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Ann Thorac Surg 2006;81:935-942
© 2006 The Society of Thoracic Surgeons

Original article: Cardiovascular

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

Sophie Jaillard, MD, PhD a , e , * , Benoît Larrue, MD b , Philippe Deruelle, MD c , e , Anne Delelis, MD c , Thameur Rakza, MD c , Ghazwan Butrous, MD d , Laurent Storme, MD c , e

a Department of Surgery, Polyclinique du Bois, Lille, France
b Department of Cardiac Surgery, CHRU de Lille, Lille, France
c Department of Perinatology, CHRU de Lille, Lille, France
d Pfizer Laboratories, Sandwich, Great Britain
e EA1049, Department of Biophysics, CHRU de Lille, Lille, France

Accepted for publication September 9, 2005.

* Address correspondence to Dr Jaillard, Department of Surgery, Polyclinique du Bois, Ave Marx Dormoy, 59000 Lille, France (Email: sjaillard{at}wanadoo.fr).


   Abstract
Top
 Abstract
Introduction
 Material and Methods
 Results
 Comment
 Acknowledgments
 References
 
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.


   Introduction
Top
 Abstract
 Introduction
Material and Methods
 Results
 Comment
 Acknowledgments
 References
 
The fetal pulmonary circulation is characterized by high pulmonary vascular resistance (PVR) and low blood flow. Owing to the high pulmonary artery pressure, pulmonary blood flow is low, with the lung receiving less than 10% of combined ventricular output during late gestation. Because of high PVR in the fetus, most of the right ventricular output crosses the ductus arteriosus into the descending aorta, thereby increasing umbilical-placental flow and gas exchange. Within minutes of birth, the normal fetus undergoes a dramatic transition including a 6-fold to a 10-fold rise in pulmonary blood flow and a decline in PVR, increased systemic vascular resistance, and functional closure of the foramen ovale and of the ductus arteriosus (DA) [1].

Persistent pulmonary hypertension of the newborn is a clinical syndrome that is associated with diverse neonatal cardiopulmonary diseases, including congenital diaphragmatic hernia [2]. Persistent pulmonary hypertension of the newborn results from the failure of the pulmonary circulation to dilate at birth. This syndrome is characterized by sustained elevation of PVR, causing extrapulmonary right-to-left shunting of blood across the DA and foramen ovale, and severe hypoxemia [2]. Persistent pulmonary hypertension of the newborn contributes to significant morbidity and mortality, indicating a need for further study on the control of perinatal pulmonary circulation [3].

Mechanisms that maintain high PVR in utero are incompletely understood, but may include low fetal PO2 [1, 4, 5], lack of a gas-liquid interface [1], and production of vasoconstrictor mediators such as endothelin 1 [6]. Although numerous vasodilator agents such as prostacyclin has been involved in the control of the fetal pulmonary circulation, past studies have demonstrated that nitric oxide is one of the main pathway modulating the pulmonary vascular tone [4, 7–9]. Nitric oxide modulates pulmonary vascular resistance in the normal fetal lung by increasing the cyclic guanosine monophosphate (cGMP) content of pulmonary vascular smooth cells. Cyclic guanosine monophosphate is a potent pulmonary vasodilator [10]. Hydrolysis of cGMP is achieved predominately by cGMP-specific phosphodiesterases-5 (PDE5). The later enzyme is selectively inhibited by sildenafil. Although PDE5 is abundantly expressed in lung tissue [11], little is known regarding the role of PDE5 on the vascular tone control in the perinatal lung.

Preliminary biochemical studies reported that lung PDE5 activity is markedly elevated during fetal life, and then rapidly falls after birth [12]. The concomitant decrease in lung PDE5 activity and in PVR in early transition suggests that PDE5 activity may be involved in the circulatory adaptation at birth [12]. Brief PDE5 inhibition was also found to cause potent pulmonary vasodilation in late gestation ovine fetus, suggesting an important role of PDE5 activity in regulating basal pulmonary vascular tone [13, 14]. Taken together, these data indicate that PDE5 may be an important regulator of perinatal pulmonary circulation. However, whether the pulmonary vascular response to PDE5 inhibition is sustained remains uncertain. Moreover, the effects of PDE5 inhibition on the pulmonary vascular reactivity are presently unknown.

