SURFACE-COAT OF SHEEP PULMONARY INTRAVASCULAR MACROPHAGES IS RECONSTITUTED FOLLOWING BREFELDIN A-MEDIATED ENDOCYTOSIS

Pulmonary intravascular macrophages (PIMs) of sheep have a globular co at on their surface which is mobilized by heparin and halothane, and i s implicated in the endocytosis of tracer particles and Escherichia co li lipopolysaccharide. Brefeldin A (BFA) was used in vivo to investiga te the effects of a pre-Golgi secretion blocker on the integrity of su rface 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% glutarald ehyde and 2.5% paraformaldehyde for 10 min. Lung tissues were processe d for routine ultrastructural examination including treatment with tan nic acid (0.1%), and for the acid phosphatase (ACPase) cytochemistry t o identify Golgi complex and enzyme-rich endocytic vesicles. Surface c oat was endocytosed by the PIMs within 10 min of BFA treatment through long ACPase-negative endocytic channels and degraded in the acid hydr olase-rich lysosomes. Golgi complex membranes were tubulated and were associated with prominent microtubules, centrioles and secretory vesic les. However, trans-Golgi network was not affected by BFA administrati on. The coat of PIMs was reconstituted within three hours of BFA-induc ed endocytosis, in concurrence with signs of enhanced biosynthetic act ivity. It seems that PIMs do not synthesize the globules, and rather s equester from the plasma to organize a surface coat. It is possible th at PIMs contribute a membrane anchor or a receptor to facilitate recon stitution of the coat to perform multiple rounds of globule-mediated c ell functions.