Dipyridamole potentiates pulmonary vasodilation induced by acetylcholine and nitric oxide in the ovine fetus

Therapies that enhance pulmonary vascular NO-signaling in the neonate

There are several pulmonary hypertensive diseases that affect the neonatal population, including persistent pulmonary hypertension of the newborn (PPHN) and bronchopulmonary dysplasia (BPD)-associated pulmonary hypertension (PH). While the indication for inhaled nitric oxide (iNO) use is for late-preterm and term neonates with PPHN, there is a suboptimal response to this pulmonary vasodilator in ~40% of patients. Additionally, there are no FDA-approved treatments for BPD-associated PH or for preterm infants with PH. Therefore, investigating mechanisms that alter the nitric oxide-signaling pathway has been at the forefront of pulmonary vascular biology research. In this review, we will discuss the various mechanistic pathways that have been targets in neonatal PH, including NO precursors, soluble guanylate cyclase modulators, phosphodiesterase inhibitors and antioxidants. We will review their role in enhancing NO-signaling at the bench, in animal models, as well as highlight their role in the treatment of neonates with PH.

Management of Pulmonary Hypertension in the Operating Room

Pulmonary artery hypertension is defined as persistent elevation of mean pulmonary artery pressure > 25 mm Hg with pulmonary capillary wedge pressure < 15 mm Hg or elevation of exercise mean pulmonary artery pressure > 35 mm Hg. Although mild pulmonary hypertension rarely impacts anesthetic management, severe pulmonary hypertension and exacerbation of moderate hypertension can lead to acute right ventricular failure and cardiogenic shock. Knowledge of anesthetic drug effects on the pulmonary circulation is essential for anesthesiologists. Intraoperative management should include prevention of exacerbating factors such as hypoxemia, hypercarbia, acidosis, hypothermia, hypervolemia, and increased intrathoracic pressure; monitoring and optimizing right ventricular function; and treatment with selective pulmonary vasodilators. Recent advances in pharmacology provide anesthesiologists with a wide variety of options for selective pulmonary vasodilatation. Pulmonary hypertension is a major determinant of perioperative morbidity and mortality in special situations such as heart and lung transplantation, pneumonectomy, and ventricular assist device placement.

Synthesis and preliminary therapeutic evaluation of copper nanoparticles against diabetes mellitus and -induced micro- (renal) and macro-vascular (vascular endothelial and cardiovascular) abnormalities in rats

Current study synthesized and investigated the effect of low-dose copper nanoparticles (CuNPs) against diabetes mellitus and -induced experimental micro- (nephropathy) and macro-vascular (cardio and endothelium) complications.

A hybrid method for prediction and repositioning of drug Anatomical Therapeutic Chemical classes

In the Anatomical Therapeutic Chemical (ATC) classification system, therapeutic drugs are divided into 14 main classes according to the organ or system on which they act and their chemical, pharmacological and therapeutic properties. This system, recommended by the World Health Organization (WHO), provides a global standard for classifying medical substances and serves as a tool for international drug utilization research to improve quality of drug use. In view of this, it is necessary to develop effective computational prediction methods to identify the ATC-class of a given drug, which thereby could facilitate further analysis of this system. In this study, we initiated an attempt to develop a prediction method and to gain insights from it by utilizing ontology information of drug compounds. Since only about one-fourth of drugs in the ATC classification system have ontology information, a hybrid prediction method combining the ontology information, chemical interaction information and chemical structure information of drug compounds was proposed for the prediction of drug ATC-classes. As a result, by using the Jackknife test, the 1st prediction accuracies for identifying the 14 main ATC-classes in the training dataset, the internal validation dataset and the external validation dataset were 75.90%, 75.70% and 66.36%, respectively. Analysis of some samples with false-positive predictions in the internal and external validation datasets indicated that some of them may even have a relationship with the false-positive predicted ATC-class, suggesting novel uses of these drugs. It was conceivable that the proposed method could be used as an efficient tool to identify ATC-classes of novel drugs or to discover novel uses of known drugs.

