute hypoxia generates pulmonary hypertension and increases the expression of inducible HO isoform (HO-1) in the vascular endothelium. Inhaled NO causes a potent pulmonary vasodilation. We hypothesized that inhaled CO might produce similar actions as NO on pulmonary vascular resistance (PVR). To test our contention, we studied the effects of inhaled CO (40 ppm) in the augmented PVR observed during hypoxemia. Five chronically instrumented German Merino sheep were submitted to a protocol consisting of 20 min of normoxemia (N), 20 min of isocapnic hypoxemia (H20), 20 min of isocapnic hypoxemia plus CO 40 ppm (H40), and 20 min of recovery (R). In the control protocol, we did not administer inhaled CO. Arterial gases and pH, percentage of carboxyhemoglobin (COHb), systemic and pulmonary arterial pressure, systemic and pulmonary vascular resistance, and cardiac output were measured during each period. During H20 period, there was a significant increase in cardiac output and PVR in sheep submitted to both protocols. The sheep treated with inhaled CO (H40 + CO) showed a modest but significant decrease (16%) in the elevated PVR. Our data indicate that inhaled CO decreases pulmonary vascular resistance associated with acute hypoxemia in adult sheep.

Homogeneous segmental profile of carbon monoxide-mediated pulmonary vasodilation in rats

Carbon monoxide (CO) has been proposed to attenuate the vasoconstrictor response to local hypoxia that contributes to pulmonary hypertension. However, the segmental response to CO, as well as its mechanism of action in the pulmonary circulation, has not been fully defined. To investigate the hemodynamic response to exogenous CO, lungs from male Sprague-Dawley rats were perfused with physiological saline solution. Measurements were made of pulmonary arterial, venous, and capillary pressures. Lungs were constricted with the thromboxane mimetic U-46619. To examine the vasodilatory response to CO, 500 microl of CO-equilibrated physiological saline solution or vehicle were injected into the arterial line. Additionally, CO and vehicle responses were examined in the presence of the soluble guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 10 microM) or the larger conductance calcium-activated K(+) (BK(Ca)) channel blockers tetraethylammonium chloride (10 mM) and iberiotoxin (100 nM). CO administration decreased vascular resistance to a similar degree in both vascular segments. This vasodilatory response was completely abolished in lungs pretreated with ODQ. Furthermore, CO administration increased whole lung cGMP content, which was prevented by ODQ. Neither tetraethylammonium chloride nor iberiotoxin affected the CO response. We conclude that exogenous CO administration causes vasodilation in the pulmonary vasculature via a soluble guanylyl cyclase-dependent mechanism that does not likely involve activation of K(Ca) channels.