Resistance to parathyroid hormone in renal failure: role of vitamin D metabolites

Acute kidney injury increases risk of kidney stones-a retrospective propensity score matched cohort study

BACKGROUND

Acute kidney injury (AKI) is common. An episode of AKI may modify the risk of developing kidney stones by potential long-term effects on urine composition. We aimed to investigate the association between AKI and the risk of kidney stone presentations.

METHODS

The retrospective cohort study used patient data (1 January 2008-31 December 2017), from an Australian Local Health District, which included AKI diagnosis, demographics, comorbidities and kidney stone admissions. Time-varying Cox proportional hazards and propensity-matched analysis were used to determine the impact of AKI on the risk of kidney stones. To address possible population inhomogeneity in comparisons between no AKI and hospitalized AKI, sub-group analysis was done comparing inpatient and outpatient AKI versus no AKI, to assess consistency of association with future stones. Sensitivity analysis was undertaken to capture the impact of a known AKI status and AKI severity.

RESULTS

Out of 137 635 patients, 23 001 (17%) had an AKI diagnosis and 2295 (2%) had kidney stone presentations. In the unadjusted analysis, AKI was associated with kidney stones, with AKI used as a time-varying exposure, [hazard ratio (HR) 1.32, 95% confidence interval (CI) 1.16-1.50)]. Both inpatient-AKI (HR 1.19, 95% CI 1.01-1.39) and outpatient-AKI (HR 1.59, 95% CI 1.30-1.94) were significantly associated with future stones compared to no AKI subjects. This association persisted in the adjusted analysis (HR 1.45, 95% CI 1.26-1.66), propensity-matched dataset (HR 1.67, 95% CI 1.40-1.99) and sensitivity analysis. There was a dose-response relationship with higher stages of AKI being associated with a greater risk of kidney stones.

CONCLUSIONS

In a large cohort of patients, AKI is associated with a greater risk of kidney stones, which increases with higher stages of AKI. This association should be examined in other cohorts and populations for verification.

Hyporesponsiveness or resistance to the action of parathyroid hormone in chronic kidney disease

Secondary hyperparathyroidism (SHPT) is an integral component of the chronic kidney disease-mineral and bone disorder (CKD-MBD). Many factors have been associated with the development and progression of SHPT but the presence of skeletal or calcemic resistance to the action of PTH in CKD has often gone unnoticed. The term hyporesponsiveness to PTH is currently preferred and, in this chapter, we will not only review the scientific timeline but also some of the molecular mechanisms behind. Moreover, the presence of resistance to the biological action of PTH is not unique in CKD since resistance to other hormones has also been described ("uremia as a receptor disease"). This hyporesponsiveness carries out important clinical implications since it explains, at least partially, not only the progressive nature of the pathogenesis of CKD-related PTH hypersecretion and parathyroid hyperplasia but also the increasing prevalence of adynamic bone disease in the CKD population. Therefore, we underline the importance of PTH control in all CKD stages, but not aiming to completely normalize PTH levels since a certain degree of SHPT may represent an adaptive clinical response. Future studies at the molecular level, i.e. on uremia or the recent description of the calcium-sensing receptor as a phosphate sensor, may become of great value beyond their significance to explain just the hyporesponsiveness to PTH in CKD.

Hyporesponsiveness or resistance to the action of parathyroid hormone in chronic kidney disease

Secondary hyperparathyroidism (SHPT) is an integral component of the chronic kidney disease-mineral and bone disorder (CKD-MBD). Many factors have been associated with the development and progression of SHPT but the presence of skeletal or calcemic resistance to the action of PTH in CKD has often gone unnoticed. The term hyporesponsiveness to PTH is currently preferred and, in this chapter, we will not only review the scientific timeline but also some of the molecular mechanisms behind. Moreover, the presence of resistance to the biological action of PTH is not unique in CKD since resistance to other hormones has also been described ("uremia as a receptor disease"). This hyporesponsiveness carries out important clinical implications since it explains, at least partially, not only the progressive nature of the pathogenesis of CKD-related PTH hypersecretion and parathyroid hyperplasia but also the increasing prevalence of adynamic bone disease in the CKD population. Therefore, we underline the importance of PTH control in all CKD stages, but not aiming to completely normalize PTH levels since a certain degree of SHPT may represent an adaptive clinical response. Future studies at the molecular level, i.e. on uremia, or the recent description of the calcium-sensing receptor as a phosphate sensor, may become of great value beyond their significance to explain just the hyporesponsiveness to PTH in CKD.

Sex hormone-binding globulin is a biomarker associated with nonvertebral fracture in men on dialysis therapy

