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.     

FGF-23 in patients with end-stage renal disease on hemodialysis

BACKGROUND: Fibroblast growth factor (FGF)-23 is a recently identified circulating factor which causes renal phosphate wasting disorders. Although the mechanism of regulation of FGF-23 secretion is unknown, plasma FGF-23 level may be regulated or affected by serum phosphate levels because of its hypophosphatemic effect. METHODS: We tested the hypothesis that plasma FGF-23 levels may be increased in hyperphosphatemia in patients with end-stage renal disease (ESRD) on maintenance hemodialysis. We measured plasma FGF-23 levels in 158 male uremic patients on maintenance hemodialysis. Plasma samples were obtained before starting dialysis sessions to determine FGF-23 levels by enzyme-linked immunosorbent assay (ELISA). RESULTS: Plasma FGF-23 level exhibited significant and positive correlations with inorganic phosphate, intact parathyroid hormone (PTH), corrected calcium, and duration of hemodialysis on simple regression analyses. All these associations remained significant in multiple regression analyses. CONCLUSION: Serum phosphate, calcium, and intact PTH could be regulators of FGF-23 levels in uremic patients on maintenance hemodialysis. Our results may provide new insights into the pathophysiologic effects of FGF-23 on calcium-phosphate homeostasis.     

Pathogenesis of refractory secondary hyperparathyroidism

Calcitriol is currently used to reduce parathyroid hormone (PTH) levels in uremic patients. However, a significant number of patients fail to respond to calcitriol therapy. The data suggest that a poor response to calcitriol can be anticipated in patients with severe hyperparathyroidism (with a high basal PTH levels) and uncontrolled serum phosphate. The abnormal parathyroid response to calcitriol in uremic patients with severe parathyroid hyperplasia may be attributed, to a large extent, to the development of nodular hyperplasia as a result of clonal transformation from a diffuse polyclonal hyperplasia. The factors involved in the development of polyclonal parathyroid hyperplasia, at earlier stages of secondary hyperparathyroidism, appear to be the same factors that stimulate PTH secretion and synthesis: hypocalcemia, hyperphosphatemia and low serum calcitriol levels. Studies performed in vitro using parathyroid tissue from uremic patients who required parathyroidectomy demonstrate that in nodular hyperplasia there is an abnormal response to calcium and calcitriol, which suggests that there are factors intrinsic to the hyperplastic cell (such as decrease in calcium sensor receptors and vitamin D receptors) responsible for an abnormal regulation of parathyroid function. Accumulation of phosphate is a key factor in the pathogenesis of secondary hyperparathyroidism and a poor response to calcitriol treatment is associated with the failure to control the serum phosphorus. High phosphate stimulates PTH secretion as demonstrated by in vivo and in vitro studies. In addition, animal studies strongly suggest that phosphate increases parathyroid cell proliferation. There are growth-related genes potentially involved in uremic hyperparathyroidism; however, changes in the expression of these genes may be the consequence rather than the cause of parathyroid hyperplasia.     

Parathyroid hormone: new assays, new receptors

Accurate measurements of parathyroid hormone (PTH) in plasma are necessary for the assessment, monitoring, and therapy of disorders of bone and mineral metabolism including renal osteodystrophy. Assays for PTH have evolved to provide 2-site immunometric assays that are highly specific for the intact 84 amino-acid peptide, PTH (1-84). With the advent of such assays, it has been shown that the prior generation of assays, thought to measure intact PTH, in fact, also detected a PTH peptide that was truncated at the N-terminus and that appeared to be similar to PTH (7-84). There has been renewed interest in such circulating PTH fragments in view of the demonstration that PTH (7-84) (and other PTH peptides) might have biologic effects. These effects include an action to oppose the calcemic effect of PTH in vivo and to inhibit bone resorption and osteoclast generation in vitro. These effects appear to be mediated by actions of a receptor for PTH peptides with specificity for the C-terminal region of PTH and distinct from the PTH receptor known to be responsible for all of the classic actions of PTH. Although the C-PTH receptor has not yet been cloned, the observations have opened a new field of research in parathyroid physiology. Clinical applications of the assay of such PTH fragments in relation to the amount of circulating PTH (1-84) concentrations are being sought actively as the new PTH assay methodology is applied to the clinical arena and as the biology of the C-PTH receptor and C-terminal PTH fragments are investigated.     