We therefore hypothesized that PDE5 is involved in the control of the pulmonary circulation in the perinatal period. To test this hypothesis, we examined the pulmonary vascular response to prolonged sildenafil infusion in chronically prepared late-gestation fetal lambs. We also studied the pulmonary hemodynamic responses to birth-related stimuli, especially to increased PaO2 and to increased shear stress with and without PDE5 inhibition.


   Material and Methods
Top
 Abstract
 Introduction
 Material and Methods
Results
 Comment
 Acknowledgments
 References
 
Animal Preparation
All animal procedures and protocols used in this study were reviewed and approved by the French "Ministère de l'Agriculture, de la Pêche et de l'Alimentation" before the studies were conducted. Thirty-two mixed-breed pregnant ewes between 130 and 132 days' gestation (term = 147 days) were fasted for 48 hours before surgery. Ewes were sedated with intravenous pentobarbital sodium (total dose, 2 to 4 g) and anesthetized with 1% bupivacaine hydrochloride (4 mg) by lumbar puncture. Under sterile conditions, the fetal lamb's left forelimb was delivered through an uterine incision. A skin incision was made. Polyvinyl catheters (20G) were advanced into the aorta and the vena cava through the axillary vessels. A left thoracotomy exposed the heart and great vessels. Catheters were inserted into the left pulmonary artery (22G), main pulmonary artery (20G), and left atrium (20G). An ultrasonic flow transducer, size 6 (Transonic Systems, Ithaca, New York), was placed around the left pulmonary artery to measure left pulmonary blood flow (right to left shunting through the DA precludes measuring pulmonary blood flow with a flow transducer placed around the main pulmonary artery). An inflatable vascular occluder (OC 10 mm; In Vivo Metric, Healdsburg, California) was placed loosely around the DA. The uteroplacental circulation was kept intact, and the fetus was gently replaced in the uterus. The flow transducer, catheters, and occluder were exteriorized through a subcutaneous tunnel to an external flank pouch. The ewes recovered rapidly from surgery, generally standing in their pens within 6 hours. Estimated weight of the fetal lambs was 3,000 g.

Catheters were maintained by daily infusions of 2 mL heparinized saline (10 U/mL). Catheter positions were checked at autopsy. Studies were performed after a recovery time of 72 hours.

Physiologic Measurements
The flow transducer cable was connected to an internally calibrated flowmeter (T201; Transonic Systems) for continuous measurements of left pulmonary artery blood flow. The output filter of the flowmeter was set at 30 Hz. The absolute value of flow was determined from the mean of phasic blood flow signals (at least 30 cardiac cycles), with zero blood flow defined as the measured flow value immediately before the beginning of systole [15]. Main pulmonary artery, aortic, left atrial, and amniotic catheters were connected to blood pressure transducer (Merlin Multi-Parameter Monitor; Philips Systèmes Médicaux, Suresnes, France). The pressure and flow signals were continuously recorded and processed on a personal computer (Pentium IV, 2,000 Mz) using an analog-to-digital converter system (Physiotrace, ESTARIS; Institut Technologie Medicale, Lille, France). Data were sampled at a rate of 200 samples/s. Pressures were referenced to the amniotic cavity pressure. Heart rate was determined from the phasic pulmonary blood flow signal. Pulmonary vascular resistance in the left lung was calculated as the difference between mean pulmonary artery and left atrial pressures divided by mean left pulmonary blood flow. Blood samples from the main pulmonary artery catheter were used for blood gas analysis and oxygen saturation measurements (OSM 3 Hemoximeter and ABL 520 Radiometer, Copenhagen, Denmark), and for plasma sildenafil concentration measurements. Plasma sildenafil concentration was measured by using liquid chromatography-tandem mass spectrometry method on silica column with aqueous-organic mobile phase.

