Physiological doses of calcium regulatory hormones do not normalize bone cells in uraemic rats

PTH(1–84)/PTH(7–84): a balance of power

This review considers many new basic and clinical aspects of parathyroid hormone (PTH). We focus especially on the identification of PTH fragments and how they may relate to renal failure, diagnosis, and treatment of secondary hyperparathyroidism and renal osteodystrophy. The biosynthesis and metabolism of PTH, measurement of circulating forms of PTH, the effects of PTH on receptor activation and turnover, the relationship between PTH levels and bone turnover in renal failure in humans, and the involvement of PTH in experimental models of renal failure are discussed. Despite these developments in understanding the etiology of renal failure and the availability of new assays for bioactive PTH, no adequate surrogate for bone biopsy and quantitative bone histomorphometry has been developed.

Downregulation of parathyroid hormone receptor gene expression and osteoblastic dysfunction associated with skeletal resistance to parathyroid hormone in a rat model of renal failure with low turnover bone

BACKGROUND

Adynamic bone disease (ABD), which is characterized by reduced bone formation and resorption, has become an increasingly common manifestation of bone abnormalities in patients with end-stage renal failure. It has been recognized that skeletal resistance to parathyroid hormone (PTH) underlies the pathogenesis of ABD; however, the mechanisms of such resistance remain unclear.

METHODS

We established a rat model simulating ABD under chronic renal failure conditions by thyroparathyroidectomy and partial nephrectomy (TPTx-Nx). TPTx-Nx rats were infused subcutaneously with a physiological dose of PTH. We analysed bone histomorphometric parameters and demonstrated gene expression using semi-quantitative reverse transcription-polymerase chain reaction.

RESULTS

Reduced bone formation was observed in this model, simulating ABD. The reduction was dependent on the degree of renal dysfunction. Bone formation rate was 6.4+/-2.7 microm3/m2/year in TPTx-5/6Nx rats and 22.7+/-7.2 microm3/m2/year in TPTx rats (P<0.05). Osteoblast surface was also significantly depressed (P<0.05) in TPTx-5/6Nx (3.8+/-2.7%) compared with TPTx rats (15.9+/-8.6). The expression of PTH/parathyroid hormone-related peptide (PTHrP) receptor and alkaline phosphatase genes was reduced significantly in TPTx-Nx compared with TPTx rats (P<0.05). Reduced bone formation in TPTx-Nx rats was ameliorated by intermittent injection of pharmacological doses of PTH.

CONCLUSIONS

Renal dysfunction without secondary hyperparathyroidism induces osteoblast dysfunction and reduces bone formation. Skeletal resistance to PTH develops in renal failure even at low or normal PTH levels, possibly through downregulation of PTH/PTHrP receptor and dysfunction of osteoblasts.

Adverse effects of hyperphosphatemia on myocardial hypertrophy, renal function, and bone in rats with renal failure

BACKGROUND

Hyperphosphatemia and disturbances in calcium or parathyroid hormone (PTH) metabolism contribute to the high incidence of cardiovascular disease and renal osteodystrophy in chronic renal failure (CRF). We evaluated the effect of hyperphosphatemia on the cardiovascular system, on renal function, and on bone in experimental uremia.

METHODS

Wistar rats were submitted to parathyroidectomy (PTx) and 5/6 nephrectomy (Nx) with minipump implantation, delivering 1-34 rat PTH (physiologic rate), or were sham-operated and received vehicle. Only phosphorus content (low-phosphorus (LP) 0.2%; high-phosphorus (HP) 1.2%) differentiated diets. We divided the groups as follows: PTx +Nx +LP; sham + LP; PTx + Nx + HP; and sham + HP. Tail-cuff pressure and weight were measured weekly. After 2 months, biochemical, arterial, and myocardial histology and bone histomorphometry were analyzed.

RESULTS

Heart weight normalized to body weight (heart weight/100 g body weight) was higher in PTx + Nx + HP rats (PTx + Nx + HP = 0.36 +/- 0.01 vs. sham + HP = 0.29 +/- 0.01, PTx + Nx + LP = 0.32 +/- 0.01, sham + LP = 0.28 +/- 0.01) (P < 0.05). Serum creatinine levels were higher in PTx + Nx + HP rats than in PTx + Nx + LP rats (1.09 +/- 0.13 vs. 0.59 +/- 0.03 mg/dL) (P < 0.05). Levels of PTH did not differ significantly between the groups. Myocardial and arterial histology detected no vascular calcification or fibrosis. Bone histomorphometry revealed an association, unrelated to uremia, between HP diets and decreased trabecular connectivity.

CONCLUSION

Myocardial hypertrophy, impaired renal function, and adverse effects on bone remodeling were associated with hyperphosphatemia and were not corrected by PTH replacement. Although no vascular calcification was observed in this model, we cannot rule out an adverse effect of hyperphosphatemia on the vascular bed. Our finding underscores the importance of phosphorus control in reducing morbidity and mortality in CRF patients.