SOD and inhaled nitric oxide normalize phosphodiesterase 5 expression and activity in neonatal lambs with persistent pulmonary hypertension

Phosphodiesterase 5 (PDE5) and soluble guanylate cyclase (sGC) are key regulators of cGMP and pulmonary vascular tone. We sought to determine the impact of mechanical ventilation with O(2) with or without inhaled nitric oxide (iNO) or recombinant human Cu/Zn SOD (rhSOD) on sGC, PDE5, and cGMP in the ovine ductal ligation model of persistent pulmonary hypertension of the newborn (PPHN). PPHN lambs were ventilated with 100% O(2) for 24 h alone or combined with either inhalation of 20 parts per million (ppm) iNO continuously or a single intratracheal dose of rhSOD (5 mg/kg). Ventilated PPHN lambs were compared with PPHN fetuses, control fetuses, and 1-day-old spontaneously breathing lambs (1DSB). In the small pulmonary arteries of 1DSB lambs, sGC expression increased, PDE5 expression decreased, and cGMP concentrations increased relative to fetal levels. In PPHN lambs ventilated with 100% O(2), sGC activity increased to levels comparable with 1DSB levels. However, PDE5 expression and activity increased, and cGMP levels remained at fetal levels. Addition of either iNO or rhSOD decreased PDE5 expression and activity in PPHN lambs and increased cGMP levels to levels comparable with 1DSB lambs. These data suggest that ventilation of PPHN lambs with 100% O(2) impairs cGMP-mediated vasodilation in part due to increased PDE5 expression and activity. The addition of either iNO or rhSOD normalized PDE5 and cGMP levels. Thus therapies designed to decrease PDE5 and increase cGMP, such as iNO and rhSOD, may prove useful in the treatment of PPHN in newborn infants.

Hyperoxia increases phosphodiesterase 5 expression and activity in ovine fetal pulmonary artery smooth muscle cells

In the pulmonary vasculature, cGMP concentrations are regulated in part by a cGMP-dependent phosphodiesterase (PDE), PDE5. Infants with persistent pulmonary hypertension of the newborn (PPHN) are often mechanically ventilated with high oxygen concentrations. The effects of hyperoxia on the developing pulmonary vasculature and PDE5 are largely unknown. Here, we demonstrate that exposure of fetal pulmonary artery smooth muscle cells (FPASMCs) to high levels of oxygen for 24 hours leads to decreased responsiveness to exogenous NO, as determined by a decreased intracellular cGMP response, increased PDE5 mRNA and protein expression, as well as increased PDE5 cGMP hydrolytic activity. We demonstrate that inhibition of PDE5 activity with sildenafil partially rescues cGMP responsiveness to exogenous NO. In FPASMCs, hyperoxia leads to increased oxidative stress without increasing cell death. Treatment of normoxic FPASMCs with H2O2 is sufficient to induce PDE5 expression and activity, suggesting that reactive oxygen species mediate the effects of hyperoxia in FPASMCs. In support of this mechanism, a chemical antioxidant, N-acetyl-cysteine, is sufficient to block the hyperoxia-mediated increase in PDE5 expression and activity and rescue cGMP responsiveness to exogenous NO. Finally, ventilation of healthy neonatal sheep with 100% O2 for 24 hours leads to increased PDE5 protein expression in the resistance pulmonary arteries and increased PDE5 activity in whole lung extracts. These data suggest that PDE5 expression and activity play a critical role in modulating neonatal pulmonary vascular tone in response to common clinical treatments for PPHN, such as oxygen and inhaled NO.

Effects of Phosphodiesterase 5 Inhibitor on Pulmonary Vascular Reactivity in the Fetal Lamb

BACKGROUND

Nitric oxide released by pulmonary vascular endothelium is a potent vasodilator related to increased cyclic guanosine monophosphate (cGMP) content. Hydrolysis of cGMP is achieved predominately by cGMP-specific phosphodiesterases. Sildenafil is a selective phosphodiesterase-5 (PDE5) inhibitor. The purpose of the study is to assess the effects of sildenafil on pulmonary vascular circulation during the perinatal period.

METHODS

Thirty-two pregnant ewes were operated on at the end of gestation, and fetal lambs were prepared with catheters placed into the aorta, vena cava, pulmonary artery, and left atrium. An ultrasonic flow transducer and an inflatable vascular occluder were placed respectively around the left pulmonary artery and the ductus arteriosus. Fetal lambs were randomly divided into two groups: (1) sildenafil group, infused continuously with sildenafil for 24 hours at a rate of 1 mg/h; or (2) control group, infused with saline for 24 hours. After 24 hours of infusion, we compared basal pulmonary vascular resistance and the pulmonary vascular responses to increase in fetal PaO2 and to acute ductus arteriosus compression causing "shear stress."

