Signal transduction in smooth muscle - Selected contribution: Roles of focal adhesion kinase and paxillin in the mechanosensitive regulation of myosin phosphorylation in smooth muscle

The increase in intracellular Ca2+ and myosin light chain (MLC) phosphorylation in response to the contractile activation of tracheal smooth muscle is greater at longer muscle lengths (21). However, MLC phosphorylation can also be stimulated by Ca2+-insensitive signaling pathways (19). The cytoskeletal proteins paxillin and focal adhesion kinase (FAK) mediate a Ca2+-independent length-sensitive signaling pathway in tracheal smooth muscle (30). We used alpha -toxin-permeabilized tracheal smooth muscle strips to determine whether the length sensitivity of MLC phosphorylation can be regulated by a Ca2+-insensitive signaling pathway and whether the length sensitivity of active tension depends on the length sensitivity of myosin activation. Although active tension remained length sensitive, ACh-induced MLC phosphorylation was the same at optimal muscle length (L-o) and 0.5 L-o when intracellular Ca2+ was maintained at pCa 7. MLC phosphorylation was also the same at Lo and 0.5 Lo in strips stimulated with 10 muM Ca2+. In contrast, the Ca2+-insensitive tyrosine phosphorylation of FAK and paxillin stimulated by ACh was higher at L-o than at 0.5 L-o. We conclude that the length-sensitivity of MLC phosphorylation depends on length-dependent changes in intracellular Ca2+ but that length-dependent changes in MLC phosphorylation are not the primary mechanism for the length sensitivity of active tension.