Losartan Blocks the Recruitment of Mast Cells in the Lungs of Rats Subjected to Fat Embolism with or without a Second Hit with LPS

Abstract

In a rat model of pulmonary fat embolism (FE) using triolein (T), we have shown that the long term histopathological effects of triolein‐induced FE extend to 10 weeks and are inhibited by losartan (LOS), an angiotensin II type 1 receptor blocker1 and augmented by a second hit with LPS2. These effects are associated with an increase in renin staining at 6 weeks3. Since other workers have implicated pulmonary mast cells (MC) in local renin‐angiotensin (RAS)‐mediated pulmonary pathology4, we examined whether FE in our model was associated with an increase in MC number and if their appearance would be blocked by LOS and modified by a second hit with LPS. 36 Sprague‐Dawley rats were treated with T (0.2 ml i.v.) or saline. After 6 weeks, half of the groups were given saline or LPS 0.1 ml (3 mg/kg) i.p., followed 1 hour later with saline or LOS (10 mg/kg i.p.), followed by LOS in the drinking water (50 mg/l). 4 weeks later (10 weeks after T), the animals were necropsied after isoflurane anesthesia. Lungs were fixed in 10% formalin and stained with H&E for morphometric scoring and for mast cells using CD11 (c‐kit). Portions of the lungs were frozen and stained for fat using Oil Red O. Two pathologists unaware of the slides’ identity took 10 photographs at random in each slide at 400× and then counted the mast cell number.MC number increased in T‐treated rats compared to saline controls (57.2 +/− 2.5 vs 20.0 +/− 2.3) (mean+/− SEM) (p=0.005). The MC count when T was followed by LPS rose to 66.8 +/− 14.5 (p=0.001 compared to saline). LPS or LOS alone did not cause a significant increase in MC (35.0 +/− 8.5 and 28.2 +/− 2.6, respectively.) The increases in MC caused by T and T+LPS were both blocked by LOS: (T+LOS 14.9 +/− 2.9; T+LPS+LOS 32.5 +/− 4.6; p=0.75 and 0.99 compared to LOS alone.The appearance of fat globules at 10 weeks was greatly reduced from what we reported for rats at 6 weeks3 but they were increased in T‐treated rats compared to saline controls.The results support suggestions that MC‐associated renin through its eventual product angiotensin II plays an important role in the histopathological changes produced by FE and the second hit with LPS. In addition, the presence of fat globules in the lungs 10 weeks after the initial challenge with T suggests that their continued presence may be important for the enhanced presence of MC in response to low grade inflammation. A role for macrophage signaling in this sequence is also supported5.Support or Funding InformationMary Katherine Geldmacher Research Foundation, St. Louis, MO