Micropuncture measurement of lung microvascular pressure during 5-HT infusion

We punctured subpleural microvessels in 19 isolated blood-perfused dog lung lobes to determine the effect of 5-hydroxytryptamine (5-HT) on the pulmonary microvascular pressure profile. Maintaining lobe airway pressure at 6 cmH2O and lobe venous pressure at 10 cmH2O we infused saline alone (10 microliter/min) in 10 control lobes and 5-HT in saline solution (100--150 micrograms/min) in 9 lobes. We measured pressures in subpleural microvessels using micropipettes connected to a servo-null system. The 5-HT infusion doubled lobar vascular resistance. In all microvessels upstream from the 150-micrometer venules, pressure was higher than in the control lobes. In contrast to the control lobes, pressure dropped markedly from the lobar artery to the 50-micrometer arterioles and from the venous capillaries to the 50-micrometer venules. We conclude that 5-HT not only constricted arterioles, but also raised lung microvascular pressure by constricting postcapillary venules.