PULMONARY INTRAVASCULAR MACROPHAGES IN HORSES AND PONIES

Seven horses (4 anesthetized and 3 awake) and 2 ponies (anesthetized) were studied to evaluate the high sensitivity of the pulmonary circula tion of the horse to various blood-borne particles, and to establish t he presence of intravascular macrophages in the lung. Pulmonary and sy stemic pressures and cardiac output before and during particle injecti on were measured in some animals. An anesthetized foal had a large inc rease in pulmonary arterial pressure (32 and 34 mm of Hg) within 1 min ute of IV administration of small test doses of radioactively labeled liposomes (2.5 mu mol/kg of body weight) or a 1% suspension of blue pi gment (0.3 ml/kg), respectively. Quantitative real time gamma camera i maging of the foal revealed high retention of the labeled liposomes du ring the first pass through the lungs; retention persisted throughout the experiment. Postmortem analysis revealed 55 and 47% lung retention of liposomes and blue pigment, respectively. The 2 anesthetized ponie s had increased pulmonary artery pressure of 34 +/- 7 mm of Hg, decrea sed cardiac output, and 42% lung retention after administration of 1% blue pigment (0.2 ml/kg), whereas 3 awake horses had increased pressur e of 28 +/- 3 mm of Hg after 1.8 x 10(8) (1.8-mu m-diameter) latex mic rospheres/kg. None of the injected particles caused vascular obstructi on, and they do not cause pulmonary vascular reactivity in species tha t lack pulmonary intravascular macrophages. Finally, 3 horses (1 anest hetized and 2 awake) were infused IV with small doses of the blue pigm ent, and their lungs were perfusion-fixed to identify specific labelin g of the pulmonary intravascular macrophages. These cells were fully d ifferentiated macrophages, contained blue pigment in phagocytes, and w ere tightly adherent to the pulmonary capillary endothelium. At this t ime, horses (order Perissodactyla) are the only species outside the ma mmalian order Artiodactyla (sheep, pig, cattle) documented to have rea ctive intravascular macrophages. Compared with other species, low dose s of particles induced marked hemodynamic responses; horses appear to be more sensitive to IV administered particles than are other species studied.