Gonadal hormones impact bone health and higher values of sex hormone-binding globulin (SHBG) have been independently associated with fracture risk in men without chronic kidney disease. People with chronic kidney disease have a greatly increased fracture risk, and gonadal dysfunction is common in men receiving dialysis treatment. Nevertheless, in these men the effect of gonadal steroids and SHBG on bone mineral density (BMD) and fracture risk is unknown. Here we investigate relationships between gonadal steroids, SHBG, BMD and fracture in men on long-term dialysis therapy, awaiting kidney or simultaneous pancreas kidney transplantation. Results of serum biochemistry, SHBG, gonadal steroids (total testosterone, calculated free testosterone and estradiol), BMD by dual-energy X-ray absorptiometry and thoracolumbar X-rays were obtained. Multivariable regression models were used to examine associations between SHBG, gonadal steroids, BMD and fracture of 321 men with a mean age of 47 years. Diabetes mellitus was present in 45% and their median dialysis vintage was 24 months. Prior fractures occurred in 42%, 18% had vertebral fracture on lateral spine X-ray, 17% had non-vertebral fragility fracture within 10 years and 7% had both. After adjustment for age, body mass index and dialysis vintage, higher SHBG levels were significantly associated with nonvertebral fractures [odds ratio 1.81 (1.30-2.53)] and remained significant after adjustment for BMD. Calculated free testosterone and estradiol values were not associated with fracture. Prevalent fracture rates were high in relatively young men on dialysis awaiting transplantation. Thus, SHBG is a novel biomarker associated with fracture, which warrants investigation in prospective studies.

Tradeoff-in-the-Nephron: A Theory to Explain the Primacy of Phosphate in the Pathogenesis of Secondary Hyperparathyroidism

Chronic kidney disease (CKD) causes secondary hyperparathyroidism (SHPT). The cardinal features of SHPT are persistence of normocalcemia as CKD progresses and dependence of the parathyroid hormone concentration ([PTH]) on phosphate influx (IP). The tradeoff-in-the-nephron hypothesis integrates these features. It states that as the glomerular filtration rate (GFR) falls, the phosphate concentration ([P]CDN) rises in the cortical distal nephron, the calcium concentration ([Ca]CDN) in that segment falls, and [PTH] rises to maintain normal calcium reabsorption per volume of filtrate (TRCa/GFR). In a clinical study, we set GFR equal to creatinine clearance (Ccr) and IP equal to the urinary excretion rate of phosphorus (EP). We employed EP/Ccr as a surrogate for [P]CDN. We showed that TRCa/Ccr was high in patients with primary hyperparathyroidism (PHPT) and normal in those with SHPT despite comparably increased [PTH] in each group. In subjects with SHPT, we examined regressions of [PTH] on EP/Ccr before and after treatment with sevelamer carbonate or a placebo. All regressions were significant, and ∆[PTH] correlated with ∆EP/Ccr in each treatment cohort. We concluded that [P]CDN determines [PTH] in CKD. This inference explains the cardinal features of SHPT, much of the evidence on which other pathogenic theories are based, and many ancillary observations.

Metabolic Bone Disease in Chronic Kidney Disease

Metabolic bone disease is a common complication of chronic kidney disease (CKD) and is part of a broad spectrum of disorders of mineral metabolism that occur in this clinical setting and result in both skeletal and extraskeletal consequences. Detailed research in that past 4 decades has uncovered many of the mechanisms that are involved in the initiation and maintenance of the disturbances of bone and mineral metabolism and has been translated successfully from "bench to bedside" so that efficient therapeutic strategies now are available to control the complications of disturbed mineral metabolism. Recent emphasis is on the need to begin therapy early in the course of CKD. Central to the assessment of disturbances in bone and mineral metabolism is the ability to make an accurate assessment of the bone disease by noninvasive means. This remains somewhat problematic, and although measurements of parathyroid hormone are essential, recently recognized difficulties with these assays make it difficult to provide precise clinical practice guidelines for the various stages of CKD at the present time. Further research and progress in this area continue to evaluate the appropriate interventions to integrate therapies for both the skeletal and extraskeletal consequences with a view toward improving patient outcomes.

The calcium-sensing receptor: a key factor in the pathogenesis of secondary hyperparathyroidism

Serum calcium levels are regulated by the action of parathyroid hormone (PTH). Major drivers of PTH hypersecretion and parathyroid cell proliferation are the hypocalcemia and hyperphosphatemia that develop in chronic kidney disease patients with secondary hyperparathyroidism (SHPT) as a result of low calcitriol levels and decreased kidney function. Increased PTH production in response to systemic hypocalcemia is mediated by the calcium-sensing receptor (CaR). Furthermore, as SHPT progresses, reduced expression of CaRs and vitamin D receptors (VDRs) in hyperplastic parathyroid glands may limit the ability of calcium and calcitriol to regulate PTH secretion. Current treatment for SHPT includes the administration of vitamin D sterols and phosphate binders. Treatment with vitamin D is initially effective, but efficacy often wanes with further disease progression. The actions of vitamin D sterols are undermined by reduced expression of VDRs in the parathyroid gland. Furthermore, the calcemic and phosphatemic actions of vitamin D mean that it has the potential to exacerbate abnormal mineral metabolism, resulting in the formation of vascular calcifications. Effective new treatments for SHPT that have a positive impact on mineral metabolism are clearly needed. Recent research shows that drugs that selectively target the CaR, calcimimetics, have the potential to meet these requirements.

Regulation of PTH1 receptor expression by uremic ultrafiltrate in UMR 106-01 osteoblast-like cells

BACKGROUND

Homologous down-regulation/desensitization of the parathyroid hormone receptor (PTH1R)/adenylate cyclase system has been demonstrated in uremia, and may contribute to parathyroid hormone (PTH) resistance; however, additional studies have shown that parathyroidectomy fails to normalize the down-regulation of the PTH1R. The present studies were designed to test directly, in vitro, the hypothesis that factors circulating in the uremic environment, other than PTH, decrease the response of osteoblastic cells to PTH.