Race is a major determinant of secondary hyperparathyroidism in uremic patients

In the general population, blacks have higher parathyroid gland mass and circulating parathyroid hormone (PTH) levels than whites. This may predispose black patients to more severe parathyroid disease when renal failure develops. Therefore, racial differences in the severity of uremic hyperparathyroidism were examined in a population of patients with end-stage renal disease (ESRD). Among ESRD patients receiving hemodialysis or peritoneal dialysis, two or more values of intact PTH (immunoradiometric assay, pg/ml) obtained at least 90 d apart were available in 1270 prevalent cases (61.1% blacks, 51% males, and 31.1% diabetic), including 466 incident cases with onset of ESRD after 1993. Maximum PTH levels were analyzed as a function of race, gender, age, diabetic status, and levels of serum calcium, phosphorus, alkaline phosphatase, and aluminum. Using a stepwise multiple regression model, the determinants of maximum PTH in the order of their importance were black race, serum phosphorus, absence of diabetes, younger age, serum calcium, and female gender. The maximum PTH levels averaged 641.7 in blacks and 346.0 in whites after adjusting for age, gender, diabetic status, serum calcium, and phosphorus (P < 0.0001). In blacks compared with whites, the odds ratio (95% confidence interval) for adynamic bone disease (maximum PTH <150 pg/ml) was 0.26 (0.17 to 0.41), whereas the odds ratio for hyperparathyroid bone disease (mean PTH >500 pg/ml) was 4.4 (2.10 to 9.25). Race is a major independent determinant of uremic secondary hyperparathyroidism. Among ESRD patients, blacks may be at an increased risk for hyperparathyroid bone disease and whites for adynamic bone disease.     

Non-aluminic adynamic bone disease in non-dialyzed uremic patients: a new type of osteopathy due to overtreatment?

Adynamic bone disease, characterized by a low bone formation rate with normal or reduced amount of unmineralized osteoid, is supposed to be the consequence of aluminum intoxication in uremic patients. However, the emergence of adynamic bone disease has been recently reported in hemodialyzed patients in the total absence of aluminum overload. This study was aimed to assess whether such a histological pattern of adynamic bone disease was already present in uremic patients not yet on dialysis. Twenty-seven asymptomatic uremic patients (mean age +/- SD 43 +/- 10 years, mean creatinine clearance 19 +/- 3 ml/mm) were studied and bone biopsies were repeated in 16 of them after 18 +/- 10 months of treatment with oral calcium carbonate (1-3 g of elemental calcium/day) and calcidiol (21 +/- 14 micrograms/day). None of the patients received aluminum hydroxide, and the search for bone aluminum deposits was negative in all patients both before and after treatment. Two patients fulfilled the criteria of adynamic bone disease on their post-treatment biopsies. They originated from patients classified as having normal bone histology before treatment. Comparison with the other patients showed that they had comparable plasma C-terminal PTH but higher plasma creatinine than patients with normal bone histology and lower plasma C-terminal PTH than patients with osteitis fibrosa but comparable plasma creatinine. The plasma levels of 1,25(OH)2D reached values above normal after treatment in these two patients. It is suggested that adynamic bone disease not related to aluminum intoxication can develop in uremic patients independently of dialysis, and is favored by a relative hypoparathyroidism for the degree of renal failure, possibly induced by elevated plasma concentrations of calcitriol.     