Drug Preparation
A solution of sildenafil at a concentration of 1 mg/mL (grant from Pfizer, Sandwich, United Kingdom) was used.

Experimental Design
The animals were randomly assigned into two groups: (1) a control group infused with saline at a rate of 1 mL/h; and (2) a sildenafil group infused continuously with sildenafil at a rate of 1 mL/h (1 mg/h). This dose was selected from previous studies that have demonstrated substantial effects of sildenafil with oral doses ranging from 100 µg/kg daily to 25 mg/kg daily in rats and with an oral dose of 150 mg/d in adult patients with pulmonary arterial hypertension [16–18]. Sildenafil or saline were infused into the venous catheter and started 72 hours after the fetal surgery. Three different experimental protocols were performed, as follows:

Protocol 1: Effects of PDE5 Inhibition on Basal Pulmonary Vascular Tone
To investigate the effects of prolonged PDE5 inhibition on the fetal pulmonary circulation, we studied the hemodynamic response to sildenafil infused for 24 hours. Drugs were infused into the venous catheter. In the sildenafil group, saline (1 mL/h) was first infused for at least 30 minutes. After 30 minutes of stable baseline measurements, sildenafil was infused at a rate of 1 mL/h (= 1 mg/h) for 24 hours. In the control group, saline (1 mL/h) was infused for 24 hours. Mean pulmonary artery pressure (PAP), mean aorta pressure (AoP), left atrial pressure, amniotic pressure, left pulmonary artery blood flow, and heart rate were recorded at 10-minute intervals for 3 hours and then at 24 hours after the beginning of saline or sildenafil infusion. Pulmonary vascular resistance in the left lung was calculated. Blood gas analysis was performed before and 30 minutes, 2 hours, and 24 hours after starting drugs infusion. Plasma sildenafil concentrations were measured 24 hours after starting sildenafil infusion. Data were compared in the two groups.

Protocol 2: Effects of PDE5 Inhibition on the Hemodynamic Response to Increased Fetal PAO2
To investigate the role of PDE5 on the pulmonary vascular reactivity to increased PaO2, we examined the hemodynamic response to 100% inhaled O2 given to the ewes (O2 = 12 L/min) for 30 minutes, with and without sildenafil. Testing was performed after 24 hours of saline (1 mL/h; control group) or sildenafil (1 mL/h = 1 mg/h; sildenafil group) infusion. Hemodynamic measurements (mean PAP, left atrial pressure, AoP, left pulmonary artery blood flow, PVR) were recorded at 5-minute intervals for 30 minutes before the test, during O2 inhalation, and for 30 minutes after O2 discontinuation. Blood gas analysis was performed before and 25 minutes after starting O2 inhalation. Data obtained in the control group and in the sildenafil group were compared.

Protocol 3: Effects of PDE5 Inhibition on the Hemodynamic Response to Partial Compression of the Ductus Arteriosus
To investigate the role of PDE5 on the pulmonary vascular reactivity to acute DA compression, we examined the hemodynamic response to partial compression of the DA with and without sildenafil. Testing was performed after 24 hours of saline (1 mL/h; control group) or sildenafil (1 mL/h [1 mg/h]; sildenafil group) infusion. Baseline hemodynamic data were recorded during 30 minutes. Then, the DA was compressed by partially inflating the DA occluder. The degree of inflation was set to increase mean PAP by 15 mm Hg from its baseline value. Compression was maintained for 100 minutes. Mean PAP was kept constant throughout the compression period by readjusting the degree of inflation of the occluder as needed. Hemodynamic measurements (mean PAP, left atrial pressure, AoP, left pulmonary artery blood flow, PVR) were recorded at 10-minute intervals. Blood gas analysis was performed before and 60 minutes after starting DA compression. Data obtained in the control group and in the sildenafil group were compared.

