Do calcium fluxes within cortical bone affect osteocyte mechanosensitivity?

Bone reacts to local mechanical environment by adapting its structure. Bone is also a key source of calcium for the body homeostasis. Osteocytes, cells located within the bone tissue, are thought to play a major role in sensing mechanical signals and regulating bone remodeling. Interestingly, osteocytes were also shown to directly participate in the calcium homeostasis by regulating dissolution and deposition of calcium in the perilacuno-pericanalicular space. However, it is not known if osteocyte's roles in mechanoregulation and calcium homeostasis have any significant crosstalk. Previously, a multi-scale mathematical model of the interstitial fluid flow through the canaliculus was developed, which took into account physicochemical phenomena including hydraulic effects, formation of electrical double layer, osmosis and electro-osmosis. We extended this model to include the directional movement of calcium from and into the bone tissue, and assessed the shear stress at the osteocyte membrane. We have found that in the bulk of the canalicular space the fluid flow due to chemical gradient generated by deposition or dissolution of calcium is negligible compared to the fluid flow due to hydraulic pressure. However, at the osteocyte proximity, the presence of calcium gradient generated sufficient fluid flow to induce significant changes in the shear stress on the osteocyte membrane. Calcium deposition and dissolution on the canalicular wall resulted in increased or decreased shear stress on the osteocyte membrane respectively. Thus, our data demonstrate that strong calcium fluxes due to whole body calcium homeostasis may affect mechanical forces experienced by osteocytes.