Mechanisms of bradykinin-induced contraction in human fetal lung fibroblasts

Bradykinin (BK) induces fibroblast contraction but the structural changes and intracellular mechanisms involved have not been completely explored. We stimulated HFL-1 fibroblasts with BK to assess: 1) fibroblast contractility; 2) the role of alpha-smooth muscle actin (SMA) in contraction by small interfering RNA (siRNA); 3) alpha-SMA protein expression; 4) alpha-SMA and F-actin structure; 5) intracellular calcium concentration ([Ca2+](i)); and 6) phosphorylated myosin light-chain (pMLC) and MLC kinase (MLCK) expression. BK triggered concentration-and time-dependent fibroblast gel contraction in conjunction with alpha-SMA over expression, but not in alpha-SMA-siRNA-treated cells. BK also increased alpha-SMA(+) and F-actin(+) cell number and stress fibre polymerisation (detectable at 5-60 min). These BK-induced changes were associated with an increase in [Ca2+](i), which peaked within 15 s, and activation of pMLC, which was detectable at 5-60 min. No MLCK content modification was observed. The different manifestations of the BK-induced fibroblast activation were downregulated at different levels (25-100%) by HOE140, a specific BK B2 receptor (B2R) antagonist and by the Ca2+ chelator, EGTA. Thus, BK-induced fibroblast contraction, associated with differentiation into alpha-SMA(+) myofibroblasts, is mediated through the activation of the B2R and involves the Ca2+/calmodulin pMLC-dependent pathway.