METHODS

Studies were conducted in confluent cultures of UMR 106-01 osteoblast-like cells. Uremic ultrafiltrate (UUF) was obtained from patients on hemodialysis. Cells were exposed to media containing 50% uremic ultrafiltrate for periods of up to 72 hours. Control cultures were exposed to a buffered salt solution containing a comparable ionic composition to that of the UUF. PTH-stimulated cyclic adenosine monophosphate (cAMP) generation was determined by radioimmunoassay (RIA), PTH binding and PTH1R mRNA levels were determined by radioligand binding and Northern analysis, respectively.

RESULTS

PTH-stimulated cAMP generation from cultures treated with uremic ultrafiltrate for 48 hours was 1385.8 +/- 183.2 pmol/culture/5 minutes, whereas control cultures generated 2389.5 +/- 271 pmol cAMP/culture/5 minutes (P < 0.05). PTH binding was decreased by 30% in cultures incubated with UUF as compared to controls. The decrease in binding induced by UUF was accompanied by a decrease in PTH1R mRNA levels.

CONCLUSION

These findings demonstrate that factors present in UUF decrease PTH-stimulated cAMP generation by a mechanism that involves a decrease in the levels of PTH1R mRNA levels. Thus, the skeletal resistance to PTH in the setting of chronic kidney disease, may be explained, at least in part, by circulating factors other than PTH.

Down-regulation of human osteoblast PTH/PTHrP receptor mRNA in end-stage renal failure

BACKGROUND

Resistance to the action of parathyroid hormone (PTH) has been demonstrated in end-stage renal failure and is considered to be important in the pathogenesis of secondary hyperparathyroidism. The mechanism of resistance is unknown. However, altered regulation of cellular PTH/PTH-related protein (PTH/PTHrP) receptor (PTH1R) has been assumed to be important.

METHODS

We have used in situ hybridization to examine PTH1R mRNA expression by osteoblasts in human bone and have compared the expression in high- and low-turnover renal bone disease, high-turnover nonrenal bone disease (healing fracture callus and Pagetic bone), and normal bone. Bone biopsies were formalin fixed, ethylenediaminetetraacetic acid decalcified, and paraffin wax embedded. A 1.8 kb PTH1R cDNA probe, labeled with 35S, was used, and the hybridization signal was revealed by autoradiography. The density of signal over osteoblasts was quantitated using a semiautomated Leica image analysis software package.

RESULTS

The mean density of PTH1R mRNA signal over osteoblasts in renal high-turnover bone was only 36% of that found in nonrenal high-turnover bone (P < 0.05) and 51% of that found in normal bone (P < 0.05). Osteoblast PTH1R mRNA signal in adynamic bone from individuals with diabetes mellitus was 28% of normal bone (P < 0.05) and 54% of that found in renal high-turnover bone (P < 0.05).

CONCLUSIONS

These results demonstrate a down-regulation of osteoblast PTH1R mRNA in end-stage renal failure in comparison to normal and high-turnover bone from otherwise healthy individuals, and provide an insight into the mechanisms of "skeletal resistance" to the actions of PTH.

Cellular mechanisms of renal osteodystrophy

Renal osteodystrophy affects all patients with end-stage renal failure, resulting in significant skeletal and extra-skeletal morbidity. The patterns of disease seen in bone are the result of changes in calcium, phosphate, parathyroid hormone (PTH), and vitamin D metabolism, as well as the effects of uremia. Standard histological techniques, however, give little insight into the altered biological activity or mechanisms of disease at the cellular level. In order to examine the cellular abnormalities in renal bone disease we have performed a series of in situ hybridization studies to examine renal bone cell expression of genes for PTH receptor (PTHR1), transforming growth factor beta (TGF-beta) and insulin growth factor 1 (IGF-I). PTHR1 mRNA was expressed predominantly by osteoblasts, but also by resorbing osteoclasts, suggesting that these cells may be stimulated directly by PTH. Semi-quantitative analysis of gene expression showed down-regulation of PTHR1 mRNA by osteoblasts in renal bone compared with normal, fracture and Pagetic bone. This may be important in the pathogenesis of skeletal resistance seen in end-stage renal failure, altering the "threshold" at which PTH has its effects on bone cells. TGF-beta and IGF-I mRNA expression was also decreased, suggesting that synthesis of these factors, postulated to be mediators of PTH, is also downregulated.

Vitamin D analogs: from renal bone disease to osteoporosis

The history of the use of vitamin D metabolites and analogs in clinical medicine began with their use in renal bone disease following the appreciation that the kidney is the major site of the 1 alpha-hydroxylase of vitamin D. In the presence of advanced renal failure, production of calcitriol is impaired and treatment is viewed conceptually as a hormone replacement therapy. Since then, there has been much interest in the rationale for the use of such compounds in osteoporosis. Arguments have been variously forwarded that osteoporosis is in part due to either a defective 1 alpha-hydroxylase occurring at menopause or in later life or to target tissue resistance to calcitriol; some argue that disturbances in vitamin D metabolism are irrelevant to osteoporosis, being either a consequence of aging or a result, not a cause, of osteoporosis itself. This paper reviews these various issues and explores the differences and similarities between the pathogenesis of renal bone disease and osteoporosis.

