Pulmonary capillary blood volume in dog: effects of 5-hydroxytryptamine

¹ Cardiovascular Research Institute, University of California School of Medicine, San Francisco, California

We studied changes in pulmonary capillary blood volume (Vc) in anesthetized dogs to determine whether 5-HT (5-hydroxytryptamine) constricts pulmonary venules. The preparation was sufficiently stable to allow reproducible measurement of pulmonary diffusing capacity (Dl). A fixed volume of CO gas mixture was introduced into the lungs by positive pressure, and an alveolar sample was obtained after 10 sec. By measuring Dl at different levels of Pa_o2, a value proportional to Vc was obtained. We cannot give absolute values for Vc since the rate at which CO replaces O₂ in red cells in dogs is unknown. In 10 of 14 dogs given 5-HT, Vc increased 6 to 133%. There were no differences in left ventricular diastolic, systemic arterial, or airway pressures. Changes in blood flow were unrelated to changes in Vc. These findings are independent evidence that 5-HT causes pulmonary venous constriction, which in turn leads to passive congestion of the pulmonary capillary bed.