Bone cell kinetics and function during experimental uremia

Role of dietary phosphorus and degree of uremia in the development of renal bone disease in rats

The remnant kidney rat model has been extensively used for the evaluation of bone changes due to uremia. The present study aimed to assess the effect of the dietary phosphorus availability and of the severity of renal failure on bone histomorphometric changes and various biochemical markers over time in this model. Chronic renal failure (CRF) was induced in male Wistar rats by 5/6th nephrectomy. Half of the number of animals received a standard rat diet (STD) (0.67% P, containing low bioavailable phosphorus of plant origin); the other animals were fed a high phosphorus diet (HPD) (0.93% P, containing inorganic phosphorus with high bioavailability). Every two weeks, blood and urine samples were collected. At sacrifice after 6 or 12 weeks, bone samples were taken for the measurement of histological and histodynamic parameters. Serum creatinine measurements indicated the development of mild to moderate renal failure in both diet groups. Phosphaturia was unexpectedly low in all animals that received the STD, indicating relative phosphorus depletion despite the normal dietary phosphorus content. In the HPD CRF group, a decrease in calcemia and a rise in phosphatemia were seen after 12 weeks of CRF, which were more pronounced in animals with higher serum creatinine. Serum iPTH levels were distinctly increased in CRF rats fed a HPD, especially those with more pronounced renal failure. Serum osteocalcin and to a lesser extend tartrate-resistant acid phosphatase and urinary pyridinoline and deoxypyridinoline crosslinks were higher in the CRF animals compared to the shams, particularly in the animals of the HPD group with more pronounced CRF. In both diet groups, the CRF animals had significantly higher amounts of osteoid compared to shams. Only the animals that received a HPD developed distinct histological signs of secondary hyperparathyroidism (sHPTH), that is, an increased bone formation rate, mineral apposition rate, osteoblast perimeter, and eroded perimeter. Again, this effect was most prominent in rats with more severe CRF. In conclusion, data of the present study indicate that in experimental studies using the remnant kidney rat model, both the dietary phosphorus bioavailability and the degree of renal failure in the development of hyperparathyroidism should be considered.

The influence of early parathyroidectomy on aluminum-induced rickets in growing uremic rats

Rats were subjected to a two-stage 5/6 nephrectomy and treated with aluminum for 2 and 4 weeks with a cumulative dose of 4.2 and 8.4 mg of aluminum, respectively. Other animals were parathyroidectomized and loaded with 8.4 mg of aluminum for 4 weeks. Histomorphometry and electron microscopy (tibiae), aluminum tissue (bone, kidney, liver) determination, serum (Ca, Mg, Zn, P, urea, creatinine, alkaline phosphatase, 1,25(OH)2D3, PTH) and urine (creatinine, A1) revealed that: (a) a dose of 8.4 mg aluminum was sufficient to induce rickets within 4 weeks of treatment and was associated with decreased serum calcitriol values and high aluminum accumulation within organs (electron-dense material was found in osteoblasts only); (b) previous parathyroidectomy prevented the occurrence of any aluminum-induced alteration of bone. It was associated with higher calcitriol and phosphorus values than in corresponding non-parathyroidectomized rats and significantly reduced aluminum accumulation within organs. The results was influenced neither by a drop in serum calcium values nor by different degrees of renal failure. We suggest that aluminum-induced rickets in growing uremic rats is prevented or delayed when previous parathyroidectomy has been performed.

Cellular basis of inflammation-induced osteopenia in growing rats

Local nonosseous inflammation provoked by four subcutaneous talc powder injections induced a marked trabecular bone loss in rats within 7 days. The disturbance included suppression of bone elongation, inefficiency and decreased number of trabecular osteoblasts, decreased osteoprogenitor cell number in tibial metaphyses, and bone marrow expansion. Neither the appearance and function of osteoblasts in the vicinity of the cortical bone nor the number of osteoclasts in the metaphysis were found to be altered. The loss of trabecular bone in granulomatosis was based on a suppression of bone elongation and a failure of osteoblasts to form normal secondary spongiosa.