Phosphorus accelerates the development of parathyroid hyperplasia and secondary hyperparathyroidism in rats with renal failure

Several studies have suggested that phosphorus (P) restriction, independent of serum levels of ionized calcium and 1,25-(OH)2D3, may prevent parathyroid hyperplasia and secondary hyperparathyroidism in patients and animals with chronic renal failure. A direct role of phosphorus in the pathogenesis of these abnormalities is, however, still controversial. Thus, studies were performed to examine the direct role of phosphorus on the development of these abnormalities. Female Sprague-Dawley rats underwent 5/6 nephrectomy or sham operation. The animals were then divided into two dietary groups (High-P: 0.8% P diet, Low-P: 0.2% P diet). Six to eight rats per group per time-point were killed at the following intervals: 0, 1, 2, and 4 days, and 1, 2, 3, and 4 weeks. Serum intact parathyroid hormone (PTH) levels in uremic rats fed the high-P diet increased 1 day after nephrectomy, and high levels persisted for the duration of the study. Parathyroid gland growth in uremic rats fed the high-P diet was apparent within 2 days of uremia and increased nearly twofold by 2 weeks. These abnormalities, however, did not develop in uremic rats fed the low-P diet. Serum P levels in uremic rats fed the high-P diet were significantly higher than those of uremic rats fed the low-P diet, but there was no significant difference in serum ionized calcium or 1,25-(OH)2D3 levels. These results demonstrate that phosphorus accelerates the development of parathyroid hyperplasia and secondary hyperparathyroidism in rats with renal failure, and that phosphorus restriction prevents these abnormalities independent of changes in serum ionized calcium and 1,25-(OH)2D3.

Abnormal skeletal response to parathyroid hormone and the expression of its receptor in chronic uremia

Chronic renal failure is characterized by a resistance to the hypercalcemic action of parathyroid hormone (PTH). This resistance probably involves several mechanisms, including a disturbance of vitamin D metabolism, a desensitization of the skeleton by high PTH levels, hyperphosphatemia, uremic toxins, and acidosis. We have explored the possibility that a downregulation of the recently cloned PTH/PTHrp receptor might also be involved. We found a marked decrease in the expression of the receptor mRNA in the kidney and the bone of uremic rats; other authors have found a decrease in the heart and the liver. The reduced expression in the kidney was accompanied by a diminished stimulability of renal adenylate cyclase activity, suggestive of a functional depression of the hormonal response in this target tissue. It is probable that the downregulation of the PTH/PTHrp receptor plays an important role in the skeletal resistance to the calcemic effect of PTII in chronic renal failure.

Secondary hyperparathyroidism in renal failure: the trade-off hypothesis revisited

Our understanding of the mechanism responsible for secondary hyperparathyroidism (HPTH) has advanced significantly since the "trade-off" hypothesis was formulated. It appears that in early renal failure a deficit of calcitriol synthesis is an important factor. However, additional factors, such as a defect of the vitamin D receptor or the newly cloned calcium sensor receptor (BoPCaR1), may be present in the parathyroid cells. As renal failure progresses, the lack of calcitriol becomes more pronounced, inducing HPTH. With advanced chronic renal failure, hyperphosphatemia is an additional important factor in worsening HPTH. In addition, resistance of the parathyroids to calcitriol due to a reduced density of calcitriol receptors also may contribute to HPTH. Finally, uremia per se not only may cause a receptor abnormality in the parathyroid but at the level of the bone it may aggravate the impaired calcemic response to PTH. In conclusion, after reviewing the "trade-off" hypothesis, although some of the original concepts may have been simplistic, most of the factors postulated 30 years ago are still operative in the pathogenesis of secondary HPTH in renal failure.

Effect of alfacalcidol on natural course of renal bone disease in mild to moderate renal failure

OBJECTIVE

To determine whether alfacalcidol--used in management of overt renal bone disease--may safely prevent renal bone disease when used earlier in course of renal failure.

DESIGN

Double blind, prospective, randomised, placebo controlled study.

SETTING

17 nephrology centres from Belgium, France, the Netherlands, and the United Kingdom.

SUBJECTS

176 patients aged 18-81 with mild to moderate chronic renal failure (creatinine clearance 15-50 ml/min) and with no clinical, biochemical, or radiographic evidence of bone disease.

INTERVENTIONS

Alfacalcidol 0.25 micrograms (titrated according to serum calcium concentration) or placebo given for two years.

MAIN OUTCOME MEASURES

Quantitative histology of bone to assess efficacy of treatment and renal function to assess safety.

RESULTS

132 patients had histological evidence of bone disease at start of study. Biochemical, radiographic, and histological indices of bone metabolism were similar for the 89 patients given alfacalcidol and the 87 controls given placebo. After treatment, mean serum alkaline phosphatase activity and intact parathyroid hormone concentration had increased by 13% and 126% respectively in controls but had not changed in patients given alfacalcidol (P < 0.001). Hypercalcaemic episodes occurred in 10 patients given alfacalcidol (but responded to decreases in drug dose) and in three controls. Histological indices of bone turnover significantly improved in patients given alfacalcidol and significantly deteriorated in controls: among patients with abnormal bone histology before treatment, bone disease resolved in 23 (42%) of those given alfacalcidol compared with two (4%) of the controls (P < 0.001). There was no difference in rate of progression of renal failure between the two groups.

