Duration and orientation of mechanical loads determine fibroblast cyto-mechanical activation: monitored by protease release.

Regulation of mechanical force on and by connective tissues is increasingly regarded as a critical factor in understanding their function. The aim of this study was to identify quantifiable characteristics of external loading to which fibroblasts were sensitive. Specific patterns of uniaxial tensile loading were applied to fibroblast populated collagen lattices through a computer controlled driver: tensioning-Culture Force Monitor. Ramp loads of 120 dynes were applied over 10 minutes and 11 hours, and the effect of high and low strain pattern changes, with orientation relative to the applied load were investigated by using two lattice configurations, high and low aspect ratios, with loads applied across the long and short axis. Both, ramp loading and lattice orientation were tested in comparison to statically loaded gels. Changes in protease production were measured to indicate which loading patterns produced cell stimulation. Greatest cell responses were with the slowest rate of ramp loading and unloading (11 dynes/hour) with the 11 hour ramp causing a 5-fold increase in MMP-9 release. Similarly, the greatest stimulation was produced in high strain, aligned High Aspect Ratio lattices with a 2.6-fold increase in MMP-9 release. In conclusion, duration over which cells are loaded rather than the rate and alignment of that loading are both critical factors in cyto-mechanical activation.