Data Analysis
The results are presented as means ± SD. The data were analyzed using repeated-measures and factorial analysis of variance (ANOVA). Intergroup differences were analyzed with the Fisher, Scheffe, and Bonferroni/Dunn least significant tests (Stat View for PC; Abacus Concepts, Berkeley, California). The Mann-Whitney test (independent values) and paired Wilcoxon rank test (paired values) were used for the comparison of groups of animals. A p value less than 0.05 was considered as statistically significant.


   Results
Top
 Abstract
 Introduction
 Material and Methods
 Results
Comment
 Acknowledgments
 References
 
Protocol 1: Effects of PDE5 Inhibition on Basal Pulmonary Vascular Tone
At baseline, mean AoP and PAP, mean left pulmonary blood flow, and mean PVR were not different between the control group (n = 6) and the sildenafil group (n = 6; Fig 1). In the control group, mean AoP and PAP, mean left pulmonary blood flow, and mean PVR did not change during the study period (Fig 1, Table 1). In the sildenafil group, mean AoP and PAP did not change significantly after starting sildenafil infusion (Fig 1). Sildenafil increased mean left pulmonary blood flow by 160% (from 75 ± 7 to 194 ± 32 mL/min; p <0.01) and decreased mean PVR by 60% (from 0.62 ± 0.04 to 0.25 ± 0.04 mm Hg · mL–1 · min–1; p < 0.01). However, both mean left pulmonary blood flow and PVR returned to baseline values after 2 hours of sildenafil infusion (Fig 1). At the end of the study period, namelay, after 24 hours of saline or sildenafil infusion, mean AoP and PAP, left pulmonary blood flow, and PVR were similar in both groups (Fig 1, Table 1). Except for heart rate at 30 minutes of sildenafil infusion, values for mean left atrial pressure (2 ± 1 mm Hg), heart rate, arterial blood gas, and pH were not different between study groups at baseline and 30 minutes, 2 hours, and 24 hours after starting saline or sildenafil infusion (Table 1). Mean plasma sildenafil concentration 24 hours after starting sildenafil infusion was 22 ± 8 ng/mL.


Figure 1
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  		Fig 1. Effects of phosphodiesterase-5 inhibition on basal pulmonary vascular tone. Mean aortic pressure and pulmonary artery pressure (PAP) did not change during the sildenafil infusion (sildenafil group [solid ovals]: n = 6) and during the saline infusion (control group [open ovals]: n = 6). During sildenafil infusion mean left pulmonary blood flow increased by 160% and mean pulmonary vascular resistance (PVR) decreased by 60%. Both values returned to baseline values after 2 hours of sildenafil infusion and remained at the baseline at the end of the study period (24 hours after the beginning of the infusion). *p < 0.01 before sildenafil infusion versus during the early phase of sildenafil infusion. Data shown as means ± SD. Gray bars represent infusion. (H = hours; min = minutes.)

 

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  		Table 1. Blood Gas, Heart Rate, and Mean Aortic Pressure, at Baseline and at 30 Minutes, 2 Hours, and 24 Hours After Saline (Control Group) or Sildenafil Infusion (Sildenafil Group)
 