CONCLUSION

Early administration of alfacalcidol can safely and beneficially alter the natural course of renal bone disease in patients with mild to moderate renal failure.

The calcemic response to PTH in the rat: effect of elevated PTH levels and uremia

Secondary hyperparathyroidism (2 degrees HPT) is a consistent finding in renal failure. A decreased calcemic response (CR) to parathyroid hormone (PTH) contributes to the development of 2 degrees HPT. Since parathyroidectomy (PTX) corrects the decreased CR to PTH in azotemic animals, down-regulation of PTH receptors induced by an elevation of PTH has been advanced as an important factor in the development of 2 degrees HPT. The goal of the study was to determine in azotemic rats whether a progressive reduction of PTH improves the CR to PTH and whether the maintenance of normal PTH levels corrects the CR to PTH. Seven groups of pair-fed rats were studied. Three groups of rats had normal renal function (NRF groups) and received either a high phosphorus (HPD-NRF), a moderate phosphorus (MPD-NRF), or a low phosphorus (LPD-NRF) diet. Three azotemic (NX) groups received similar diets (HPD-NX, MPD-NX and LPD-NX groups) in order to vary the magnitude of 2 degrees HPT. A PTX was performed in a fourth azotemic group (PTX-NX) to induce the complete absence of PTH. After 14 to 16 days on the maintenance diets, the CR to PTH was determined with a 48 hour infusion of 1-34 rat PTH.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcemic response to parathyroid hormone in renal failure: role of calcitriol and the effect of parathyroidectomy

Hyperparathyroidism due to renal failure begins in the early stages of renal insufficiency and is in part secondary to skeletal resistance to the calcemic action of parathyroid hormone (PTH). Factors which have been reported to reduce the calcemic response to PTH include: decreased calcitriol levels, hyperphosphatemia and down regulation of PTH receptors in bone. While hyperphosphatemia may directly decrease the calcemic response to PTH, it may also act indirectly by a suppression of calcitriol synthesis. In this study, the effect of calcitriol on the calcemic response to PTH was evaluated in normal rats and in rats with moderate and advanced renal failure. To determine the combined effect of calcitriol and phosphorus on the calcemic response to PTH, rats receiving calcitriol were fed either a high (1.0%) or low (0.2%) phosphorus diet during a 48-hour PTH infusion. In advanced renal failure, calcitriol administration increased the calcemic response to PTH independent of the dietary phosphorus intake. During ingestion of a low phosphorus diet, a 48 hour PTH infusion resulted in a serum calcium level of 13.7 +/- 0.5 and 12.1 +/- 0.2 mg/dl (P less than 0.02) with and without calcitriol administration, respectively. In normal rats and in rats with moderate renal failure, calcitriol administration improved the calcemic response only during a high phosphorus intake. After a 48-hour PTH infusion in normal rats, the serum calcium levels with and without calcitriol were 16.1 +/- 0.9 and 14.8 +/- 0.6 mg/dl, P less than 0.01 respectively; in rats with moderate renal failure, calcitriol administration increased serum calcium, 13.2 +/- 0.5 versus 11.2 +/- 0.4 mg/dl, P less than 0.01.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcitonin, an important factor in the calcemic response to parathyroid hormone in the rat

The role of calcitonin on the calcemic response to parathyroid hormone (PTH) in renal failure has not been evaluated previously. Often animal studies evaluating the calcemic response to PTH in renal failure are performed in thyroparathyroidectomized (TPTX) animals, and thus eliminate any potential physiologic effect of calcitonin. In addition, parathyroidectomy (PTX), presumably by reduction of high PTH levels, has corrected the calcemic response to PTH in animals with renal failure. The present study was designed to evaluate the effect of endogenous calcitonin production on the calcemic response to PTH in rats with renal failure and secondary hyperparathyroidism, and in rats with normal renal function with diet induced hyperparathyroidism. Four groups of rats were evaluated: 1) chronic renal failure plus TPTX with autotransplant of the parathyroid gland, (CT-) CRF; 2) chronic renal failure plus selective PTX with autotransplant of the parathyroid gland, (CT+) CRF; 3) normal renal function plus TPTX with autotransplant of the parathyroid gland, (CT-) NRF; and 4) normal renal function plus selective PTX with autotransplant of the parathyroid gland, (CT+) NRF. Renal failure was surgically induced by a two-stage 5/6 nephrectomy, and exogenous thyroxine was administered to the two thyroidectomized (CT-) groups. Hyperparathyroidism was induced with a high phosphate diet (1.2%), and thus at the time of PTH infusion, PTH levels were (CT-) CRF 84 +/- 16, (CT+) CRF 89 +/- 21, (CT-) NRF 37 +/- 7, and (CT+) NRF 31 +/- 4 pg/ml, respectively (normal 21 +/- 3 pg/ml). Rat 1-34 PTH (2.6 U/hr) was infused for 48 hours via a subcutaneously implanted Alzet pump.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of calcium antagonists on renal function and secondary hyperparathyroidism in acute renal failure in rats

Glycerol-induced acute renal failure (ARF) is characterized by an increase in serum creatinine, urea, and phosphate concentration, and severe impairment of creatinine clearance. Secondary hyperparathyroidism develops rapidly during ARF. The calcium antagonist nisoldipine clearly improves renal function, which becomes evident by an improvement of creatinine clearance and attenuation of the increase of serum creatinine, urea, and phosphate concentrations. Further secondary hyperparathyroidism is ameliorated by nisoldipine treatment. In spite of normalization of the hypocalcemia in ARF by nisoldipine, weak hyperparathyroidism persists, suggesting that hypocalcemia is not exclusively responsible for elevated parathyroid hormone serum levels in ARF.

