In vivo attenuation of endothelium-dependent pulmonary vasodilation by methylene blue

Effects of lack of pulsatility on pulmonary endothelial function in the Fontan circulation

OBJECTIVES

Continuous flow in the Fontan circulation results in impairment of pulmonary artery endothelial function, increased pulmonary arterial resistance, and, potentially, late failure of Fontan circulation. We investigated the mechanisms of vascular remodeling and altered vascular reactivity associated with chronic privation of pulsatility on pulmonary vasculature.

METHODS

A total of 30 pigs were evenly distributed in 3 groups: 10 underwent a sham procedure (group I) and 20 underwent a cavopulmonary shunt between the superior vena cava and right pulmonary artery--10 with complete ligation of the proximal right pulmonary artery (group II, nonpulsatile) and 10 with partial ligation (group III, micropulsatile). At 3 months postoperatively, the in vivo hemodynamics, in vitro vasomotricity (concentration response curves on pulmonary artery isolated rings), and endothelial nitric oxide synthase protein level were assessed. A comparison between group and between the right and left lung in each group was performed.

RESULTS

Group II developed right pulmonary hypertension and increased right pulmonary resistance. Endothelial function was altered in group II, as reflected by a decrease in the vasodilation response to acetylcholine and ionophoric calcium but preservation of the nonendothelial-dependent response to sodium nitroprusside. Group III micropulsatility attenuated pulmonary hypertension but did not prevent impairment of the endothelial-dependant relaxation response. Right lung Western blotting revealed decreased endothelial nitric oxide synthase in group II (0.941 ± 0.149 vs sham 1.536 ± 0.222, P = .045) that was preserved in group III (1.275 ± 0.236, P = .39).

CONCLUSIONS

In a chronic model of unilateral cavopulmonary shunt, pulsatility loss resulted in an altered endothelial-dependant vasorelaxation response of the pulmonary arteries. Micropulsatility limited the effects of pulsatility loss. These results are of importance for potential therapies against pulmonary hypertension in the nonpulsatile Fontan circulation, by retaining accessory pulmonary flow or pharmaceutical modulation of nonendothelial-dependant pulmonary vasorelaxation.

Treatment of obstructive sleep apnoea leads to enhanced pulmonary vascular nitric oxide release

BACKGROUND

Obstructive sleep apnoea (OSA) is associated with pulmonary hypertension, however neither the pathogenesis of pulmonary vascular disease nor the effect of successful treatment of OSA on pulmonary vascular physiology has been characterised.

METHODS

Seven subjects aged 52 (range 36-63) years with moderate to severe obstructive sleep apnoea (apnoea-hypopnoea index>15/h) had detailed pulmonary vascular reactivity studies, before and after 3 months of successful treatment with nasal continuous positive airways pressure (CPAP). On both occasions, we measured pulmonary pressure, flow velocity, flow and resistance, at baseline and in response to acetylcholine (an endothelium-dependent dilator), sodium nitroprusside (an endothelium-independent dilator), l-NMMA (an antagonist of nitric oxide synthesis) and l-Arginine (the substrate of nitric oxide).

RESULTS

At baseline, pulmonary flow increased in response to acetylcholine and nitroprusside and fell in response to l-NMMA. Following CPAP treatment, the decrease in flow to l-NMMA was significantly greater (to 62+/-6% of control value vs 85+/-6% of pre-treatment; p=0.01), consistent with enhanced basal release of nitric oxide. The acetylcholine response tended to be greater after treatment (174+/-26% of control vs 147+/-12% of pre-CPAP, p=0.22), however the nitroprusside response was unchanged.

CONCLUSION

Successful treatment of obstructive sleep apnoeic episodes in sleep results in enhanced nitric oxide release by the pulmonary microvascular circulation.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Shear stress regulation of endothelial NOS in fetal pulmonary arterial endothelial cells involves PKC

We have shown that increased pulmonary blood flow at birth increases the activity and expression of endothelial nitric oxide (NO) synthase (eNOS). However, the signal transduction pathway regulating this process is unclear. Because protein kinase C (PKC) has been shown to be activated in response to shear stress, we undertook a study to examine its role in mediating shear stress effects on eNOS. Initial experiments demonstrated that PKC activity increased in response to shear stress. NO production in response to shear stress was found to be biphasic, with an increase in NO release up to 1 h, a plateau phase until 4 h, and another increase between 4 and 8 h. PKC inhibition reduced the initial rise in NO release by 50% and the second increase by 70%. eNOS mRNA and protein levels were also increased in response to shear stress, whereas PKC inhibition prevented this increase. The stimulation of PKC activity with phorbol ester increased eNOS gene expression without increasing NO release. These results suggest that PKC may play different roles in shear stress-mediated release of NO and increased eNOS gene expression.

