Anesth Analg 2000;90:790-794
© 2000 International Anesthesia Research Society
CARDIOVASCULAR ANESTHESIA
Milrinone Attenuates Serotonin-Induced Pulmonary Hypertension and Bronchoconstriction in Dogs
Eiji Hashiba, MD,
Kazuyoshi Hirota, MD,
Hideki Yoshioka, MD,
Yoshio Hashimoto, MD,
Tsuyoshi Kudo, PhD,
Tetsumi Sato, MD, and
Akitomo Matsuki, MD
Department of Anesthesiology, University of Hirosaki School of Medicine, Hirosaki, Japan
Address correspondence and reprint requests to K. Hirota, MD, Department of Anesthesiology, University of Hirosaki School of Medicine, Hirosaki 036-8562, Japan.
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Abstract
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We determined whether milrinone, a phosphodiesterase III inhibitor, attenuates serotonin-induced (5-hydroxytryptamine [5HT]) pulmonary hypertension (PH) and bronchoconstriction. Dogs were anesthetized with pentobarbital (30 mg/kg + 2 mg · kg-1 · h-1). Bronchoconstriction and PH were elicited by 5HT (10 µg/kg + 1.0 mg · kg-1 · h-1). Pulmonary vascular resistance was used to assess PH. Bronchoconstriction was also assessed by changes in bronchial cross-sectional area obtained from our bronchoscopic method. At 30 min after 5HT infusion started, seven dogs were given milrinone: 0 (saline), 5, 50, 500, and 5000 µg/kg at 10-min intervals. The other 12 dogs were given milrinone 5000 µg/kg 30 min after 5HT infusion, and 5 min later were given propranolol 0.2 mg/kg (n = 6) or saline (n = 6) IV. The 5HT significantly increased percentage of pulmonary vascular resistance to 208% ± 27% and decreased percentage of bronchial cross-sectional area to 52% ± 5% of the basal. Milrinone significantly attenuated both PH and bronchoconstriction in a dose-dependent manner. However, -log 50% effective concentration (mean ED50 in µg/kg) of milrinone for bronchoconstriction: 4.32 ± 0.13 (47.6) was significantly smaller than that for PH: 3.84 ± 0.29 (144.9) (P < 0.01). In addition, the spasmolytic effects of milrinone (5000 µg/kg) were not antagonized by propranolol, although this dose significantly increased plasma catecholamines. In conclusion, milrinone attenuates 5HT-induced PH and bronchoconstriction; however, this drug may be more sensitive to phosphodiesterase III in the airway smooth muscle than in pulmonary vascular smooth muscle. In addition, the relaxant effects could not be caused by ß-adrenoceptor activation because ß-blocker did not antagonize.
Implications: We studied the effects of milrinone on serotonin-induced pulmonary hypertension and bronchoconstriction in dogs. Milrinone produces pulmonary vasodilation and bronchodilation, whose effects may not be caused by ß-adrenoceptor activation. In addition, this drug may be more sensitive to phosphodiesterase III in the airway smooth muscle than that in pulmonary vascular smooth muscle.
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Introduction
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Increasingly, patients with co-existing cardiac and reactive airway diseases undergo surgery. However, these patients sometimes suffer severe circulatory depression or bronchospasm caused by various mechanical stimuli, drugs, and imbalance of fluid homeostasis during perioperative periods. The cyclic adenosine monophosphate (cAMP)-specific phosphodiesterase III (PDE III) isoenzyme inhibitors have clinically been used to treat congestive heart failure (1,2) and are used intraoperatively to prevent acute heart failure in cardiac patients (3) by improving left ventricular systolic function and concurrent systemic vasodilation.
As PDE III exists in vascular (4,5) and airway smooth muscles (4,6,7), PDE III inhibitors increase contractile force in cardiac muscle and relax vascular (4,5,8,9) and airway smooth muscle (6,7,10,11) by increasing intracellular cAMP concentrations. Therefore, these drugs may improve pulmonary hypertension and bronchoconstriction. However, Silver et al. (4) reported that, although cardiac and vascular smooth muscle phosphodiesterase isozymes are pharmacologically similar, there is pharmacological and substrate heterogeneity of PDE III in aortic, compared with tracheal, smooth muscle in the same species.