Calcimimetics, mechanisms of action and therapeutic applications

The extracellular calcium-sensing receptor (CaR) on the parathyroid cell surface negatively regulates secretion of parathyroid hormone (PTH). Its activation by small changes in the extracellular concentration of ionized calcium (ec[Ca2+]) decreases PTH secretion and secondarily bone turnover. CaR is an ideal target for compounds that may be developed to modulate its activity - activating calcimimetics and inhibiting calcilytics. Calcimimetics can amplify the sensitivity of the CaR to ec(Ca2+), thereby suppressing PTH levels and in turn reducing blood Ca++. They dose-dependently reduce the secretion of PTH in cultured parathyroid cells, in animal models and in humans. In uremic animals, these compounds prevent parathyroid cell hyperplasia when given at the onset of the disease and stop cell proliferation if they are administered afterwards, when the hyperplasia already exists. They normalize plasma PTH levels and bone remodeling. In uremic patients undergoing hemodialysis, calcimimetics reduce plasma PTH concentrations in the short (12 weeks) and long (2 years) terms. They also reduce serum levels of calcium-phosphorus product. Calcimimetics are therefore an alternative for the treatment of secondary hyperparathyroidism, particularly in dialysis patients, when increased serum levels of calcium-phosphorus product, the attendant risk of cardiovascular calcification, and its lack of efficacy limit use of the standard treatment.     

Parathyroid hormone effect on the fragility of human young and old red blood cells in uremia

Parathyroid hormone (PTH) is elevated in patients with chronic renal failure (CRF) and was suggested to be one of the factors responsible for the anemic syndrome of these patients because it raises the osmotic fragility of the red blood cells (RBC). In the present study, the youngest and oldest RBC were separated from circulating erythrocytes by high-speed centrifugation. The age distribution was described, and the effect of PTH on the different age groups was investigated. Median density (MD) and glutamic-oxaloacetic transaminase (GOT) activity were chosen as age markers. MD (1.0985 +/- 0.00087) and GOT activity (12.49 +/- 2.083 IU/g Hb) of the young uremic cells did not differ significantly from the values of young normal cells (1.0987 +/- 0.00046 and 10.36 +/- 1.174 IU/g Hb, respectively). The MD of the oldest cells, however, was lower (1.1048 +/- 0.00054) and GOT was higher (6.60 +/- 1.1019 IU/g Hb) in the uremic than in the control cells (1.1093 +/- 0.00175 and 3.77 +/- 0.233 IU/g Hb, respectively). These results indicate that the life span of RBC in uremics is shorter than normal and that an enrichment of circulating RBC by young cells occurs in uremic patients. The median osmotic fragility (MOF) of the young cells was lower in both uremic (0.376 +/- 0.006) and control patients (0.378 +/- 0.003) than the MOF of old cells (0.402 +/- 0.005 and 0.392 +/- 0.004, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)     

The metabolism and radioimmunoassay of parathyroid hormone

Parathyroid hormone (PTH), a single-chain polypeptide of 84 amino acids, plays a critical role in the regulation of mineral metabolism. An assessment of the activity of the parathyroid glands is frequently required in clinical medicine. Assay of PTH in serum is complicated by the fact that circulating PTH is immunoheterogeneous. Thus, circulating immunoreactive PTH consists of a mixture of intact hormone and smaller molecular weight hormone fragments. These hormone fragments arise as a consequence of direct secretion from the parathyroid glands and also from peripheral metabolism of PTH. The principle organs involved in the metabolism of PTH are the liver and the kidney. The liver metabolizes the intact PTH by proteolysis in an endosomal compartment of Kupffer cells and returns small molecular weight fragments to the circulation. Metabolism of PTH by the kidney involves uptake of biologically active PTH molecules at peritubular sites and glomerular filtration of both active and inactive forms of PTH. Intact PTH and biologically active amino-terminal PTH fragments have extremely short half-lives in serum, while the biologically inactive PTH fragments from the mid-region and C-terminal portion of PTH molecule, which depend upon glomerular filtration for clearance, have prolonged half-lives. Radioimmunoassays for PTH in blood may yield widely differing results depending on the portion of the PTH molecule at which the particular antiserum is directed. Accordingly, for the understanding and interpretation of PTH assay results, it is necessary to have an understanding of the characteritics of any given PTH assay. PTH assays may be categorized into (a) amino-terminal (N-terminal) assays which recognize intact PTH as well as immunoterminal PTH fragments; (b) assays directed towards the C-terminal or midregion of PTH which recognize intact PTH and any hormone fragments containing these regions of the PTH molecule. Recently assays have been developed which recognize two regions of PTH simultaneously and using this immunoradiometric (IRMA) technique intact PTH can be measured. Results of assays for amino-terminal PTH or the intact PTH molecule have the advantage of having a similar “normal range” regardless of renal function. These assays have the disadvantages of the ability of N-terminal or intact PTH in serum and of showing a transient or pulsatile changes in PTH levels, depending upon the conditions under which the samples were obtained and assayed. C-terminal or mid region assays have the advantage of rapid assay times, lack of effect of acute variations in PTH secretion, have extensive correlations with bone histology and may reflect an integrated index of PTH secretion. Because of the role of the kidney in the clearance of these fragments, extremely high values may be obtained in patients with severe hyperparathyroidism in renal failure. Thus, a different “normal range” is required for patients with advanced renal failure. However, with the use of a particular assay such experience is rapidly obtained. Thus, these types of assays have proven to be extremely valuable in that assessment of disorders of mineral metabolism. Thus, knowledge of the metabolism of PTH and the characteristics of PTH assays, a reliable assessment of the activity of parathyroid gland can be obtained in diverse clinical circumstances.     