Alterations in systemic and local colonic hemodynamic variables associated with intravenous infusion of ATP-MgCl2 in healthy anesthetized horses

OBJECTIVE

To characterize alterations in systemic and local colonic hemodynamic variables associated with IV infusion of ATP-MgCl2 in healthy anesthetized horses.

ANIMALS

12 adult horses.

PROCEDURE

Six horses were given ATP-MgCl2, IV, beginning at a rate of 0.1 mg of ATP/kg of body weight/min with incremental increases until a rate of 1.0 mg/kg/min was achieved. The remaining 6 horses were given an equivalent volume of saline (0.9% NaCl) solution over the same time period. Colonic and systemic hemodynamic variables and colonic plasma nitric oxide (NO) concentrations were determined before, during, and after infusion.

RESULTS

Infusion of ATP-MgCl2 caused a rate-dependent decrease in systemic and colonic vascular resistance, principally via its vasodilatory effects. A rate of 0.3 mg of ATP/kg/min caused a significant decrease in systemic and colonic arterial pressure and colonic vascular resistance without a significant corresponding decrease in colonic arterial blood flow. Consistent alterations in NO concentrations of plasma obtained from colonic vasculature were not detected, despite profound vasodilatation of the colonic arterial vasculature.

CONCLUSIONS AND CLINICAL RELEVANCE

Results revealed that IV infusion of ATP-MgCl2 may be beneficial in maintaining colonic perfusion in horses with ischemia of the gastrointestinal tract, provided a sufficient pressure gradient exists to maintain blood flow.

Bidirectional cavopulmonary shunt with right ventricular outflow patency: the impact of pulsatility on pulmonary endothelial function

OBJECTIVE

Although in vitro studies have suggested the importance of flow pulsatility in endothelial function, few reports have focused on pulmonary endothelial function under decreased pulsatile flow after a bidirectional cavopulmonary shunt with or without an additional pulmonary flow source. The purpose of the present study was to assess the pulmonary endothelial function after bidirectional cavopulmonary shunt.

METHODS AND RESULTS

Pulmonary vasodilating response was evaluated in 10 patients 0.4 to 7.0 years (median 1.6 years) after bidirectional cavopulmonary shunt who were provided an additional flow source by retaining the pulmonary outflow tract and in 8 control subjects. Average pulmonary flow velocity was measured with a Doppler flow wire placed in the segmental lower lobe pulmonary artery during incremental infusion of acetylcholine (10(-8), 10(-7), 10(-6), and 10(-5) mol/L) and then of nitroglycerin (0.5 and 1.0 microg. kg(-1). min(-1)) after recovery. In the control subjects, a dose-dependent increase in flow velocity was observed in response to acetylcholine (maximum increase was 155% +/- 17% of baseline) and to nitroglycerin (maximum increase was 151% +/- 20% of baseline). In contrast, patients showed a significantly impaired response to acetylcholine (maximum increase was 124% +/- 17% of baseline; P <.01 vs control), whereas the response to nitroglycerin was preserved (138% +/- 12% of baseline; P =.09 vs control). In addition, the maximum response to acetylcholine correlated significantly with the pulmonary pulse pressure (r = 0.89, P <.01) and with the pulmonary flow pulsatility (r = 0.88, P <.01).

CONCLUSIONS

These results clearly suggest that patients after bidirectional cavopulmonary shunt show pulmonary endothelial functional attenuation and, of more importance, that decreased pulsatility of cavopulmonary flow is mainly responsible for this endothelial abnormality.

Novel catheterization technique for the in vivo measurement of pulmonary vascular responses in rats

A novel cardiac catheterization technique was devised to investigate the pulmonary arterial pressure-blood flow relationship in intact spontaneously breathing rats (ISBR) under physiological conditions with constant left atrial pressure and controlled blood flow within the normal range. Observations using this new technique in vivo were contrasted with data derived with isolated perfused rat lungs in vitro. Unlike results in in vitro isolated perfused rat lungs, the pressure-flow curves in vivo were curvilinear, with pulmonary artery pressure increasing more rapidly at low pulmonary blood flows of 4-8 ml/min and less rapidly at higher flow rates. Pressure-flow curves were reproducible and were not altered by 1-1.5 h of arrested perfusion, cyclooxygenase blockade, or perfusion with aortic or mixed venous blood. In contrast to results in in vitro isolated perfused rat lungs, NG-nitro-L-arginine methyl ester (L-NAME) increased pulmonary arterial pressure at all but the lowest flow rates with a slight effect on the curvilinear pressure-flow relationship. L-NAME reversed pulmonary vasodilator responses to acetylcholine and bradykinin and enhanced the pulmonary vasodilator response to nitroglycerin. The present data suggest that actively induced pulmonary hypertension is under greater control by endothelium-derived relaxing factor (EDRF). Unlike previous results in in vitro perfused rat lungs, results in ISBR demonstrate that the pulmonary vasodilator response to adrenomedullin-(13-52) is not mediated by calcitonin gene-related peptide receptors, which are not coupled to the release of EDRF. These results indicate that this novel technique may provide a useful model for the study of the pulmonary circulation in the intact chest rat.

