Species differences in the longitudinal distribution of pulmonary vascular resistance (PVR) in response to 5-hydroxytryptamine (5-HT) or histamine (His) may be attributed to variations in the distribution of functional smooth muscle between arteries and veins estimated by the response to KCl. Isolated dog, guinea pig, or rabbit lungs were perfused at a constant flow = 55-75 ml . min-1 . kg body wt-1. Pulmonary arterial (Ppa); arterial, double, and venous occlusion (Po,a; Pdo; Po,v, respectively); and pulmonary venous (Ppv) pressures were measured before and after increasing PVR by infusing His, 5-HT, or KCl. 5-HT and His increased Ppa - Pdo in rabbits but Pdo - Ppv in guinea pigs. In dogs, 5-HT increased Ppa - Po,a, but His increased Pdo - Ppv. Dynamic (Co,v) and static vascular compliance (CP-Q), as well as critical closing pressure (Pcc, the y-intercept of pressure-flow curves), were also measured. At baseline, Co,v was the same among species. However, CP-Q was higher than Co,v in all lungs and was significantly different among species in order of (in ml . cmH2O-1 . 100 g-1) rabbit (4.54 +/- 0.28) > guinea pig (3.31 +/- 0.18) > dog (2.21 +/- 0.13). Increases in Pcc correlated with increases in microvascular resistance (Po,a - Po,v) but not with increases in PVR after agonist infusion. KCl responses suggest that guinea pigs and rabbits have relatively more functional smooth muscle in venous and arterial microvessels, respectively, whereas dogs have approximately equal amounts. These results suggest that differences in longitudinal vasoreactivity may be due in part to differences in the distribution of functional smooth muscle mass between pulmonary arteries and veins in distinct species.