RESULTS

Sildenafil infusion did not change mean aortic and pulmonary artery pressures, increased mean left pulmonary blood flow by 160%, and decreased pulmonary vascular resistance by 60% (p < 0.05). However, both mean flow (Q) and pulmonary vascular resistance returned to baseline values after 2 hours of sildenafil infusion. Despite similar baseline values, pulmonary vascular resistance during maternal O2 inhalation was lower in the sildenafil group than in the control group (0.21 +/- 0.03 versus 0.33 +/- 0.03 mm Hg.mL(-1).min(-1), respectively; p < 0.01). Furthermore, drop in pulmonary vascular resistance during acute ductus arteriosus compression was greater in the sildenafil group (from 0.56 +/- 0.06 to 0.26 +/- 0.04 mm Hg.mL(-1).min(-1)) than in the control group (from 0.55 +/- 0.05 to 0.39 +/- 0.03 mm Hg.mL(-1).min(-1); p < 0.01).

CONCLUSIONS

Although sildenafil induces a transient pulmonary vasodilation, it mediates a sustained change in vascular reactivity, especially to birth-related stimuli in the ovine fetal lung. These data suggest that PDE5 is involved in the regulation of pulmonary vascular reactivity during the perinatal period and may potentiate birth-related pulmonary vasodilator stimuli.

Combined Administration of Intravenous Dipyridamole and Inhaled Nitric Oxide to Assess Reversibility of Pulmonary Arterial Hypertension in Potential Cardiac Transplant Recipients

BACKGROUND

Irreversible, severe pulmonary hypertension (PH) can produce right heart failure and early mortality after cardiac transplantation. We hypothesized that dipyridamole, an inhibitor of Type 5 phosphodiesterase, would augment the ability of inhaled nitric oxide (NO) to identify reversibility of PH.

METHODS

In 9 patients with congestive heart failure (CHF) and severe PH who were breathing 100% oxygen during right heart catheterization, we administered inhaled NO (80 ppm) alone and in combination with intravenous dipyridamole (0.2-mg/kg bolus, with an infusion of 0.0375 mg/kg/min).

RESULTS

Compared with breathing oxygen alone, NO inhalation decreased pulmonary artery pressure and pulmonary vascular resistance (PVR) (by 10 +/- 4% and 26 +/- 12% [mean +/- SEM], respectively; both p < 0.05). The combination of NO and dipyridamole reduced PVR (43 +/- 7%; p < 0.05) to a greater extent than did administration of NO alone, and increased the duration of pulmonary vasodilation produced by NO inhalation. Combined administration of inhaled NO and intravenous dipyridamole increased cardiac index (by 23 +/- 10%) and reduced SVR (by 19 +/- 6%, both p < 0.05) without changing systemic arterial pressure. NO inhalation reduced PVR to <200 dyne x s/cm5 in 3 of 7 patients who had a PVR of >200 dyne x s/cm5 when breathing oxygen alone, whereas the combination of NO and dipyridamole decreased PVR to <200 dyne.s/cm(5) in 2 additional patients.

CONCLUSIONS

Intravenous dipyridamole augments and prolongs the pulmonary vasodilator effects of inhaled NO in CHF patients with severe PH and, when administered in combination with NO inhalation, can identify PH reversibility in potential cardiac transplant recipients in whom a pulmonary vasodilator response to inhalation of NO alone is not observed.

Pulmonary vascular effects of sildenafil on the development of chronic pulmonary hypertension in the ovine fetus

We investigated the pulmonary vascular effects of prophylactic use of sildenafil, a specific phosphodiesterase-5 inhibitor, in late-gestation fetal lambs with chronic pulmonary hypertension. Fetal lambs were operated on at 129 +/- 1 days gestation (term = 147 days). Ductus arteriosus (DA) was compressed for 8 days to cause chronic pulmonary hypertension. Fetuses were treated with sildenafil (24 mg/day) or saline. Pulmonary vascular responses to increase in shear stress and in fetal PaO2 were studied at, respectively, day 4 and 6. Percent wall thickness of small pulmonary arteries (%WT) and the right ventricle-to-left ventricle plus septum ratio (RVH) were measured after completion of the study. In the control group, DA compression increased PA pressure (48 +/- 5 to 72 +/- 8 mmHg, P < 0.01) and pulmonary vascular resistance (PVR) (0.62 +/- 0.08 to 1.15 +/- 0.11 mmHg x ml(-1) x min(-1), P < 0.05). Similar increase in PAP was observed in the sildenafil group, but PVR did not change significantly (0.54 +/- 0.06 to 0.64 +/- 0.09 mmHg x ml(-1) x min(-1)). Acute DA compression, after brief decompression, elevated PVR 25% in controls and decreased PVR 35% in the sildenafil group. Increased fetal PaO2 did not change PVR in controls but decreased PVR 60% in the sildenafil group. %WT and RVH were not different between groups. Prophylactic sildenafil treatment prevents the rise in pulmonary vascular tone and altered vasoreactivity caused by DA compression in fetal lambs. These results support the hypothesis that elevated PDE5 activity is involved in the consequences of chronic pulmonary hypertension in the perinatal lung.

