The molecular mechanisms by which bradykinin induces excessive airway obstruction in asthmatics remain unknown. Transforming Growth Factor (TGF-) has been involved in regulating airway inflammation and remodeling in asthma, although it is not known whether TGF- can modulate bradykinin-associated bronchial hyper-responsiveness. To test whether TGF- directly modulates airway smooth muscle (ASM) responsiveness to bradykinin, isolated murine tracheal rings were used to assess whether TGF- alters ASM contractile responsiveness to bradykinin. Interestingly, we found that TGF--treated murine rings (12.5 ng/ml, 18h), exhibited increased expression of B2 receptors, became hyperreactive to bradykinin as shown by increases in the maximal contractile responses and receptor distribution. We investigated the effect of TGF- on bradykinin-evoked calcium signals since intracellular calcium is a key molecule regulating airway smooth muscle (ASM) excitation-contraction coupling. We reported that TGF-, in a dose- (0.5 to 10.0ng/ml) and time (2-24h)-dependent manner, increased mRNA and protein expression of the bradykinin 2 (B2) receptor in cultured human ASM cells. Maximal B2 receptor protein expression, which co-localized with CD44 a marker of membrane cell surface, occurred after 18h of TGF- treatment and was further confirmed using fluorescence microscopy. TGF- (2.5ng/ml, 18h) also increased bradykinin-induced intracellular calcium mobilization in Fura-2 loaded ASM cells. TGF--mediated enhancement of calcium mobilization was not attenuated with indomethacin, a cyclooxygenase (COX) inhibitor. These data demonstrated for the first time that TGF- may play a role in mediating airway hyperresponsiveness to bradykinin seen in asthmatics by enhancing ASM contractile responsiveness to bradykinin, possibly as a result of increased B2 receptor expression and signaling.