Impaired Growth , Delayed Ossification , and Reduced Osteoclastic Activity in the Growth Plate of Calcium-Supplemented Rats with Renal Failure *

Linear growth is reduced in prepubertal children with adynamic renal osteodystrophy, suggesting that the proliferation and/or differentiation of epiphyseal growth plate chondrocytes is abnormal in this disorder. To examine this issue, in situ hybridization and histochemistry were used to measure selected markers of endochondral bone formation and bone resorption in the proximal tibia of subtotally nephrectomized rats fed a high calcium diet to induce biochemical changes consistent with adynamic osteodystrophy. Blood ionized calcium concentrations were higher and serum PTH levels were lower in nephrectomized, calcium-supplemented rats than in either intact or nephrectomized control animals. Linear growth and tibial length were reduced, but messenger RNA levels for type II collagen, type X collagen, and the PTH/PTHrP receptor did not differ from control values in nephrectomized rats given supplemental calcium. In contrast, both the width of epiphyseal cartilage and the height of the zone of hypertrophic chondrocytes were greater in calcium-supplemented nephrectomized rats. These morphological changes were associated with decreases in histochemical staining for tartrate-resistant acid phosphatase and lower levels of messenger RNA expression for the matrix metalloproteinase MMP-9/gelatinase B immediately adjacent to the epiphyseal growth plate. Diminished chondroclastic/osteoclastic activity alters growth plate morphology and adversely affects linear bone growth in calcium-supplemented, nephrectomized rats. (Endocrinology 141: 1536–1544, 2000) I THE PAST, chronic renal failure almost invariably led to the development of secondary hyperparathyroidism, but adynamic renal osteodystrophy is now a common skeletal disorder in patients with end-stage renal disease (1, 2). Serum PTH are normal or only modestly elevated in those with adynamic skeletal lesions, and bone formation is diminished (1, 2). In contrast, serum PTH levels are high in patients with secondary hyperparathyroidism, and the rates of bone formation and turnover are elevated (1, 2). The long-term consequences of adynamic renal osteodystrophy have yet to be determined. Hypercalcemia is more common, however, in patients with adynamic bone than in those with secondary hyperparathyroidism (3, 4). Preliminary work also suggests that the risk of fracture is greater in adult hemodialysis patients with adynamic lesions (5, 6). In the pediatric age group, linear growth has recently been reported to decline in prepubertal children who develop adynamic renal osteodystrophy during the treatment of secondary hyperparathyroidism with large intermittent doses of calcitriol (7). Such findings suggest that adynamic bone is associated with abnormalities in chondrocyte proliferation and/or differentiation within the epiphyseal growth plate at the ends of long bones in the developing skeleton. Whether mediated through direct or indirect mechanisms, disturbances in chondrocyte function may aggravate the growth retardation that characterizes children with renal failure (8, 9). Factors associated with low serum PTH levels and adynamic renal osteodystrophy in adults include previous parathyroidectomy, age-related or postmenopausal osteoporosis, diabetes mellitus, and the use of large doses of calcitriol to treat secondary hyperparathyroidism (10, 11). In most patients, however, including children, the ingestion of large amounts of calcium to diminish intestinal phosphorus absorption and the use of high dialysate calcium concentrations are major contributors (10). The current study was undertaken, therefore, to test the hypothesis that a high dietary calcium intake lowers serum PTH levels and modifies chondrocyte proliferation and/or differentiation in the epiphyseal growth plate of growing animals with renal failure and to determine whether these changes are associated with impaired linear bone growth. Materials and Methods Forty-four weanling male Sprague Dawley rats weighing 60–70 grams (Harlan Sprague Dawley Laboratories, Indianapolis, IN) were obtained at 3 weeks of age and housed in cages at constant room temperature with a 12-h light, 12-h dark cycle. All study procedures were reviewed and approved by the Animal Subjects Protection Committee at UCLA. Animals were given free access to food and drinking water, and they were maintained on standard rat chow containing 23.4% Received September 17, 1999. Address all correspondence and requests for reprints to: William G. Goodman, M.D., Division of Nephrology, 7–155 Factor Building, UCLA Medical Center, 10833 Le Conte Avenue, Los Angeles, California 90095. E-mail: wgoodman@ucla.edu. * This work was supported, in part, by USPHS Grants DK-52905, DK-35423, DK-56688, RR-00865, and M01-RR-00865, by a grant from the Genentech Foundation for Growth and Development, and by funds from the Casey Lee Ball Foundation. † Current address: Room 3590 MSC, 1300 University Avenue, University of Wisconsin School of Medicine, Madison, Wisconsin 53706. 0013-7227/00/$03.00/0 Vol. 141, No. 4 Endocrinology Printed in U.S.A. Copyright © 2000 by The Endocrine Society