Medical therapy for pulmonary arterial hypertension: ACCP evidence-based clinical practice guidelines

Pulmonary arterial hypertension (PAH) is often difficult to diagnose and challenging to treat. Untreated, it is characterized by a progressive increase in pulmonary vascular resistance leading to right ventricular failure and death. The past decade has seen remarkable improvements in therapy, driven largely by the conduct of randomized controlled trials. Still, the selection of most appropriate therapy is complex, and requires familiarity with the disease process, evidence from treatment trials, complicated drug delivery systems, dosing regimens, side effects, and complications. This chapter will provide evidence-based treatment recommendations for physicians involved in the care of these complex patients. Due to the complexity of the diagnostic evaluation required, and the treatment options available, it is strongly recommended that consideration be given to referral of patients with PAH to a specialized center.

Intermittent nitric oxide combined with intravenous dipyridamole in a piglet model of acute pulmonary hypertension

Continuous administration of inhaled nitric oxide is now widely used as a potent and selective pulmonary vasodilator. We have evaluated the effects of IV dipyridamole, a cyclic guanosine monophosphate (cGMP) phosphodiesterase inhibitor, on the magnitude and duration of action of inhaled nitric oxide (NO)-mediated pulmonary vasodilation. We hypothesized that inhibition of cGMP degradation could augment and prolong the pulmonary vasodilating effects of NO and allow for intermittent NO inhalation. In eight anesthetized and mechanically ventilated piglets, IV U-46619, a thromboxane A(2) analog, was used to induce pulmonary hypertension. The effects of 2, 5, and 10 ppm of NO, delivered during 4 min for each concentration and followed by a 10-min NO-free interval after each NO concentration, were evaluated without and with dipyridamole. Pulmonary vascular resistance decreased from 825 +/- 49 dynes. s. cm(-5) (U-46619) to 533 +/- 48 dynes. s. cm(-5) (10 ppm NO) (P < 0.05 versus U-46619) and 396 +/- 42 dynes. s. cm(-5) (dipyridamole 10 microg kg-1x min-1 and 10 ppm NO) (P <0.05 versus NO), and cardiac output increased from 1.93 +/- 0.09 L/min to 2.03 +/- 0.13 L/min and 2.60 +/- 0.30 L/min (P < 0.05 versus NO). Mean arterial blood pressure decreased from 90 +/- 5 mm Hg (10 ppm NO) to 75 +/- 3 mm Hg (dipyridamole plus 10 ppm NO) (P < 0.01). The pulmonary vasodilation obtained with NO alone could be prolonged from 12 to 42 min when inhaled NO was combined with IV dipyridamole, accounting for a time-weighted reduction in NO exposure by 72%. We conclude that dipyridamole augments the effects of NO on right ventricular afterload, allows for intermittent NO inhalation, and can significantly reduce exposure to NO.

IMPLICATIONS

IV dipyridamole prolongs the action of inhaled nitric oxide (NO) in a piglet model of acute pulmonary hypertension. Intermittent NO inhalation combined with IV dipyridamole decreases pulmonary artery pressure for a prolonged period of time and reduces exposure to NO.

Intravenous dipyridamole enhances the effects of inhaled nitric oxide and prevents rebound pulmonary hypertension in piglets

Inhaled nitric oxide (NO) is increasingly used in the treatment of pulmonary hypertension, despite its potential toxicity and the risk of life-threatening rebound pulmonary hypertension upon its discontinuation. We investigated whether i.v. dipyridamole, a cGMP phosphodiesterase inhibitor, increased the effects of inhaled NO and prevented rebound pulmonary hypertension. In 14 anesthetized and mechanically ventilated piglets, pulmonary hypertension was induced with U-46619, a thromboxane A(2) analogue. Response to NO and rebound pulmonary hypertension were evaluated without and with i.v. dipyridamole. Low-dose dipyridamole (10 micro g/kg/min) increased cardiac output and augmented the effects of inhaled NO on pulmonary vascular resistance, with marginal additive effect on mean pulmonary artery pressure. Pulmonary vascular resistance decreased from 904 to 511 (20 parts per million NO) (p < 0.0005) and 358 dyne s cm(-5) (20 parts per million NO + dipyridamole) (p < 0.001 versus NO alone), and mean pulmonary artery pressure decreased from 29.0 to 20.5 (p < 0.0001) and 19.3 mm Hg (NS versus NO), respectively. Mean arterial pressure decreased from 85 to 74 mm Hg (dipyridamole + NO) (p < 0.01). High-dose dipyridamole (100 micro g/kg/min) with inhaled NO reduced pulmonary vascular resistance to 334 dyne s cm(-5) but also decreased mean arterial pressure to 57 mm Hg. Eight piglets developed rebound pulmonary hypertension. Two died of acute right ventricular failure and, in five, rebound pulmonary hypertension was prevented by low-dose dipyridamole. In conclusion, low-dose i.v. dipyridamole augments the effects of inhaled NO on right ventricular afterload with moderate changes in systemic hemodynamics, and can prevent rebound pulmonary hypertension.

