Mechanical strain increases contractile enzyme activity in cultured airway smooth muscle cells

Smooth muscle hypertrophy is often found in tissue subjected to abnormal physical stress. To determine if physical stress (strain) per se could increase the contractile potential of airway smooth muscle (ASM), we compared cultured ASM cells subjected to strain to control cells (no strain) for rates of 1) myosin light chain kinase (MLCK)-mediated myosin light chain (LC20) phosphorylation, 2) actin-activated myosin ATPase, and 3) myosin light chain phosphatase-mediated myosin dephosphorylation. Lysates from strained cells showed increases in both LC20 phosphorylation activity and actomyosin ATPase activity but decreased rates of phosphatase-dependent myosin dephosphorylation. The increased LC20 phosphorylation activity and ATPase activity of the strained cells were accompanied by increases in cellular content of MLCK and myosin, respectively, compared with control. Because the cultured ASM cells exposed to strain expressed higher MLCK activity and actomyosin ATPase activity but lower myosin light chain phosphatase activity, these data suggest that physical stress in part determines ASM potential for contractile state.