Tibial loading increases osteogenic gene expression and cortical bone volume in mature and middle-aged mice.

There are conflicting data on whether age reduces the response of the skeleton to mechanical stimuli. We examined this question in female BALB/c mice of different ages, ranging from young to middle-aged (2, 4, 7, 12 months). We first assessed markers of bone turnover in control (non-loaded) mice. Serum osteocalcin and CTX declined significantly from 2 to 4 months (p<0.001). There were similar age-related declines in tibial mRNA expression of osteoblast- and osteoclast-related genes, most notably in late osteoblast/matrix genes. For example, Col1a1 expression declined 90% from 2 to 7 months (p<0.001). We then assessed tibial responses to mechanical loading using age-specific forces to produce similar peak strains (-1300 µÎµ endocortical; -2350 µÎµ periosteal). Axial tibial compression was applied to the right leg for 60 cycles/day on alternate days for 1 or 6 weeks. qPCR after 1 week revealed no effect of loading in young (2-month) mice, but significant increases in osteoblast/matrix genes in older mice. For example, in 12-month old mice Col1a1 was increased 6-fold in loaded tibias vs. controls (p = 0.001). In vivo microCT after 6 weeks revealed that loaded tibias in each age group had greater cortical bone volume (BV) than contralateral control tibias (p<0.05), due to relative periosteal expansion. The loading-induced increase in cortical BV was greatest in 4-month old mice (+13%; p<0.05 vs. other ages). In summary, non-loaded female BALB/c mice exhibit an age-related decline in measures related to bone formation. Yet when subjected to tibial compression, mice from 2-12 months have an increase in cortical bone volume. Older mice respond with an upregulation of osteoblast/matrix genes, which increase to levels comparable to young mice. We conclude that mechanical loading of the tibia is anabolic for cortical bone in young and middle-aged female BALB/c mice.

Low-magnitude whole-body vibration does not enhance the anabolic skeletal effects of intermittent PTH in adult mice.

Whole-body vibration (WBV) is a low-magnitude mechanical stimulus that may be anabolic for bone, yet we recently found that WBV did not improve bone properties in adult mice. Because intermittent parathyroid hormone (PTH) enhances the anabolic effects of high-magnitude skeletal loading, we sought to determine the skeletal effects of WBV in combination with PTH. Seven-month-old male BALB/c mice were assigned to six groups (n = 13-14/group) based on magnitude of applied acceleration (0 or 0.3 G) and PTH dose (0, 10, or 40 µg/kg/day). Mice were exposed to WBV (0.3 G, 90 Hz, sine wave) or sham loading (0 G) for 15 min/day, 5 days/week for 8 weeks. Vehicle or hPTH (1-34) was administered prior to each WBV session. Whole-body bone mineral content increased by ~ 5% from 0 to 8 weeks in the 40 µg/kg PTH group only, independent of WBV loading. Similarly, PTH treatment increased tibial cortical bone volume by ~5% from 0 to 8 weeks, independent of WBV loading. Neither PTH nor WBV stimulated trabecular bone formation. Consistent with the cortical bone effect, tibias from the 40 µg/kg PTH group had significantly greater ultimate force and energy to failure than tibias in the 0 and 10 µg/kg PTH groups, independent of WBV treatment. In summary, 8 weeks of intermittent PTH treatment increased cortical bone volume and strength in adult male BALB/c mice. Daily exposure to low-magnitude WBV by itself did not improve skeletal properties and did not enhance the PTH effect. No WBV-PTH synergy was found in this preclinical study.

Skeletal effects of whole-body vibration in adult and aged mice.

Low-amplitude, whole-body vibration (WBV) may be anabolic for bone. Animal studies of WBV have not evaluated skeletal effects in aged animals. We exposed 75 male BALB/c mice (7 month/young-adult; 22 month/aged) to 5 weeks of daily WBV (15 min/day, 5 day/wk; 90 Hz sine wave) at acceleration amplitudes of 0 (sham), 0.3, or 1.0 g. Whole-body bone mineral content (BMC) increased with time in 7 month (p < 0.001) but not 22 month (p = 0.34) mice, independent of WBV (p = 0.60). In 7 month mice, lower-leg BMC increased with time in 0.3 and 1.0 g groups (p < 0.005) but not in the sham group (p = 0.09), indicating a positive WBV effect. In 22 month mice, there were no changes with time in lower-leg BMC (p = 0.11). WBV did not affect tibial trabecular or cortical bone structure (by microCT), dynamic indices of trabecular or cortical bone formation, trabecular osteoclast surface, or the mass of the reproductive fat pad (p > 0.05). Each of these outcomes was diminished in 7 month versus 22 month animals (p < 0.05). In summary, 5 weeks of daily exposure to low-amplitude WBV had no skeletal effects in aged male mice. The potential of WBV to enhance bone mass in age-related osteoporosis is not supported in this preclinical study.

Type 1 diabetes in young rats leads to progressive trabecular bone loss, cessation of cortical bone growth, and diminished whole bone strength and fatigue life.

People with diabetes have increased risk of fracture disproportionate to BMD, suggesting reduced material strength (quality). We quantified the skeletal effects of type 1 diabetes in the rat. Fischer 344 and Sprague-Dawley rats (12 wk of age) were injected with either vehicle (Control) or streptozotocin (Diabetic). Forelimbs were scanned at 0, 4, 8, and 12 wk using pQCT. Rats were killed after 12 wk. We observed progressive osteopenia in diabetic rats. Trabecular osteopenia was caused by bone loss: volumetric BMD decreased progressively with time in diabetic rats but was constant in controls. Cortical osteopenia was caused by premature arrest of cortical expansion: cortical area did not increase after 4-8 wk in diabetic rats but continued to increase in controls. Postmortem muCT showed a 60% reduction in proximal tibial trabecular BV/TV in diabetic versus control rats, whereas moments of inertia of the ulnar and femoral diaphysis were reduced approximately 30%. Monotonic bending tests indicated that ulna and femora from diabetic animals were approximately 25% less stiff and strong versus controls. Estimates of material properties indicated no changes in elastic modulus or ultimate stress but modest ( approximately 10%) declines in yield stress for diabetic bone. These changes were associated with a approximately 50% increase in the nonenzymatic collagen cross-link pentosidine. Last, cyclic testing showed diminished fatigue life in diabetic bones at the structural (force) level but not at the material (stress) level. In summary, type 1 diabetes, left untreated, causes trabecular bone loss and a reduction in diaphyseal growth. Diabetic bone has greatly increased nonenzymatic collagen cross-links but only modestly reduced material properties. The loss of whole bone strength under both monotonic and fatigue loading is attributed mainly to reduced bone size.