EFFECT OF INHALED AND INTRAVENOUS ACETYLCHOLINE ON BRONCHIAL BLOOD-FLOW IN ANESTHETIZED SHEEP

The reported effects of cholinergic agonists on bronchial blood flow ( Qbr) have been inconsistent. The aim of the present study was to deter mine whether the inconsistency could be due to the mode of agonist adm inistration (systemic vs. aerosol) or the anatomic site of blood flow in the bronchus (mucosal vs. deep wall). In 10 anesthetized mechanical ly ventilated adult sheep, we measured Qbr in main bronchi by color-co ded microspheres, systemic and pulmonary arterial pressures, cardiac o utput, and lung resistance (RL) before and after acetylcholine (ACh) a dministered either as an aerosol (nebulized dose 100 mu g) or as an in travenous bolus (2 mu g/kg). Before drug administration, 72% of mean Q br was distributed to the bronchial mucosa and the remainder was distr ibuted to the deep bronchial wall. For a comparable increase in mean R L (150% for intravenous ACh and 205% for aerosol ACh), mean total Qbr normalized for systemic arterial pressure increased by 291% after intr avenous ACh (P < 0.05) and decreased by 9% after aerosol ACh (not sign ificant). Mucosal and deep wall Qbr increased proportionally. Atropine (0.2 mu g/kg) prevented the changes in Qbr and RL after intravenous A Ch. Thus intravenous but not aerosol ACh increased blood flow in the m ucosa and deep wall of extrapulmonary bronchi. This suggests that the muscarinic receptors mediating vasodilation are more accessible to int ravascular than intrabronchial ACh.