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.     

Chronic renal failure, parathyroid hormone and fatty acids oxidation in skeletal muscle

Fatty acids are an important source of skeletal muscle energy, and certain data suggest oxidation of long-chain fatty acids (LCFA) may be impaired in uremia. This abnormality may in part be responsible for uremic myopathy. Uremia is associated with hyperparathyroidism and PTH affects muscle metabolism; PTH enhances muscle proteolysis and impairs muscle bioenergetics, and it is possible that PTH also affects fatty acids oxidation. The present study examined in rats the effects of 4 days administration PTH and of 21 days of chronic renal failure (CRF) with and without excess PTH on oxidation of LCFA and short-chain fatty acids (SCFA). Both 1-84 and 1-34 PTH impaired oxidation of LCFA but not of a SCFA (beta-hydroxybutyric acid) and reduced the activity of carnitine palmitoyl transferase (CPT). Inactivation of the PTH abolished its effects. CRF rats with intact parathyroid glands had also impaired oxidation of LCFA and of CPT activity. Parathyroidectomy in CRF rats normalized these abnormalities. Carnitine contents of muscle were not altered. The data show that PTH excess in normal or in CRF rats is associated with impaired oxidation of LCFA and this effect is due to reduction in the activity of CPT, a key enzyme for the transport of LCFA to mitochondrial matrix for beta-oxidation. The data demonstrate another toxic effect of PTH on muscle in CRF and provide an additional pathogenic mechanism for uremic myopathy.     

Endothelin-1 regulates parathyroid hormone expression of human parathyroid cells

OBJECTIVE: Parathyroid cells synthesize and release endothelin-1 (ET-1). ET-1 displays an in vitro inhibitory effect on basal parathyroid hormone (PTH) secretion and also counteracts PTH hypersecretion stimulated by low calcium. Such effects are further demonstrated in vivo, independent of the changes in calcitonin. We propose that ET-1 may regulate the pathogenesis of uremic hyperparathyroidism. However, this was not directly demonstrated in human parathyroid glands. DESIGN: Hyperplastic parathyroid glands are obtained from the surgical operation for uremic hyperparathyroidism. Cells are isolated by enzyme digestion and treated with ET-1, and are assessed for PTH mRNA expression. PTH in the plasma and the medium is measured by a newly developed method to detect the whole PTH (1-84). PATIENTS: Uremic patients with severe secondary hyperparathyroidism and ultrasonography-proved hypertrophy of parathyroid glands received elective surgical approaches under general anesthesia. The resected glands were immediately taken to the laboratory for fresh isolation. MEASUREMENTS: Following ET-1 treatment, PTH mRNA expression is evaluated by RT-PCR method. ET-1 is detected with radioimmunoassay kit and PTH is measured by a new commercially available Duo PTH kit. RESULTS: ET-1 exhibited a dose-dependent inhibitory effect (from 10(-12) - 10(-7) M) on PTH mRNA expression of parathyroid cells, either in the basal or in the low-calcium-stimulated states. Release of PTH into the medium is also gradually inhibited by the increase in ET-1 concentrations. CONCLUSIONS: Our results demonstrate that ET-1 attenuates PTH mRNA expression in freshly isolated human parathyroid cells, and PTH release is also decreased. This result is consistent with our previously reported in vitro and in vivo experiments.     

