Effect of phenylephrine on histamine-induced bronchoconstriction in dogs

Although an α-adrenoceptor has been suggested to be involved in the mechanism of asthma, the effect of α₁-agonist on the airway is still unclear. In this study we evaluated the effect of phenylephrine on the airway with a direct visualization method using a superfine fiberoptic bronchoscope (SFB).

Seven mongrel dogs were anesthetized with pentobarbital (30 mg·kg^-1 IV) and paralyzed by pancuronium (0.2mg·kg^-1·h^-1). The trachea was intubated with an endotracheal tube (ID 7 mm) that has a second lumen for insertion of a SFB (OD 2.2 mm) to monitor the bronchial cross-sectional area (BCA) continuously. The tip of a SFB was placed at the level between the second and third bronchial bifurcation. To assess hemodynamics, the direct arterial blood pressure (ABP) and pulmonary arterial pressure (PAP) were monitored via a femoral arterial catheter and Swan-Granz catheter. Bronchoconstriction was elicited by histamine (10 μg·kg^-1+ 500 μg·kg^-1·h^-1_. At 30 min after the histamine was started, saline or phenylephrine (1, 10, and 100μg·kg^-1) was given intravenously. The BCA and hemodynamic variables were assessed before (basal) and 30 min after the histamine was started and 5 min after saline and each phenylephrine dose.

Histamine reduced BCA by 40.3±6.3%. Phenylephrine at 10 and 100 μg·kg^-1 significantly increased the ABP and PAP; and it significantly decreased the BCA, by 6.5±6.9% and 14.2±7.9%, respectively. Plasma epinephrine and norepinephrine were also significantly reduced following phenylephrine 100 μg·kg^-1 IV.

The dose of phenylephrine that produced vasopressive actions worsened the histamine-induced bronchoconstriction slightly but significantly. Therefore, phenylephrine should be used with caution in asthmatic patients.