Renal osteodystrophy: some new questions on an old disorder

The two major lesions of renal osteodystrophy are osteitis fibrosa cystica (OFC) and osteomalacia (OM). OFC is the characteristic bone lesion of uremic hyperparathyroidism. Although renal failure causes predictable parathyroid hyperplasia, the precise pathogenetic mechanism is still not defined. The "hyperphosphatemia-hypocalcemia-parathyroid hormone (PTH) hypersecretion" sequence of events is no longer an adequate model for the pathogenesis of uremic hyperparathyroidism. Other abnormalities associated with uremia include reduced 1,25-dihydroxyvitamin D (1,25D) synthesis, changes in intracellular phosphorus content or transcellular phosphate fluxes, or alteration in PTH metabolism, eg, change in set-point for PTH secretion. Each abnormality interacts with others and contributes to PTH hypersecretion, but none can completely account for the development and persistence of hyperparathyroidism in renal failure. The possibility that uremia may directly cause parathyroid hyperplasia remains open. It is also possible that factor(s) that initiate hyperparathyroidism may turn out to be quite different from that which sustains the hyperparathyroid state. Although both vitamin D-deficient and vitamin D-resistant OM may develop in patients with renal failure, the majority of uremic OM seen currently is "vitamin D-refractory." Although now there is persuasive evidence implicating aluminum (Al) accumulation as the major pathogenetic cause for the mineralization defect seen in this disorder, additional disturbances may play important contributory roles. Such factors would include extraskeletal effects of Al, differences in host-susceptibility to this element, the localization of Al within bone, uremia per se, and the participation of other metals and toxins. Finally, possible interactions between hyperparathyroidism and OM of uremia are speculated on.

Role of 1,25-dihydroxyvitamin D on the skeletal resistance to parathyroid hormone

Hypocalcemia in chronic renal failure (CRF) has been attributed in part to a skeletal resistance (S.R.) to the calcemic action of parathyroid hormone (PTH) as a consequence of low levels of 1,25(OH)2D3. To further elucidate the role of 1,25(OH)2D3 in the genesis of S.R., the calcemic effect of infusion of synthetic b-PTH 1-34 was examined in dogs before and after 7, 90 and 180 days of CRF. The maximum increment in ionized calcium after the infusion of PTH in the normal dogs was 1.15 +/- 0.13 mg/dl, decreased to 0.84 +/- 0.09 after 7 days, 0.68 +/- 0.1 after 90 days and to 0.66 +/- 0.11 mg/dl after 180 days of CRF. Thereafter, the dogs received 1,25(OH)2D3, 0.5 microgram daily for seven days and the studies were repeated. No improvement in the calcemic response to PTH was observed (0.57 +/- 0.26 mg/100 ml). Subsequently, a parathyroidectomy (PTX) was performed and 24 hours later the studies were repeated. After the infusion of PTH the calcemic response returned to normal (0.93 +/- 0.14 mg/dl). Further studies were performed in a group of four uremic dogs receiving no 1,25(OH)2D3, in which a PTX was performed 24 hours prior to the infusion of PTH. For this group, the increase in serum ionized calcium was 0.99 +/- 0.11 mg/dl. These values were not statistically different from normal dogs. In summary: (1) renal insufficiency was characterized by an abnormal calcemic response to PTH; (2) the administration of 1,25(OH)2D3 did not correct the S.R. to PTH; and (3) PTX performed 24 hours before the infusion of PTH restored the calcemic response to normal. These data suggest that high levels of endogenous parathyroid hormone desensitized the skeleton to the administration of exogenous PTH. These studies indicate that low values of 1,25(OH)2D3 are not directly responsible for the skeletal resistance to PTH in chronic renal failure.

Hypocalcemia may not be essential for the development of secondary hyperparathyroidism in chronic renal failure

Hypocalcemia is the main factor responsible for the genesis of secondary hyperparathyroidism in chronic renal disease. Studies with parathyroid cells obtained from uremic patients indicate that there is a shift in the set point for calcium-regulated hormone (parathyroid hormone [PTH] secretion. Studies were performed in dogs to further clarify this new potential mechanism. Hypocalcemia was prevented in uremic dogs by the administration of a high calcium diet. Initially, ionized calcium was 4.79 +/- 0.09 mg/dl and gradually increased up to 5.30 +/- 0.05 mg/dl. Despite a moderate increase in ionized calcium, immunoreactive PTH (iPTH) increased from 64 +/- 7.7 to 118 +/- 21 pg/ml. Serum 1,25(OH)2D3 decreased from 25.4 +/- 3.8 to 12.2 +/- 3.6 pg/ml. Further studies were performed in two other groups of dogs. One group received 150-200 ng and the second group 75-100 ng of 1,25(OH)2D3 twice daily. The levels of 1,25(OH)2D3 increased from 32.8 +/- 3.5 to a maximum of 69.6 +/- 4.4 pg/ml. In the second group the levels of serum 1,25(OH)2D3 after nephrectomy remained normal during the study. Amino-terminal iPTH did not increase in either of the two groups treated with 1,25(OH)2D3. In summary, the dogs at no time developed hypocalcemia; however, there was an 84% increase in iPTH levels, suggesting that hypocalcemia, per se, may not be the only factor responsible for the genesis of secondary hyperparathyroidism.

