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 α-smooth muscle actin (SMA) in contraction by small interfering RNA (siRNA); 3) α-SMA protein expression; 4) α-SMA and F-actin structure; 5) intracellular calcium concentration ([Ca ²⁺]_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 α-SMA over expression, but not in α-SMA-siRNA-treated cells. BK also increased α-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 α-SMA⁺ myofibroblasts, is mediated through the activation of the B2R and involves the Ca²⁺/calmodulin pMLC-dependent pathway. Copyright