This study aims to investigate the role of Rho-kinase in phenotype switching and proliferation of bovine tracheal smooth muscle. To induce different phenotypic states, bovine tracheal smooth muscle strips were cultured (8 days) in 10% foetal bovine serum (foetal bovine serum, less contractile phenotype) or insulin (1 microM, hypercontractile phenotype) and compared to strips cultured in serum-free medium. In contraction experiments, the Rho-kinase inhibitor (+)-(R)-trans-4-(1-aminoethyl)-N-(4-pyridyl) cyclohexane carboxamide (Y-27632, 1 microM) decreased sensitivity to methacholine and KCl and lowered maximal responsiveness to KCl in all strips irrespective of the phenotype present. To investigate the effects of Rho-kinase bovine tracheal smooth muscle phenotypic regulation, strips were pretreated with Y-27632 (1 microM) for 8 days. This resulted in a decreased maximal contractility to both methacholine and KCl, quantitatively comparable to the decrease in contractility induced by platelet-derived growth factor (PDGF, 10 ng/ml). The combination of Y-27632 and PDGF responded additively. Y-27632 did not affect basal or PDGF-induced bovine tracheal smooth muscle cell proliferation, determined both as increases in [3H]thymidine incorporation and cell number. Inhibitors of the p42/p44 mitogen-activated protein kinase (MAPK) pathway, the p38 MAPK pathway and the phosphatidyl inositol (PI) 3-kinase pathway all inhibited PDGF-induced proliferation and phenotype changes. These results show that the functional contribution of Rho-kinase to bovine tracheal smooth muscle contraction is not dependent on phenotypic state. In addition, Rho-kinase is not involved in phenotypic modulation or proliferation induced by PDGF, whereas p42/p44 MAPK, p38 MAPK and PI 3-kinase are. Rho-kinase is, however, a major regulator involved in the basal maintenance of contractility in bovine tracheal smooth muscle.