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Pulmonary Hemodynamic Responses to Intravenous Prostaglandin E2 in Broiler Chickens
Pulmonary Hemodynamic Responses to Intravenous Prostaglandin E2 in Broiler Chickens S. Stebel and R. F. Wideman1 Department of Poultry Science, University of Arkansas, Fayetteville 72701 compared with preinfusion control values, PGE2 reduced CO from 140 ± 6 to 111 ± 5 mL/kg of BW × min (P < 0.001), reduced PAP from 25 ± 2 to 21 ± 1 mmHg (P < 0.001), and reduced RR from 49 ± 4 to 35 ± 4 breaths/ min (P < 0.001). The reduction in CO was caused by a reduction in HR from 305 ± 9 to 260 ± 9 beats/min without a significant reduction in stroke volume (control: 0.46 ± 0.02 mL/kg of BW × beat; PGE2: 0.43 ± 0.02 mL/kg of BW × beat; P = 0.158). After the PGE2 infusion ceased the CO, PAP, RR, and HR recovered within 9 min to levels that did not differ from preinfusion control values. The PVR, calculated as PAP/CO, was not altered by PGE2 (control: 0.18 ± 0.01 relative resistance units; PGE2: 0.20 ± 0.02 relative resistance units; P > 0.723). These results indicate that in broilers PGE2 reduced PAP by reducing CO rather than by acting as a pulmonary vasodilator to lower PVR. The PGE2-induced reductions in PAP would benefit broilers that are susceptible to pulmonary hypertension syndrome by reducing their right ventricular overload; however, the reductions in CO and RR combined with the onset of systemic arterial hypoxemia would accelerate the pathophysiological progression leading to terminal pulmonary hypertension syndrome.
Key words: respiration, pulmonary hypertension, cardiac output 2008 Poultry Science 87:138–145 doi:10.3382/ps.2007-00334
binds to 1 of 4 specific E-prostanoid receptor types (EP1 to EP4) to elicit biological responses (Menconi et al., 1984; Chilton et al., 1997; Audoly et al., 1999; Ivanov and Romanovsky, 2004). Depending on the EP receptors present in organs or tissues, PGE2 can affect diverse biological functions including inflammatory responses (Brigham et al., 1988; Downey et al., 1988; Wakabayashi et al., 2002; Goulet et al., 2004), platelet aggregation (Fabre et al., 2001; Gross et al., 2007); febrile responses to LPS (Ivanov et al., 2002, 2003; Ivanov and Romanovsky, 2004; Li et al., 2006); airway tone and respiratory function (Brigham et al., 1988; Guerra et al., 1988; Savich et al., 1995; Hartney et al., 2006; Tilley et al., 2003), cardiac function (Hintze and Kaley, 1984; Panzenbeck et al., 1989; Klein et al., 2004), and systemic and regional (renal, pulmonary) blood pressure and flow (Cassin et al., 1979; Lock et al., 1980; Downey et al., 1988; Guerra et al., 1988;
INTRODUCTION Inflammatory stimuli such as bacterial lipopolysaccharide (LPS) and physical stimuli such as membrane stretch or shear stress can cause leukocytes, platelets and endothelial cells to express the enzyme phospholipase A2 which cleaves arachidonic acid (AA) from cell membrane phospholipids. Free AA is converted by lipooxygenase into leukotrienes, or by constitutive and inducible cyclooxygenases (COX-1 and COX-2, respectively) into prostaglandin H2 (PGH2). Prostaglandin E synthase converts PGH2 into prostaglandin E2 (PGE2), which
©2008 Poultry Science Association Inc. Received August 13, 2007. Accepted October 8, 2007. 1 Corresponding author: [email protected]
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ABSTRACT Prostaglandin E2 (PGE2) affects pulmonary arterial pressure (PAP), pulmonary vascular resistance (PVR), and respiratory rate (RR) in mammals, but no information previously was available regarding avian pulmonary responses to PGE2. Two experiments were conducted in which 45- to 55-d-old male broiler chickens were infused i.v. with PGE2 at the lowest rate (30 g/ min for 4 min) that reliably reduced PAP during pilot studies. When compared with preinfusion (control) values in experiment 1, PGE2 reduced PAP from 19 ± 1 to 16 ± 1 mmHg (P < 0.001) and reduced mean systemic arterial pressure from 111 ± 6 to 81 ± 5 mmHg (P < 0.001) but did not significantly reduce heart rate (HR; control: 338 ± 9 beats/min; PGE2: 320 ± 12 beats/min; P > 0.05). Infusing PGE2 also reduced the RR from 57 ± 2 to 46 ± 4 breaths/min (P < 0.001) and reduced the percentage saturation of hemoglobin with oxygen (%HbO2) from 85 ± 2 to 77 ± 3%HbO2 (P < 0.001). After the PGE2 infusion ceased, the PAP, mean systemic arterial pressure, RR, and %HbO2 recovered within 8 min to levels that did not differ from preinfusion control values. In experiment 2, an ultrasonic flow probe was surgically implanted on 1 pulmonary artery to measure cardiac output (CO). When
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MATERIALS AND METHODS Experiment 1 Male broiler chicks from a commercial hatchery were transported on the day of hatch to the Poultry Environmental Research Lab at the University of Arkansas Poultry Research Farm, where they were wing-banded and placed on fresh wood shavings litter in an environmental chamber (8 m2 floor space). Chicks were brooded at 32°C on d 1 to 3, 31°C on d 4 to 6, 29°C on d 6 to 10, and 24°C thereafter. They were fed a 23% CP corn-soybean meal-based broiler ration formulated to meet NRC (1994) standards. Feed and water were provided for ad libitum consumption. Lights were on for 24 h/d through d 5
and for 12 h/d thereafter. Between 45 and 55 d of age, 10 broilers (3,675 ± 85 g of BW, mean ± SEM) were prepared for surgery as described previously (Wideman et al., 2001; Chapman and Wideman, 2002). All birds had full access to feed and water until they were anesthetized. A surgical plane of anesthesia was induced with intramuscular injections of allobarbital (5,5-diallyl-barbituric acid; 16 mg/kg of BW, Sigma Chemical Co., St. Louis, MO) and ketamine HCl (45 mg/kg of BW; Henry Schein Inc., Melville, NY). Silastic Tubing (0.012 in I.D., 0.037 in O.D.; Konigsberg Instruments Inc., Pasadena, CA) filled with heparinized saline (150 units of ammonium heparin/mL of 0.9% NaCl, Sigma Chemical Co.) was inserted through the left basilica (wing) vein. The distal end of the cannula was attached to a blood pressure transducer interfaced through a Transbrige preamplifier (World Precision Instruments, Sarasota, FL) to a Biopac MP100 data acquisition system using AcqKnowledge software (Biopac Systems Inc., Santa Barbara, CA). The Silastic cannula was advanced into a pulmonary artery by monitoring the characteristic pulse pressures to identify its location (Chapman and Wideman, 2001). Polyethylene tubing (PE-50) filled with heparinized saline was inserted into the left basilica vein, and was advanced to a position near the inferior vena cava for i.v. infusion. To monitor mean systemic arterial pressure (MAP), PE-50 tubing filled with heparinized saline was inserted into the left brachial artery, and the distal end was attached to a blood pressure transducer. A universal “C” sensor attached to a Vet/Ox 4403 pulse oximeter (Heska Corp., Ft. Collins, CO) was positioned on the right wing to illuminate the tissue between the radius and ulna and record the percentage saturation of hemoglobin with oxygen (%HbO2) and HR (Wideman and Kirby, 1995). When all surgical preparations were complete and a stabilization period of 10 min had elapsed, control data were recorded for 5 min. The birds then received a constant i.v. infusion of PGE2 that was freshly prepared immediately prior to infusion by diluting an aliquot of a 250 g/mL of PGE2 stock solution (Cayman Chemical, Ann Arbor, MI) with 0.9% saline. A syringe infusion pump (Harvard Apparatus Inc., Holliston, MA) was used to infuse the PGE2 for 4 min at a rate of 30 g/min (75 g of PGE2/mL infused at 0.4 mL/min). Pilot studies conducted using 4 broilers (not shown) indicated the 30 g/min infusion rate was the lowest rate (range: 5 to 50 g of PGE2/min) that reliably reduced PAP by approximately 2 mmHg. Data collection continued for 10 min after the PGE2 infusion ceased. Experiments were terminated by injecting birds i.v. with 10 mL of 0.1 M KCl.