Protocol 2: Effects of PDE5 Inhibition on the Hemodynamic Response to Increased Fetal PAO2
Before starting the O2 challenge, namely, after 20 hours of saline or sildenafil infusion, mean AoP and PAP, left pulmonary blood flow, and PVR were similar in both groups (Fig 2, Table 2). Maternal inhalation of 100% O2 for 30 minutes increased fetal PaO2 from 16 ± 2 to 23 ± 4 mm Hg and from 17 ± 2 to 25 ± 5 mm Hg, respectively, in the control group (n = 5) and the sildenafil group (n = 5; p < 0.05). In the control group (n = 5), increased fetal PaO2 did not change mean AoP and PAP, and increased mean pulmonary artery blood flow by 100% (from 74 ± 13 to 152 ± 12 mL/min; p < 0.0001; Fig 2). Mean PVR decreased by 40% (from 0.56 ± 0.09 to 0.33 ± 0.03 mm Hg · mL–1 · min–1; p < 0.05; Fig 2). Maternal O2 inhalation did not alter AoP, heart rate, left atrial pressure (2 ± 1 mm Hg), pH, and PaCO2 (Table 2). In the sildenafil group (n = 5), increased fetal PaO2 did not change mean AoP and PAP, and increased the mean left pulmonary blood flow by 190% (from 84 ± 22 to 226 ± 50 mL/min; p < 0.0001; Fig 2). Mean PVR dropped by 65% (from 0.61 ± 0.07 to 0.21 ± 0.03 mm Hg · mL–1 · min–1; p < 0.0001; Fig 2). Maternal O2 inhalation did not alter significantly heart rate, left atrial pressure (2 ± 1 mm Hg), pH, and PaCO2 in the two groups (Table 2). Increase in left pulmonary blood flow and decrease in PVR resulting from O2 inhalation were greater in the sildenafil group than in the control group (p < 0.05). Mean plasma sildenafil concentration 24 hours after starting sildenafil infusion was 24 ± 6 ng/mL.


Figure 2
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  		Fig 2. Effects of phosphodiesterase-5 inhibition on the hemodynamic response to increased fetal PaO2. Increase in left pulmonary blood flow and decrease in pulmonary vascular resistance (PVR [40% versus 65%]) resulting from O2 inhalation were greater in the sildenafil group (n = 5 [solid ovals]) than in the control group (n = 5 [open ovals]). *p < 0.05 sildenafil group versus control group. Data shown as means ± SD. Gray bars represent maternal O2 inhalation. (min = minutes; PAP = pulmonary artery pressure.)

 

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  		Table 2. Blood Gas, Heart Rate, and Mean Aortic Pressure, at Baseline and at 25 Minutes After Starting Maternal O2 Inhalation in Control and Sildenafil Groups
 
Protocol 3: Effects of PDE5 Inhibition on the Hemodynamic Response to Partial Compression of the Ductus Arteriosus
Just before the DA compression, namely, after 20 hours of saline or sildenafil infusion, mean AoP and PAP, left pulmonary blood flow, and PVR were similar in both groups (Fig 3, Table 3). In the control group (n = 5), DA compression increased mean PAP from 47 ± 4 to 62 ± 4 mm Hg (p < 0.0001), and increased left pulmonary blood flow by 75% (from 83 ± 12 to 144 ± 34 mL/min; p < 0.0001). Pulmonary vascular resistance fell steadily during this compression period by 30% (from 0.55 ± 0.05 to 0.39 ± 0.03 mm Hg · mL–1 · min–1; p < 0.0001). However, both mean left pulmonary artery blood flow and PVR returned to baseline values after 2 hours of DA compression (Fig 3). In the sildenafil group (n = 5), DA compression increased PAP from 46 ± 4 to 59 ± 4 mm Hg (p < 0.0001), and increased left pulmonary blood flow 20 minutes after compression by 160% (from 84 ± 11 to 220 ± 20 mL/min; p < 0.0001). The PVR dropped by 50% (from 0.56 ± 0.06 to 0.26 ± 0.04 mm Hg · mL–1 · min–1; p < 0.0001). Mean values for heart rate, left atrial pressure (2 ± 1 mm Hg), and blood gas parameters were not different between the two groups (Table 3). Increase in pulmonary blood flow, and decrease in PVR resulting from DA compression were greater and more prolonged in the sildenafil group than in the control group (p < 0.05). Mean plasma sildenafil concentration 24 hours after starting sildenafil infusion was 20 ± 5 ng/mL.


