Calcium-regulated parathyroid hormone release in patients with mild or advanced secondary hyperparathyroidism

Usefulness and feasibility of measuring ionized calcium in haemodialysis patients

Background

Measuring blood calcium level is recommended in haemodialysis (HD) patients. The Kidney Disease Improving Global Outcomes position states that the measurement of ionized calcium (ICa) level is preferred, but in the clinical setting, due to technical difficulties, total calcium (tCa) level is preferred to ICa.

Aim

The aim of this study was to test the possibility of delayed ICa analysis using frozen serum, and so to identify the factors associated with predialysis ICa level and compare the ability of tCa and Alb-Ca to predict ICa level and finally to compare the survival rate according to the three calcium measurements.

Methods

All prevalent HD patients, dialysed by a native AV fistula in a 3 × 4 to 3 × 8 h schedule, had their predialysis ICa, tCa and Alb-Ca levels and usual mid-week biology recorded. Intergroup comparisons between ICa quartile were performed. Bland–Altman plots and linear regression were used to assess the differences between 30 fresh and frozen samples. Survival analyses were performed using ICa and tCa levels.

Results

Comparing fresh blood and frozen serum samples, linear regression (y = 0.98 + 0.02, r = 0.961) showed that the two methods were quite identical with the same mean ICa value (1.1 ± 0.1 mmol/L, P = 0.45). A total of 160 HD patients were included in the study. Hypocalcaemia, using ICa values, was highly prevalent in our population (40%) whereas hypercalcaemia was observed only in three cases (1.8%). In predicting ICa hypocalcaemia (<1.12 mmol/L, n = 64), the use of tCa was accurate in 48.4% of patients, and the use of Alb-Ca was accurate in only 17.2% of patients; tCa was not a predictive factor for hypercalcaemia (ICa > 1.32 mmol/L, n = 3); Alb-Ca value predicted hypercalcaemia in 2/3 of the patients. In predicting normocalcaemia, the use of tCa values was correct in 92.4% of patients and the use of Alb-Ca values in 88.1% of patients; only younger age (P = 0.03) and female sex (P = 0.01) were associated with higher ICa quartile. None of the three calcium measures was significantly associated with survival rate using log-rank and Cox models adjusted for age, dialysis vintage, diabetes and sex.

Conclusion

In the present study, we report that (1) delayed ICa measure is feasible in dialysis patients using a freezing technique, (2) hypocalcaemia is highly prevalent in HD patients and poorly predicted by Alb-Ca level, (3) the main factor associated with ICa level is sex of the individual and (4) calcaemia is not associated with survival rate using any of the three methods.

Development and progression of secondary hyperparathyroidism in chronic kidney disease: lessons from molecular genetics

The identification of the calcium-sensing receptor (CaSR) and the clarification of its role as the major regulator of parathyroid gland function have important implications for understanding the pathogenesis and evolution of secondary hyperthyroidism in chronic kidney disease (CKD). Signaling through the CaSR has direct effects on three discrete components of parathyroid gland function, which include parathyroid hormone (PTH) secretion, PTH synthesis, and parathyroid gland hyperplasia. Disturbances in calcium and vitamin D metabolism that arise owing to CKD diminish the level of activation of the CaSR, leading to increases in PTH secretion, PTH synthesis, and parathyroid gland hyperplasia. Each represents a physiological adaptive response by the parathyroid glands to maintain plasma calcium homeostasis. Studies of genetically modified mice indicate that signal transduction via the CaSR is a key determinant of parathyroid cell proliferation and parathyroid gland hyperplasia. Because enlargement of the parathyroid glands has important implications for disease progression and disease severity, it is possible that clinical management strategies that maintain adequate calcium-dependent signaling through the CaSR will ultimately prove useful in diminishing parathyroid gland hyperplasia and in modifying disease progression.

