PLATELET-ACTIVATING-FACTOR AUGMENTS LIPOPOLYSACCHARIDE-INDUCED NITRIC-OXIDE FORMATION BY RAT KUPFFER CELLS

Acute endotoxic shock is accompanied by an increase in the production of nitric oxide (NO) by several different hepatic cell types. Platelet -activating factor (PAF) is a potent proinflammatory mediator with man y pathophysiological actions and, in fact, elevated plasma and tissue levels of PAF are observed in animal models of endotoxic shock. The cu rrent study demonstrates that PAF induced nitrite formation, the end p roduct of nitric oxide synthesis, by Kupffer cells in a dose- and time -dependent manner. Moreover, PAF was seen to initiate NO synthase gene expression and protein synthesis. PAF augmented lipopolysaccharide (L PS)-induced expression of inducible nitric oxide synthase messenger RN A (mRNA), protein, nitrite and cyclic guanosine monophosphate (cGMP) l evels in Kupffer cells. Treatment of Kupffer cells with actinomycin D or cycloheximide inhibited PAF- and LPS-stimulated nitrite and nitric oxide synthase protein formation confirming that de novo synthesis of the enzyme occurred. In Kupffer cells, the presence of an arginine ana log, N-G-methyl-L-arginine, attenuated nitrite formation induced by PA F and LPS alone or in combination. L-arginine is the principal substra te for nitric oxide synthase. PAF and LPS individually and in combinat ion induced a time-dependent uptake of L-[H-3]-arginine into the Kupff er cell, and this response was sensitive to cycloheximide. The current study indicates that exogenous PAF contributes to the induction of ni tric oxide synthase by LPS in cultured rat Kupffer cells.