Pharmacological properties of canine intrapulmonary blood vessels

The contractile response of ring segments of large, medium, and small pulmonary arteries and veins of the dog to histamine, norepinephrine, and serotonin have been studied. The maximum contractile response to these drugs was normalized with respect to the maximal response obtained in stimulation with 127 mM K+. The small pulmonary artery was more reactive to histamine, norepinephrine, and serotonin when compared with large and medium pulmonary arteries. The medium and large pulmonary artery showed no difference in reactivity to histamine. However, the mean effective dose (ED50) values for these agonists among the different segments of pulmonary arteries showed no significant difference. The small and medium pulmonary veins demonstrated increased reactivity to histamine, but not norepinephrine and serotonin. The ED50 values also indicated that both small and medium veins were more sensitive to histamine when compared with the large pulmonary vein. The log concentration percent response curves for both small and medium pulmonary veins were displaced leftward (increased sensitivity) with respect to that for the large pulmonary vein. However, the reactivity and sensitivity to histamine between medium and small pulmonary veins were no different. The reactivity and sensitivity of different segments of pulmonary veins to norepinephrine and serotonin showed no significant differences among them. We conclude that histamine and other vasoactive substances, which are directly or indirectly related to mast cell degranulation, exert pharmacological effects on the pulmonary vasculature which possesses differential responsiveness at various levels of the vascular tree.