Effects of 1 alpha,25- and 24R,25-dihydroxyvitamin D3 on aluminum-induced rickets in growing uremic rats

Rats were subjected to a two-stage subtotal nephrectomy or sham operation, and treated with aluminum (Al) or both aluminum and vitamin D3 metabolites for 5 weeks with a cumulative dose of 13.6 mg aluminum. Animals were injected with 3H-thymidine and 3H-proline. The following analyses were performed: quantitative histology of tibial metaphyses and cytomorphometric electron microscopy of osteoclasts, quantitative (ICP-spectroscopy) and qualitative determination (histochemical staining) of aluminum within organs, and serum biochemistry (Ca, P, Mg, vitamin D3 metabolites, alkaline phosphatase, urea). The following new facts of the aluminum-related bone disease became evident: (a) Application of aluminum to growing uremic rats induced rickets, whose major epiphyseal growth plate changes were 1 alpha,25(OH)2D3-dependent. Addition of 1 alpha,25(OH)2D3 prevented the formation of rachitic metaphysis, but failed to prevent osteoid accumulation on epiphyseal and metaphyseal trabecular surfaces. Moreover, calcitriol produced hyperosteoidosis and osteosclerosis in the same rats. Aluminum did not alter the function of osteoblasts, while osteoclasts seemed inactivated. (b) The development of rickets was associated with suppressed serum levels of 1,25(OH)2D3, reduced phosphorus level and the high content of aluminum in the bone, kidney, and liver. The addition of 24R,25(OH)2D3 markedly exaggerated the reduction of serum levels of calcitriol. We suggested that aluminum induces rickets in growing uremic rats, which consists of two components: vitamin D refractory osteomalacia and 1 alpha,25(OH)2D3-dependent epiphyseal growth plate changes.

The role of bone cells in increasing metaphyseal hard tissue in rapidly growing rats treated with prostaglandin E2

The skeletal effects of graded doses of prostaglandin E2 (PGE2) given to weanling Sprague-Dawley rats for 3 weeks were investigated to elucidate the role of bone cells in increasing hard tissue mass. Decalcified (3 micron) sections were quantified in the light microscope by point hit and intersect counting using a Merz grid. Hard tissue mass (bone and calcified cartilage) and osteoblast, osteoclast and osteoprogenitor cell numbers were counted in metaphyseal tissue bands 0.24, 0.48, 0.72, 1.20, 1.68, 2.16, 2.64, 3.12, 3.60 and 4.08 mm from the growth plate metaphyseal junction. Changes were different and more marked in the secondary spongiosa than the primary spongiosa of the proximal tibial metaphysis of treated rats. In the primary spongiosa of rats treated with 3 or 6 mg PGE2/kg/d (1) an increase in bone and hard tissue masses and (2) a decrease in osteoclasts, osteoprogenitor cell numbers and surface to volume ratio was observed. In the secondary spongiosa (lower metaphysis) of rats treated with 2 same dose levels (1) an increase in bone mass, calcified cartilage cores, and hard tissue mass and perimeter, an elevation of osteoprogenitor cell and osteoblast numbers, a depression of osteoclast, osteoclast nuclei numbers and surface to volume ratio and new sites of intramembranous ossification (woven bone formation) originating from the cortico-endosteal envelope was observed. In this growing rat skeletal model, we showed that PGE2 increases metaphyseal calcified tissue mass by depressing hard tissue resorption and stimulating the replication and differentiation of osteoblast precursors to form new foci of woven bone.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of 16,16-dimethyl prostaglandin E2 methyl ester on weanling rat skeleton: daily and systemic administration

The effects of 0, 0.3, 1.0, and 3.0 mg of 16,16-dimethyl prostaglandin E2 methyl ester (Di-M-PGE2) per kilogram per day administered subcutaneously for 21 days to fluorescent-labeled weanling rats were studied, by single-photon absorptiometric and static and dynamic histomorphometric techniques, to determine possible alterations in growth and mineralized tissue mass and their mechanisms of response. Specimens of femurs, proximal tibia, and tibial shaft were analyzed. Di-M-PGE2 caused a reduction in bone elongation and a dramatic accumulation in metaphyseal trabecular hard tissue mass. At high doses, the growth cartilage exhibited reduced thickness and degenerative cell size and cell production rate. The increased metaphyseal trabecular hard tissue mass was restricted to the secondary spongiosa region and was observed at all dose levels. The metaphysis was further characterized by an increase in bone and calcified cartilage cores, a marked elevation in osteoblast and osteoclast numbers, in osteoblast-to-osteoclast ratios, and in ratios of differentiated cells to osteoprogenitor cells. These findings were consistent with the interpretations that Di-M-PGE2 depressed bone elongation by delaying the division and maturation of growth plate chondrocytes; stimulated the differentiation of osteoblasts and osteoclasts, thus generating more differentiated bone cells but suppressing their activities; and increased metaphyseal trabecular hard tissue by creating an imbalance in osteoblasts over osteoclasts and suppressing hard tissue resorption.