Inhaled nitric oxide in ARDS

The role of nitric oxide (NO) in numerous physiologic systems only recently has been discovered. When used as a gas, inhaled NO (iNO) has many unique properties that cause immediate improvements in pulmonary hemodynamics and oxygenation. Acute benefits in physiologic parameters have been demonstrated in numerous studies of iNO in acute respiratory distress syndrome (ARDS), but recent randomized controlled trials have failed to show improvement in outcome. The addition of other treatments that prolong or enhance the affect of iNO or its use with other ventilator modalities such as prone positioning or high-frequency ventilation offer hope that iNO may be beneficial in select groups of patients.

Combination of nonspecific PDE inhibitors with inhaled prostacyclin in experimental pulmonary hypertension

Inhalation of aerosolized prostacyclin (PGI(2)) exerts selective pulmonary vasodilation, but its effect is rapidly lost after termination of nebulization. Amplification of the vasodilatory response to inhaled PGI(2) might be achieved by phosphodiesterase (PDE) inhibitors to stabilize its second messenger, cAMP. We established stable pulmonary hypertension in perfused rabbit lungs by continuous infusion of U-46619. Short-term (10-min) aerosolization maneuvers of PGI(2) effected a rapid, moderate decrease in pulmonary arterial pressure, with post-PGI(2) vasorelaxation being lost within 10-15 min, accompanied by a marginal reduction in shunt flow. Preceding administration of subthreshold doses of the PDE inhibitors theophylline, dipyridamole, and pentoxifylline via the intravascular or inhalational route, which per se did not influence pulmonary hemodynamics, caused more than doubling of the immediate pulmonary arterial pressure drop in response to PGI(2) and marked prolongation of the post-PGI(2) vasorelaxation to >60 min (all PDE inhibitors via both routes of application). This was accompanied by a reduction in shunt flow in the case of aerosolized theophylline (27.5%), pentoxifylline (30.5%), and dipyridamole (33.4%). Coaerosolization of PGI(2) and PDE inhibitors may be considered as a therapeutic strategy in pulmonary hypertension.

The use of dipyridamole to wean from inhaled nitric oxide in congenital diaphragmatic hernia

A full-term neonate with a left-sided congenital diaphragmatic hernia (CDH) was ventilated mechanically by high-frequency oscillatory ventilation (HFOV). Despite inhaled nitric oxide (iNO) at a dose of 15 ppm, the neonate had severe respiratory acidosis and was placed on extracorporeal membrane oxygenation (ECMO) for 2 days. On day 7 of life, surgical repair of the CDH was performed. After the intervention, iNO (20 ppm) had to be restarted because of severe pulmonary hypertension (PHT). Ventilatory support and iNO then were weaned progressively. However, each daily attempt to discontinue iNO (from 2 ppm to 0 ppm), led to severe desaturation with significant right-to-left shunting. At the age of 33 days, dipyridamole (persantin) was administered intravenously at a dose of 0,4 mg/kg/min over 10 minutes and repeated every 12 hours for a total of 3 doses. After the second administration of dipyridamole, iNO could be stopped without rebound of PHT, and the neonate was extubated 1 week later. The authors report the use of dipyridamole for successful withdrawal of iNO. By inhibition of phosphodiesterase type 5, dipyridamole has the potential to increase the level of cyclic guanosine monophosphate in vascular smooth muscle cells, permitting vasodilation and restoration of endogenous NO. J Pediatr Surg 36:1864-1865.

Nebulized sildenafil is a selective pulmonary vasodilator in lambs with acute pulmonary hypertension

OBJECTIVE

To determine whether inhalation of aerosolized sildenafil with and without inhaled nitric oxide (NO) causes selective pulmonary vasodilation in a sheep model of pulmonary hypertension.

DESIGN

A controlled laboratory study in instrumented, awake, spontaneously breathing lambs.

SETTING

Animal research laboratory affiliated with a university hospital.

SUBJECT

Twenty Suffolk lambs.