Serotonin (5-hydroxytryptamine [5HT]) produces a biphasic vasoactive effect (12). At small concentrations, 5HT acts directly on a specific 5HT receptor, especially 5HT2, whereas at large concentrations, it produces vasoconstriction by activating 
-adrenoceptors (13,14). In addition, 5HT also produces bronchoconstriction via 5HT2 (15). We investigated whether milrinone, one of the PDE III inhibitors, attenuates 5HT-induced pulmonary hypertension and bronchoconstriction in dogs.
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Methods
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This study was approved by our university animal experiment committee. A total of 19 mongrel dogs (8–12 kg) were anesthetized with IV pentobarbital (30 mg/kg + 2 mg · kg-1 · h-1), and muscle relaxation was produced with IV pancuronium 200 µg · kg-1 · h-1. The trachea was intubated with a special endotracheal tube (internal diameter, 7.0 mm) (Univent tube®, Fuji System, Tokyo, Japan) with a second lumen for insertion of a superfine fiberoptic bronchoscope (outer diameter, 2.2 mm). The lungs were ventilated and end-tidal carbon dioxide concentration was maintained at 4.0% to 4.5%.
A pulmonary artery catheter (continuous cardiac output thermodilution catheter) was inserted via the femoral vein to monitor pulmonary artery pressure, pulmonary capillary wedge pressure, central venous pressure, and cardiac output, and to administer 5-HT. Cardiac output was continuously measured by using a Vigilance® (Model VGSSYS; Baxter Healthcare Co., Irvine, CA) monitor. The femoral artery was also cannulated to monitor systemic arterial pressure and to collect blood samples. In addition, lactated Ringer’s solution was infused at 4 mL · kg-1 · h-1, and drugs were administered via the pulmonary artery catheter sheath. Bronchoconstriction and pulmonary hypertension were elicited by IV 5HT (10 µg/kg + 1.0 mg · kg · h-1) via a peripheral vein.
Pulmonary vascular resistance (PVR), calculated from cardiac output, mean pulmonary artery pressure, and pulmonary capillary wedge pressure, was used for assessment of pulmonary hypertension. Bronchoconstriction was assessed by changes in a bronchial cross-sectional area (BCA) obtained by a bronchoscopic method as previously reported (16–20). Bronchial cross-sectional area at the third bifurcation was monitored continuously by the bronchoscope. The area was printed by a video printer during the end-expiratory pause, and then, measured by using a image analyzing software (NIH Image program; written by Wayne Rasband at the National Institutes of Health and available from the Internet by anonymous ftp from zippy.nimh.nih.gov or on floppy disk from NTIS, 5258 Port Royal Rd., Springfield, VA 22161, part no. PB93–504858). PVR and BCA were expressed as percentages of the baseline values.
The first study (n = 7) was designed to determine the spasmolytic effects of milrinone on 5HT-induced bronchoconstriction and pulmonary hypertension. At 30 min after the 5HT infusion was started, when a stable level of bronchoconstriction and pulmonary hypertension had been achieved, the dogs were sequentially given the following doses of milrinone at 10-min intervals: 0 (saline), 5, 50, 500, and 5000 µg/kg. Bronchial cross-sectional area and PVR were measured before and 30 min after the start of 5HT infusion, and 5 min after each dose of milrinone.
The second study (n = 12) was designed to determine whether relaxant effects of milrinone are mediated through ß-adrenoceptors. Dogs were given 5000 µg/kg of milrinone 30 min after the start of 5HT infusion, and then, 5 min later they were given 0.2 mg/kg propranolol (n = 6) or saline (n = 6).
Arterial blood (6 mL) was collected via the femoral artery catheter into EDTA syringes at the same time as the measurements of BCA and PVR. The blood was centrifuged immediately at 3000 rpm for 10 min at -10°C. The plasma was separated and kept frozen at -70°C until assayed for catecholamines. Plasma concentrations of epinephrine and norepinephrine were measured by using a high pressure liquid chromatography (21). The assay coefficients of variation were 6.0% for epinephrine and 3.3% for norepinephrine.
The doses (ED50) of milrinone that attenuated pulmonary hypertension and bronchoconstriction by 50% were calculated by computer-assisted curve fitting (Sigmoidal dose-response, GraphPad Prism 1.0, San Diego, CA) from individual curves. Repeated Measures analysis of variance, followed by Fisher’s protected least significant difference test and paired and unpaired t-test, were performed for intragroup and intergroup comparison, respectively. Percentages of baseline values were used for statistical analysis. A P < 0.05 was considered significant. All data were shown as mean ± SEM.
