Calcium response in bone cells at different osteogenic stages under unidirectional or oscillatory flow

It was found that preosteoblast MC3T3-E1 cells were less responsive in calcium signaling than mature osteocyte MLO-Y4 cells when a steady fluid flow was exerted on a micropatterned cell network. However, the effect of fluid flow on the calcium response in preosteocyte MLO-A5 was seldom investigated. In the present study, MLO-A5 as well as MC3T3-E1 and MLO-Y4 cells were cultured on a regular substrate with high or low density under unidirectional or oscillatory fluid flow. The results showed that calcium oscillation in the cells during late osteogenesis was significantly stronger than during early osteogenesis regardless of the fluid flow type or the presence of a physical cell-cell connection. Calcium oscillation produced by the oscillatory flow in the three types of cells was stronger than that produced by the unidirectional flow, but MC3T3-E1 and MLO-A5 cells exhibited limited potential for calcium oscillation compared with MLO-Y4 cells. After suramin was used to block the binding of extracellular adenosine triphosphate (ATP) to the membrane P2 receptor, the calcium oscillation in the three types of bone cells with or without physical connections was significantly suppressed as a single responsive peak under unidirectional flow. For the ATP-blocking group of low-density cells under oscillatory flow, the number of oscillation peaks in three types of cells was still more than two. It indicates that besides the ATP pathway, other mechanosensitive calcium pathways may exist under oscillatory flow. The present study provided further evidence for the osteogenic stage-dependent calcium response of bone cells under unidirectional or oscillatory fluid flow.