Effects of immobilization, three forms of remobilization, and subsequent deconditioning on bone mineral content and density in rat femora.

Disuse is associated with bone loss, which may not be recoverable. It is not known whether intensified remobilization is beneficial in restoring disuse-related bone loss nor if any such benefit would depend upon continuing mobilization for its maintenance. After an immobilization period of 3 weeks, the effects of free remobilization (11 weeks), and low-and high-intensity treadmill running (11 weeks) with and without subsequent deconditioning (18 weeks) on the bone mineral content (BMC) and density (BMD) of the hindlimb femora of Sprague-Dawley rats (n = 98) were studied using a dual-energy X-ray absorptiometric (DXA) scanner. Our hypothesis was that intensified remobilization is beneficial in restoring the BMC and BMD from disuse to normal while subsequent deconditioning is deleterious to these parameters. Immobilization for 3 weeks produced a significant BMC and BMD loss in the immobilized left femur (range -4.4 to -12.8%; p < 0.05-0.001). In the groups with free remobilization (free cage activity), the body weight-adjusted BMCs and BMDs always remained below those in the controls (range -2.3 to -12.1%; p values ranging from NS to < 0.01). Both low- and high-intensity running restored BMC and BMD in the immobilized limb, the effect being better in the latter group. In both of these groups, the values of the immobilized left limbs and those of the free right limbs exclusively exceeded the corresponding values of the age-matched control rats (left limb values 3.0-21.1% higher with p values ranging from NS to < 0.01; right limb values 7.9-21.4% higher with p < 0.05-0.01). However, after the deconditioning period of 18 weeks, the above described beneficial effects of low- and high-intensity running were lost, the left and right limb BMC and BMD values being lower than those in the age-matched controls (range -3.8 to -8.7%; p values ranging from NS to < 0.05). In conclusion, this study clearly indicates the need for greater than normal activity to restore the BMC and BMD after disuse to normal levels. However, the benefits of intensified remobilization are lost if the activity is terminated, and therefore, after immobilization and disuse, bone loading activities should be continued, perhaps indefinitely.

Expression of osteocalcin in the patella of experimentally immobilized and remobilized rats.

The exact mechanisms by which mechanical loading-unloading affects bone tissue are mostly unknown. Recently, osteocalcin, a direct product of osteoblasts, has been shown to reflect the activity of the mineralization phase of the newly formed bone matrix, and therefore, the in situ detection of osteocalcin could be used for studying the effects of physical activity-inactivity on the osteoblast function or bone formation in the target bone. In this study, the effect of various loading states (immobilization and three forms of subsequent remobilization) on the in situ expression of osteocalcin in the rat patellas and their osteotendinous junctions was studied immunohistochemically using a polyclonal rat antiosteocalcin as the primary antibody. Following immobilization for 3 weeks, the immunoreactivity of osteocalcin was markedly decreased or was completely absent in all the patellar areas which normally show intense reaction as a sign of mineralization of the newly formed bone, that is, in the subperiosteal and subchondral regions, in the osteoid tissue that lies on the surface of the trabecular bone, and around the cortical lacunae. The same was true in the mineralized fibrocartilage zone of the osteotendinous junction of the quadriceps tendon. Free remobilization for 8 weeks (free cage activity) could not improve the situation, but after intensified remobilization of the same duration (low and especially high intensity treadmill running) high osteocalcin expression was observed in the above-mentioned anatomic sites. These findings indicate that formation of new bone tissue is rapidly regulated by the loading states of the bone. Higher than normal activity seems to be needed to restore the bone formation from the disuseinduced depression to normal.

Vascular density at the myotendinous junction of the rat gastrocnemius muscle after immobilization and remobilization.

Muscle injuries often occur at or near the myotendinous junction. Immobilization decreases the tensile strength of the myotendinous junction and predisposes it to strain injury. However, there are no data available on whether physical training or remobilization can lower the susceptibility of the myotendinous junction to strain injuries. We investigated the effects of three different remobilization programs (8 weeks) after immobilization (3 weeks) by evaluating the vascular density at the myotendinous junction of the rat gastrocnemius muscle. The myotendinous junctions had a portal system vascularity, or capillary-arteriole-capillary system, which probably protects the blood supply against pathologic conditions. The vascular density at the myotendinous junction decreased about 30% after immobilization (P < 0.001). After free cage remobilization for 8 weeks, the mean vascular density returned to the level of the controls. After progressively increasing running programs the vascular density was slightly higher in the immobilized myotendinous junction and about 50% higher than controls in the contralateral myotendinous junction (P < 0.001). The capacity of the vascular bed of the rat myotendinous junction to recover from immobilization atrophy seems to be good. Progressively increasing physical training improves the process of revascularization and probably protects an injured and immobilized muscle from reinjury.