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Results
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Serotonin significantly increased PVR to 208% ± 27% of baseline and significantly decreased BCA to 52% ± 5% of baseline. Milrinone decreased PVR and increased BCA, in a dose-dependent manner (Figure 1). Other hemodynamic variables are shown in Table 1. Clinically relevant doses of milrinone (50 µg/kg) did not change plasma catecholamine concentrations, although doses exceeding 500 µg/kg significantly increased both plasma epinephrine and norepinephrine (Table 2). The pED50 (mean ED50) of milrinone for pulmonary hypertension and bronchoconstriction were 3.84 ± 0.29 (144.9 µg/kg) and 4.32 ± 0.13 (47.6 µg/kg), respectively. The relaxant effects of milrinone were not affected by block of ß-adrenoceptor activity (Figure 2). 
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Figure 1. Spasmolytic effects of milrinone on A, 5HT-induced pulmonary hypertension and B, bronchoconstriction. Data are mean ± SEM. 5HT30 = 30 min after serotonin infusion started. *P < 0.05, **P < 0.01 versus 5HT30.
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Figure 2. Effects of propranolol on milrinone-induced A, pulmonary vasodilation and B, bronchodilation. Data are mean ± SEM. 5HT30 = 30 min after serotonin infusion started, milrinone = 5000 µg/kg of milrinone IV, prop = 0.2 mg/kg of propranolol. *P < 0.05, **P < 0.01 versus 5HT30.
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Discussion
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We found that milrinone, in clinically relevant doses (50 µg/kg), improved 5HT-induced pulmonary hypertension and bronchoconstriction. Phosphodiesterase III exists in vascular (4,5) and airway smooth muscles (4,6,7). Therefore, it is likely that PDE III inhibitors produce bronchodilation and pulmonary vasodilation. Several in vitro studies have shown that PDE III inhibitors relax vascular and airway smooth muscles by increasing intracellular cAMP concentrations (4–11). Clinical reports also suggest that milrinone could produce pulmonary vasodilation (1–3) and bronchodilation (22).
However, we found that the ED50 of milrinone for bronchoconstriction was significantly smaller than that for pulmonary hypertension. Silver et al. (4) reported a pharmacological and substrate heterogeneity of PDE III between aortic and tracheal smooth muscle in the same species, although cardiac and vascular smooth muscle phosphodiesterase isozymes are pharmacologically similar. Therefore, milrinone may be more specific to PDE III in the airway than that in pulmonary vessels.
Plasma levels of catecholamine significantly increased after more than 500 µg/kg of milrinone IV. As milrinone has vasorelaxant effects, the catecholamine increase may be caused by baroreceptor reflex induced by the vasodilation. Sympathetic modulation of airway smooth muscle tone is mediated mainly via circulating catecholamines, as the direct sympathetic nerve supply to the lung is limited (23). Therefore, the catecholamine release might partially contribute to bronchodilating effects of milrinone similar to aminophylline (24). However, as propranolol did not antagonize the bronchodilation, an increase in plasma catecholamines may not be involved in the mechanism of relaxation by milrinone.
The bronchodilating effect of milrinone was assessed by a direct visualization method with a superfine fiberoptic bronchoscope. As we previously reported (16,18), the bronchoscopic method is more specific to measure bronchial caliber than indirect methods, such as peak airway pressure, airway resistance, or compliance. Similarly, Brown et al. (25) reported that the direct visualization method by using high resolution computed tomography is more reliable than indirect methods.
In conclusion, we have shown that the PDE III inhibitor, milrinone, in a clinically relevant dose, significantly attenuates not only pulmonary hypertension but also bronchoconstriction. However, as the ED50 for bronchoconstriction is significantly smaller than that for pulmonary hypertension, this drug may be more specific to PDE III in airway smooth muscle than in pulmonary vascular smooth muscle. In addition, this relaxation may not be mediated through ß-adrenoceptors, because propranolol did not antagonize it.
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Acknowledgments
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The authors thank Prof. G. Smith, University Department of Anesthesia and Pain Management, Leicester Royal Infirmary, Leicester, UK, for his valuable comments.
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Accepted for publication December 8, 1999.
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Abstract
Introduction