Determinants of serum 1,25(OH)2D levels in renal disease

Serum 1,25(OH)2D and factors related to its production were studied in 39 patients with various degrees of renal insufficiency. Serum 1,25(OH)2D levels correlated positively with 1/serum creatinine values (r = 0.54, P less than 0.001) and negatively with serum phosphorus (r = -0.39, P less than 0.02) and age (r = -0.33, P less than 0.05). There was no significant correlation between 1,25(OH)2D levels and serum calcium or calcitonin or PTH, although the logarithm of PTH correlated inversely with 1,25(OH)2D levels (r = -0.47, P less than 0.01). Patients who had normal or supranormal 1,25(OH)2D levels despite low GFR tended to have low serum phosphorus values. Serum levels of bone Gla protein (BGP), a biochemical marker for bone metabolism, correlated negatively with 1/serum creatinine (r = -0.39, P less than 0.02) and positively with PTH (r = 0.57, P less than 0.001) and age (r = 0.33, P less than 0.05). Prophylaxis with 1,25(OH)2D should be considered in patients with significantly decreased serum 1,25(OH)2D levels, as seem to occur when serum creatinine is greater than 4.0 mg/dl. However, despite the statistically significant correlation between serum 1,25(OH)2D and 1/serum creatinine, direct measurement should be used to ascertain the serum concentration of 1,25(OH)2D in chronic renal insufficiency.

Effects of vitamin D metabolites on protein catabolism of muscle from uremic rats

Epitrochlearis muscles obtained from normal male Sprague-Dawley rats used as controls (C) and rats with reduced renal mass (Nx) were incubated for 1 hr in Krebs-Ringer buffer containing 5 mM glucose with or without insulin, 25-hydroxycholecalciferol [25(OH)D3] or 1,25 dihydroxycholecalciferol [1,25(OH)2D3]. Plasma levels of 25(OH)D3 were unaffected by reduction in renal mass. Alanine (ALA), tyrosine (TYR), and phenylalanine (PHE) release rates from muscles of Nx rats were increased 40% above C values. Addition of 100 ng/ml of 25(OH)D3 to the incubating media reduced these release rates to C values within 1 hr of incubation. No additive effects with insulin were seen. Addition of 1 ng/ml of 1,25(OH)2D3 did not affect these results. Reduction of renal mass or the addition of insulin or 25(OH)D3 did not affect tissue concentrations of ATP or phosphocreatine. On the other hand, tissue levels of TYR and PHE were increased significantly (approximately equal to 20 to 25%) in muscles from Nx rats compared to C values and were reduced to control values by the addition of 25(OH)D3. The addition of insulin to the incubating media reduced the tissue levels of TYR and PHE in muscles of C rats by approximately equal to 20%, but reduced these levels in muscles of Nx rats by approximately equal to 55%. 25(OH)D3 did not affect tissue levels of cyclic AMP in muscles from either C or Nx rats. Protein synthetic rates were reduced significantly in muscles from Nx rats and returned to C values after 3 hr of incubation but were unaffected by 25(OH)D3. Muscle uptake of 3H,25(OH)D3 was reduced by approximately equal to 30% in muscles from Nx rats compared to C rats. These data suggest that increased muscle protein catabolism exists in rats with reduced renal mass which can be reduced to C values by 25(OH)D3 and does not appear to be mediated through stimulation of adenylate cyclase. 25(OH)D3 did not affect muscle protein synthetic rates. Reduced uptake of 3H,25(OH)D3 by muscles of Nx rats suggests that resistance to this vitamin metabolite may exist at the level of muscle in uremia.

Calcemic and phosphaturic response to parathyroid hormone in normal and chronically uremic dogs

It is well established that the calcemic response to parathyroid hormone (PTH) is blunted in chronic uremia and is corrected partially by 1,25(OH)2D treatment. Recent evidence suggests that PTH(1-34) and not PTH(1-84) may be the actual calcemic fragment. Equivalent doses of both peptides were infused into five normal dogs (GFR = 51 ml/min) and eight dogs with a remnant kidney and chronic renal insufficiency (GFR = 15 ml/min). Both the calcemic and phosphaturic responses were studied. Remnant dogs had a blunted calcemic response to bPTH(1-84). The increase in fractional phosphate excretion was similar. In contrast, the calcemic response to bPTH(1-34) was equivalent in remnant and normal dogs. Treatment of uremic dogs with 400 ng 1,25(OH)2D daily for 3 days restored the effect of bPTH(1-84) on serum calcium and increased the control value for tubular phosphate reabsorption from 28 +/- 3 micrograms/ml GFR to 37 +/- 4 micrograms/ml GFR (P less than 0.01). These results suggest that there is an impaired conversion of PTH(1-84) to PTH(1-34) in chronic renal insufficiency and that 1,25 (OH)2D may be involved in metabolism of PTH(1-84). In addition, the effect of PTH on fractional phosphate excretion is not magnified in nonparathyroidectomized uremic dogs.