Experiment 2 Male broilers were reared as described for experiment 1. Between 45 and 50 d of age, 10 broilers (3,259 ± 88 g of BW) were anesthetized and fastened in dorsal recumbency on a surgical board; then, an incision was made to open the thoracic inlet, and a Transonic ultrasonic
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Audoly et al., 1999; Tod et al., 1992; Gao et al., 1996). The responses elicited by PGE2 can vary dramatically depending on the pre- or postnatal stage of development and maturation (Tai and Adamson, 2000). Broiler chickens are susceptible to the onset of pulmonary arterial hypertension leading to pulmonary hypertension syndrome (PHS, ascites) when they possess an inadequate pulmonary vascular capacity characterized by an elevated pulmonary vascular resistance (PVR) and systemic arterial hypoxemia (under-saturation of hemoglobin with oxygen in arterial blood). It currently is our hypothesis that susceptibility to PHS is a consequence of an anatomically inadequate pulmonary vascular capacity combined with the functional predominance of vasoconstriction (increased PVR) attributable to serotonin over vasodilation (reduced PVR) and immune modulation attributable to nitric oxide (Wideman and Bottje, 1993; Wideman, 2000, 2001; Wideman et al., 2004, 2005b, 2007). The contributions of vasoactive eicosanoids (oxygenated metabolites of AA) such as thromboxane A2 (TxA2), prostacyclin (PGI2), and PGE2 remain uncertain. Previous studies demonstrated the capacity of potent TxA2 analogs to constrict the pulmonary and systemic vasculature of broilers (Wideman et al., 1999, 2001), and TxA2 may contribute to the increased PVR during the pulmonary hypertensive response to LPS (Wideman et al., 2004). Prostacyclin does not appear to dilate the pulmonary vasculature in broilers, but when infused i.v. PGI2 reduced the pulmonary arterial pressure (PAP) by reducing the cardiac output (CO; Wideman et al., 2005a). The impact of PGE2 on pulmonary hemodynamics and respiratory function previously had not been evaluated in broilers, but in various mammalian species PGE2 dilated the pulmonary vasculature (Cassin et al., 1979; Lock et al., 1980; Downey et al., 1988; Gao et al., 1996), reduced the heart rate (HR) and CO (Cassin et al., 1979; Hintze and Kaley, 1984; Guerra et al., 1988; Panzenbeck et al., 1989), inhibited the respiratory rate (RR; Guerra et al., 1988; Savich et al., 1995), and modulated the pulmonary response to LPS (Brigham et al., 1988). The present study was designed to assess the respiratory and hemodynamic responses of broilers to i.v. infusions of PGE2.
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Data Analysis In both experiments data were averaged electronically at 3, 2, and 1 min before the start of the infusion (control sample intervals C1, C2, C3), at the start of PGE2 infusion, and at 30 s intervals thereafter until the infusion was stopped (PGE2 infusion sample intervals E-Start, E1 to E8, E-Stop), and during the postinfusion recovery period at 1-min intervals (recovery sample intervals R1 to R10). The protocol used for data averaging accommodates the influences of pulse pressure and respiratory cycles on pulmonary and systemic arterial pressures (Wideman et al., 1996). The primary data recorded included PAP (mmHg), MAP (mmHg), and blood flow through the left pulmonary artery (mL/min). Heart rate (beats/min) was recorded directly from the pulse oximeter in experiment 1 or by counting systolic peaks over time in the PAP recordings in experiment 2. In both experiments the RR (breaths/min) was obtained by counting the wave cycles associated with respiratory movement that constitute an integral part of the PAP recording (Wideman et al., 1999). Based on the assumption that CO (mL/min) normally is divided approximately equally between the lungs, CO was calculated as 2 × blood flow through the left pulmonary artery. Because of the strong correlation between CO and BW (Wideman, 1999), the CO was normalized per kg of BW on an individual bird basis (CO/kg of BW). The CO is the product of HR (beats/min) × stroke volume (SV, mL/beat); consequently, SV was calculated as the CO (per kg of BW)/HR. Assuming atrial pressures remain close to zero, as assessed by direct cannulation or wedge pressure measurements (Chapman and Wideman, 2001; Lorenzoni et al., 2008), then the pressure gradient across the pulmonary circulation is essentially equal to PAP, permitting the PVR (relative resistance units) to be estimated as PVR = PAP/CO (per kg of BW; Wideman et al., 1996). Data were analyzed over time (across sample intervals) using the SigmaStat Repeated Measures ANOVA procedure, and means were differentiated by the Tukey Test (Jandel Scientific, 1994). The threshold for significance in all cases was P ≤ 0.05.
RESULTS Values for PAP, MAP, HR, %HbO2, and RR before, during, and after the i.v. infusion of PGE2 in experiment 1 are shown in Figure 1. During control sample intervals C1 to C3, none of the variables differed over time. Comparing sample intervals C3 vs. E8, PGE2 infusion reduced PAP from 19.3 ± 1 to 16.2 ± 1 mmHg and reduced MAP from 111 ± 6 to 81 ± 5 mmHg but did not significantly reduce HR (control: 338 ± 9 beats/min; PGE2: 320 ± 12 beats/min). Infusing PGE2 also reduced the percentage saturation of hemoglobin with oxygen (%HbO2) from 85 ± 2 to 77 ± 3%, and reduced the RR from 57 ± 2 to 46 ± 4 breaths/min. After the PGE2 infusion ceased the PAP, MAP, RR, and %HbO2 recovered within 8 min to levels that no longer were significantly lower than their respective preinfusion control values. The responses to PGE2 infusion in experiment 2 are shown in Figure 2. During control sample intervals C1 to C3 none of the variables differed over time. Comparing sample intervals C3 vs. E8, PGE2 infusion reduced PAP from 25 ± 2 to 21 ± 1 mmHg, reduced CO from 140 ± 6 to 111 ± 5 mL/kg of BW × min, and reduced RR from 49 ± 4 to 35 ± 4 breaths/min. The PVR, calculated as PAP/CO, was not affected by the PGE2 infusion (control: 0.18 ± 0.01 relative resistance units; PGE2: 0.20 ± 0.02 relative resistance units; P > 0.723). After the PGE2 infusion ceased, the PAP, CO, and RR recovered within 9 min to levels that did not differ from preinfusion control values (Figure 2). The HR (not shown) was determined by counting systolic peaks within the PAP recording during sample intervals C1 to C3, E7 to E-Stop, and R8 to R10. The HR averaged 305.5 ± 8.6 beats/min during the preinfusion control intervals, declined to 259.7 ± 9.1 beats/min by the final PGE2 infusion intervals (P < 0.001), and then returned to 287.5 ± 8.0 beats/min by postinfusion intervals R8 to R10. Contemporaneous values calculated for SV averaged 0.46 ± 0.02 mL/kg of BW × beat during sample intervals C1 to C3, 0.43 ± 0.02 mL/ kg of BW × beat during intervals E7 to E-Stop, and 0.48 ± 0.04 mL/kg of BW × beat during intervals R8 to R10 (P = 0.158).
