Local Anabolic Effects of Growth Hormone on Intact Bone and Healing Fractures in Rats

The local effects of rat growth hormone (rGH) injected at the surfaces of intact tibial diaphyses and healing tibial diaphysial fractures were investigated in 10-month-old female rats. Intact diaphysial bones: rats were injected daily for 14 days with vehicle, 2 µg rGH, or 20 µg rGH at the surface of the right tibial diaphysis. After 10 days of injection the animals were labeled with calcein. At the rGH-injected location, increased external diaphysial bone dimensions and increased calcein-labeled area were seen, and the responses to rGH were dose dependent. The new bone formed at the periosteal surface was woven bone. At the opposite left tibia, no systemic effect of rGH was found. rGH did not influence body weight changes during the treatment, or muscle mass or serum IGF-I at the end of the treatment. Healing diaphysial fractures: a closed fracture was made in the right tibial diaphysis, and stabilized by medullary nailing. The fractures were tested after 21 days and 98 days of healing. During the first 21 days of healing, all rats were injected daily with either vehicle or 20 µg rGH at the surface of the fracture line. In the 21-day healing rGH group, ultimate load, ultimate stiffness, external callus dimensions, and external callus volume of the fractures were increased. rGH did not affect body weight changes during this healing period or serum IGF-I at the end of the healing period. In the 98-day healing rGH group, ultimate load was still increased compared with the vehicle group, although a ninefold increase took place in the vehicle group between days 21 and 98 of healing. External callus dimensions of the fractures were increased in the rGH group, whereas body weight changes during the healing period were not affected.