The effects of downhill running on bone ash, calcium concentration, and total calcium in mice

Abstract

Exercise is essential for reducing age‐related bone mineral loss by providing mechanical loading. Due to greater shear strain forces generated, downhill walking or running at slower speeds may provide similar stimuli to faster running, sprinting, or jumping on flat surfaces. Results from previous studies are mixed when comparing changes in morphological and mechanical properties from slower, downhill running to level or uphill running and sedentary controls. The purpose of this study was to determine if downhill running (DHR) could improve bone mineral content, calcium concentration, and total calcium in the limbs of mice. This study was approved by the MSU IACUC and FVB mice (Charles River) were housed according the Guide for the Care and Use of Laboratory Animals. Forty‐five days after birth (PN45), mice were allocated to either a sedentary or exercised group. Exercised mice began running on a treadmill set at a −16% gradient five times per week for 12 weeks in 30 minute bouts. After a two‐minute warm‐up speeds were increased from 10 m·min−1 to 17 m·min−1 for the remaining 28 minutes. At PN129, mice were euthanized, and fore‐ and hindlimbs were collected as these were the bones most likely to respond to exercise. Bones were cleaned, ether‐extracted, and ashed as a measure of bone mineral content. Ash was recorded and then microwave digested in nitric acid for calcium concentration analysis via atomic absorption spectrophotometry. Total calcium was calculated from multiplying calcium concentration by amount of ash for each sample. Due to the quantity of ash needed for digestion and the small size of mice, bones were pooled. Sedentary (n=7) and exercised (n=6) mouse data were found to be normally distributed and analyzed using the mixed model procedure in SAS 9.4 for effects of sex, exercise, and their interaction on ash, calcium concentration, and total calcium. There was no interaction between sex and exercise as well as no effect of sex nor exercise (P > 0.10) on any of the parameters measured (Table 1). Surprisingly, DHR in this study did not produce greater bone ash or total calcium when compared to sedentary controls. Though DHR can produce compressive strain and high shear strain rates, the slower speed may not be enough for bone to respond as it does to higher speeds or larger impacts. Additionally, these mice were analogous to human adults when they began training, and since younger bone responds to exercise better, this age may have limited the mice’s ability to accrue greater bone mineral. In this study, DHR failed to stimulate bone to accrue or even maintain mineral above sedentary values and therefore may not be an appropriate exercise recommendation to reduce age‐related mineral loss. Ash as a measure of bone mineral content, calcium concentration, and total calcium from the limbs of either downhill running (DHR) or sedentary (Sed) mice. Neither sex nor exercise had an effect on any of the values (P > 0.10). Sex Exercise Ash (mg) Calcium Concentration (μg·ml−1) Total Calcium (mg) Female DHR 445 ± 26 360 ± 6 160 ± 11 Sed 441 ± 26 352 ± 6 155 ± 11 Male DHR 442 ± 26 345 ± 6 152 ± 11 Sed 423 ± 23 352 ± 5 150 ± 9