Pulmonary intravascular macrophages appear in rats after long-term administration of lipid emulsion and amino acid solution. An ultrastructural morphometric study.

Intravascular macrophages have rarely been seen in normal lungs of humans and rats, but in rats endotoxaemia has induced their presence. To study whether substrates used for parenteral nutrition could have a similar stimulatory effect on mononuclear phagocytes, rats were given lipid emulsion (n=5), amino acid solution (n=5), or isotonic saline (n=5) through central venous catheters for 3 weeks. Structural changes in the lung microvessels were evaluated using electron microscopy. The areal fraction of pulmonary intravascular mononuclear phagocytes was 19.6% (SD=8.2) in rats given lipid emulsion (p<0.05) and 8.2% (SD=8.2) in rats given amino acid solution n.s. compared to 2.4% (SD= 4.0) in rats given saline. The increase in areal fraction was mainly due to an increase in cell numbers. In rats given lipid emulsion the intravascular phagocytes were only slightly larger than in rats given saline, but had the morphological features of mature macrophages. The study demonstrates that lipid emulsion recruits pulmonary intravascular macrophages in rats, indicating a stimulatory effect on the mononuclear phagocyte system. The effect was less pronounced with amino acid solution.