1α-Hydroxyvitamin D2 and 1α-hydroxyvitamin D3 have anabolic effects on cortical bone, but induce intracortical remodeling at toxic doses in ovariectomized rats
Introduction
It is well established that vitamin D metabolites possess antiresorptive [1], [2], [3], and, at higher doses, anabolic properties [3], [4], [5], [6], [7], [8] on cancellous bone in rat models of estrogen deficiency-induced bone loss. However, the effects of vitamin D analogs on cortical bone are less well characterized. Although clinical studies in osteoporotic patients have given ambiguous results, with some studies showing positive effects on bone mass and fracture incidence [9], [10], [11], [12], whereas other trials reported negative results [13], [14], an improved understanding of the overall skeletal effects of active vitamin D metabolites is desirable because this substance class may be of interest for the development of future osteoporosis treatments. Vitamin D compounds are orally available, augment osteoblast team performance, suppress bone resorption and stimulate intestinal calcium absorption [15]. Thus, these drugs have a favorable pharmacological profile for osteoporosis treatment. However, the wide use of vitamin D metabolites in osteoporotic patients, especially at higher doses, has been hampered by the major side effects of this compound class, hypercalcemia and hypercalciuria, due to the excessive stimulation of intestinal calcium absorption. Therefore, the development of bone selective vitamin D analogs is a prerequisite for a potential bone anabolic treatment of osteoporotic patients with these substances, especially in countries with a higher calcium intake.
Rats lack intracortical remodeling activity under physiological conditions [16]. Therefore, the utility of the rat as a model for drug effects on cortical bone is certainly limited. Nevertheless, intracortical remodeling can be initiated in rats by various stimuli such as fatigue loading of long bones [17] or treatment with bone anabolic substances such as parathyroid hormone (PTH), insulin-like growth factor-I (IGF-I) or prostaglandins [18], [19], [20]. It is still controversial whether rats lose cortical bone after ovariectomy. Much of this controversy may arise from differences in the age at ovariectomy and the strain of rats used. Ovariectomy of young rats increases periosteal bone formation in long bones, resulting in bones of increased radial dimensions [21]. The stimulation of periosteal bone formation is much less pronounced when rats are ovariectomized (OVX) at later ages, although this finding may be strain dependent. In a well-powered experiment with 9-month-old Sprague–Dawley rats, Sato et al. [22] reported unchanged cross-sectional area in the tibial diaphysis of OVX rats 6 months postsurgery. Oxlund et al. [23] showed that ovariectomy of 12-month-old Wistar rats increased periosteal bone formation rate, but tibial cross-sectional area remained unchanged, 3 months postsurgery. Previous results from our laboratory have demonstrated that when female Fischer 344 rats are ovariectomized at 6 months of age, they clearly lose cortical bone within 4 months postsurgery [24].
Data on cortical bone effects of vitamin D analogs are scarce. The studies by Shiraishi et al. [3], [25] in OVX rats showed an increase in endocortical bone formation, increased bone mineral density (BMD) and improved mechanical properties of the femoral diaphysis in response to treatment with the synthetic vitamin D analog 1α-hydroxyvitamin D3 (1α(OH)D3). 1α(OH)D3 is a prodrug that is activated in the liver or in other tissues by 25-hydroxylation to form the naturally occurring vitamin D hormone 1α,25-dihydroxyvitamin D3 (1α,25(OH)2D3) [26]. Many animal studies have suggested that vitamin D2 and 1α-hydroxyvitamin D2 (1α(OH)D2) are less toxic in comparison with their corresponding vitamin D3 compounds, vitamin D3 and 1α(OH)D3 [27], [28], [29], [30]. In a study previously conducted in our laboratory [8] aimed at comparing the efficacy and toxicity of 1α(OH)D2 and 1α(OH)D3, we found a dose-dependent increase in total and cortical BMD of the tibial diaphysis in OVX rats treated with 1α(OH)D2 and 1α(OH)D3 at doses of up to 0.1 μg/kg/day, using peripheral quantitative computed tomography (pQCT). However, at the highest dose of 0.2 μg/kg/day, which was associated with hypercalcemia and hypercalciuria in the abovementioned study, cortical BMD declined to the vehicle-treated OVX control level, especially in 1α(OH)D3-treated rats. This finding prompted us to perform a detailed histological analysis of the tibial diaphysis in this study. The data provided by the current investigation show that 1α(OH)D2 and 1α(OH)D3 have anabolic properties on cortical bone in OVX rats, but activate intracortical remodeling when administered at toxic doses.
