PLATELET-ACTIVATING-FACTOR (PAF) INDUCES A CONTRACTION OF ISOLATED SMOOTH-MUSCLE CELLS FROM GUINEA-PIG ILEUM - INTRACELLULAR PATHWAY INVOLVED

This study was designed to evaluate the effect of platelet-activating factor (PAF) on isolated smooth muscle cells from guinea pig ileum cir cular layer, to characterize the PAF receptors involved in this effect and to determine the intracellular pathways triggered by PAF. Cells d ispersed by enzymatic digestion were incubated for 30 sec in the prese nce of PAF and fixed by glutaraldehyde. When inhibitors or antagonists were tested, cells were preincubated with them for 1 min. Then PAF wa s added for 30 sec, and the cells were fixed. Contraction was assessed by measuring the length of 50 cells and was expressed as the percenta ge decrease in cell length from controls. The relaxing effect of inhib itors was expressed as the percentage of the maximal contraction obser ved in their absence. PAF induced a cell contraction in a concentratio n-dependent manner. Maximal contraction (24.2 +/- 4.2%) was obtained f or a PAF concentration of 10 nM (EC50 = 10 pM). PAF-induced contractio n was inhibited by the PAF receptor antagonists BN52021, L659.989 and SR27417. Contraction induced by 10 nM PAF was inhibited when cells wer e incubated in Ca++-free medium with or without 2 mM EGTA or in a 1 mM Ca++ medium to which 100 nM nifedipine was added. When cells were pre incubated with concentrations ranging from 0.01 pM to 10 muM of relaxi ng agents (vasoactive intestinal polypeptide, forkolin, 8 Bromo cAMP) known to increase the intracellular level of cAMP, PAF-induced contrac tion was inhibited. Moreover, when cells were preincubated with pertus sis toxin (200 ng/ml) or cholera toxin (8.4 ng/ml), contraction induce d by PAF was also inhibited. PAF induces a contraction of isolated smo oth muscle circular cells from guinea pig ileum by interacting with a specific receptor. The effect of PAF is mediated through the activatio n of a pertussis toxin-sensitive G protein and through the triggering of an influx of Ca++ into the cell.