This study was designed to evaluate the effect of platelet-activating factor (PAF) on isolated smooth muscle cells from guinea pig ileum circular layer, to characterize the PAF receptors involved in this effect and to determine the intracellular pathways triggered by PAF. Cells dispersed by enzymatic digestion were incubated for 30 sec in the presence of PAF and fixed by glutaraldehyde. When inhibitors or antagonists were tested, cells were preincubated with them for 1 min. Then PAF was added for 30 sec, and the cells were fixed. Contraction was assessed by measuring the length of 50 cells and was expressed as the percentage decrease in cell length from controls. The relaxing effect of inhibitors was expressed as the percentage of the maximal contraction observed in their absence. PAF induced a cell contraction in a concentration-dependent manner. Maximal contraction (24.2 +/- 4.2%) was obtained for a PAF concentration of 10 nM (EC50 = 10 pM). PAF-induced contraction was inhibited by the PAF receptor antagonists BN52021, L659.989 and SR27417. Contraction induced by 10 nM PAF was inhibited when cells were 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 preincubated with concentrations ranging from 0.01 pM to 10 microM of relaxing agents (vasoactive intestinal polypeptide, forskolin, 8 Bromo cAMP) known to increase the intracellular level of cAMP, PAF-induced contraction was inhibited. Moreover, when cells were preincubated with pertussis toxin (200 ng/ml) or cholera toxin (8.4 ng/ml), contraction induced by PAF was also inhibited.(ABSTRACT TRUNCATED AT 250 WORDS)