Models of persistent pulmonary hypertension of the newborn (PPHN) and the role of cyclic guanosine monophosphate (GMP) in pulmonary vasorelaxation

At birth, a marked decrease in pulmonary vascular resistance allows the lung to establish gas exchange. Persistent pulmonary hypertension of the newborn (PPHN) occurs when this normal adaptation of gas exchange does not occur. We review animal models used to study the pathogenesis and treatment of PPHN. Both acute models, such as acute hypoxia and infusion of vasoconstrictors, and chronic models of PPHN created both before and immediately after birth are described. Inhaled nitric oxide is an important emerging therapy for PPHN. We review nitric oxide receptor mechanisms, including soluble guanylate cyclase, which produces cGMP when stimulated by nitric oxide, and phosphodiesterases, which control the intensity and duration of cGMP signal transduction. A better understanding of these mechanisms of regulation of vascular tone may lead to safer use of nitric oxide and improved clinical outcomes.

Ventilation and oxygenation induce endothelial nitric oxide synthase gene expression in the lungs of fetal lambs

At birth, ventilation and oxygenation immediately decrease pulmonary vascular resistance (PVR) and increase pulmonary blood flow (PBF); more gradual changes occur over the next several hours. Nitric oxide, produced by endothelial nitric oxide synthase (eNOS), mediates these gradual changes. To determine how ventilation and oxygenation affect eNOS gene expression, 12 fetal lambs were ventilated for 8 h without changing fetal descending aortic blood gases or pH (rhythmic distension) or with 100% oxygen (O2 ventilation). Vascular pressures and PBF were measured. Total RNA, protein, and tissue sections were prepared from lung tissue for RNase protection assays, Western blotting, and in situ hybridization. O2 ventilation increased PBF and decreased PVR more than rhythmic distension (P < 0.05). Rhythmic distension increased eNOS mRNA expression; O2 ventilation increased eNOS mRNA expression more and increased eNOS protein expression (P < 0.05). To define the mechanisms responsible for these changes, ovine fetal pulmonary arterial endothelial cells were exposed to 1, 21, or 95% O2 or to shear stress. 95% O2 increased eNOS mRNA and protein expression (P < 0.05). Shear stress increased eNOS mRNA and protein expression (P < 0.05). Increased oxygenation but more importantly increased PBF with increased shear stress induce eNOS gene expression and contribute to pulmonary vasodilation after birth.

Pulmonary vasoconstriction and hypertension in mice with targeted disruption of the endothelial nitric oxide synthase (NOS 3) gene

NO, synthesized in endothelial cells by endothelial NO synthase (NOS 3), is believed to be an important endogenous pulmonary vasodilator substance that contributes to the normal low pulmonary vascular resistance. To selectively investigate the role of NOS 3 in the pulmonary circulation, mice with targeted disruption of the NOS 3 gene were studied. Pulmonary hemodynamics were studied by measuring pulmonary artery pressure, left ventricular end-diastolic pressure, and lower thoracic aortic flow by using a novel open-chest technique. Transient partial occlusion of the inferior vena cava was used to assess the pulmonary artery pressure-flow relationship. Tension developed by isolated pulmonary artery segments after acetylcholine stimulation was measured in vitro. The histological appearance of NOS 3-deficient and wild-type murine lungs was compared. NOS 3-deficient mice (n = 27), when compared with wild-type mice (n = 32), had pulmonary hypertension (pulmonary artery pressure, 19.0 +/- 0.8 versus 16.4 +/- 0.6 mm Hg [mean +/- SE]; P < .05) that was due to an increased total pulmonary resistance (62 +/- 6 versus 33 +/- 2 mm Hg.min.g.mL-1; P < .001). In vitro, acetylcholine induced vasodilation in the main pulmonary arteries of wild-type but not NOS 3-deficient mice. The morphology of the lungs of NOS 3-deficient mice did not differ from that of wild-type mice. We conclude that NOS 3 is a key enzyme responsible for providing basal pulmonary NO release. Congenital NOS 3 deficiency produces mild pulmonary hypertension in mice.