DISCUSSION Intravenous infusions of PGE2 reduced the PAP in clinically healthy broilers. The magnitude of the pressure gradient across the pulmonary circulation is determined by multiplying the PVR and CO (Wideman and Bottje, 1993; Wideman, 2000; Lorenzoni et al., 2008). The PVR was not reduced by PGE2 in experiment 2; consequently, PGE2 did not dilate the pulmonary vasculature but instead reduced the PAP by reducing the CO. The CO is determined by multiplying the HR and SV. Infusing PGE2 did not significantly affect HR in experiment 1 but markedly reduced the HR and numerically lowered the SV (P = 0.158) in experiment 2. These responses to PGE2 infusion suggest a multiplicative effect whereby the combined reductions in HR and SV triggered a substan-
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flow probe (Transonic Systems Inc., Ithaca, NY) was positioned on the left pulmonary artery. The probe was connected to a Transonic T206 blood flow meter to confirm signal acquisition and record blood flow through the pulmonary artery; then, the skin of the thoracic inlet was sealed airtight with surgical wound clips. A Silastic cannula was inserted into a pulmonary artery to record PAP, and PE-50 polyethylene tubing filled with heparinized saline was inserted into the left basilica vein for i.v. infusion. The MAP was not recorded. After a stabilization period of 10 min had elapsed, control data were recorded for 5 min. The birds then received a constant i.v. infusion of PGE2 for 4 min at a rate of 30 g/min (75 g of PGE2/mL infused at 0.4 mL/min). Data collection continued for 10 min after the PGE2 infusion ceased. Birds were euthanized by i.v. injections of 0.1 M KCl.
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Figure 1. Pulmonary arterial pressure (PAP), mean systemic arterial pressure (MAP), heart rate (HR), percentage saturation of hemoglobin with oxygen (HbO2), and respiratory rate (RR) for male broilers in experiment l (mean ± SEM; n = 10). Data were averaged electronically at 3, 2, and 1 min prior to the start of the infusion (control sample intervals C1, C2, C3), at the start of PGE2 infusion and at 30-s intervals thereafter until the infusion was stopped (PGE2 infusion sample intervals E-Start, E1 to E8, E-Stop), and during the postinfusion recovery period at 1 min intervals (recovery sample intervals R1 to R10). *Denotes all values that differed when compared with sample interval C3 by repeated measures ANOVA (P ≤ 0.05).
tial decrease in CO that in turn contributed to contemporaneous reductions in PAP and MAP. Qualitatively, similar pulmonary hemodynamic responses previously were observed when broilers were infused i.v. with PGI2, which also did not reduce the PVR but instead consistently reduced the PAP by lowering the CO. The impact of PGI2 on CO was associated with a reduction in SV
but not HR in one experiment, or with reductions in SV and HR in 2 additional experiments (Wideman et al., 2005a). In mammals both PGE2 and PGI2 appear to elicit reductions in HR and CO by stimulating ventricular receptors linked to reflex vagal inhibition of cardiac function (Chapple et al., 1980; Chiba and Malik, 1980; Hintze and Kaley, 1984; Panzenbeck et al., 1988, 1989). Stimulat-
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Figure 2. Pulmonary arterial pressure (PAP), cardiac output normalized for BW (CO), pulmonary vascular resistance normalized for BW (PVR), and respiratory rate (RR) for male broilers in experiment 2 (mean ± SEM; n = 10). Data were averaged electronically at 3, 2, and 1 min prior to the start of the infusion (control sample intervals C1, C2, C3), at the start of PGE2 infusion and at 30-s intervals thereafter until the infusion was stopped (PGE2 infusion sample intervals E-Start, E1 to E8, E-Stop), and during the postinfusion recovery period at 1-min intervals (recovery sample intervals R1 to R10). *Denotes all values that differed when compared with sample interval C3 by repeated measures ANOVA (P ≤ 0.05).
ing the cardiac vagal (parasympathetic) innervation of domestic fowl reduces the CO by reducing the HR (negative chronotropic effect) and the SV (negative inotropic effect; Smith et al., 2000). The cardio-inhibitory effects of PGE2 evidently overrode the reflexive increase in CO that normally would occur to restore adequate O2 delivery to the tissues in response to systemic arterial hypo-
xemia (Wideman and Tackett, 2000; Wideman et al., 2000). The negative inotropic and negative chronotropic responses to PGE2 also are consistent with a significant reduction in venous return to the heart. If PGE2 dilates the systemic venules and large veins of broilers, then the resulting increased pooling of blood within the venous volume reservoir would reduce atrial and ventricular
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vasodilation (a reduction in total peripheral resistance) that, when combined with the reduction in CO, caused the 30 mmHg decline in mean arterial pressure. Systemic arterial hypoxemia reduces the systemic vascular tone but does not alter the pulmonary vascular tone in broilers (Wideman et al., 1996, 2000; Wideman and Tackett, 2000). In summary, i.v. infusions of PGE2 successfully reduced the PAP in broilers but did not dilate the pulmonary vasculature. Instead, reductions in PAP were directly attributable to reductions in CO. Clearly the PAP of broilers can be directly affected by changes in CO in the absence of significant changes in PVR. The observed reductions in CO may be caused by PGE2-mediated inhibition of cardiac function or by reduced venous return secondary to PGE2-mediated dilation (expansion) of the venous volume reservoir. Both PGE2 (present study) and PGI2 (Wideman et al., 2005a) failed to reduce the PVR but elicited reductions in PAP by lowering the CO. Both eicosanoids also reduced the MAP by reducing the CO and, presumably, by dilating the systemic vasculature. Within the context of the pathogenesis of PHS, eicosanoid-induced reductions in the PAP would benefit susceptible broilers by reducing their right ventricular overload, whereas reductions in CO and MAP (reduced O2 delivery to the tissues) and systemic venous dilation (enhanced central venous congestion) would accelerate the onset of PHS. Inhibition of respiratory drive and the ensuing systemic arterial hypoxemia induced by PGE2 also would accelerate the pathophysiological progression leading to terminal PHS (Wideman, 2000, 2001). Accordingly, most of the cardio-pulmonary hemodynamic responses to PGE2 appear to be detrimental rather than beneficial for broilers developing PHS. Nitric oxide remains the principal vasoactive mediator that is capable of dilating the pulmonary vasculature in broilers (Wideman and Chapman, 2004; Wideman et al., 2005b, 2007; Tan et al., 2007).
ACKNOWLEDGMENTS This work was supported by USDA/CSREES/NRI grant #2003-35204-13392.
