Bone and Aletered Mechanical Loading: Basic Science Approaches
Bone's mechanosensitivity tends to decline soon after the initiation of dynamic mechanical loading and as a result the osteogenic response of bone saturates. This suggests the existence of a recovery period that would restore bone cell responsiveness to mechanical stimuli.
To expose MC3T3-E1 osteoblastic cells to rest inserted oscillatory fluid flow and track the real time intracellular calcium mobilization to help elucidate whether the recovery phenomenon stems from a Ca2+ regulated mechanism.
Intracellular levels of Ca2+ in MC3T3-E1 osteoblastic cells were quantified using a ratiometric imaging technique. Cells were exposed to a total of 3 minutes of oscillating fluid flow with rest periods of 0,5,10, and 15 seconds inserted every 10 loading cycles. A cell response was defined as a transient increase in Ca2+ of at least 4 times the maximum oscillation recorded during the 3 min pre-flow baseline period. One-way ANOVA and Fisher's Protected Least Significant Difference was utilized for statistical analysis.
Insertion of 10 and 15s rest periods but not 5s resulted in Ca2+ response magnitudes that were significantly higher than control (mean ± SE, 10s = 231 ± 13.6, 15s = 181 ± 16.6 vs. control = 108 ± 17.8 nM p < 0.005). Among cells that responded to flow, the average # of Ca2+ responses during flow was significantly higher in the 10s and 15s groups but not the 5s group compared to control (10s = 1.27 ± .035, 15s = 1.282 ± 047 vs. control = 1.065 ± 024 p < .005).
Since previous studies have shown that the insertion of recovery periods of similar length enhance bone formation in animal models, our results suggest the possibility that downstream events in the bone adaptation process may be regulated by the frequency and magnitude of Ca2+ oscillations that a cell or group of cells undergo. Support from NIH grant AR45989, US Army Medical Research Award DAMD 17-98-1-8509