Parathyroid hormone and the altered vascular response to norepinephrine in uremia

Patients with chronic renal failure manifest reduced pressor response to norepinephrine (NE); this abnormality is at least partly responsible for the autonomic nervous system dysfunction observed in these patients. Since uremia is associated with increased levels of parathyroid hormone (PTH) and since PTH blunts the pressor effect of NE most likely via activation of prostaglandins, we have studied the relationship between blood levels of PTH and the reduced pressor response to NE in 17 uremic patients and we examined the effect of treatment with indomethacin on the response to NE in 9 uremic and 5 normal subjects. There was a significant negative correlation (r = -0.63, p less than 0.01) between the changes in blood pressure and the blood levels of PTH in uremic patients. Treatment with indomethacin was followed by significant improvement or normalization of the pressor response to NE in uremic patients. These data are consistent with the notion that the decreased pressor response to NE in uremia is due to increased production of prostaglandins induced by excess PTH and provide a therapeutic tool for the treatment of some of the manifestations of autonomic nervous system dysfunction in uremia.     

The parathyroid polyhormone hypothesis revisited

The parathyroid polyhormone hypothesis holds that peptides derived from the metabolism of parathyroid hormone (PTH) (so-called C-terminal fragments) are themselves biologically active and that their effects are mediated by a novel 'C-terminal receptor.' The evidence supporting these assertions is extensive but remains inconclusive. This Commentary focuses on in vivo pharmacology studies that provide information relevant to understanding the physiological significance of C-terminal fragments. The more recent studies of this sort provide compelling evidence that the bioactivity of C-terminal fragments is likely to become physiologically relevant in settings of secondary hyperparathyroidism. In this condition, circulating levels of C-terminal fragments greatly exceed those of PTH. There is convincing evidence that the hypocalcemic effect of C-terminal fragments results from direct actions on the skeleton that inhibit bone resorption. On the other hand, there are few if any results of in vivo studies suggesting a role for C-terminal fragments in more physiological settings, at least when parameters associated with systemic calcium homeostasis are assessed.     

Hyperplasia of the parathyroid gland without secondary hyperparathyroidism

BACKGROUND: Low dietary phosphorus (P) prevents parathyroid gland (PTG) hyperplasia and the development of secondary hyperparathyroidism (SH) in uremic rats. The present study explores the effects of P restriction on parathyroid hormone (PTH) synthesis and secretion and PT cell growth in rats with established SH and PTG hyperplasia. METHODS: Normal and 5/6 nephrectomized rats were fed a high P (0.8%) diet. After two weeks, the normal rats and half of the uremic rats were sacrificed (U-HP) while the remaining uremic rats were switched to a low P (0.2%) diet (U-HP-LP). RESULTS: High dietary P induced a significant increase in serum P, PTH, and PTG weight, but not ionized calcium compared to normal animals fed the same diet (N-HP). P restriction returned serum P and PTH to normal levels by one week. In contrast, PTG size did not regress and glands remained enlarged for up to eight weeks with no evidence of apoptosis. Ribonuclease protection assay and metabolic labeling studies demonstrated similar PTH/actin mRNA ratios and 35S-labeled PTH among the three groups. Intracellular intact PTH was higher in U-HP and U-HP-LP rats compared to N-HP animals with no differences between the two uremic groups. PTG-PTH content correlated only with PTG weight, and serum PTH only with serum P. The PTG secretory response to calcium remained intact. CONCLUSIONS: In established chief-cell hyperplasia, P restriction restores normal serum PTH levels without affecting PTG hyperplasia, PTH synthesis, PTG cytosolic PTH or the PTH secretory response to calcium, suggesting an impaired exocytosis of PTH.     