Figure 3
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  		Fig 3. Effects of phosphodiesterase-5 inhibition on the hemodynamic response to ductus arteriosus compression. Increase in pulmonary blood flow, and decrease in pulmonary vascular resistance (PVR [30% versus 50%]) resulting from ductus arteriosus compression were greater and more prolonged in the sildenafil group (n = 5 [solid ovals]) than in the control group (n = 5 [open ovals]). *p < 0.05 sildenafil group versus control group. Data shown as means ± SD. (min = minutes; PAP = pulmonary artery pressure.)

 

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  		Table 3. Blood Gas, Heart Rate, and Mean Aortic Pressure, at Baseline and 1 Hour After Starting Ductus Arteriosus Compression in Control and Sildenafil Group
 

   Comment
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
Acknowledgments
 References
 
In this in vivo study, we tested the hypothesis that PDE5 is involved in the control of the pulmonary circulation in the perinatal period. We studied the pulmonary vascular response to prolonged sildenafil infusion (24 hours) in near-term fetal lambs. We found that sildenafil transiently decreased PVR. However, pulmonary vascular tone returned to baseline after 2 hours of sildenafil infusion. Moreover, we have found that sildenafil increased the pulmonary hemodynamic responses to increased fetal PaO2 and to acute DA compression. These results support the hypothesis that sildenafil enhances pulmonary vascular reactivity to birth-related events.

Many studies investigated the safety and functional role of the PDE5-cGMP-nitric oxide pathway in the cardiovascular system. Among the cGMP-specific PDEs, PDE5 is quantitatively prevalent in lung tissue, and many recent reports have emerged from clinical experience with the use of PDE5 inhibitors in adult patients with pulmonary hypertension [18–22]. Previous investigations suggested that PDE5 is also involved in the control of the perinatal lung circulation. Past study has demonstrated that the gene encoding PDE5 is abundantly expressed in the lungs of perinatal rats, and is available to participate in the mammalian pulmonary vascular transition to extrauterine life [23]. In parallel, Hanson and colleagues [12] have demonstrated that within 1 hour after birth, PDE5 activity, protein, and mRNA levels decrease in ovine and mouse species, in a manner that correlates with known decreases in pulmonary vascular resistance in early transition [12]. However, from 4 to 7 days after birth, a secondary increase in PDE5 activity, protein, and mRNA occurs in both ovine and mouse lung. In piglets, both total and PDE5-dependent cGMP hydrolytic activity and PDE5 protein expression increased with postnatal age [24].

The functional role of PDE5 has been assessed by testing the pulmonary vascular response to PDE5 inhibitors in the pulmonary vascular bed of various experimental models. Brief infusions (2 hours) of dipyridamole, a nonselective PDE inhibitor, and zaprinast, a selective PDE5 inhibitor, mediate a pulmonary vasodilation dependent on endogenous (basal) nitric oxide production in the ovine fetal pulmonary circulation [14, 25]. The role of PDE5 on the pulmonary vascular reactivity has not been fully investigated. Dipyiridamole enhances acetylcholine- and inhaled nitric oxide–mediated pulmonary vasodilation in the ovine fetus [14].

Our study provides new information regarding the role of PDE5 during perinatal life. We found that sildenafil causes a potent but transient pulmonary vasodilation in the fetus. Furthermore, we found that sildenafil enhances the pulmonary vasodilator responses to birth-related stimuli, especially to increase in PaO2 and to acute increase in blood flow. Indeed, partial compression of the DA causes a rise in mean pulmonary artery pressure and pulmonary artery blood flow, and a drop in PVR [26]. The decrease in PVR is considered as a consequence of mechanical increase in blood flow and in shear stress. Thus, PDE5 inhibition potentiates the pulmonary vascular response to shear-stress. Finally, although both dipyridamole and zaprinast decrease systemic pressure, no change in aortic pressure was found during prolonged sildenafil infusion [13, 14, 27–29].