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filling, thereby causing simultaneous intrinsic reductions in HR, SV, and CO (Sturkie, 1986; Wideman et al., 2005a). Two of the 4 EP receptor types have been cloned and characterized in domestic fowl, but the vascular and cardiac distributions of EP receptors have not been demonstrated (Kwok et al., 2007). In mice the anatomical distributions of the EP receptor types determine the vascular responsiveness to PGE2 (Audoly et al., 1999), and in newborn lambs both PGE2 and PGI2 act primarily to dilate pulmonary veins rather than pulmonary arteries (Gao et al., 1996). Wedge pressure studies have consistently demonstrated that the PVR of broilers overwhelmingly reflects the tone (degree of partial contracture) of the precapillary arterioles rather than the resistance of the postcapillary vasculature (Chapman and Wideman, 2001; Lorenzoni et al., 2008). Accordingly, if receptors for PGE2 and PGI2 are primarily located on broilers’ pulmonary veins rather than arteries, then eicosanoidinduced reductions in postcapillary vascular tone would have a minimal impact on the PVR. In newborn lambs PGE2 was more potent in reducing systemic rather than pulmonary vascular resistance, whereas the opposite was true for PGI2 (Lock et al., 1980). In experiment 1 of the present study, PGE2 reduced the MAP by 30 mmHg but only reduced the PAP by 3 mmHg, suggesting PGE2 may be a more potent vasodilator of the systemic circulation than of the pulmonary circulation in broilers. The CO was not recorded in experiment 1 and the MAP was not recorded in experiment 2; therefore, the systemic vascular resistance (total peripheral resistance) could not be calculated. Intravenous PGE2 infusions reduced the RR and %HbO2 in broilers, and these variables readily returned to control levels after the PGE2 infusion ceased. The respiratory inhibition elicited by PGE2 in 45- to 55-d-old broilers resembles the responses elicited by PGE2 in fetal and neonatal mammals. Prostaglandin E2 profoundly reduces spontaneous breathing movements in fetal mammals and reduces the RR, tidal volume, and blood oxygenation in neonatal mammals (Kitterman et al., 1983; Murai et al., 1987; Guerra et al., 1988; Savich et al., 1995; Tai and Adamson, 2000; Hofstetter et al., 2007). The hypoventilation induced by PGE2 does not consistently involve peripheral chemoreceptors (carotid and aortic bodies) but instead is mediated primarily through receptors located within the central nervous system (Murai et al., 1987; Guerra et al., 1988; Tai and Adamson, 2000; Hofstetter et al., 2007). Hypoventilatory responses to PGE2 tend to wane as neonatal mammals mature (Tai and Adamson, 2000). To the best of our knowledge, the present study constitutes the first demonstration that PGE2 inhibits respiration in 6- to 8-wk-old domestic fowl, even to the extent of overriding the enhanced ventilatory drive that normally is triggered by hypoxemia (Wideman et al., 1996; Wideman and Tackett, 2000). Pulmonary vasoconstriction was not observed during the hypoxemic response to PGE2 (PVR did not increase); however, hypoxemia may have contributed to systemic
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STEBEL AND WIDEMAN function in adult rat ventricular cardiocytes. Pharmacol. Res. 49:99–103. Kwok, A. H. Y., C. Y. Wang, Y. Wang, and F. C. Leung. 2007. Cloning and characterization of chicken prostaglandin E subtypes 2 and 4 (EP2 and EP4). Poult. Sci. 86(Suppl. 1):535. (Abstr.) Li, Z., V. Perlik, C. Feleder, Y. Tang, and C. M. Blatteis. 2006. Kupfer cell-generated PGE2 triggers the febrile response of guinea pigs to intravenously injected LPS. Am. J. Physiol. 290:1262–1270. Lock, J. E., P. M. Olley, and F. Coceani. 1980. Direct pulmonary vascular responses to prostaglandins in the conscious newborn lamb. Am. J. Physiol. 238:H631–H638. Lorenzoni, A. G., N. B. Anthony, and R. F. Wideman. 2008. Transpulmonary pressure gradient verifies pulmonary hypertension is initiated by increased arterial resistance in broilers. Poult. Sci. 87:125–132. Menconi, M., G. Hahn, and P. Polgar. 1984. Prostaglandin synthesis in cells comprising the calf pulmonary artery. J. Cell. Physiol. 120:163–168. Murai, D. T., L. D. Wallen, C.-C. H. Lee, R. I. Clyman, F. Mauray, and J. H. Kitterman. 1987. Effects of prostaglandins on fetal breathing do not involve peripheral chemoreceptors. J. Appl. Physiol. 62:271–277. National Research Council (NRC). 1994. Nutrient Requirements of Poultry. 9th rev. ed. Natl. Acad. Press, Washington, DC. Panzenbeck, M. J., W. Tan, M. A. Hajdu, K. G. Cornish, and I. H. Zucker. 1989. PGE2 and arachidonate inhibit the baroreflex in conscious dogs via cardiac receptors. Am. J. Physiol. 256:H999–H1005. Panzenbeck, M. J., W. Tan, M. A. Hajdu, and I. H. Zucker. 1988. Intracoronary infusion of prostaglandin I2 attenuates the arterial baroreflex control of heart rate in conscious dogs. Circ. Res. 63:860–868. Savich, R. D., F. A. Guerra, C. C. Lee, and J. A. Kitterman. 1995. Prostaglandin E2 decreases fetal breathing movements, but not pulmonary blood flow, in fetal sheep. J. Appl. Physiol. 78:1477–1484. Smith, F. M., N. H. West, and D. R. Jones. 2000. The cardiovascular system. Pages 141–231 in Sturkie’s Avian Physiology, 5th ed. G. C. Whittow, ed. Acad. Press, San Diego, CA. Sturkie, P. D. 1986. Heart and circulation: Anatomy, hemodynamics, blood pressure, blood flow. Pages 130–166 in Avian Physiology, 4th ed. P. J. Sturkie, ed. Springer-Verlag, New York, NY. Tai, T. C., and S. L. Adamson. 2000. Developmental changes in respiratory, febrile, and cardiovascular responses to PGE2 in newborn lambs. Am. J. Physiol. 278:R1460–R1473. Tan, X., S.-H. Hu, and X.-L. Wang. 2007. Possible role of nitric oxide in the pathogenesis of pulmonary hypertension in broilers: A synopsis. Avian Pathol. 36:261–267. Tilley, S. L., J. M. Hartney, C. J. Erikson, C. Jania, M. Nguyn, J. McNeisch, C. Valancius, R. A. Panettieri, R. B. Penn, and B. H. Koller. 2003. Receptors and pathways mediating the effects of prostaglandin E2 on airway tone. Am. J. Physiol. 284:L599–L606. Tod, M. L., C.-M. Tseng, and L. J. Rubin. 1992. The role of eicosanoids in the pulmonary circulation. Coron. Artery Dis. 3:77–83. Wakabayashi, I., K. Yasui, and M. Negoro. 2002. Diverse effects of ethanol on the pathway of inducible prostaglandin E2 production in macrophages. Prostaglandins Other Lipid Mediat. 67:61–74. Wideman, R. F. 1999. Cardiac output in four-, five- and sixweek-old broilers, and hemodynamic responses to intravenous injections of epinephrine. Poult. Sci. 78:392–403. Wideman, R. F. 2000. Cardio-pulmonary hemodynamics and ascites in broiler chickens. Poult. Avian Biol. Rev. 11:21–43. Wideman, R. F. 2001. Pathophysiology of heart/lung disorders: Pulmonary hypertension syndrome in broiler chickens. World’s Poult. Sci. J. 57:289–307.