INTERVENTIONS

Lambs were instrumented with a carotid artery catheter, a pulmonary artery catheter, and a tracheostomy tube and studied awake. After baseline measurements, pulmonary hypertension was induced by the continuous infusion of U46619, a thromboxane A2 analog. After breathing three concentrations of inhaled NO (2, 5, and 20 ppm), lambs were divided into two groups. Group 1 (n = 7) breathed aerosols containing 1, 10, and 30 mg of sildenafil alone, and group 2 (n = 4) simultaneously breathed NO (2 and 5 ppm) and aerosols containing 10 mg of sildenafil. Hemodynamic measurements were obtained before and at the end of each drug administration. Venous admixture was calculated, and plasma cyclic guanosine monophosphate and sildenafil concentrations were measured.

MEASUREMENTS AND MAIN RESULTS

Aerosols containing 10 mg and 30 mg of sildenafil selectively decreased the pulmonary artery pressure by 21% +/- 3% and 26% +/- 3%, respectively (p < .05 vs. baseline pulmonary hypertension). When 10 mg of sildenafil was inhaled while simultaneously breathing 2 ppm and 5 ppm NO, the pulmonary artery pressure decreased by 35% +/- 3% and 43% +/- 2% (p < .05 vs. baseline pulmonary hypertension). Inhaled sildenafil did not impair systemic oxygenation, increase right-to-left intrapulmonary shunting, or impair the ability of inhaled NO to reduce right-to-left shunting.

CONCLUSIONS

Nebulized sildenafil is a selective pulmonary vasodilator that can potentiate the pulmonary vasodilating effects of inhaled NO.

Cyclic nucleotide phosphodiesterases: relating structure and function

Cyclic nucleotide phosphodiesterases (PDEs) comprise a superfamily of metallophosphohydrolases that specifically cleave the 3',5'-cyclic phosphate moiety of cAMP and/or cGMP to produce the corresponding 5'-nucleotide. PDEs are critical determinants for modulation of cellular levels of cAMP and/or cGMP by many stimuli. Eleven families of PDEs with varying selectivities for cAMP or cGMP have been identified in mammalian tissues. Within these families, multiple isoforms are expressed either as products of different genes or as products of the same gene through alternative splicing. Regulation of PDEs is important for controlling myriad physiological functions, including the visual response, smooth muscle relaxation, platelet aggregation, fluid homeostasis, immune responses, and cardiac contractility. PDEs are critically involved in feedback control of cellular cAMP and cGMP levels. Activities of the various PDEs are highly regulated by a panoply of processes, including phosphorylation events, interaction with small molecules such as cGMP or phosphatidic acid, subcellular localization, and association with specific protein partners. The PDE superfamily continues to be a major target for pharmacological intervention in a number of medically important maladies.

Combination therapy with inhaled nitric oxide and intravenous dobutamine during pulmonary hypertension in the rabbit

Combination therapy with an intravenous inovasodilator and inhaled nitric oxide (NO) may be appropriate in patients with pulmonary hypertension and associated right ventricular failure. We examined whether dobutamine and inhaled NO would have additive pulmonary vasodilator effects in experimental pulmonary hypertension. Pulmonary hypertension was produced in anesthetized, mechanically ventilated rabbits by infusion of U46619, a thromboxane analogue. Dobutamine was administered in increasing doses (2.5-20 microg/kg/min) with and without inhaled NO (40 ppm). Dobutamine produced dose-dependent decreases in pulmonary vascular resistance (PVR) and mean arterial pressure (MAP) and increases in cardiac output (CO). Inhaled NO alone decreased pulmonary artery pressure (PAP) and PVR with no effect on MAP or CO. The effects of dobutamine and inhaled NO were additive, so that at each dose of dobutamine, inhaled NO decreased PAP and PVR with no effect on systemic hemodynamics. This study suggests that the combination of dobutamine and inhaled NO should produce additive pulmonary vasodilation in patients with pulmonary hypertension and associated right ventricular dysfunction.

Combined therapy with zaprinast and inhaled nitric oxide abolishes hypoxic pulmonary hypertension

OBJECTIVE

To determine whether the combination of the phosphodiesterase 5 inhibitor zaprinast and inhaled nitric oxide (NO) decreases hypoxic pulmonary hypertension in the rat.

DESIGN

Prospective, experimental study.

SETTING

Animal laboratory of a university medical center.

SUBJECTS

Male Sprague-Dawley rats.

INTERVENTIONS

Anesthetized rats were mechanically ventilated and instrumented for measurement of mean systemic arterial pressure, pulmonary arterial pressure, and cardiac output. In group 1, four acute hypoxic challenges (FIO2 = 0.17 for 5 mins) were performed: initial, during 40 ppm inhaled NO, immediately after discontinuation of 5 mins of inhaled NO, and final. In group 2 rats, an initial hypoxic challenge was performed and rats then received zaprinast (3 mg/kg bolus followed by 0.3 mg/kg/min infusion). Four hypoxic challenges analogous to group 1 were then performed during zaprinast administration.