Oxygen-induced pulmonary vasodilation is mediated by adenosine triphosphate in newborn lambs

In the fetal lamb, oxygen-induced pulmonary vasodilation is attenuated by the combined use of purinergic receptor P1 and P2y antagonists, which block the effect of adenosine and adenosine triphosphate (ATP), respectively, and by N(omega)-nitro-L-arginine [an inhibitor of endothelium-derived nitric oxide (EDNO) synthesis]. In the newborn lamb, oxygen-induced pulmonary vasodilation is not blocked by N(omega)-nitro-L-arginine. We investigated the role of ATP and adenosine in oxygen-induced pulmonary vasodilation in eight newborn lambs with pulmonary hypertension induced by the thromboxane mimic, U46619. The hemodynamic effects of hyperoxia, ATP, adenosine, sodium nitroprusside (SNP), and acetylcholine (ACh) were compared before and after purinergic receptor blockade with Cibacron blue (CB, a P2y-receptor antagonist) and 8-phenyltheophylline (8PT, a P1-receptor antagonist) individually, together, and on a separate day, after infusion of N(omega)-nitro-L-arginine. During pulmonary hypertension, combined pretreatment with 8PT and CB attenuated the decrease in pulmonary arterial pressure caused by hyperoxia (11.3 vs. 35.2%), ATP (10.6 vs. 32.2%), and adenosine (1.9 vs. 33.7%) without change in the effect of ACh or SNP (p < 0.05). N(omega)-Nitro-L-arginine attenuated the pulmonary vasodilation caused by ATP and ACh but not by hyperoxia, adenosine, or SNP. In the newborn lamb, the pulmonary vasodilating effect of both oxygen and ATP are attenuated by combined P1 and P2y purinergic-receptor antagonists. Postnatally, oxygen-induced pulmonary vasodilation appears to be mediated by ATP through purinergic receptors.

Effect of sodium nitroprusside on compound action potential thresholds in the gerbil cochlea

The presence of active nitric oxide synthase (NOS) in the spiral ganglion cells of the cochlea suggests that the neuromodulator nitric oxide (NO) may play a role in hearing. This study investigated the effects of sodium nitroprusside (SNP), an NO donor, upon cochlear function mediated through its activation of guanylate cyclase. In gerbils, cochlear compound action potential (CAP) thresholds were recorded after cochlear perfusions of control and test solutions in four experimental groups. Perfusions were performed using the following: artificial perilymph solution (APS); the NO donor SNP; the guanylate cyclase inhibitor methylene blue (MB); and sodium dodecyl sulfate (SDS), which facilitates MB entrance into cells. SNP caused significant elevations of CAP thresholds from baseline (25 dB SPL +/- 1.54 dB to 64.3 dB SPL +/- 2.54 dB). SNP with MB also resulted in significant CAP threshold elevations (29.4 dB SPL +/- 4.27 dB to 38.1 dB SPL +/- 4.0 dB); however, these elevations were significantly lower than those seen in SNP perfusions without MB. Drilling perfusion holes and perfusion of APS, APS/SDS, and MB/SDS/APS solutions did not significantly affect CAP thresholds. These results suggest that the NO donor nitroprusside does affect cochlear neuromodulation and effects this mediation in part through NO activation of guanylate cyclase.

Differential responses of ovine intrapulmonary arteries and veins to acetylcholine

We compared the effects of acetylcholine (Ach) in intrapulmonary arteries and veins of adult sheep. Preconstricted arterial rings with endothelium relaxed with Ach whereas arteries without endothelium or pretreated with NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide (NO) synthesis, did not dilate. Venous rings with or without endothelium, whether under resting tension or preconstricted, always contracted with Ach. However, preconstricted veins dilated with bradykinin, another endothelium-dependent vasodilator. Preconstricted veins pretreated with indomethacin or SQ29548, a prostaglandin H2 (PGH2)/TxA2 receptor blocker, did not constrict but rather dilated with Ach. This dilation was abolished with removal of endothelium or treatment with L-NAME, indicating that endothelium-derived NO (EDNO) was mediating the dilation. We conclude that Ach is an endothelium-dependent vasodilator in ovine intrapulmonary arteries, whereas in veins, Ach elicits two responses: EDNO-mediated vasodilation and vasoconstriction mediated by TxA2/PGH2.

Methylene blue does not neutralize heparin after cardiopulmonary bypass

OBJECTIVE

It was hypothesized that methylene blue could neutralize heparin in patients after cardiopulmonary bypass and become an alternative to protamine.

DESIGN

This was a prospective unblinded study with patients serving as their own controls. SETTING PARTICIPANTS: A single, large university hospital. This study was conducted in patients scheduled for elective aortocoronary bypass grafting.

INTERVENTIONS

When heparin was to be neutralized, patients received methylene blue, 2 to 12 mg/kg, diluted in 50 mL and infused over 20 minutes. If a clot was not observed or the activated coagulation time (ACT) remained elevated, protamine, 250 mg, was administered and the ACT was repeated.

MEASUREMENTS AND MAIN RESULTS

No patient demonstrated clot after methylene blue infusion. ACT did not return to preheparin values in any patient. All patients required protamine to establish hemostasis. Protamine restored the ACT to preheparin values in every patient. The one patient who received 12 mg/kg experienced severe pulmonary hypertension.

CONCLUSIONS

Methylene blue does not neutralize heparin after cardiopulmonary bypass.