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Chapman, M. E., and R. F. Wideman. 2002. Hemodynamic responses of broiler pulmonary vasculature to intravenously infused serotonin. Poult. Sci. 81:231–238. Chapple, D. J., G. J. Dusting, R. Hughes, and J. R. Vane. 1980. Some direct and reflex cardiovascular actions of prostacyclin (PGI2) and prostaglandin E2 in anesthetized dogs. Br. J. Pharmacol. 68:437–447. Chiba, S., and K. Malik. 1980. Mechanism of the chronotropic effects of prostacyclin in the dog: Comparison of the actions of prostaglandin E2. J. Pharmacol. Exp. Ther. 213:261–266. Chilton, F. H., A. N. Fonteh, M. M. Johnson, and M. E. Surette. 1997. Metabolism of arachidonic acid. Pages 77–88 in The Lung, 2nd ed. R. G. Crystal, J. B. West, E. R. Weibel, and P. J. Barnes, ed. Lippencott–Raven, New York, NY. Downey, G. P., R. S. Gumbay, D. E. Doherty, J. F. LaBrecque, J. E. Henson, P. M. Henson, and G. S. Worthen. 1988. Enhancement of pulmonary inflammation by PGE2: Evidence for a vasodilator effect. J. Appl. Physiol. 64:728–741. Fabre, J.-E., M. T. Nguyen, K. Athirakul, K. Coggins, J. D. McNeish, S. Austin, L. K. Parise, G. A. FitzGerald, T. M. Coffman, and B. H. Koller. 2001. Activation of the murine EP3 receptor for PGE2 inhibits cAMP production and promotes platelet aggregation. J. Clin. Invest. 107:603–610. Gao, Y., H. Zhou, B. O. Ibe, and J. U. Raj. 1996. Prostaglandins E2 and I2 cause greater relaxations in pulmonary veins than in arteries of newborn lambs. J. Appl. Physiol. 81:2534–2539. Goulet, J. L., A. J. Pace, M. L. Key, R. S. Byrum, M. Nguyn, S. L. Tilley, S. G. Morham, R. Langenbach, J. L. Stock, J. D. McNeish, O. Smithies, T. M. Coffman, and B. H. Koller. 2004. E-prostanoid receptors mediate the proinflammatory actions of prostaglandin E2 in acute cutaneous inflammation. J. Immunol. 173:1321–1326. Gross, S., P. Tilly, D. Hentsch, L.-L. Vonesch, and J.-E. Fabre. 2007. Vascular wall-produced prostaglandin E2 exacerbates arterial thrombosis and atherothrombosis through platelet EP3 receptors. J. Exp. Med. 204:311–320. Guerra, F. A., R. D. Savich, L. D. Wallen, C. H. Lee, R. I. Clyman, F. E. Mauray, and J. A. Kitterman. 1988. Prostaglandin E2 causes hypoventilation and apnea in newborn lambs. J. Appl. Physiol. 64:2160–2166. Hartney, J. M., K. G. Coggins, S. L. Tilley, L. A. Jania, A. K. Lovgren, L. P. Audoly, and B. H. Koller. 2006. Prostaglandin E2 protects lower airways against bronchoconstriction. Am. J. Physiol. 290:L105–L113. Hintze, T. H., and G. Kaley. 1984. Ventricular receptors activated following myocardial prostaglandin synthesis initiate reflex hypotension, reduction in heart rate, and redistribution of cardiac output in the dog. Circ. Res. 54:239–247. Hofstetter, A. O., S. Saha, V. Siljehav, P.-J. Jakobsson, and E. Herlenius. 2007. The induced prostaglandin E2 pathway is a key regulator of the respiratory response to infection and hypoxia in neonates. Proc. Natl. Acad. Sci. USA 104:9894– 9899. Ivanov, A. I., R. S. Pero, A. C. Scheck, and A. A. Romanovsky. 2002. Prostaglandin E2-synthesizing enzymes in fever: Differential transcriptional regulation. Am. J. Physiol. 283:R1104–R1117. Ivanov, A. I., A. C. Scheck, and A. A. Romanocsky. 2003. Expression of genes controlling transport and catabolism of prostaglandin E2 in lipopolysaccharide fever. Am. J. Physiol. 284:698–706. Ivanov, A. I., and A. A. Romanovsky. 2004. Prostaglandin E2 as a mediator of fever: Synthesis and catabolism. Front. Biosci. 9:1977–1993. Jandel Scientific. 1994. SigmaStat Statistical Software User’s Manual. Jandel Sci. Software, San Rafael. CA. Kitterman, J. A., G. C. Liggens, J. E. Fewell, and W. H. Tooley. 1983. Inhibition of breathing movements in fetal sheep by prostaglandins. J. Appl. Physiol. 54:687–692. Klein, A. L., L. E. Wold, and J. Ren. 2004. The cyclooxygenase2 product prostaglandin E2 modulates cardiac contractile
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Wideman, R. F., G. F. Erf, and M. E. Chapman. 2005b. NωNitroL-Arginine Methyl Ester (L-NAME) amplifies the pulmonary hypertensive response to micro-particle injections in broilers. Poult. Sci. 84:1077–1091. Wideman, R. F., M. R. Fedde, C. D. Tackett, and G. E. Weigle. 2000. Cardio-pulmonary in preascitic (hypoxemic) or normal broilers inhaling ambient air or 100% oxygen. Poult. Sci. 79:415–425. Wideman, R. F., and Y. K. Kirby. 1995. Evidence of a ventilation-perfusion mismatch during acute unilateral pulmonary arterial occlusion in broilers. Poult. Sci. 74:1209–1217. Wideman, R. F., Y. K. Kirby, C. D. Tackett, N. E. Marson, C. J. Tressler, and R. W. McNew. 1996. Independent and simultaneous unilateral occlusion of the pulmonary artery and extra-pulmonary primary bronchus in broilers. Poult. Sci. 75:1417–1427. Wideman, R. F., P. Maynard, and W. G. Bottje. 1999. Thromboxane mimics the pulmonary but not systemic vascular responses to bolus HCl injections in broiler chickens. Poult. Sci. 78:714–721. Wideman, R. F., and C. Tackett. 2000. Cardio-pulmonary function in broilers reared at warm or cold temperatures: Effect of acute inhalation of 100% oxygen. Poult. Sci. 79:257–264.
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Wideman, R. F., and W. G. Bottje. 1993. Current understanding of the ascites syndrome and future research directions. Pages 1–20 in Nutr. Tech. Symp. Proc. Novus Int. Inc., St. Louis, MO. Wideman, R. F., and M. E. Chapman. 2004. Nω-Nitro-L-Arginine Methyl Ester (L-NAME) amplifies the pulmonary hypertensive response to endotoxin in broilers. Poult. Sci. 82:485–494. Wideman, R. F., M. E. Chapman, and G. F. Erf. 2005a. Pulmonary and systemic hemodynamic responses to intravenous prostacyclin (PGI2) in broilers. Poult. Sci. 84:442–453. Wideman, R. F., M. E. Chapman, K. R. Hamal, O. T. Bowen, A. G. Lorenzoni, G. F. Erf, and N. B. Anthony. 2007. An inadequate pulmonary vascular capacity and susceptibility to pulmonary arterial hypertension in broilers. Poult. Sci. 86:984–998. Wideman, R. F., M. E. Chapman, W. Wang, and G. F. Erf. 2004. Immune modulation of the pulmonary hypertensive response to bacterial lipopolysaccharide (endotoxin) in broilers. Poult. Sci. 83:624–634. Wideman, R. F., G. F. Erf, and M. E. Chapman. 2001. Intravenous endotoxin triggers pulmonary vasoconstriction and pulmonary hypertension in broiler chickens. Poult. Sci. 80:647–655.