Comparison of peripheral bone and body axis skeleton in a rat model of mild-to-moderate renal failure in the presence of physiological serum levels of calcitropic hormones

The skeleton is characterized by anatomic heterogeneity of metabolic turnover. Site-dependent differences in hormonal effects seem likely. Hyporesponsiveness of osteoclasts to parathyroid hormone (PTH) and probably calcitriol was recently demonstrated in the fifth lumbar vertebra of a rat model with moderate renal failure. We compared histomorphometric findings of the tibial head to these data. Histomorphometric measurements were carried out in sections of the right tibial head of pair-fed male Sprague-Dawley rats. Subtotally nephrectomized (SNx), parathyroidectomized (PTx), rats, which received either solvent or rat PTH(1-34) (100 ng/kg per hour) + calcitriol (5 pmol/kg per hour) via osmotic minipumps were compared with sham-operated controls. Results were compared with data from the fifth lumbar vertebra reported recently. Osteoclast numerical density and osteoclast surface density were lower in the tibial head and the lumbar vertebra of solvent-treated SNxPTx rats than in sham-operated controls (p < 0.05), and could not be returned to normal by the substitution of PTH + calcitriol (p < 0.05). On the other hand, there were differences between interventional effects on the tibial head and on the lumbar vertebra concerning parameters describing osteoblasts and trabecular bone volume. In the tibial head, osteoblast surface density was nearly unchanged in both interventions. Nevertheless, bone volume increased after SNxPTx without substitution of PTH + calcitriol (p < 0.05), and no further changes occurred after hormonal replacement. In contrast, osteoblast surface density in the lumbar vertebra was decreased slightly compared with values in sham-operated rats; a clear but nonsignificant increase occurred after the administration of calcitropic hormones. This was accompanied by unchanged trabecular bone volume after SNxPTx. Hormonal replacement, however, caused an increase in trabecular bone volume (p < 0.05), which represents an anabolic effect, which contrasts with findings from the tibial head. The different interventional effects on the lumbar spine and on peripheral bone, regarding the parameters reflecting the condition of osteoblasts, may be intrinsic to the uremic syndrome itself as well as to dissimilar growth manner, function, and mechanical requirements. The findings substantiate the site dependence of bone surface cell metabolism in renal failure and should be the subject of further study. Corresponding results with regard to bone resorption argue for a bone-site-independent, diminished response of osteoclasts to calcitropic hormones.

Regulation of bone remodeling: impact of novel therapies

The kidneys serve as both an endocrine organ and as a target of endocrine action, with the aim of controlling mineral and water balance. Hormones and other key metabolites regulate mineral homeostasis by altering gene function directly or by initiating a sequence of events, leading ultimately to a change in enzyme function. Two of these hormones, parathyroid hormone (PTH) and calcitriol (the active form of vitamin D), interact in multiple tissues in the body to regulate the flux of calcium and phosphorus between extra- and intracellular compartments. Changes in the concentration of PTH and vitamin D, or the interaction of these with other factors, lead to the aberrant regulation of calcium and phosphorus. Among other effects, this aberrant regulation leads to pathologic changes in bone metabolism. The pathology of renal bone disease varies along a spectrum from disorders of low turnover to those of high turnover. This spectrum reflects the results of therapeutic intervention, hormone balances, and other causes. Effective management of renal bone disease therefore requires thorough evaluation of relevant risk factors, measurement of biochemical markers of bone remodeling, and determination of the physical status of bone tissue either by bone mineral density or bone biopsy. Subsequent therapeutic intervention with newer vitamin D compounds, novel phosphate binders, calcimimetics, and the use of alternative dialysis modalities offer hope in normalizing bone remodeling and mineral balance. The human skeleton functions in two capacities: the storage of minerals and structural support of the body. It is the only tissue that behaves as both a major source and a sink of calcium (Ca) and phosphorus (P). Healthy bone is a composite of a collagenous matrix embedded with crystals of hydroxyapatite. On the surface of bone and within the calcified matrix are specialized cells that build and maintain the tissue, and facilitate the movement of Ca and P into and out of serum. Bone undergoes remodeling in response to either damage from mechanical strain or as part of the normal cycle of bone renewal. The process involves distinct steps of cellular activation, bone resorption, and subsequent bone formation. It is a relatively slow process that takes several months, and at any one time occurs at many different sites along the bone surface. Systemic factors, such as PTH and vitamin D, regulate the resorption and formation of bone, and thus the systemic movement of Ca and P. However, during conditions of stress and disease, other factors may also play a role. In patients with chronic renal disease, the balance of Ca and P is profoundly disturbed. This disruption and the compensatory changes that occur in response alter the normal processes of bone metabolism.

Osteodystrophy in the millennium

Despite three decades of intensive research on the derangements of calcium phosphate metabolism of renal failure, several unresolved issues are still with us at the turn of the millennium: poor control of hyperphosphatemia, relative inefficacy of active vitamin D to prevent progressive parathyroid hyperplasia, and persistence of bone disease despite lowering of parathyroid hormone (PTH) and administration of active vitamin D. Although predictions are problematic, it is not unreasonable to hope that, barring unforeseen side effects, calcimimetics will prove to be valuable for suppressing or even preventing hyperparathyroidism, thus potentially replacing, at least in part, active vitamin D. There is also reason to hope that more effective phosphate binders with fewer side effects will become available and that controlled studies will provide a rationale for the administration of estrogens to dialyzed women. As regards understanding the pathological mechanisms, one can anticipate that the disturbances leading to autonomous growth of parathyroid cells will be elucidated and the signals involved in osteoclast/osteoblast differentiation pathways and osteoclast/osteoblast coupling will be clarified, with obvious impact on patient management.