Cinacalcet reduces the set point of the PTH-calcium curve

The calcimimetic cinacalcet increases the sensitivity of the parathyroid calcium-sensing receptor to calcium and therefore should produce a decrease in the set point of the parathyroid hormone (PTH)-calcium curve. For investigation of this hypothesis, nine long-term hemodialysis patients with secondary hyperparathyroidism were given cinacalcet for 2 mo, the dosage was titrated per a protocol based on intact PTH and plasma calcium concentrations. Dialysis against low- and high-calcium (0.75 and 1.75 mM) dialysate was used to generate curves describing the relationship between PTH and calcium. Compared with precinacalcet levels, cinacalcet significantly reduced mean serum calcium, intact PTH and whole PTH (wPTH; all P < 0.001). The set points for PTH-calcium curves were significantly reduced, and both maximum and minimum levels of PTH (intact and whole) were significantly decreased. The calcium-mediated inhibition of PTH secretion was more marked after cinacalcet treatment. In addition, cinacalcet shifted the inverse sigmoidal curve of wPTH/non-wPTH ratio versus calcium to the left (i.e., less calcium was required to reduce the wPTH/non-wPTH ratio). In conclusion, cinacalcet increases the sensitivity of the parathyroids to calcium, causing a marked reduction in the set point of the PTH-calcium curve, in hemodialysis patients with secondary hyperparathyroidism.

Calcium-mediated parathyroid hormone release changes in patients treated with the calcimimetic agent cinacalcet

BACKGROUND

The parathyroid-calcium (Ca(2+)-PTH) curve expresses modulation of parathyroid hormone (PTH) secretion by the parathyroid gland as a function of changing extracellular Ca(2+) concentration. Patients with hyperparathyroidism (HPT) show a rightward shift of the curve compared with controls, suggesting a reduced sensitivity of parathyroid cells to Ca(2+). Increasing the sensitivity of the parathyroid gland to extracellular Ca(2+) by manipulation of the Ca(2+)-sensing receptor (CaR) may have therapeutic potential. Calcimimetics allosterically modify CaR and render it more sensitive to extracellular Ca(2+), accounting for the simultaneous reduction of Ca(2+) and PTH seen in most patients.

METHODS

The Ca(2+)-PTH curve was evaluated in 10 haemodialysis patients, with baseline intact PTH levels >300 pg/ml in two haemodialysis sessions, one before and the other after (range, 9-22 weeks) cinacalcet treatment. In each session a 2-h low-dialysate Ca(2+) concentration was used to induce hypocalcaemia and maximally stimulate PTH secretion, followed immediately by a 2-h high-dialysate Ca(2+) concentration to induce hypercalcaemia and maximally inhibit PTH secretion.

RESULTS

Significant decreases in ionized Ca(2+) and intact PTH were observed following cinacalcet treatment. Cinacalcet treatment also led to a decrease in the set point for Ca(2+) and to a leftward shift of the Ca(2+)-PTH curve. Significant differences were present in all segments of the Ca(2+)-PTH curves.

CONCLUSION

The pathological rightward shift of the Ca(2+)-PTH curve seen in many HPT patients may be reversed by cinacalcet treatment.

Renal Osteodystrophy in Peritoneal Dialysis: Special Considerations

Bone disease is one of the most challenging complications in patients with chronic kidney disease. Today, it is considered to be part of a complex systemic disorder manifested by disturbances of mineral metabolism and vascular calcifications called chronic kidney disease – mineral bone disorder (CKD-MBD).

The term renal osteodystrophy is reserved to define the specific bone lesion in CKD-MBD, whose spectrum ranges from high turnover to low turnover disease. Phosphate retention, decreased serum calcium, and 1,25-dihydroxy vitamin D synthesis are involved in the pathogenesis of high bone turnover. However, the various therapeutic approaches (calcium supplements, phosphate binders, and vitamin D metabolites, among others), the renal replacement modality (hemodialysis or continuous ambulatory peritoneal dialysis), and the types of patients to whom dialysis is offered (more patients who are diabetic or older, or both) may influence the evolution of the bone disorder. As a result, recent studies have reported a greater prevalence of adynamic forms of renal osteodystrophy, especially in diabetic and peritoneal dialysis patients.

The present article reviews, for patients treated with peritoneal dialysis, the pathophysiologic mechanisms involved in the evolution and perpetuation of this bone disease and the therapeutic modalities for treating and preventing adynamic bone.

