GROWTH-HORMONE AND MILD EXERCISE IN COMBINATION MARKEDLY ENHANCE CORTICAL BONE-FORMATION AND STRENGTH IN OLD RATS

The effects of a combination of mild exercise and GH injections on bon e were studied in old female rats. Biosynthetic human GH, 2.7 mg/kg/da y, was injected sc for 73 days. Exercised rats ran 8 m/min on a treadm ill for 1 h/day. All rats (age 21 months old) were labeled with a tetr acycline injection 56 days and a calcein injection 11 days before kill ing. The GH injections resulted in an 11-fold increase in femoral midd iaphyseal bone formation rate and a 12% increase in cross-sectional ar ea compared with the saline-injected group. The mild exercise doubled the mineralizing surface but did not influence the bone formation rate significantly. The combination of GH injections plus exercise, howeve r, resulted in a further increase of 39% in bone formation rate, prima rily at the anterolateral aspects, and an increase of 5% in cross-sect ional area compared with the group injected with GH only. The femur ul timate breaking load was increased by 37% and the stiffness by 42% in the group injected with GH compared with the saline-injected group. Ex ercise alone did not influence the femur mechanical properties. The co mbination of GH injections plus exercise induced a 4% further increase in ultimate breaking load and 7% further increase in stiffness compar ed with the group injected with GH alone. The GH injections induced a 117% increase in serum insulin-like growth factor I. The GH-insulin-li ke growth factor I axis stimulates recruitment of osteoblast precursor cells, resulting in increased bone formation at the periosteal surfac e. GH injections and mild excercise in combination modulate and increa se further the formation and strength of cortical bone in old female r ats.