1. The present study aims to investigate whether phenotypic changes, reported to occur in cultured isolated airway smooth muscle (ASM) cells, are of relevance to intact ASM. Moreover, we aimed to gain insight into the signalling pathways involved. 2. Culturing of bovine tracheal smooth muscle (BTSM) strips for up to 8 days in the presence of 10% foetal bovine serum caused a time-dependent (t(1/2)=2.8 days) decrease in maximal contraction (E(max)) to methacholine compared to serum-deprived controls (E(max)=74+/-4% at day 8). A reduced E(max) was also found using insulin-like growth factor-1 (30 ng ml(-1)) and platelet-derived growth factor (30 ng ml(-1)), but not using epidermal growth factor (10 ng ml(-1)) (E(max)=83+/-3, 67+/-8, 100+/-4%, respectively). Similar serum and growth factor-induced changes in E(max) were found for KCl-induced contraction (65+/-9, 80+/-7, 64+/-11% and 107+/-2%, respectively). 3. Strong correlations were found between the growth factor-induced reductions in E(max) and their proliferative responses, assessed by [(3)H]-thymidine-incorporation, in BTSM cells. (r=0.97, P=0.002 for methacholine and r=0.93, P=0.007 for KCl). 4. The PDGF-induced reduction in E(max) was inhibited completely by combined treatment with either PD 98059 (30 micro M) or LY 294002 (10 micro M). 5. These results indicate that serum and growth factors may cause a functional shift towards a less contractile phenotype in intact BTSM, which is associated with their proliferative response and dependent on signalling pathways involving the mitogen-activated protein kinase pathway and the phosphatidylinositol-3-kinase pathway.