In vivo anabolic effects of parathyroid hormone (PTH) 28-48 and N-terminal fragments of PTH and PTH-related protein on neonatal mouse bones.

We developed a neonatal mouse model to investigate in vivo anabolic effects of intact PTH (1-84) and its two fragments PTH (1-34) and PTH (28-48) and of the N-terminal fragment of PTH-related peptide [PTHrP (1-34)]. Two-day-old mice were injected with low-dose (0.05 microg/g body weight) and high-dose (0.2 microg/g body weight) of each of these peptides daily for 6 or 16 consecutive days. Long bones (tibias and femurs) and mandibular condylar cartilages were harvested. Total DNA and protein were analyzed as parameters for anabolic effects. DNA was increased significantly in tibias only by low doses of PTH (1-84) and PTH (1-34), but by both doses of PTH (28-48). In the cartilages of the mandibular condyles, both doses of all three peptides increased DNA. Total protein was increased in the tibia by the low dose of the three peptides, whereas in the condylar cartilage high doses of PTH (1-34) and PTH (28-48) also caused a 2- to 4-fold increase. When the effects of PTH (1-34) and PTHrP (1-34) on the tibias were compared, it became apparent that PTH (1-34) was more effective than PTHrP (1-34) when injected in low doses, but the latter caused a severalfold increase in DNA and protein at both doses. The outstanding anabolic effect of PTH (28-48) was further investigated using [3H]thymidine autoradiography, analysis of insulin-like growth factor I (IGF-I) protein, and localization of IGF-I messenger RNA (mRNA) by in situ hybridization. PTH (28-48) increased by 3-fold the number of [3H]thymidine-labeled cells in the epiphyseal cartilage of tibias removed from 8-day-old injected mice, and in the proliferative zone of the epiphyseal growth plate of tibias removed from 18-day-old injected mice. Femurs from the latter showed a 20% increase in their IGF-I content. In parallel, only tibias from 18-day-old injected mice showed IGF-I mRNA localization in proliferating chondrocytes, whereas those from vehicle-injected control mice did not exhibit IGF-I mRNA. In summary, our study showed that the neonatal mouse is a sensitive model to examine anabolic effects of different PTH and PTHrP fragments. It also revealed that PTH (28-48) has strong anabolic effects on this model, and suggests that IGF-I might mediate the anabolic effects of PTH (28-48).