Author(s): CHEN, Y.C., OGA, R., NAKAGAWA, K., TAMAKI, H., Institution: NATIONAL INSTITUTE OF FITNESS AND SPORTS IN KANOYA, Country: JAPAN, Abstract-ID: 1475

INTRODUCTION:
It is well known that exercise brings multiple benefits, and osteocalcin (Ocn), a bone-derived hormone, is considered one of the beneficial factors involved in the process. It has been shown to help improve insulin sensitivity, glucose and fat metabolism, and cognitive development. However, for those who are unable to do voluntary exercise, passive exercise triggered by external electrical stimulation (ES) may be a solution. Nevertheless, whether even a relatively small bone strain from ES-induced muscle contraction force can stimulate osteocytes to promote Ocn secretion, and how long any such increase in Ocn secretion lasts, remains unclear. In this study, therefore, we aimed to investigate the effects of ES-induced muscle contraction on Ocn secretion and changes in this relationship over time.
METHODS:
Sixteen male ten-week-old Fischer 344 rats were randomly assigned to two groups: (1) low-frequency ES at 10 Hz (LF, n = 8); (2) high-frequency ES at 100 Hz (HF, n = 8). An acute 30 min ES intervention was applied specifically to the left tibialis anterior (TA) muscle under isoflurane inhalational anesthesia. We recorded in vivo tibia bone strain caused by ES-induced muscle contraction force at both ES frequencies using a strain gauge. Blood samples were collected from the caudal vein just before, 0, 0.25, 0.5, 1, 3, 5, 7, and 14 days after the ES intervention, and fasting blood glucose (FBG) was measured at the same time. Serum was extracted to analyze the concentration of Ocn at each timepoint using enzyme-linked immunosorbent assay (ELISA). The changes in Ocn and FBG concentration over time and the correlation between them were analyzed.
RESULTS:
The in vivo tibia bone strain during ES was 35 µε at 10 Hz and 264 µε at 100 Hz. However, there was no significant difference between the groups in Ocn and FBG concentrations at any timepoint. The Ocn concentration in the LF group increased significantly at 0.25, 0.5, 1, 5, and 14 days after the ES intervention. In the HF group, the concentration decreased immediately after the intervention and then increased significantly at 0.25, 0.5, 1, 3, and 14 days after the ES intervention. There were significant decreases in FBG at all timepoints following the ES intervention in both groups. Further, significant negative correlations were observed between Ocn and FBG concentrations, as well as between the changes in Ocn and the changes in FBG.
CONCLUSION:
Electrical-stimulation–induced muscle contraction force can promote an increase in Ocn secretion for up to five days, as well as a sustained reduction in FBG for two weeks. The elevation in Ocn secretion correlates with lower FBG levels.