Milrinone attenuates serotonin-induced pulmonary hypertension and bronchoconstriction in dogs

We determined whether milrinone, a phosphodiesterase III inhibitor, attenuates serotonin-induced (5-hydroxy-tryptamine [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 mu g/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 mu g/kg at 10-min intervals. The other 12 dogs were given milrinone 5000 mu g/kg 30 min after SPIT 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 mu g/kg) of milrinone for bronchoconstriction: 4.32 +/- 0.13 (47.6) was significantly smaller than that for PEI: 3.84 +/- 0.29 (144.9) (P < 0.01). In addition, the spasmolytic effects of milrinone (5000 mu g/kg) were not antagonized by propranolol, although this dose significantly increased plasma catecholamines. In conclusion, milrinone attenuates 5HT-induced PPI 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 beta-adrenoceptor activation because beta-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 beta-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.