Evaluation of the skeletal effects of combined mild dietary calcium restriction and ovariectomy in sinclair S-1 minipigs: A pilot study
Authors: Mosekilde Lis , Weisbrode Steven E. , Safron Joseph A. , Stills Harold F. , Jankowsky Maryann L. , Ebert Deb C. , Danielsen Carl Christian , Søgaard Charlotte H. , Franks Alan F. , Stevens Mary L. , Paddock Carol L. , Boyce Rogely Waite 
Published: February 2010

Abstract
A pilot study was conducted to investigate the combined effects of ovariectomy (OVX) with preceding and concomitant mild dietary calcium restriction on the minipig skeleton. Minipigs 4 months old were fed diets containing 0.9, 0.75, or 0.5% calcium (Ca). At 10 months, the 0.75 and 0.5% pigs were OVX and the 0.9% were either sham operated or OVX. All pigs were maintained on their respective diets for an additional 6 months. Excised lumbar vertebrae and long bones were evaluated by densitometry and histomorphometry, and vertebral cancellous bone samples were tested biomechanically. In pigs fed the 0.9% Ca diet, OVX alone effected decreases of 6% in vertebral bone mineral density (BMD), 15% in trabecular bone volume (BV/TV), and 13% in trabecular number (Tb.N), an increase of 15% in trabecular separation (Tb.Sp), and a nonsignificant increase (p &lt; 0.056) in vertebral cancellous final erosion depth (F.E.De) compared with the 0.9% Ca sham-operated group. Decreasing dietary Ca to 0.5% in combination with OVX effected an 8% reduction in vertebral BMD that was not associated with any significant alterations in parameters of vertebral cancellous bone microstructure or remodeling compared with the 0.9% Ca sham-operated pigs. Increases in serum PTH noted in the 0.5% Ca OVX group were generally paralleled by increases in calcitriol. In OVX pigs fed a diet containing 0.75% Ca, a 10% reduction in vertebral BMD was observed. This was associated with significant increases in F.E.De and vertebral marrow star volume (Ma.St.V) compared with the 0.9% Ca sham-operated pigs and the other OVX groups. In addition, Tb.Sp was increased and Tb.N decreased compared with the 0.9% Ca sham-operated pigs. Increases in serum PTH in this group were not accompanied by increases in calcitriol. Midradial and midfemoral BMD values were reduced in the 0.75 and 0.5% Ca OVX groups compared with the 0.9% Ca sham-operated pigs. Histomorphometric analyses of cortical bone suggested the reduction in cortical bone mass in the 0.75% Ca OVX group may have been largely due to net loss on the endocortical surface versus possible failure to accrue bone in the 0.5% Ca OVX group. Ash density and biomechanical parameters for vertebral cancellous bone decreased progressively in the 0.9% sham-operated, 0.9% Ca OVX, and 0.75% Ca OVX groups and then increased in the 0.5% Ca OVX group. After normalization for bone mass (ash), mechanical changes were still apparent, particularly for the 0.75% Ca OVX group compared with other OVX groups, reflecting that structural changes had taken place in the trabecular network. In minipigs, OVX results in increases in resorptive cell function at the level of the remodeling unit that effect minor alterations in cancellous bone microstructure. Imposing moderate calcium restriction (0.5%) in combination with OVX appears to blunt rather than exaggerate resorptive cell function in vertebral cancellous bone, essentially preserving microstructure; this effect may be a consequence of the combined increase in PTH and calcitriol. OVX in combination with mild dietary calcium restriction (0.75%) results in excessive resorptive cell function at the level of the remodeling unit associated with vertebral cancellous osteopenia and reduced biomechanical competence of vertebral cancellous bone. The reduction in cancellous bone in this group appears to be due, at least in part, to trabecular perforation. The alterations in remodeling may be a consequence of elevated PTH unopposed by calcitriol. The calcium-restricted (0.75%) Sinclair S-1 OVX minipig appears to be a model of primarily cancellous osteopenia associated with increased resorptive cell function at the level of the remodeling unit and trabecular plate perforation. This animal model may be valuable for studying the consequences of alterations of remodeling and resorptive cell function on cancellous bone microstructure and strength and may be useful for modeling certain aspects of perimenopausal bone loss in women.