It has been shown that activation of protein tyrosine kinases is the earliest detectable signalling response to FcepsilonRI cross-linking on mast cell. Following tyrosine kinase activation, a family of mitogen-activated protein kinases (MAPKs) was found to be activated as well. The present study examined the role of MAPK signalling cascade in in vitro model of allergic asthma using a specific MAPK kinase inhibitor PD 098059. 2. Guinea-pigs were passively sensitized with IgG antibody raised against ovalbumin (OA). Effects of PD 098059 on OA-induced anaphylactic contraction of isolated bronchi and release of histamine and peptidoleukotrienes from chopped lung preparations were studied. 3. PD 098059 (10-50 microM) produced only minor reduction of maximal OA-induced bronchial contraction. In contrast, the rate of relaxation of OA-induced bronchial contraction was markedly faster in the presence of PD 098059 than the vehicle control in a concentration-dependent manner. 4. These observations corroborate well with the inability of PD 098059 (5-50 microM) to substantially block the OA-induced release of histamine and with marked inhibition of OA-induced release of peptidoleukotrienes from lung fragments in the presence of PD 098059. Exogenous arachidonic acid-induced release of peptidoleukotrienes from lung fragments was not blocked by PD 098059. 5. In immunoblotting study, we found that p42MAPK was constitutively expressed in guinea-pig bronchi. However, treatment with OA, histamine or LTD4 did not cause activation of p42MAPK. These findings together with the lack of inhibitory effects of PD 098059 on bronchial contraction induced by histamine or LTD4 suggest that histamine- and LTD4-induced bronchial contractions are not mediated by p42MAPK activation. 6. Taken together, our findings show that inhibition of MAPK signalling cascade by PD 098059 significantly reduced the OA-triggered release of peptidoleukotrienes leading to rapid relaxation of anaphylactic bronchial contraction. On the other hand, p42MAPK did not play a role in histamine- or LTD4-induced bronchial smooth muscle contraction suggesting that PD 098059 exerts its inhibitory effects on OA-induced bronchial contraction primarily through inhibition of peptidoleukotrienes release from mast cells.
