Bone's early responses to mechanical loading differ in distinct genetic strains of chick: Selection for enhanced growth reduces skeletal adaptability

Bone's functional competence is established and maintained, at least partly , by mechanisms involving appropriate adaptation to mechanical loading. The se appear to fail in chickens selectively bred either for maximum egg (Egg- type) or meat (Meat-type) production, which show high rates of fracture and skeletal abnormality, respectively. By measuring several early strain-indu ced responses in cultured embryonic tibiotarsi from commercially bred (Egg- type and Meat-type) and wild-type (Wild-type) chicks, we have investigated the possibility that these skeletal failures are the product of a compromis ed ability to respond appropriately to loading-induced mechanical strain. A xial loads engendering peak dynamic (1 Hz) longitudinal strains of between -1300 mu epsilon and -1500 mu epsilon (for 10 minutes) in vitro in tibiotar si from the three types of 18-day-old chicks increased periosteal osteoblas t glucose 6-phosphate dehydrogenase (G6PD) activity in both Wild-type (26%, p < 0.01) and Egg-type (49%, p < 0.001) chicks in situ, while Meat-type ch icks did not show any significant changes (11%). Load-induced increases in medium nitrite accumulation (stable nitric oxide [NO] metabolite) were prod uced in Egg-type and Wild-type tibiotarsi (82 +/- 12%, p < 0.01; 39 +/- 8%, p < 0.01), respectively. In contrast, loading, produced no change in NO re lease from Meat-type chick tibiotarsi. These changes in NO release correlat ed with load-related increases in G6PD activity (R-2 = 0.98, p < 0.05) in t he different chick types. Wild-type and Meat-type tibiotarsal periosteal os teoblasts responded in a biphasic manner to exogenous prostacyclin (PGI(2)) , with maximal stimulation of G6PD activity at 10(-7) M and 10(-6) M PGI(2) . However, Egg-type chick osteoblasts showed smaller, progressive increases up to 10(-5) M PGI(2). These results indicate that early phases of the ada ptive response to loading differ in different genetic strains of embryonic chick; that skeletal abnormalities which develop in genetically selected, h igh growth rate chicks may reflect a compromised ability to respond to load ; and that load-induced increases in osteoblastic G6PD. activity appear to be closely associated with increased rates of NO release. It is probable th at similar genetically related differences in bones' responsiveness to mech anical loading occur in other species.