Calvarial and limb bone cells in organ and monolayer culture do not show the same early responses to dynamic mechanical strain

Abstract

Responses to mechanical strain in calvaria and limb bone organ cultures were compared by measuring cellular glucose 6-phosphate dehydrogenase (G6PD) activity in situ and prostaglandin release. Normal functional strains were recorded in the ulnae (1000 μϵ) and calvarium (30 μϵ) in vivo in 110 g rats. Organ cultures of ulnae and calvaria from similar animals were loaded to produce dynamic strains (600 cycles, 1 Hz) of 1000 μϵ in the ulna, and 100 or 1000 μϵ in calvaria. In ulnae, both PGE₂ and PGI₂ were released and resident osteocytes and osteoblasts showed increased G6PD activity. Neither response was seen in calvaria. However, exogenous PGI₂ (10⁻⁵−10⁻⁹ M) stimulated G6PD activity in osteocytes and osteoblasts in organ cultures of both calvaria and ulnae. In ulnar cells the response was linear, in calvarial cells it was biphasic with maximum activity at 10⁻⁷ M. Osteoblasts derived from ulnae and cultured on plastic plates subjected to dynamic strain (600 cycles, 1 Hz, 4000 μϵ) showed increased G6PD activity. There was no such response in similarly treated calvarial-derived cells. Calvarial bone cells differ from those of the ulna in that they do not respond to physiological strains in their locality with increased prostanoid release or G6PD activity either in situ or when seeded onto dynamically strained plastic plates. Cells from both sites in organ culture show increased G6PD activity in response to exogenous PGI₂, but their dose: Responses differ in shape. These differences may reflect the extent to which functional loading influences bone architecture in these two sites.