Role of nitric oxide in the local regulation of pulmonary vascular resistance in humans

BACKGROUND

Endothelium-derived nitric oxide (NO) may be an important mediator of vascular resistance in the pulmonary circulation. We tested the hypotheses that in conscious adults the endothelium, through NO production, is important in maintaining basal pulmonary vascular resistance and that it can increase NO production further in response to receptor-mediated stimulation, leading to further vasodilation.

METHODS AND RESULTS

Pulmonary arterial resistance vessel function was studied within the distribution of a segmental lower lobe pulmonary artery in eight conscious adults 37 to 76 years old who were undergoing cardiac catheterization. Segmental blood flow was determined with use of a Doppler-tip guide wire and quantitative angiography. Drugs were administered locally within the segmental artery through an infusion catheter. NG-Monomethyl-L-arginine (L-NMMA) was used as a specific inhibitor of NO production, whereas acetylcholine (ACh) was used to test receptor-mediated vasodilation. To demonstrate that vasodilation to ACh was NO dependent, ACh response was tested alone, in the presence of L-NMMA, and in the presence of a control constrictor phenylephrine. Basal pulmonary vascular resistance was NO dependent because L-NMMA infusion resulted in a dose-dependent decrease in local flow velocity (P < .005), with flow decreasing 33% at the highest dose of L-NMMA. ACh infusion resulted in a dose-dependent increase in flow velocity (P = .001). The ACh response was at least in part NO dependent because it was diminished by the presence of L-NMMA (P < .05). The effect of L-NMMA on the ACh response was not due to nonspecific preconstriction because L-NMMA diminished the ACh response significantly more than did the endothelium-independent constrictor phenylephrine (P < .05) despite comparable preconstriction.

CONCLUSIONS

In healthy conscious adults, (1) normal basal pulmonary resistance is maintained in part by continuous local production of NO and (2) the local NO production is responsive to receptor-mediated stimulation, leading to further vasodilation, and can be tested with ACh.

Effects of methylene blue on oxygen availability and regional blood flow during endotoxic shock

OBJECTIVE

We hypothesized that methylene blue, by inhibiting the activation of soluble guanylate cyclase mediated by nitric oxide, may reverse systemic hypotension, enhance myocardial function, and improve peripheral distribution of blood flow during endotoxic shock.

DESIGN

Randomized, controlled, acute intervention study.

SETTING

University intensive care laboratory.

SUBJECTS

Twenty-one healthy, anesthetized, mongrel dogs, weighing 26 +/- 4 kg.

INTERVENTIONS

Groups 1 (n = 7) and 2 (n = 7) received endotoxin (2 mg/kg iv) alone combined with increasing doses of 2.5, 5, 10, and 20 mg/kg iv of methylene blue. Each dose was administrated for 30 mins with a free interval of 30 mins. Group 3 (n = 7) served as a control group, receiving the same doses of methylene blue in the absence of endotoxin. All animals were given normal saline to keep cardiac filling pressures constant. Blood flow probes were placed around the superior mesenteric, renal, and femoral arteries to measure regional blood flow by ultrasonic technique. Data were collected every 30 mins during the study.

MEASUREMENTS AND MAIN RESULTS

After endotoxemia, methylene blue increased systemic and pulmonary arterial pressure and vascular resistances in a dose-dependent manner up to 10 mg/kg, but had no effect on cardiac index. At the highest dose, methylene blue decreased arterial pressure and systemic vascular resistance. At doses of methylene blue of < or = 10 mg/kg, mesenteric and femoral blood artery flow increased. At the highest dose of 20 mg/kg, femoral artery blood flow further increased, but mesenteric blood flow decreased. Renal artery blood flow was unaffected by methylene blue. In the absence of endotoxin, methylene blue at doses of 2.5 or 5 mg/kg did not alter mean arterial pressure, but reduced cardiac index, indicating an increase in systemic vascular resistance. In contrast, the higher doses of 10 or 20 mg/kg of methylene blue decreased mean arterial pressure and systemic vascular resistance. However, pulmonary arterial pressure and pulmonary vascular resistance increased in a dose-dependent manner. Mesenteric and renal artery blood flow decreased but femoral blood flow increased. As in the presence of endotoxin, methylene blue induced dose-related increases in oxygen uptake and oxygen extraction ratio, but did not alter oxygen delivery. Methylene blue largely attenuated the endotoxin-induced increase in plasma nitrite concentrations.

CONCLUSIONS

Low and moderate doses of methylene blue can significantly increase arterial blood pressure but not cardiac index during endotoxic shock. Methylene blue infusion may selectively increase mesenteric blood flow. High doses of methylene blue can worsen systemic hypotension, myocardial depression, and pulmonary hypertension after endotoxemia.

