Metabolic modulation of disuse osteopenia: endocrine-dependent site specificity of bone remodeling.

The remodeling response of bone tissue to disuse in four normal adult male turkeys and four adult males metabolically altered by castration was compared by functionally isolating the left ulna of each animal via transverse epiphyseal osteotomies. The right ulna in each animal was left intact and served as a control. After 8 weeks, the animals were euthanized, the ulnae harvested, and 100 microns undecalcified cross sections of the midshaft microradiographed. Areal properties, osteon mineral apposition rates from in vivo fluorochrome labels, and the number and ratios of bone-forming and bone-resorbing foci were quantitated. Compared to their control ulnae, the magnitude of bone resorbed from the functionally isolated ulnae of normal versus castrated males was not significantly different (-12.8 +/- 3.7 versus -10.7 +/- 3.5%, respectively). However, in the functionally isolated ulnae of normal birds, 94% of the total bone loss resulted from expansion of the corticoendosteal envelope, and 97% of the decrease in cross-sectional areas of the ulnae in the castrated birds was due to intracortical porosity. Furthermore, there was a significant interaction between disuse and castration, increasing the total number of intracortical remodeling events (9.4 +/- 0.9) when compared to disuse alone (4.7 +/- 1.4, p less than 0.01), or to the intact ulnae of castrated (2.1 +/- 0.5) and normal adult males (2.0 +/- 1.1). This work emphasizes that the manner in which the bone tissue responds to local changes in its physical environment is directly dependent on the status of the organism's metabolic milieu.