Enhanced osteoblast development after continuous infusion of hPTH(1-84) inthe rat

Rats and humans respond to intermittent treatment with parathyroid hormone (PTH) with increased bone density and cancellous bone volume. In the rat, o steoblast expression of insulin-like growth factor-I (IGF-I) is elevated by intermittent PTH, We examined the effect of continuous infusion of rhPTH(1 -84), a bone catabolic regime, on the IGF system in rat pelvis. Female Spra gue-Dawley rats (12 weeks, 250 g) were randomly assigned to receive 0, 0.1, 1, or 5 mu g/100 g body weight (b.w.) rhPTH(1-84) (0, 0.106, 1.06, or 5.30 5 nmol/kg) in vehicle (1% normal rat serum in saline) delivered by subcutan eous Alzet minipump, After 7 days, blood was taken for serum chemistry and pelvises were processed for immunocytochemistry. Sections of pelvis from ra ts continuously infused with 0.1 or 1 mu g/100 g b.w. rhPTH(1-84) for 7 day s did not differ significantly from those of the vehicle-treated controls. However, continuous infusion of 5 mu g/100 g b.w. rhPTH(1-84) resulted in a dramatic increase in cellular development, with trabeculae surrounded by m any layers of large, plump osteoblasts. All pelvis osteoblasts expressed os teocalcin, but only those from rats that received 0, 0,1, or 1 mu g/100 g b .w. rhPTH(1-84) showed positive staining for IGF-I. The extra-abundant oste oblasts from rats that received 5 mu g/100 g b.w. rhPTH(1-84) did not stain for IGF-I. However, although all osteoblasts stained positively for IGF bi nding proteins (IGFBPs)-3, -4, and -5, staining for these IGFBPs increased as the dose of rhPTH(1-84) (and osteoblast number) increased. These results suggest that continuous infusion of PTH has a direct effect on osteoblast development (either recruitment or proliferation), decreases the expression of IGF-I, and enhances the expression of IGFBPs in pelvis, factors which m ay interact to bring about negative bone balance. (Bone 24:89-94; 1999) (C) 1999 by Elsevier Science Inc. All rights reserved.