Vascular pharmacology of methylene blue in vitro and in vivo: a comparison with NG-nitro-L-arginine and diphenyleneiodonium

1. The vascular effects of the soluble guanylyl cyclase inhibitor, methylene blue as well as the nitric oxide (NO) synthase inhibitors, NG-nitro-L-arginine (L-NOARG) and diphenyleneiodonium (DPI) were studied in rat isolated aortic rings and conscious, unrestrained rats. 2. Acetylcholine (ACh) and sodium nitroprusside (SNP) caused concentration-dependent relaxation of preconstricted aortic rings. Both methylene blue (1 x 10(-5) M) and L-NOARG (3 x 10(-5) M) abolished ACh-induced relaxation; however, methylene blue but not L-NOARG shifted the concentration-response curve of SNP to the right. 3. In conscious rats, i.v. infusion of methylene blue (1.1 x 10(-5) mol kg-1 min-1), at a concentration which reduced the aortic tissue level of cyclic GMP by 50%, did not significantly alter mean arterial pressure (MAP) and heart rate (HR). In contrast, i.v. bolus injection of L-NOARG (1.5 x 10(-4) mol kg-1) markedly increased MAP and decreased HR. 4. Both ACh and SNP dose-dependently decreased MAP in conscious rats. Methylene blue did not alter the magnitude or duration of ACh- or SNP-induced depressor responses. L-NOARG, on the other hand, significantly though incompletely, reduced the magnitude and duration of the depressor response to ACh but not SNP. The depressor response to ACh or SNP was not altered by pretreatment with indomethacin (1.4 x 10(-5) mol kg-1) or capsaicin (3.3 x 10(-4) mol kg-1). 5. NG-nitro-L-arginine methyl ester (L-NAME) also caused dose-dependent increases in MAP in conscious rats. Both methylene blue and DPI (1 x 10-5 mol kg-1) selectively shifted the dose-pressor response curve of L-NAME to the right.6. These results suggest that: (1) the inhibition of endogenous NO biosynthesis does not necessarily lead to pressor response in vivo, (2) L-NOARG may not produce pressor response solely via the inhibition of endogenous endothelial NO biosynthesis, and (3) the depressor responses to ACh and SNP may not involve the release of NO or prostanoids or afferent nerve transmitters.

Cardiovascular effects of inhaled nitric oxide in patients with left ventricular dysfunction

BACKGROUND

Pulmonary vascular resistance (PVR) is frequently elevated in patients with advanced heart failure. Nitric oxide (NO), which contributes to the activity of endothelium-derived relaxing factor, causes relaxation of pulmonary arteries and veins in vitro. Inhalation of NO gas causes pulmonary vasodilation in patients with primary and secondary forms of pulmonary hypertension.

METHODS AND RESULTS

To test the hypothesis that inhalation of NO gas lowers PVR in patients with heart failure, we studied the hemodynamic effects of a 10-minute inhalation of NO (80 ppm) in 19 patients with New York Heart Association class III (n = 5) and class IV (n = 14) heart failure due to left ventricular (LV) dysfunction. Although inhalation of NO had no effect on pulmonary artery pressures, the PVR decreased by 31 +/- 7% (P < .001) due to a 23 +/- 7% increase (P < .001) in pulmonary artery wedge pressure and despite a 4 +/- 2% (P < .05) decrease in cardiac index. The magnitude of the decrease in PVR with inhaled NO was inversely related (r = -.713; P < .001) to the baseline PVR. Inhaled NO had no effect on heart rate, systemic arterial pressure, systemic vascular resistance, or LV peak +dP/dt or -dP/dt.

CONCLUSIONS

In patients with heart failure due to LV dysfunction, inhalation of NO causes a decrease in the PVR associated with an increase in LV filling pressure. These findings predict that inhaled NO, if used alone at this dose (80 ppm), may have adverse effects in patients with LV failure.

Effect of inhibitors of nitric oxide release and action on vascular tone in isolated lungs of pig, sheep, dog and man

1. The actions of inhibitors of the release or action of nitric oxide (NO) on pulmonary vascular resistance (PVR) were investigated in lungs isolated from pig, sheep, dog and man. 2. In pig, sheep and human lungs perfused with Krebs-dextran solution, both N omega-nitro-L-arginine methyl ester (L-NAME; 10(-5) M) and Methylene Blue (10(-4) M) increased basal PVR. This increase was reversed by sodium nitroprusside (10(-5) M). In pig lungs N omega-monomethyl-L-arginine (10(-4) M) increased PVR by 154%. This increase was partially reversed by L-arginine (10(-3) M). L-NAME had no effect in dog lungs. 3. Pulmonary artery pressure-flow (PPA/Q) relationships were studied over a wide range of flows. In pigs, sheep and human lungs perfused with Krebs-dextran solution, L-NAME increased the PPA/Q slope. This increase was reversed by sodium nitroprusside. In dog lungs L-NAME had no effect. 4. In blood-perfused lungs, the respective responses to L-NAME were similar to those observed with saline. Acute hypoxia in pig and dog lungs increased intercept pressure. Addition of L-NAME during hypoxia increased the PPA/Q slope in both species. 5. In the human, there was no difference in the absolute increase of PVR or PPA/Q slope elicited by L-NAME between hypertensive and control lungs. 6. We conclude that NO is continuously released in the pulmonary vascular bed of pig, sheep and humans under normoxic conditions. In dog lungs inhibition of NO synthesis increases PVR only under hypoxic conditions. In human lungs with pulmonary hypertension, NO is still released under basal conditions.

