Evidence that phospholipase A2 (PLA2) inhibitors can selectively block PLA2-mediated contractions of guinea pig lung pleural strips

We have demonstrated previously that porcine pancreatic Phospholipase A2 (PLA2)-induced contractions of guinea pig lung pleural strips can be abated by inhibitors of cyclooxygenase and 5-lipoxygenase pathways, suggesting the liberation of arachidonic acid. To validate further the involvement of a PLA2-related mechanism, the effects of three known inhibitors of PLA2 were evaluated. Manoalogue, an irreversible inhibitor of PLA2, parabromophenacyl bromide, an irreversible, active site directed inhibitor and a transition-state analog, a competitive inhibitor of PLA2 were used. Transition-state analog (3-30 microM), added to the tissues for 30 min before PLA2, shifted the PLA2 curves to the right in a concentration-related manner. In contrast, the reported inactive enantiomer of transition-state analog failed to alter the PLA2 curves. Manoalogue and para-bromophenacyl bromide, at concentrations up to 40- and 50-fold higher than the enzyme concentration, respectively, were incubated in Krebs' buffer with the enzyme for 24 hr before challenging the tissues. Under these conditions, the PLA2-induced contractions were suppressed markedly. In contrast, neither reduced nor methylated manoalogue, incubated at a 20-fold molar excess with PLA2 for 24 hr, suppressed the maximal PLA2-induced responses. These results demonstrate that inhibitors of secretory PLA2, acting by different mechanisms, can alter the contractile responses induced by PLA2 on pleural strips from guinea pig lung.