Section snippets
Animal procedures
Housing, feeding and administration of vitamin D metabolites were performed as previously described [8]. Eighty female 6-month-old Fischer-344 rats were ovariectomized and eight rats were sham-operated (SHAM). The animals were kept in pairs at 24°C with a 12-h light–dark cycle and were fed a powdered standard laboratory diet (Altromin, Lage, Germany) containing 0.9% calcium, 0.75% phosphorus and 600 IU/kg vitamin D3. The 80 OVX rats were allocated into nine weight-matched groups, receiving
Effects of ovariectomy
By the end of the experiment, that is, 3 months postsurgery, vehicle-treated OVX rats showed a significant loss of tibial cortical bone relative to SHAM controls. The relative cortical bone area (%Ct.Ar) was reduced in OVX rats, mainly due to an expansion of the marrow cavity as measured by an absolute and relative (%Ma.Ar) increase in marrow area (Table 1, Fig. 1, Fig. 2). Total cross-sectional area, as well as periosteal and endocortical BFR did not significantly differ between SHAM and
Discussion
The results of the current study have confirmed previous findings that aged OVX rats lose cortical bone by increased endocortical bone resorption [22], [23], [24]. Although post-OVX enlargement of the marrow cavity is a universal finding independent of the age at ovariectomy [21], [22], [23], [24], true loss of cortical bone occurs only in aged OVX rats, when the periosteal modeling drifts present in young rats have subsided. In young OVX rats, the endocortical bone loss is compensated by
Acknowledgements
The authors wish to thank Claudia Bergow and Siglinde Hirmer for excellent technical assistance. 1α(OH)D2 and 1α(OH)D3 were a gift of Dr. Joyce C. Knutson, Bone Care International, Madison, WI, USA.
References (36)
- et al.
The ultra long-term treatment of senile osteoporosis with 1α-hydroxyvitamin D3
Bone Miner
(1993) - et al.
Intracortical remodeling in adult rat long bones after fatigue loading
Bone
(1998) - et al.
Comparison of the effects of intermittent and continuous administration of human parathyroid hormone(1–34) on rat bone
Bone
(1995) - et al.
Low-intensity, high-frequency vibration appears to prevent the decrease in strength of the femur and tibia associated with ovariectomy of adult rats
Bone
(2003) - et al.
A comparison of the toxicity of ergocalciferol and cholecalciferol in rhesus monkeys (Macaca mulatta)
J. Nutr
(1972) - et al.
Vitamin D metabolites prevent vertebral osteopenia in ovariectomized rats
Calcif. Tissue Int
(1992) - et al.
Short-term prophylaxis against estrogen depletion-induced bone loss with calcitriol does not provide long-term beneficial effects on cancellous bone mass or structure in ovariectomized rats
Osteoporos. Int
(1998) - et al.
Alfacalcidol inhibits bone resorption and stimulates formation in an ovariectomized rat model of osteoporosis: distinct actions from estrogen
J. Bone Miner. Res
(2000) - et al.
Prophylactic effects of 1,24,25-trihydroxyvitamin D3 on ovariectomy-induced cancellous bone loss in the rat
Calcif. Tissue Int
(1997) - et al.
Short-term treatment of rats with high dose 1,25-dihydroxyvitamin D3 stimulates bone formation and increases the number of osteoblast precursor cells in bone marrow
Endocrinology
(1997)
Therapeutic efficacy of 1α,25-dihydroxyvitamin D3 and calcium in osteopenic ovariectomized rats: evidence for a direct anabolic effect of 1α,25-dihydroxyvitamin D3 on bone
Endocrinology
Prevention of bone loss in ovariectomized rats by combined treatment with risedronate and 1α,25-dihydroxyvitamin D3
J. Bone Miner. Res
1α-hydroxyvitamin D2 is less toxic but not bone selective relative to 1α-hydroxyvitamin D3 in ovariectomized rats
J. Bone Miner. Res
Treatment of postmenopausal osteoporosis with calcitriol or calcium
N. Engl. J. Med
Treatment of postmenopausal osteoporosis with high doses of synthetic calcitriol. A randomized controlled study
Ann. Intern. Med
Combination treatment with estrogen and calcitriol in the prevention of age-related bone loss
J. Clin. Endocrinol. Metab
Calcitriol treatment is not effective in postmenopausal osteoporosis
Ann. Intern. Med
Postmenopausal osteoporosis: no effect of three years treatment with 1,25-dihydroxycholecalciferol
Acta Med. Scand
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