Use of ATP-MgCl2 in the evaluation and treatment of children with pulmonary hypertension secondary to congenital heart defects

BACKGROUND

Pulmonary hypertension results in increased morbidity and mortality in children after surgical repair of congenital heart defects. Various vasodilators have been unsuccessful in providing preferential pulmonary vasodilation in these patients. Identification of a more preferential pulmonary vasodilator would improve the assessment, management, and outcome of these children. To determine whether ATP-MgCl2 is a preferential pulmonary vasodilator in children with pulmonary hypertension secondary to congenital heart defects, ATP-MgCl2 was administered during routine cardiac catheterization, and the effects were compared with tolazoline. In addition, ATP-MgCl2 was infused intravenously during episodes of postoperative pulmonary hypertension.

METHODS AND RESULTS

During cardiac catheterization in 28 children, the effect of ATP-MgCl2 on the pulmonary artery pressure (PAP) and pulmonary vascular resistance index (Rp) was compared with tolazoline. ATP-MgCl2 (0.1 mg of ATP per kilogram per minute) decreased mean PAP by 24% (P < .05) and Rp by 47% (P < .05) without changing mean systemic arterial pressure or systemic vascular resistance. These effects were comparable to those of tolazoline (1 mg/kg). ATP-MgCl2 produced no significant side effects; tolazoline caused tachycardia, nausea, and vomiting. After cardiac surgery in 7 patients, ATP-MgCl2 decreased PAP by 14% (P < .05) and systemic arterial pressure by 6% (P < .05) and eliminated pulmonary hypertensive crises in 3 of 3 patients.

CONCLUSIONS

ATP-MgCl2 is a safe, effective, and preferential pulmonary vasodilator in children with pulmonary hypertension secondary to congenital heart defects. It is useful for evaluating pulmonary vasoreactivity during cardiac catheterization and for treating pulmonary hypertension after cardiac surgery.

Adrenergic, purinergic, and endothelial mediators and modulators of norepinephrine-induced mesenteric autoregulatory escape

We evaluated the effects of potential factors in autoregulatory escape from norepinephrine-induced vasoconstriction in rat anterior mesenteric artery. We determined mesenteric artery blood flow velocity with a pulsed Doppler, sonic flowmeter, and systemic arterial blood pressure with a transducer. A 4-min norepinephrine infusion (0.125-1.0 x 10(-8) M/min) intravenously evoked a dose-dependent, initial vasoconstriction that was followed by rapid escape of blood flow toward or above the control value during sustained norepinephrine administration. Neonatal capsaicin treatment enhanced vasoconstrictor responses to norepinephrine but failed to affect escape parameters. Propranolol decreased norepinephrine-induced escape dose dependently. Adenosine deaminase attenuated escape, and the combination of this enzyme plus propranolol nearly abolished escape from norepinephrine-induced vasoconstriction. Methylene blue also diminished autoregulatory escape. These findings suggest that norepinephrine-induced autoregulatory escape involves simultaneous beta-adrenoceptor, purinergic, and endothelial mediation. Norepinephrine-evoked mesenteric vasoconstriction appears to involve predominantly alpha 2-adrenoceptors and is modulated by peptidergic sensory nerves and adenosine.

Nitric oxide regulates basal systemic and pulmonary vascular resistance in healthy humans

BACKGROUND

The endothelium synthesizes and releases a relaxing factor with the physiochemical properties of nitric oxide (NO). However, the role of endothelium-derived NO in the basal regulation of systemic and pulmonary vascular resistance in humans is not known. Our primary objectives were to determine the effects of inhibiting NO synthesis on blood pressure and systemic vascular resistance and to establish the role of endothelium-derived NO in the regulation of normoxic pulmonary vascular tone.