Dynamics of PTH secretion in hemodialysis patients as determined by the intact and whole PTH assays

BACKGROUND

Renal hyperparathyroidism is assessed by measurement of parathyroid hormone (PTH) levels. The intact PTH assay (I-PTH) not only reacts with 1-84 PTH but also with large, truncated fragments of non-1-84 PTH. Because the whole PTH assay (W-PTH) is specific for 1-84 PTH, non-1-84 PTH is determined by subtracting W-PTH from I-PTH values. These large circulating PTH fragments may exert a hypocalcemic effect by contributing to skeletal resistance to 1-84 PTH.

METHODS

The dynamic secretion of both 1-84 PTH and non-1-84 PTH was evaluated during the induction of hypo- and hypercalcemia in eight hemodialysis patients.

RESULTS

The basal ionized calcium concentration was 1.23 +/- 0.03 mmol/L at which time I-PTH, W-PTH, and non-1-84 PTH values were 276 +/- 78 pg/mL, 164 +/- 48 pg/mL, and 102 +/- 28 pg/mL, respectively. The induction of hypo- and hypercalcemic changes resulted in a sigmoidal response for all three PTH moieties, I-PTH, W-PTH, and non-1-84 PTH. During hypocalcemia, maximal values of W-PTH were greater than those of non-1-84 PTH. But during hypercalcemia, minimal values of W-PTH and non-1-84 PTH were similar. Neither the set points nor the basal/maximal ratios for W-PTH, I-PTH, and non-1-84 PTH were different. At the baseline ionized calcium concentration, the W-PTH (1-84 PTH)/non-1-84 PTH ratio was 1.53 +/- 0.15. Changes in ionized calcium resulted in a sigmoidal relationship with hypocalcemia, increasing this ratio to a maximum of 2.01 +/- 0.30 and hypercalcemia decreasing this ratio to a minimum of 1.18 +/- 0.15 (P < 0.01 vs baseline for both hypo- and hypercalcemia).

CONCLUSION

Although acute changes in serum calcium produce similar secretory responses in 1-84 PTH and non-1-84 PTH, the secretory responses are not proportional for these PTH moieties. Changes in the serum calcium concentration modulate the ratio of 1-84 PTH/non-1-84 PTH in a sigmoidal pattern with hypocalcemia maximizing this ratio. Whether changes in the 1-84 PTH/non-1-84 PTH ratio specifically modulate the calcemic action and other biologic effects of 1-84 PTH remain to be determined.

A new method for in vivo analysis of parathyroid hormone-calcium set point in mice

Although methods for measuring the parathyroid hormone (PTH)-calcium set point in vivo in humans and large animals exist, translating such methods to the increasingly important mouse model poses considerable challenges. We also found that manipulation of dietary calcium does not yield sufficiently high or low serum calcium levels to achieve the minimum and maximum PTH levels needed for set point estimations. Therefore, we developed a new method for in vivo evaluation of the relative set point in mice. Intraperitoneal injection of calcium gluconate caused progressive increases in serum calcium over 120 minutes, with corresponding decreases in serum PTH levels. Intraperitoneal injection of Na2-EGTA caused a nadir of serum calcium at 30 minutes and a corresponding peak value of serum PTH. The lowest and highest serum calcium concentrations achieved were 6.7 mg/dl and 10.0 mg/dl, respectively. Linear regression analyses indicated high correlation coefficients (serum calcium vs. serum PTH; r = 0.969). To obtain the additional data points needed for set point estimation, blood was collected 30 minutes after intraperitoneal injection by tail nicking from three mice once a day, on 8 consecutive days, using multiple doses of calcium gluconate or Na2-EGTA. The lowest and highest serum calcium concentrations achieved were 5.0 mg/dl and 11.4 mg/dl. Maximum and minimum PTH levels were indeed observed, and a sigmoidal curve with a set point of 7.8 mg/dl was readily generated by the four-parameter model that was fit using a nonlinear mixed effects statistical approach. This protocol for in vivo set point analysis should be applicable in the future study of multiple genetically engineered and pharmacologically treated mouse models.