MEASUREMENTS AND MAIN RESULTS

Initial hypoxic challenge produced similar increases in pulmonary arterial pressure in both groups. In group 1, inhaled NO either only before or only during hypoxia decreased the pulmonary hypertensive response to hypoxia. In group 2, zaprinast administration did not alter hemodynamics. Zaprinast alone decreased the pulmonary hypertensive response to hypoxia. The combination of zaprinast and inhaled NO (either before or during hypoxia) abolished the pulmonary hypertensive response to hypoxia.

CONCLUSIONS

Treatment with inhaled NO for 5 mins before but not during hypoxia is as effective as inhaled NO during hypoxia. Inhaled NO and zaprinast both decrease the pulmonary hypertensive response to hypoxia, and the combination abolishes the response. The combination of a phosphodiesterase 5 inhibitor and inhaled NO may have clinical applicability in the treatment of pulmonary hypertension.

Additive effects of inhaled nitric oxide and intravenous milrinone in experimental pulmonary hypertension

OBJECTIVE

To determine whether inhaled nitric oxide (IN0) and intravenous milrinone have additive pulmonary vasodilator effects in a rat model of pulmonary hypertension.

DESIGN

Prospective, experimental study.

SETTING

Animal laboratory of a university medical center.

SUBJECTS

Male New Zealand White rabbits.

INTERVENTIONS

Anesthetized rabbits were mechanically ventilated and instrumented for measurement of systemic mean arterial pressure (MAP), pulmonary artery pressure (PAP), left atrial pressure, and cardiac output (CO). After baseline measurements, the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (30 mg/kg iv) was administered. Pulmonary hypertension was produced by the continuous infusion of U46619, a thromboxane A2 mimetic. INO (40 ppm) was added to the inspired gas, and hemodynamic measurements were obtained before and after INO. Milrinone was administered sequentially as a 30-mg/kg bolus followed by a 3-microg/kg/min infusion, a 100-mg/kg bolus followed by a 10-microg/kg/min infusion, and a 300-mg/kg bolus followed by a 30-microg/kg/min infusion (M3). Hemodynamic measurements were obtained with and without INO at each dose of milrinone.

MEASUREMENTS AND MAIN RESULTS

During U46619-induced pulmonary hypertension, INO decreased PAP and pulmonary vascular resistance (PVR) but did not affect MAP, systemic vascular resistance (SVR), or CO. Milrinone dose dependently decreased PAP, PVR, MAP, and SVR and increased CO. At each dose of milrinone, INO further decreased PVR but not SVR. M3 decreased PVR 49%, and the addition of INO decreased PVR an additional 19% so that PAP and PVR decreased to baseline values.

CONCLUSIONS

Milrinone and INO both decrease pulmonary hypertension individually, and the combination produces additive effects. Combination therapy may produce potent and selective pulmonary vasodilation during the treatment of pulmonary hypertension.

The cGMP-specific phosphodiesterase inhibitor E4021 dilates the pulmonary circulation

We investigated the pulmonary vascular effects of E4021, a potent inhibitor of cGMP-specific phosphodiesterase, in control late-gestation fetal lambs, and in newborn lambs with persistent pulmonary hypertension (PPHN) after prenatal ligation of the ductus arteriosus. E4021 alone significantly relaxed fifth-generation pulmonary arteries isolated from control fetal lambs, an effect completely blocked after inhibition of nitric oxide synthase (NOS). In contrast, E4021 did not relax pulmonary arteries isolated from hypertensive lambs. Pretreatment with E4021 (10(-7) M) significantly enhanced relaxations to the NO donor S-nitrosyl-acetyl-penicilamine (SNAP) in arteries from both control and hypertensive lambs. In control, fully instrumented fetal lambs, infusions of E4021 (31 microgram/min) selectively dilated the pulmonary circulation, an effect again blocked after inhibition of NO synthase. Further studies were performed in newborn lambs with PPHN to study the vascular effects of E4021 alone, and in combination with inhaled NO. E4021 alone (1 to 100 microgram/kg/min) decreased pulmonary artery pressure (Ppa) in a dose-dependent fashion, and had minimal effect on systemic pressure. At the highest dose (100 microgram/kg/min), the dilation was selective for the pulmonary circulation. In subsequent protocols, E4021 (10 microgram/kg/min) significantly decreased Ppa and pulmonary vascular resistance (PVR), but these pulmonary vascular effects were not enhanced after NO inhalation at 0.5 or 5 ppm. We speculate that the lack of enhancement was due to the dramatic effects of E4021 alone. Potent, specific phosphodiesterase inhibitors such as E4021 may prove to be useful in the treatment of PPHN.