METHODS AND RESULTS

We studied the systemic and pulmonary hemodynamic effects of NG-monomethyl-L-arginine (L-NMMA, 0.03 to 1.0 mg.kg-1.min-1 IV), an NO synthase inhibitor, in 11 healthy volunteers, aged 33 +/- 2 years. An arterial cannula and a pulmonary artery catheter were placed in each subject to measure blood pressure, pulmonary artery pressure, and pulmonary capillary wedge pressure. Cardiac output was determined by the Fick technique, and systemic and pulmonary vascular resistances were calculated. Serum NO levels (free and protein bound) were measured by chemiluminescence in 5 subjects. Six of the subjects also received phenylephrine (25 to 100 micrograms/min IV) to compare the cardiac hemodynamic effects of L-NMMA with those of a direct-acting vasoconstrictor. L-NMMA caused dose-dependent increases in both blood pressure and systemic vascular resistance. At the highest dose of L-NMMA, there was a 15.5 +/- 1.3% increase in mean blood pressure and a 63.4 +/- 8.2% increase in systemic vascular resistance (each P < .01). Pulmonary vascular resistance increased 39.8 +/- 9.4% (P < .01), whereas mean pulmonary artery pressure did not change. Administration of L-NMMA also reduced cardiac output by 27.8 +/- 2.9% and stroke volume by 15.4 +/- 3.5% (each P < .01). Serum NO levels decreased 65 +/- 10% from basal values (P < .05), confirming inhibition of endogenous NO production. Phenylephrine increased blood pressure to a level comparable to that observed with L-NMMA. The decline in stroke volume was greater with L-NMMA than with phenylephrine (P < .01).

CONCLUSIONS

This study demonstrates that basal release of endothelium-derived NO is directly involved in the determination of systemic vascular resistance and, therefore, blood pressure in healthy humans. In addition, NO regulates basal normoxic pulmonary vascular tone. The complex hemodynamic effects of NO are composite properties of its actions on systemic and pulmonary vascular resistance and cardiac function.

Oxygen or low concentrations of nitric oxide reverse pulmonary vasoconstriction induced by nitric oxide synthesis inhibition in rabbits

The objective of this study was to investigate the role of nitric oxide and oxygen in the regulation of pulmonary vascular resistance, especially by means of substitution with nitric oxide after inhibition of endogenous nitric oxide formation. In artificially ventilated open-chest rabbits pulmonary vascular resistance at normoxic ventilation (FIO2 = 21%) was 56 +/- 6 cmH2O ml-1 min-1 1000-1 (mRUL). N omega-nitro-L-arginine methyl ester (L-NAME, 30 mg kg-1), an inhibitor of NO synthase, increased pulmonary vascular resistance to 122 +/- 17 mRUL at normoxic ventilation. In response to L-NAME there was also an increase in mean arterial blood pressure. Exogenous nitric oxide (0.014-9 p.p.m. in the inhaled air) dose-dependently and reversibly counteracted the effect of L-NAME on pulmonary vascular resistance at normoxic ventilation, without affecting systemic blood pressure. In addition, the L-NAME-induced vasoconstriction was critically dependent on oxygen. Thus, during hypoxic ventilation (FIO2 = 10%) the pulmonary vascular resistance was increased approximately four-fold by the presence of L-NAME (30 mg kg-1), and increments in FIO2 (21-100%) dose-dependently and reversibly counteracted the effect of L-NAME on pulmonary vascular resistance. Taken together these findings demonstrate that inhalation of low doses of NO may act as a replacement when endogenous NO synthesis is inhibited, and that pulmonary vasoconstriction induced by NO synthesis inhibition is likely to be the result of interference with oxygen-dependent regulatory mechanisms. Endogenous NO co-operates with oxygen to evoke a vasodilator component of the pulmonary hypoxic pressor response, balancing a hitherto unknown constrictor mechanism.

Endothelium-dependent pulmonary artery responses in chronic heart failure: influence of pulmonary hypertension

OBJECTIVES

The purpose of this study was to determine whether pulmonary artery responses to acetylcholine are abnormal in patients with chronic heart failure.

BACKGROUND

Defective pulmonary artery endothelium-dependent responses have been observed in chronic heart failure models in animals. However, pulmonary artery endothelial responses in humans with chronic heart failure are unknown.

METHODS

Twenty-two patients with chronic treated heart failure (12 with secondary pulmonary hypertension, Group I; 10 with normal pulmonary artery pressure, Group II) and 8 control patients constituted the study groups. Intravascular ultrasound measurements of pulmonary artery area just beyond the tip of an 8F infusion sheath were obtained in response to acetylcholine (10(-6), 10(-5) and 10(-4) mol/liter). The 10(-6) mol/liter infusion was repeated after methylene blue infusion. Indomethacin (5 micrograms/ml) was sequentially added to this combination in 17 patients.

RESULTS

There were no significant differences among the three groups in vascular area responses to the lowest concentration (10(-6) and 10(-5) mol/liter) of acetylcholine, but the 10(-4) mol/liter infusion resulted in significant constriction in Group II patients (p < 0.05, analysis of variance [ANOVA]). Pretreatment with methylene blue in Group II also resulted in significant pulmonary artery vasoconstriction to even the 10(-6) mol/liter acetylcholine infusion (10.4 +/- 7.8% in Group II vs. 1.7 +/- 3.9% in the control group and 0.1 +/- 4.3% in Group I, p < 0.05, ANOVA). The addition of indomethacin resulted in reversal of the constriction in Group II patients.

CONCLUSIONS

These responses indicate that the pulmonary artery endothelium may play a significant role in inhibiting vasoconstriction in patients with chronic heart failure who maintain normal pulmonary artery pressure.