Influence of hypoxia and serotonin on small pulmonary vessels

X-ray angiograms obtained from isolated perfused dog lungs were used to measure changes in the internal diameter of small intraparenchymal pulmonary arteries (150-1,600 μm) and veins (200-1,000 μm) in response to hypoxia or intra-arterial serotonin [5-hydroxytryptamine (5-HT)] infusion. The diameter changes in response to the two stimuli were measured over a range of stimulus-induced increases (ΔPa) in the total arteriovenous pressure drop. When the resulting ΔPa was small, all arteries in the diameter range studied constricted in response to either stimuli. The maximum decrease in diameter was ~25% with hypoxia and 36% with 5-HT. However, when ΔPa was large, arteries with a control diameter larger than ~800 μm distended with hypoxia. On the other hand, 5-HT constricted all the arteries in the size range studied regardless of the resulting magnitude of ΔPa. Hypoxia caused a small (~9%) constriction in all veins in the diameter range studied independent of diameter or the magnitude of ΔPa, whereas in the concentration range studied 5-HT had no significant influence on these veins. An analysis of the potential impact of these vessels on total pulmonary vascular resistance suggested that although vessels in the size range studied contributed significantly to the total response to these two stimuli, vessels smaller than those studied also made a major contribution to the total response.