DISTINCT BIOCHEMICAL RESPONSES OF HEPATIC MACROPHAGES AND ENDOTHELIAL-CELLS TO PLATELET-ACTIVATING-FACTOR DURING ENDOTOXEMIA

Acute endotoxemia is associated with activation of hepatic macrophages and endothelial cells. These cells release a variety of inflammatory mediators that have been implicated in tissue injury. In the present s tudies, we analyzed the biochemical responses of these cells to platel et-activating factor (PAF), a lipid autacoid released during hepatic i nflammatory responses. To induce acute endotoxemia, rats were injected intravenously with lipopolysaccharide (LPS). Using the calcium sensit ive fluorescent indicator dye Indo-1, we found that PAF induced a rapi d and transient increase in intracellular calcium in both hepatic macr ophages and endothelial cells. Induction of acute endotoxemia resulted in an increase in the amount of calcium mobilized by both cell types. Although endothelial cells from control rats were less responsive to PAF than macrophages, these cells were more sensitive to in vivo endot oxin. PAF was also found to cause a rapid decrease in intracellular pH in hepatic macrophages that was quantified by fluorescence image anal ysis using the pH sensitive dye SNAFL-calcein. This decrease occurred more rapidly in macrophages from endotoxemic rats. In cells from both control and endotoxemic rats, the effects of PAF on intracellular pH w ere inhibited by the specific PAF antagonist triazolam. In contrast to hepatic macrophages, PAF had no effect on intracellular pH in endothe lial cells from either control or endotoxemic rats. Ligand binding stu dies demonstrated that both hepatic macrophages and endothelial cells possess high affinity binding sites for PAF. Macrophages expressed 6- to 7-fold more binding sites/cell than endothelial cells and exhibited a higher K-d. Whereas treatment of rats with LPS had no effect on the K-d for PAF binding to macrophages or on the number of binding sites, a significant increase in both of these receptor characteristics was observed in endothelial cells. Taken together, the present data sugges t that the biochemical responses of endothelial cells and macrophages to PAF are distinct. Furthermore, cellular activation induced by PAF i n endothelial cells appears to be independent of changes in intracellu lar