A novel mechanism for skeletal resistance in uremia

BACKGROUND: In treating secondary hyperparathyroidism, the target level of serum intact parathyroid hormone (I-PTH) should be three to five times normal to prevent adynamic bone disease. In circulation, there is a non-(1-84) PTH-truncated fragment, likely 7-84, which, in addition to PTH 1-84, is measured by most I-PTH immunoradiometric (IRMA) assays, giving erroneously high I-PTH values. We have developed a new IRMA assay in which the labeled antibody recognizes only the first six amino acids of the PTH molecule. Thus, this new IRMA assay (Whole PTH) measures only the biologically active 1-84 PTH molecule. METHODS: Using this new IRMA assay (Whole PTH) and the Nichols "intact" PTH assay, we compared the ability of each assay to recognize human PTH (hPTH) 1-84 and hPTH 7-84 and examined the percentage of non-1-84 PTH in circulation and in parathyroid glands. Possible antagonistic effects of the 7-84 PTH fragment on the biological activity of 1-84 PTH in rats were also tested. RESULTS: In 28 uremic patients, PTH values measured with the Nichols assay, representing a combined measurement of both hPTH 1-84 and hPTH 7-84, were 34% higher than with the Whole assay (hPTH 1-84 only); the median PTH was 523 versus 318 pg/mL (P < 0.001). Similar results were found in 14 renal transplant patients. In osteoblast-like cells, ROS 17.2, 1-84 PTH (10-8 mol/L) increased cAMP from 18.1 +/- 1.25 to 738 +/- 4.13 mmol/well. Conversely, the same concentration of 7-84 PTH had no effect. In parathyroidectomized rats fed a calcium-deficient diet, 7-84 PTH was not only biologically inactive, but had antagonistic effects on 1-84 PTH in bone. Plasma calcium was increased (0.65 mg/dL) two hours after 1-84 PTH treatment, while 7-84 PTH had no effect. When 1-84 PTH and 7-84 PTH were given simultaneously in a 1:1 molar ratio, the calcemic response to 1-84 PTH was decreased by 94%. In normal rats, the administration of 1-84 PTH increased renal fractional excretion of phosphate (11.9 to 27.7%, P < 0.001). However, when 1-84 PTH and 7-84 PTH were given simultaneously, the 7-84 PTH decreased the phosphaturic response by 50.2% (P < 0.005). Finally, in surgically excised parathyroid glands from six uremic patients, we found that 44.1% of the total intracellular PTH was the non-PTH (1-84), most likely PTH 7-84. CONCLUSION: In patients with chronic renal failure, the presence of high circulating levels of non-1-84 PTH fragments (most likely 7-84 PTH) detected by the "intact" assay and the antagonistic effects of 7-84 PTH on the biological activity of 1-84 PTH explain the need of higher levels of "intact" PTH to prevent adynamic bone disease.     

The consequences of uncontrolled secondary hyperparathyroidism and its treatment in chronic kidney disease

Secondary hyperparathyroidism (HPT) is a common complication of chronic kidney disease (CKD) and a frequent cause of clinically significant bone disease. Soft-tissue and vascular calcification, cardiovascular disease, and calcific uremic arteriolopathy (CUA) are additional serious consequences of the disorder that may contribute directly to cardiovascular morbidity and mortality in patients with CKD. Less widely appreciated manifestations include neurological disturbances, hematological abnormalities, and endocrine dysfunction. Secondary HPT arises from alterations in calcium, phosphorus, and vitamin D metabolism that develop early in the course of CKD and become more pronounced as kidney function declines. Treatment is often delayed, however, until the disease is well established. Current therapeutic strategies rely largely on the use of vitamin D sterols to diminish excess parathyroid hormone (PTH) synthesis and to lower serum or plasma PTH levels, but their use is often confounded by increases in serum calcium and phosphorus concentrations, changes that can aggravate soft-tissue and vascular calcification. As such, there is a need for new therapeutic interventions that can effectively lower serum or plasma PTH levels without producing untoward side effects. The current review summarizes the diverse manifestations of secondary HPT in patients with CKD. The consequences of inadequately controlled secondary HPT and the adverse effects of selected therapeutic interventions for the disorder on vascular calcification and cardiovascular disease in those with CKD are discussed.     

