Early strain-related changes in enzyme activity in osteocytes following bone loading in vivo

Abstract

The skeleton's architecture is matched to the changing loads to which it is subjected because mechanical loading directly or indirectly influences the activity of cell populations to deposit, maintain, or remove bone tissue as appropriate. This responsiveness to load bearing presupposes that certain cells are sensitive to load itself or to its consequences within the tissue. The nature of this effect and the cells concerned have not yet been determined. In this series of experiments, bones were exposed in vivo to a single short period of dynamic loading, which if repeated daily had been shown to result in increased new bone formation. There was an increase in the activity of glucose 6-phosphate dehydrogenase (G6PD) in the periosteal cells adjacent to the bone surface 6 min after this single period of loading. This increase was proportional to the strain magnitude in the bone tissue in the immediate vicinity of the cells. In osteocytes, although the G6PD activity in each individual cell was unchanged by loading, the number of cells displaying activity was increased. This increase was also directly proportional to the applied strain in that area of the cortex (52% compared with 26% active osteocytes at a strain of 0.002). Activation of G6PD was unaccompanied by any equivalent changes in the activities of either glyceraldehyde 3-phosphate dehydrogenase (GA3PD) or lactate dehydrogenase (LDH). This finding is consistent with loading increasing the activity of the oxidative part of the pentose monophosphate shunt pathway. It is also consistent with stimulation of a synthetic process, such as the production of RNA from ribose 5-phosphate. These results show that intermittent loading of bone tissue produces rapid strain-related effects on the metabolism of both osteocytes and periosteal cells. This change is consistent with an increase in their synthetic activity.