Abnormal skeletal response to parathyroid hormone in dogs with chronic uremia

The release of cyclic AMP from bone in response to stimulation with PTH 1-34 was examined in 20 dogs with long-term chronic renal failure (CRF) produced by unilateral nephrectomy and contralateral partial renal artery ligation. After 9 to 15 months of uremia, the tibiae were removed and perfused in vitro. Seven dogs with CRF served as controls, 7 dogs with CRF were treated with 24,25(OH)2D3 - 2.5 micrograms per day, and 6 CRF dogs underwent thyroparathyroidectomy (TPTX) 42 h before they were sacrificed. The release of cyclic AMP from bone in response to PTH 1-34 in the CRF dogs was severely reduced compared to the response observed in 7 dogs with normal renal function (net accumulation of cyclic AMP release 86 +/- 8.5 versus 426 +/- 59.0 pmol/30 min). Long-term treatment of uremic dogs with 24,25(OH)2D3 had no effect on the release of cyclic AMP by bone. However, the release of cyclic AMP was restored to normal levels in the CRF dogs that underwent thyroparathyroidectomy. All CRF dogs had secondary hyperparathyroidism and the fact that TPTX returned the cyclic AMP response to normal values suggests that desensitization to PTH of the adenylate cyclase system of bone exists in chronic uremia.

[Hyperparathyroidism in hemodialysis patients: studies on parathyroid regulation and classification problems (author's transl)]

In a series of 22 patients undergoing chronic intermittent hemodialysis the response in secretion of parathyroid hormone (PTH) to an acute increase in serum calcium (Ca) concentration was studied. All patients had raised levels of predialysis PTH. Ca concentrations were measured by means of atomic absorption spectrophotometry and PTH concentrations were determined by radioimmunoassay. A close, significant, linear, negative correlation was demonstrated by regression analysis when Ca concentrations were correlated to changes in PTH, expressed as a percentage of the initial predialysis PTH value (100%). This finding supports the assumption that the function of the parathyroid gland at cellular level is similar in all patients, irrespective of very different predialysis PTH concentrations in peripheral blood. The results together with findings in literature were used to describe secondary hyperparathyroidism as a disease which shows typical phases during progression. Some still obscure phenomena of secondary hyperparathyroidism can be explained by discriminating between parathyroid function at a cellular and an organ level. The classification of hyperparathyroidism in regulative and autonomous forms only seems to be applicable when referred to the cellular level.

The pathophysiology of altered calcium metabolism in rhabdomyolysis-induced acute renal failure. Interactions of parathyroid hormone, 25-hydroxycholecalciferol, and 1,25-dihydroxycholecalciferol

We studied six oliguric patients with rhabdomyolysis-induced acute renal failure. On admission, all had marked hyperphosphatemia and hypocalcemia associated with low levels of 1,25-dihydroxycholecalciferol [1,25(OH)2D]. During the early polyuric phase, moderate hypercalcemia was accompanied by marked elevations in plasma 1,25(OH)2D and persistent elevations in parathyroid hormone (both amino and carboxy terminals). During the late polyuric phase, the levels of serum calcium and 1,25(OH)2D reverted to normal. Thus, in rhabdomyolysis-induced acute renal failure, the hypocalcemia of the oliguric phase may be secondary to decreased synthesis of 1,25(OH)2D; severe hyperphosphatemia may also have a major role. The hypercalcemia of the polyuric phase may be partly due to increased synthesis of 1,25(OH)2D, resulting from the high parathyroid hormone levels and recovery of renal function.

Evidence that resistance to the calcemic action of parathyroid hormone in rats with acute uremia is caused by phosphate retention

An animal model was developed to examine the cause of resistance to the calcemic action of PTH in renal failure. Thyroparathyroidectomized (TPTX) rats were repeatedly reinfused with their excreted urine, over a 5-hour period, to produce an acute uremic animal with normal kidneys. Nonuremic controls were infused with equivalent volumes of a simple electrolyte solution. Using this model, we have demonstrated impaired calcemic response to PTH (Lilly Parathyroid Extract, 80 U/100 g) in urine-infused rats compared with electrolyte-infused rats. The final plasma calcium concentrations were 12.0 +/- 0.3 and 14.9 +/- 0.3 mg/dl, respectively (P less than 0.001). The cause of this impaired calcemic response was investigated by reinfusing rats with their own urine that had been pretreated with either activated charcoal or zirconium oxide in two different anionic forms, or urine that had been ultrafiltrated through an Amicon membrane of which the stated molecular-weight cut-off of the smallest pore-size membrane was 500 daltons. It was found that charcoal and ultrafiltration techniques had no effect, whereas the zirconium oxide treatment completely corrected the impaired calcemic response and returned the plasma phosphorus to a concentration similar to that in nonuremic controls. This can be explained by the fact that the principal effect of zirconium oxide on urine is to remove inorganic phosphate. Other groups of TPTX rats given PTH extract were infused with an electrolyte solution containing varying amounts of phosphate up to a maximum similar to the amount that a urine-infused rat would receive. A highly significant inverse relationship was found between the dose of phosphate infused with the electrolyte solution and the measured calcemic response to PTH. This relationship is represented by the following equation: calcium (mg/dl) = 14.84 - 0.139 inorganic phosphate; r = 0.915, P less than 0.001. From these series of studies, we conclude that phosphate retention is the cause of resistance to the calcemic action of PTH extract in this acute uremic model.