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binds to 1 of 4 specific E-prostanoid receptor types (EP1 to EP4) to elicit biological responses (Menconi et al., 1984; Chilton et al., 1997; Audoly et al., 1999; Ivanov and Romanovsky, 2004). Depending on the EP receptors present in organs or tissues, PGE2 can affect diverse biological functions including inflammatory responses (Brigham et al., 1988; Downey et al., 1988; Wakabayashi et al., 2002; Goulet et al., 2004), platelet aggregation (Fabre et al., 2001; Gross et al., 2007); febrile responses to LPS (Ivanov et al., 2002, 2003; Ivanov and Romanovsky, 2004; Li et al., 2006); airway tone and respiratory function (Brigham et al., 1988; Guerra et al., 1988; Savich et al., 1995; Hartney et al., 2006; Tilley et al., 2003), cardiac function (Hintze and Kaley, 1984; Panzenbeck et al., 1989; Klein et al., 2004), and systemic and regional (renal, pulmonary) blood pressure and flow (Cassin et al., 1979; Lock et al., 1980; Downey et al., 1988; Guerra et al., 1988;
INTRODUCTION Inflammatory stimuli such as bacterial lipopolysaccharide (LPS) and physical stimuli such as membrane stretch or shear stress can cause leukocytes, platelets and endothelial cells to express the enzyme phospholipase A2 which cleaves arachidonic acid (AA) from cell membrane phospholipids. Free AA is converted by lipooxygenase into leukotrienes, or by constitutive and inducible cyclooxygenases (COX-1 and COX-2, respectively) into prostaglandin H2 (PGH2). Prostaglandin E synthase converts PGH2 into prostaglandin E2 (PGE2), which
©2008 Poultry Science Association Inc. Received August 13, 2007. Accepted October 8, 2007. 1 Corresponding author: [email protected]
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ABSTRACT Prostaglandin E2 (PGE2) affects pulmonary arterial pressure (PAP), pulmonary vascular resistance (PVR), and respiratory rate (RR) in mammals, but no information previously was available regarding avian pulmonary responses to PGE2. Two experiments were conducted in which 45- to 55-d-old male broiler chickens were infused i.v. with PGE2 at the lowest rate (30 g/ min for 4 min) that reliably reduced PAP during pilot studies. When compared with preinfusion (control) values in experiment 1, PGE2 reduced PAP from 19 ± 1 to 16 ± 1 mmHg (P < 0.001) and reduced mean systemic arterial pressure from 111 ± 6 to 81 ± 5 mmHg (P < 0.001) but did not significantly reduce heart rate (HR; control: 338 ± 9 beats/min; PGE2: 320 ± 12 beats/min; P > 0.05). Infusing PGE2 also reduced the RR from 57 ± 2 to 46 ± 4 breaths/min (P < 0.001) and reduced the percentage saturation of hemoglobin with oxygen (%HbO2) from 85 ± 2 to 77 ± 3%HbO2 (P < 0.001). After the PGE2 infusion ceased, the PAP, mean systemic arterial pressure, RR, and %HbO2 recovered within 8 min to levels that did not differ from preinfusion control values. In experiment 2, an ultrasonic flow probe was surgically implanted on 1 pulmonary artery to measure cardiac output (CO). When
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MATERIALS AND METHODS Experiment 1 Male broiler chicks from a commercial hatchery were transported on the day of hatch to the Poultry Environmental Research Lab at the University of Arkansas Poultry Research Farm, where they were wing-banded and placed on fresh wood shavings litter in an environmental chamber (8 m2 floor space). Chicks were brooded at 32°C on d 1 to 3, 31°C on d 4 to 6, 29°C on d 6 to 10, and 24°C thereafter. They were fed a 23% CP corn-soybean meal-based broiler ration formulated to meet NRC (1994) standards. Feed and water were provided for ad libitum consumption. Lights were on for 24 h/d through d 5
and for 12 h/d thereafter. Between 45 and 55 d of age, 10 broilers (3,675 ± 85 g of BW, mean ± SEM) were prepared for surgery as described previously (Wideman et al., 2001; Chapman and Wideman, 2002). All birds had full access to feed and water until they were anesthetized. A surgical plane of anesthesia was induced with intramuscular injections of allobarbital (5,5-diallyl-barbituric acid; 16 mg/kg of BW, Sigma Chemical Co., St. Louis, MO) and ketamine HCl (45 mg/kg of BW; Henry Schein Inc., Melville, NY). Silastic Tubing (0.012 in I.D., 0.037 in O.D.; Konigsberg Instruments Inc., Pasadena, CA) filled with heparinized saline (150 units of ammonium heparin/mL of 0.9% NaCl, Sigma Chemical Co.) was inserted through the left basilica (wing) vein. The distal end of the cannula was attached to a blood pressure transducer interfaced through a Transbrige preamplifier (World Precision Instruments, Sarasota, FL) to a Biopac MP100 data acquisition system using AcqKnowledge software (Biopac Systems Inc., Santa Barbara, CA). The Silastic cannula was advanced into a pulmonary artery by monitoring the characteristic pulse pressures to identify its location (Chapman and Wideman, 2001). Polyethylene tubing (PE-50) filled with heparinized saline was inserted into the left basilica vein, and was advanced to a position near the inferior vena cava for i.v. infusion. To monitor mean systemic arterial pressure (MAP), PE-50 tubing filled with heparinized saline was inserted into the left brachial artery, and the distal end was attached to a blood pressure transducer. A universal “C” sensor attached to a Vet/Ox 4403 pulse oximeter (Heska Corp., Ft. Collins, CO) was positioned on the right wing to illuminate the tissue between the radius and ulna and record the percentage saturation of hemoglobin with oxygen (%HbO2) and HR (Wideman and Kirby, 1995). When all surgical preparations were complete and a stabilization period of 10 min had elapsed, control data were recorded for 5 min. The birds then received a constant i.v. infusion of PGE2 that was freshly prepared immediately prior to infusion by diluting an aliquot of a 250 g/mL of PGE2 stock solution (Cayman Chemical, Ann Arbor, MI) with 0.9% saline. A syringe infusion pump (Harvard Apparatus Inc., Holliston, MA) was used to infuse the PGE2 for 4 min at a rate of 30 g/min (75 g of PGE2/mL infused at 0.4 mL/min). Pilot studies conducted using 4 broilers (not shown) indicated the 30 g/min infusion rate was the lowest rate (range: 5 to 50 g of PGE2/min) that reliably reduced PAP by approximately 2 mmHg. Data collection continued for 10 min after the PGE2 infusion ceased. Experiments were terminated by injecting birds i.v. with 10 mL of 0.1 M KCl.
Experiment 2 Male broilers were reared as described for experiment 1. Between 45 and 50 d of age, 10 broilers (3,259 ± 88 g of BW) were anesthetized and fastened in dorsal recumbency on a surgical board; then, an incision was made to open the thoracic inlet, and a Transonic ultrasonic
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Audoly et al., 1999; Tod et al., 1992; Gao et al., 1996). The responses elicited by PGE2 can vary dramatically depending on the pre- or postnatal stage of development and maturation (Tai and Adamson, 2000). Broiler chickens are susceptible to the onset of pulmonary arterial hypertension leading to pulmonary hypertension syndrome (PHS, ascites) when they possess an inadequate pulmonary vascular capacity characterized by an elevated pulmonary vascular resistance (PVR) and systemic arterial hypoxemia (under-saturation of hemoglobin with oxygen in arterial blood). It currently is our hypothesis that susceptibility to PHS is a consequence of an anatomically inadequate pulmonary vascular capacity combined with the functional predominance of vasoconstriction (increased PVR) attributable to serotonin over vasodilation (reduced PVR) and immune modulation attributable to nitric oxide (Wideman and Bottje, 1993; Wideman, 2000, 2001; Wideman et al., 2004, 2005b, 2007). The contributions of vasoactive eicosanoids (oxygenated metabolites of AA) such as thromboxane A2 (TxA2), prostacyclin (PGI2), and PGE2 remain uncertain. Previous studies demonstrated the capacity of potent TxA2 analogs to constrict the pulmonary and systemic vasculature of broilers (Wideman et al., 1999, 2001), and TxA2 may contribute to the increased PVR during the pulmonary hypertensive response to LPS (Wideman et al., 2004). Prostacyclin does not appear to dilate the pulmonary vasculature in broilers, but when infused i.v. PGI2 reduced the pulmonary arterial pressure (PAP) by reducing the cardiac output (CO; Wideman et al., 2005a). The impact of PGE2 on pulmonary hemodynamics and respiratory function previously had not been evaluated in broilers, but in various mammalian species PGE2 dilated the pulmonary vasculature (Cassin et al., 1979; Lock et al., 1980; Downey et al., 1988; Gao et al., 1996), reduced the heart rate (HR) and CO (Cassin et al., 1979; Hintze and Kaley, 1984; Guerra et al., 1988; Panzenbeck et al., 1989), inhibited the respiratory rate (RR; Guerra et al., 1988; Savich et al., 1995), and modulated the pulmonary response to LPS (Brigham et al., 1988). The present study was designed to assess the respiratory and hemodynamic responses of broilers to i.v. infusions of PGE2.