Pulmonary and systemic effects of the phosphodiesterase inhibitor dipyridamole in newborn lambs with persistent pulmonary hypertension

Nitric oxide (NO) relaxes vascular smooth muscle by increasing the intracellular concentration of cGMP. In the pulmonary circulation, cGMP is inactivated by specific phosphodiesterases (PDE5). Dipyridamole, a clinically approved drug, has inhibitory activity against PDE5 and has been reported to augment the response to inhaled NO in persistent pulmonary hypertension of the newborn (PPHN). We wished to determine whether dipyridamole alone, or in combination with NO, can be used to treat a newborn lamb model of PPHN. In newborn lambs with PPHN, dipyridamole infused at 0.02 mg/kg/min for 45 min alone, or in combination with 5 ppm of inhaled NO for the final 15 min, significantly decreased pulmonary and systemic blood pressure, decreased pulmonary vascular resistance, and increased pulmonary blood flow. There was no significant difference between the pulmonary vascular effects of 5 ppm NO alone compared with the effects of NO combined with dipyridamole. In control lambs, the 45-min infusion of dipyridamole did not change pulmonary pressure whereas systemic pressure decreased by 28 +/- 3%. These systemic effects in control lambs persisted 90 min after discontinuing the dipyridamole infusion. Systemic arteries isolated from both control and PPHN lambs were significantly more sensitive to dipyridamole than pulmonary arteries. We conclude that dipyridamole has significant hemodynamic effects in both the pulmonary and systemic circulations of newborn lambs with pulmonary hypertension as well as in the systemic circulation of newborn control lambs. The pronounced effects of dipyridamole on the systemic circulation limits its utility as an adjunct to inhaled NO in the treatment of PPHN.

Effects of dipyridamole and inhaled nitric oxide in pediatric patients with pulmonary hypertension

Inhaled nitric oxide (iNO) causes selective pulmonary vasodilation by increasing pulmonary vascular levels of cyclic guanosine monophosphate (cGMP). Dipyridamole, a drug with several putative vasodilator mechanisms, is an inhibitor of cGMP-specific phosphodiesterases (PDE5); it therefore has the potential to increase pulmonary vascular cGMP levels, lower pulmonary vascular resistance, augment iNO-induced pulmonary vasodilation, and attenuate excessive pulmonary vasoreactivity. To test dipyridamole in the pulmonary circulation, we studied pediatric patients undergoing cardiac catheterization who had severe resting pulmonary hypertension (Group 1; n = 11) or exaggerated acute hypoxia-induced pulmonary vasoconstriction (Group 2; n = 4). In Group 1, we compared the effects of iNO (20 ppm), dipyridamole (0.6 mg/kg), and combined treatments (iNO + dipyridamole) on pulmonary and systemic hemodynamics. In Group 2 we measured the pulmonary and systemic effects of dipyridamole while the patients were breathing room air and hypoxic gas mixtures (FIO2 = 0.16). One patient in Group 1 had a hypotensive response to dipyridamole and was exluded from study. In the remaining 12 studies done on 10 patients, iNO caused a selective decrease in mean pulmonary artery pressure (Ppa) and indexed pulmonary vascular resistance (PVRI) without affecting mean aortic pressure (Pao) or indexed systemic vascular resistance (SVRI). Dipyridamole decreased PVRI to similar values as did iNO, but this effect was primarily due to an increase in cardiac index (CI), and was not associated with any change in Ppa, and was associated with a decrease in Pao and SVRI. In comparison with individual treatments, combined therapy (iNO + dipyridamole) did not augment pulmonary vasodilation in the group as a whole; however, in 50% of patients, combined therapy decreased PVRI by 20% more than did iNO or dipyridamole alone. In Group 2, Ppa and the pulmonary-to-systemic resistance ratio (Rp/Rs) increased to suprasystemic levels during acute hypoxia. Pretreatment with dipyridamole blunted the increase in Ppa and Rp/Rs during repeat hypoxia, keeping Ppa at a subsystemic level and Rp/Rs < 1. We conclude that: (1) dipyridamole nonselectively reduces PVRI, primarily through an increase in CI; (2) in combination with iNO, dipyridamole augments the decrease in PVRI in some patients; and (3) dipyridamole blunts the severity of acute hypoxic pulmonary vasoconstriction in children with exaggerated hypoxic pressor responses.