QuiCk-IntraOperative Bio-Intact PTH Assay at Parathyroidectomy for Secondary Hyperparathyroidism

Background In uremic patients, metabolism of 1-84 parathyroid hormone (PTH) and fragments are delayed, and in these patients, the usefulness of intraoperative PTH assay may be problematic. We evaluated the usefulness of the QuiCk-IntraOperative Bio-Intact PTH (QPTH) assay for uremic patients with secondary hyperparathyroidism who required total parathyroidectomy (PTx) with forearm autograft. The purpose of our study was to recognize whether QPTH in uremic patients was useful to determine during operation whether complete PTx had been achieved. Methods Forty-four patients who underwent initial PTx were enrolled in this study. Blood samples were drawn just after induction of general anesthesia (basal samples), immediately after removal of the last gland, and at 5, 10, 15, and 30 minutes, and at the first morning after PTx. The assay was performed immediately after sample collection. Reductions of PTH levels were evaluated and expressed in percentage of basal levels. Results The mean PTH levels in 41 patients, excluding 3 in whom the PTH level did not drop significantly (>60 pg/ml), measured by QPTH at anesthesia, 0, 5, 10, 15, and 30 minutes were 734.3, 104.7, 58.8, 37.4, 27.0, 16.3 pg/ml, corresponding to 100%, 17.1%, 9.3%, 5.8%, 4.1%, 2.4% of the preexcision values, respectively. If the cutoff value was defined as 10.8% at 10 minutes, the sensitivity was 100% and specificity 90%. When the QPTH level dropped to under 10.8% at 10 minutes, we could consider that all glands were removed. Conclusions QPTH in uremic patients is very useful to determine whether complete PTx is achieved during operation.     

Predictors of bone mineral density in patients on hemodialysis

Renal osteodystrophy is a universal complication of uremia. Renal failure patients are at risk for low bone mineral density (BMD) and fractures. Parathyroid hormone (PTH) plays a pivotal role in the pathophysiology of uremic bone disease. Histomorphometric studies suggest that the maintenance of PTH levels between two and four times the upper limit of normal is associated with the lowest prevalence of two common forms of osteodystrophy: osteitis fibrosa cystica and adynamic bone disease. The purpose of this study was to investigate whether the above recommendation for PTH levels in dialysis patients corresponds to a more optimal BMD with a special emphasis on diabetic versus nondiabetic subjects. Twenty-eight patients with chronic renal failure on hemodialysis underwent measurement of PTH levels, as well as BMD at the lumbar spine, hip, and forearm. They were divided into three groups based on the mean PTH level over the 5 years prior to having BMD measured. Osteoporosis was diagnosed in 55% of men and 87% of women on dialysis. Predictors of BMD were gender, duration on hemodialysis, and diabetes. Our study supports the histomorphometry-based studies suggesting that the maintenance of intact PTH levels two to four times the upper limit of normal may be associated with better skeletal health in uremic patients on hemodialysis, and that the diabetic subgroup is at particular risk for low BMD.     

Plasma insulin-like growth factors and bone formation in uremic hyperparathyroidism

Bone formation in uremia is considered to be regulated in part by parathyroid hormone (PTH). However, while low levels of immunoreactive PTH are usually associated with low rates of bone formation in uremia, elevated PTH levels do not always correlate with increased bone formation. In an attempt to identify other factors that may regulate bone formation in uremic patients, we measured plasma immunoreactive insulin-like growth factors (IGF-I and IGF-II) in 15 patients who did not have aluminum-associated reductions in bone formation. Plasma levels of IGF-I but not PTH, were significantly higher in patients with high rates of bone formation when compared to patients with low or normal bone formation (P less than 0.02). While the bone formation rate at the tissue level correlated significantly with plasma PTH (r = 0.53, P less than 0.05) and IGF-I (r = 0.67, P less than 0.01), only for plasma IGF-I were there significant correlations with bone apposition (r = 0.57, P less than 0.05) and bone formation rate at the BMU level (r = 0.62, P less than 0.02), parameters which reflect mineralization activity at the cellular level. Among the static histologic parameters, osteoblastic osteoid correlated only with plasma PTH (r = 0.76, P less than 0.001), while osteoclast number correlated with both PTH (r = 0.56, P less than 0.05) and IGF-I (r = 0.67, P less than 0.01). There were no correlations between IGF-II levels and bone histology. From these data we suggest that IGF-I may promote bone formation in uremic patients with hyperparathyroidism.(ABSTRACT TRUNCATED AT 250 WORDS)