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Data Analysis In both experiments data were averaged electronically at 3, 2, and 1 min before the start of the infusion (control sample intervals C1, C2, C3), at the start of PGE2 infusion, and at 30 s intervals thereafter until the infusion was stopped (PGE2 infusion sample intervals E-Start, E1 to E8, E-Stop), and during the postinfusion recovery period at 1-min intervals (recovery sample intervals R1 to R10). The protocol used for data averaging accommodates the influences of pulse pressure and respiratory cycles on pulmonary and systemic arterial pressures (Wideman et al., 1996). The primary data recorded included PAP (mmHg), MAP (mmHg), and blood flow through the left pulmonary artery (mL/min). Heart rate (beats/min) was recorded directly from the pulse oximeter in experiment 1 or by counting systolic peaks over time in the PAP recordings in experiment 2. In both experiments the RR (breaths/min) was obtained by counting the wave cycles associated with respiratory movement that constitute an integral part of the PAP recording (Wideman et al., 1999). Based on the assumption that CO (mL/min) normally is divided approximately equally between the lungs, CO was calculated as 2 × blood flow through the left pulmonary artery. Because of the strong correlation between CO and BW (Wideman, 1999), the CO was normalized per kg of BW on an individual bird basis (CO/kg of BW). The CO is the product of HR (beats/min) × stroke volume (SV, mL/beat); consequently, SV was calculated as the CO (per kg of BW)/HR. Assuming atrial pressures remain close to zero, as assessed by direct cannulation or wedge pressure measurements (Chapman and Wideman, 2001; Lorenzoni et al., 2008), then the pressure gradient across the pulmonary circulation is essentially equal to PAP, permitting the PVR (relative resistance units) to be estimated as PVR = PAP/CO (per kg of BW; Wideman et al., 1996). Data were analyzed over time (across sample intervals) using the SigmaStat Repeated Measures ANOVA procedure, and means were differentiated by the Tukey Test (Jandel Scientific, 1994). The threshold for significance in all cases was P ≤ 0.05.
RESULTS Values for PAP, MAP, HR, %HbO2, and RR before, during, and after the i.v. infusion of PGE2 in experiment 1 are shown in Figure 1. During control sample intervals C1 to C3, none of the variables differed over time. Comparing sample intervals C3 vs. E8, PGE2 infusion reduced PAP from 19.3 ± 1 to 16.2 ± 1 mmHg and reduced MAP from 111 ± 6 to 81 ± 5 mmHg but did not significantly reduce HR (control: 338 ± 9 beats/min; PGE2: 320 ± 12 beats/min). Infusing PGE2 also reduced the percentage saturation of hemoglobin with oxygen (%HbO2) from 85 ± 2 to 77 ± 3%, and reduced the RR from 57 ± 2 to 46 ± 4 breaths/min. After the PGE2 infusion ceased the PAP, MAP, RR, and %HbO2 recovered within 8 min to levels that no longer were significantly lower than their respective preinfusion control values. The responses to PGE2 infusion in experiment 2 are shown in Figure 2. During control sample intervals C1 to C3 none of the variables differed over time. Comparing sample intervals C3 vs. E8, PGE2 infusion reduced PAP from 25 ± 2 to 21 ± 1 mmHg, reduced CO from 140 ± 6 to 111 ± 5 mL/kg of BW × min, and reduced RR from 49 ± 4 to 35 ± 4 breaths/min. The PVR, calculated as PAP/CO, was not affected by the PGE2 infusion (control: 0.18 ± 0.01 relative resistance units; PGE2: 0.20 ± 0.02 relative resistance units; P > 0.723). After the PGE2 infusion ceased, the PAP, CO, and RR recovered within 9 min to levels that did not differ from preinfusion control values (Figure 2). The HR (not shown) was determined by counting systolic peaks within the PAP recording during sample intervals C1 to C3, E7 to E-Stop, and R8 to R10. The HR averaged 305.5 ± 8.6 beats/min during the preinfusion control intervals, declined to 259.7 ± 9.1 beats/min by the final PGE2 infusion intervals (P < 0.001), and then returned to 287.5 ± 8.0 beats/min by postinfusion intervals R8 to R10. Contemporaneous values calculated for SV averaged 0.46 ± 0.02 mL/kg of BW × beat during sample intervals C1 to C3, 0.43 ± 0.02 mL/ kg of BW × beat during intervals E7 to E-Stop, and 0.48 ± 0.04 mL/kg of BW × beat during intervals R8 to R10 (P = 0.158).
DISCUSSION Intravenous infusions of PGE2 reduced the PAP in clinically healthy broilers. The magnitude of the pressure gradient across the pulmonary circulation is determined by multiplying the PVR and CO (Wideman and Bottje, 1993; Wideman, 2000; Lorenzoni et al., 2008). The PVR was not reduced by PGE2 in experiment 2; consequently, PGE2 did not dilate the pulmonary vasculature but instead reduced the PAP by reducing the CO. The CO is determined by multiplying the HR and SV. Infusing PGE2 did not significantly affect HR in experiment 1 but markedly reduced the HR and numerically lowered the SV (P = 0.158) in experiment 2. These responses to PGE2 infusion suggest a multiplicative effect whereby the combined reductions in HR and SV triggered a substan-
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flow probe (Transonic Systems Inc., Ithaca, NY) was positioned on the left pulmonary artery. The probe was connected to a Transonic T206 blood flow meter to confirm signal acquisition and record blood flow through the pulmonary artery; then, the skin of the thoracic inlet was sealed airtight with surgical wound clips. A Silastic cannula was inserted into a pulmonary artery to record PAP, and PE-50 polyethylene tubing filled with heparinized saline was inserted into the left basilica vein for i.v. infusion. The MAP was not recorded. After a stabilization period of 10 min had elapsed, control data were recorded for 5 min. The birds then received a constant i.v. infusion of PGE2 for 4 min at a rate of 30 g/min (75 g of PGE2/mL infused at 0.4 mL/min). Data collection continued for 10 min after the PGE2 infusion ceased. Birds were euthanized by i.v. injections of 0.1 M KCl.