The selectivity and the potency of PDE5 inhibitors are very variable. Sildenafil inhibits the human PDE5 enzyme in vitro by 50% at a concentration of 3.5 nmol/L, which is 100 times lower than the concentration of dipyridamole and zaprinast required to obtain the same effect (IC50: 745 and 330 nmol/L, respectively) [30]. No change in fetal blood gases was observed with sildenafil. Although pulmonary vasodilation may diminish placental blood flow, unchanged blood gases suggests that sildenafil did not alter placental blood flow. Mechanism is unclear, but may include a vasodilation of the umbilical and placental circulation, or a more global increase in systemic blood flow. In accordance with this hypothesis, sildenafil increased heart rate. As the combined ventricular output in the fetus is largely rate dependent, sildenafil may have increased systemic blood flow. Taken together, these data show that sildenafil induces a transient pulmonary vasodilation without change in aortic pressure and potentiates the vascular response to O2 and shear stress in the fetal lung. The results suggest that PDE5 is a key regulator of the pulmonary circulation during the perinatal period.

Phosphodiesterase-5 inactivates cGMP, a potent pulmonary vasodilator [10]. As sildenafil selectively inhibits PDE5, it increases cGMP content in vascular smooth muscle cells [31]. Sildenafil-mediated pulmonary vasodilation may be explained by an increase in cGMP in pulmonary arteries, because PDE5 is abundantly expressed in perinatal lung tissue [12, 23]. Surprisingly, sildenafil-induced pulmonary vasodilation was not sustained. Such transient response was found in the fetus with other vasodilator stimuli. Prolonged exposure to most of the pulmonary vasodilator stimuli (acetylcholine, bradykinin, shear stress, O2, and so forth) mediates a time-dependent pulmonary vasodilation with hemodynamic variables returning to baseline despite continued exposure to the vasodilating stimulus [14, 26, 32]. Teleologically, this active opposition to prolonged pulmonary vasodilation in the fetal lung prevents a "steal" of blood flow from the placenta, the fetal organ of gas exchange. However, endothelium-independent agents that directly increase smooth muscle cell cGMP levels were found to cause sustained pulmonary vasodilation during prolonged infusions [33, 34]. Mechanisms contributing to this time-dependent autoregulatory response are unknown, but may include decreased production of endogenous nitric oxide by the endothelial and smooth muscle cells, down-regulation of guanylate cyclase, or increased release of vasoconstricting mediators such as endothelin-1.

Conclusion and Perspectives
In conclusion, we found that sildenafil induces a potent but transient pulmonary vasodilation in the fetus. Although the effects of sildenafil on the basal pulmonary vascular tone are transient, sildenafil causes prolonged changes in pulmonary vasoreactivity. Especially, sildenafil increases the pulmonary hemodynamic responses to increased fetal PaO2 and to acute DA compression. These results support the hypothesis that PDE5 is involved in the control of the basal pulmonary vascular tone and in the regulation of the pulmonary vascular reactivity to birth-related events during the perinatal life.

Pulmonary hypertension is associated with elevated pulmonary vascular tone and abnormal pulmonary vasoreactivity. Especially, the pulmonary vascular response to increased O2 tension and to change in hemodynamic forces are markedly impaired in persistent pulmonary hypertension of the newborn [7, 9]. Chronic pulmonary hypertension impairs flow-induced vasodilation and augments a myogenic response, causing a paradoxical vasoconstriction to increase in pulmonary pressure [9]. As sildenafil increases the pulmonary vascular response to birth-related stimuli, we speculate that PDE5 inhibitor treatment may have a therapeutic impact on perinatal pulmonary hypertension.


   Acknowledgments
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
 Acknowledgments
References
 
This work was supported by Grants from Délégation à la Recherche du CHRU de Lille, Faculté de Médecine de Lille, Université de Lille II, Fondation de France, Fondation de l'Avenir, and by Journées Francophones de Recherche en Néonatologie.


   References
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
 Acknowledgments
 References
 

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