Surface coat of sheep pulmonary intravascular macrophages is reconstituted following brefeldin A-mediated endocytosis.

Pulmonary intravascular macrophages (PIMs) of sheep have a globular coat on their surface which is mobilized by heparin and halothane, and is implicated in the endocytosis of tracer particles and Escherichia coli lipopolysaccharide. Brefeldin A (BFA) was used in vivo to investigate the effects of a pre-Golgi secretion blocker on the integrity of surface coat, and to know whether PIMs produce coat globules. Sheep were injected intravenously with BFA to reach a one time concentration of 2-5 microgram/ml of plasma, and euthanised at 10, 30, 45, 120 and 180 min (n = 1) post-treatment. Lungs were fixed in situ with 2% glutaraldehyde and 2.5% paraformaldehyde for 30 min. Lung tissues were processed for routine ultrastructural examination including treatment with tannic acid (0.1%), and for the acid phosphatase (ACPase) cytochemistry to identify Golgi complex and enzyme-rich endocytic vesicles. Surface coat was endocytosed by the PIMs within 10 min of BFA treatment through long ACPase-negative endocytic channels and degraded in the acid hydrolase-rich lysosomes. Golgi complex membranes were tubulated and were associated with prominent microtubules, centrioles and secretory vesicles. However, trans-Golgi network was not affected by BFA administration. The coat of PIMs was reconstituted within three hours of BFA-induced endocytosis, in concurrence with signs of enhanced biosynthetic activity. It seems that PIMs do not synthesize the globules, and rather sequester from the plasma to organize a surface coat. It is possible that PIMs contribute a membrane anchor or a receptor to facilitate reconstitution of the coat to perform multiple rounds of globule-mediated cell functions.