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Figure 1. Pulmonary arterial pressure (PAP), mean systemic arterial pressure (MAP), heart rate (HR), percentage saturation of hemoglobin with oxygen (HbO2), and respiratory rate (RR) for male broilers in experiment l (mean ± SEM; n = 10). Data were averaged electronically at 3, 2, and 1 min prior to the start of the infusion (control sample intervals C1, C2, C3), at the start of PGE2 infusion and at 30-s intervals thereafter until the infusion was stopped (PGE2 infusion sample intervals E-Start, E1 to E8, E-Stop), and during the postinfusion recovery period at 1 min intervals (recovery sample intervals R1 to R10). *Denotes all values that differed when compared with sample interval C3 by repeated measures ANOVA (P ≤ 0.05).
tial decrease in CO that in turn contributed to contemporaneous reductions in PAP and MAP. Qualitatively, similar pulmonary hemodynamic responses previously were observed when broilers were infused i.v. with PGI2, which also did not reduce the PVR but instead consistently reduced the PAP by lowering the CO. The impact of PGI2 on CO was associated with a reduction in SV
but not HR in one experiment, or with reductions in SV and HR in 2 additional experiments (Wideman et al., 2005a). In mammals both PGE2 and PGI2 appear to elicit reductions in HR and CO by stimulating ventricular receptors linked to reflex vagal inhibition of cardiac function (Chapple et al., 1980; Chiba and Malik, 1980; Hintze and Kaley, 1984; Panzenbeck et al., 1988, 1989). Stimulat-
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Figure 2. Pulmonary arterial pressure (PAP), cardiac output normalized for BW (CO), pulmonary vascular resistance normalized for BW (PVR), and respiratory rate (RR) for male broilers in experiment 2 (mean ± SEM; n = 10). Data were averaged electronically at 3, 2, and 1 min prior to the start of the infusion (control sample intervals C1, C2, C3), at the start of PGE2 infusion and at 30-s intervals thereafter until the infusion was stopped (PGE2 infusion sample intervals E-Start, E1 to E8, E-Stop), and during the postinfusion recovery period at 1-min intervals (recovery sample intervals R1 to R10). *Denotes all values that differed when compared with sample interval C3 by repeated measures ANOVA (P ≤ 0.05).
ing the cardiac vagal (parasympathetic) innervation of domestic fowl reduces the CO by reducing the HR (negative chronotropic effect) and the SV (negative inotropic effect; Smith et al., 2000). The cardio-inhibitory effects of PGE2 evidently overrode the reflexive increase in CO that normally would occur to restore adequate O2 delivery to the tissues in response to systemic arterial hypo-
xemia (Wideman and Tackett, 2000; Wideman et al., 2000). The negative inotropic and negative chronotropic responses to PGE2 also are consistent with a significant reduction in venous return to the heart. If PGE2 dilates the systemic venules and large veins of broilers, then the resulting increased pooling of blood within the venous volume reservoir would reduce atrial and ventricular
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vasodilation (a reduction in total peripheral resistance) that, when combined with the reduction in CO, caused the 30 mmHg decline in mean arterial pressure. Systemic arterial hypoxemia reduces the systemic vascular tone but does not alter the pulmonary vascular tone in broilers (Wideman et al., 1996, 2000; Wideman and Tackett, 2000). In summary, i.v. infusions of PGE2 successfully reduced the PAP in broilers but did not dilate the pulmonary vasculature. Instead, reductions in PAP were directly attributable to reductions in CO. Clearly the PAP of broilers can be directly affected by changes in CO in the absence of significant changes in PVR. The observed reductions in CO may be caused by PGE2-mediated inhibition of cardiac function or by reduced venous return secondary to PGE2-mediated dilation (expansion) of the venous volume reservoir. Both PGE2 (present study) and PGI2 (Wideman et al., 2005a) failed to reduce the PVR but elicited reductions in PAP by lowering the CO. Both eicosanoids also reduced the MAP by reducing the CO and, presumably, by dilating the systemic vasculature. Within the context of the pathogenesis of PHS, eicosanoid-induced reductions in the PAP would benefit susceptible broilers by reducing their right ventricular overload, whereas reductions in CO and MAP (reduced O2 delivery to the tissues) and systemic venous dilation (enhanced central venous congestion) would accelerate the onset of PHS. Inhibition of respiratory drive and the ensuing systemic arterial hypoxemia induced by PGE2 also would accelerate the pathophysiological progression leading to terminal PHS (Wideman, 2000, 2001). Accordingly, most of the cardio-pulmonary hemodynamic responses to PGE2 appear to be detrimental rather than beneficial for broilers developing PHS. Nitric oxide remains the principal vasoactive mediator that is capable of dilating the pulmonary vasculature in broilers (Wideman and Chapman, 2004; Wideman et al., 2005b, 2007; Tan et al., 2007).
ACKNOWLEDGMENTS This work was supported by USDA/CSREES/NRI grant #2003-35204-13392.
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filling, thereby causing simultaneous intrinsic reductions in HR, SV, and CO (Sturkie, 1986; Wideman et al., 2005a). Two of the 4 EP receptor types have been cloned and characterized in domestic fowl, but the vascular and cardiac distributions of EP receptors have not been demonstrated (Kwok et al., 2007). In mice the anatomical distributions of the EP receptor types determine the vascular responsiveness to PGE2 (Audoly et al., 1999), and in newborn lambs both PGE2 and PGI2 act primarily to dilate pulmonary veins rather than pulmonary arteries (Gao et al., 1996). Wedge pressure studies have consistently demonstrated that the PVR of broilers overwhelmingly reflects the tone (degree of partial contracture) of the precapillary arterioles rather than the resistance of the postcapillary vasculature (Chapman and Wideman, 2001; Lorenzoni et al., 2008). Accordingly, if receptors for PGE2 and PGI2 are primarily located on broilers’ pulmonary veins rather than arteries, then eicosanoidinduced reductions in postcapillary vascular tone would have a minimal impact on the PVR. In newborn lambs PGE2 was more potent in reducing systemic rather than pulmonary vascular resistance, whereas the opposite was true for PGI2 (Lock et al., 1980). In experiment 1 of the present study, PGE2 reduced the MAP by 30 mmHg but only reduced the PAP by 3 mmHg, suggesting PGE2 may be a more potent vasodilator of the systemic circulation than of the pulmonary circulation in broilers. The CO was not recorded in experiment 1 and the MAP was not recorded in experiment 2; therefore, the systemic vascular resistance (total peripheral resistance) could not be calculated. Intravenous PGE2 infusions reduced the RR and %HbO2 in broilers, and these variables readily returned to control levels after the PGE2 infusion ceased. The respiratory inhibition elicited by PGE2 in 45- to 55-d-old broilers resembles the responses elicited by PGE2 in fetal and neonatal mammals. Prostaglandin E2 profoundly reduces spontaneous breathing movements in fetal mammals and reduces the RR, tidal volume, and blood oxygenation in neonatal mammals (Kitterman et al., 1983; Murai et al., 1987; Guerra et al., 1988; Savich et al., 1995; Tai and Adamson, 2000; Hofstetter et al., 2007). The hypoventilation induced by PGE2 does not consistently involve peripheral chemoreceptors (carotid and aortic bodies) but instead is mediated primarily through receptors located within the central nervous system (Murai et al., 1987; Guerra et al., 1988; Tai and Adamson, 2000; Hofstetter et al., 2007). Hypoventilatory responses to PGE2 tend to wane as neonatal mammals mature (Tai and Adamson, 2000). To the best of our knowledge, the present study constitutes the first demonstration that PGE2 inhibits respiration in 6- to 8-wk-old domestic fowl, even to the extent of overriding the enhanced ventilatory drive that normally is triggered by hypoxemia (Wideman et al., 1996; Wideman and Tackett, 2000). Pulmonary vasoconstriction was not observed during the hypoxemic response to PGE2 (PVR did not increase); however, hypoxemia may have contributed to systemic
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