THE EFFECTS OF 1,25-DIHYDROXYCHOLECALCIFEROL, PARATHYROID-HORMONE, AND THYROXINE ON TRABECULAR BONE REMODELING IN ADULT DOGS - A HISTOMORPHOMETRIC STUDY

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  • 1 January 1981
    • journal article
    • research article
    • Vol. 105  (3) , 279-287
Abstract
The effects of 1,25-dihydroxycholecalciferol (1,25-(OH)2D3), parathyroid hormone (PTH), and L-thyroxine (T4) on trabecular bone remodeling were evaluated by histomorphometric methods in adult female beagle dogs. 1,25-(OH)2D3 (1.25 .mu.g/day, i.v., in equally divided doses) was administered intermittently for 6 days and withdrawn 14 days for 3 complete cycles. PTH was administered i.v. (2.5 U/kg per day) in divided doses 6 h apart for 60 days. T4 was given orally (1.0 mg/kg per day) in divided doses for a similar interval. Static and dynamic changes were evaluated using tetracycline and DCAF (2,4 BIS) N,N'',-Di(carboxymethyl)aminomethyl fluorescein in vivo double labeling of bone from the iliac crest taken before treatment and after 60 days. The intermittent administration of 1,25-(OH)2D3 stimulated the bone resorption rate and depressed the formation rate. 1,25-(OH)2D3 increased trabecular resorption surfaces; osteoid surface, volume, and thickness; mineralization lag time; and osteoblast number but decreased the bone volume. Multiple small daily doses of PTH resulted in an overall negative balance in trabecular bone. This was associated with an increased trabecular surace-to-volume ratio, bone resorption and formation rates, active forming surfaces, osteoid volume and surface, life span of bone forming and resorbing sites, and the number of osteoclast nuclei. T4 appeared to increase bone mass by enhancing the switch-over from the resorptive to the formative phase of remodeling. Coupling between osteoid apposition and mineralization was increased by recruiting more forming sites and prolonging their life span. T4 increased bone resorption and formation rates, trabecular bone volume and balance, number of osteoclast nuclei, and life span of bone forming sites. The osteoid seam thickness and mineralization lag time were decreased. The present study demonstrated that 1,25-(OH)2D3, PTH and T4 at the dose and schedule used markedly altered stimulators of remodeling in trabecular bone of adult dogs.