Histamine-induced constriction of canine peripheral lung: An airway or tissue response?

We compared the histamine responsiveness of peripheral airways (less than 6.0 mm diam) and parenchymal tissues in eight anesthetized paralyzed open-chest mongrel dogs. We measured pressure in a peripheral bronchus by using an antegrade wedged catheter and pressure in the alveolar region subtended by the wedged bronchus by using an alveolar capsule. Sinusoidal volume oscillations at a frequency of 0.5 Hz were delivered by a linear motor pump into the segment through the wedged catheter. We calculated the resistance of the segment (Rseg) and partitioned Rseg into tissue viscance (i.e., proportional to the resistive pressure drop between the alveolus and the pleura) and peripheral airway resistance. Measurements were taken under baseline conditions and after delivery of increasing concentrations of aerosolized histamine (0.1 micrograms/ml to 100.0 mg/ml) into the segment. We found that the histamine responsiveness of the peripheral airways and lung tissues varied markedly within a given dog. In four of eight dogs the airways were more responsive to histamine, in three of eight the tissues were more responsive, and in one of eight the response was equivalent at the two sites. We conclude that in a given animal, there is marked heterogeneity in the histamine responsiveness of the peripheral airways and parenchymal tissues and that either may dominate responsiveness in the peripheral lung.