Clonal chromosomal defects in the molecular pathogenesis of refractory hyperparathyroidism of uremia

Indirect X chromosome-inactivation analyses have demonstrated that most parathyroid glands from patients with uremic refractory secondary/tertiary hyperparathyroidism are monoclonal neoplasms. However, little is known regarding the specific acquired genetic abnormalities that must underlie such clonal expansion or the molecular pathogenetic features of this disorder, compared with primary parathyroid adenomas. To address these issues in a uniquely powerful manner, both comparative genomic hybridization (CGH) and genome-wide molecular allelotyping were performed with a large group of uremia-associated parathyroid tumors. As indicated by CGH, one or more chromosomal changes were present in 24% of the tumors, which is markedly different from the value for common sporadic adenomas (72%). Two recurrent abnormalities that had not been previously described for sporadic parathyroid adenomas were noted with CGH, i.e., gains on chromosomes 7 (9%) and 12 (11%). Losses on chromosome 11 occurred in only one of the 46 uremia-associated tumors (2%); the tumor also contained a somatic mutation of the remaining MEN1 allele (221del18). A total of 13% of tumors demonstrated recurrent allelic loss on 18q, with 18q21.1-q21.2 being defined as the putative tumor suppressor-containing region. In conclusion, the powerful combination of genome-wide molecular allelotyping and CGH has identified recurrent clonal DNA abnormalities that suggest the existence and locations of genes important in uremic hyperparathyroidism. In addition, genome-wide patterns of somatic DNA alterations, including disparate roles for MEN1 gene inactivation, indicate that markedly different molecular pathogenetic processes exist for clonal outgrowth in severe uremic hyperparathyroidism versus common parathyroid adenomas.

A calcimimetic agent lowers plasma parathyroid hormone levels in patients with secondary hyperparathyroidism

BACKGROUND

The calcimimetic agent R-568 lowers plasma parathyroid hormone (PTH) levels in hemodialysis patients with mild secondary hyperparathyroidism, but its efficacy in those with more severe secondary hyperparathyroidism has not been studied.

METHODS

Twenty-one patients undergoing hemodialysis three times per week with plasma PTH levels between 300 and 1200 pg/mL were randomly assigned to 15 days of treatment with either 100 mg of R-568 (N = 16) or placebo (N = 5). Plasma PTH and blood ionized calcium levels were measured at intervals of up to 24 hours after oral doses on days 1, 2, 3, 5, 8, 11, 12, and 15.

RESULTS

Pretreatment PTH levels were 599 +/- 105 (mean +/- SE) and 600 +/- 90 pg/mL in subjects given R-568 or placebo, respectively, and values on the first day of treatment did not change in those given placebo. In contrast, PTH levels fell by 66 +/- 5%, 78 +/- 3%, and 70 +/- 3% at one, two, and four hours, respectively, after initial doses of R-568, remaining below pretreatment values for 24 hours. Blood ionized calcium levels also decreased after the first dose of R-568 but did not change in patients given placebo. Despite lower ionized calcium concentrations on both the second and third days of treatment, predose PTH levels were 422 +/- 70 and 443 +/- 105 pg/mL, respectively, in patients given R-568, and values fell each day by more than 50% two hours after drug administration. Predose PTH levels declined progressively over the first nine days of treatment with R-568 and remained below pretreatment levels for the duration of study. Serum total and blood ionized calcium concentrations decreased from pretreatment levels in patients given R-568, whereas values were unchanged in those given placebo. Blood ionized calcium levels fell below 1.0 mmol/L in 7 of 16 patients receiving R-568; five patients withdrew from study after developing symptoms of hypocalcemia, whereas three completed treatment after the dose of R-568 was reduced.

CONCLUSIONS

The calcimimetic R-568 rapidly and markedly lowers plasma PTH levels in patients with secondary hyperparathyroidism caused by end-stage renal disease.

Effect of a low calcium dialysate on parathyroid hormone secretion in diabetic patients on maintenance hemodialysis

Diabetic patients on maintenance dialysis often are characterized by a relative parathyroid hormone (PTH) deficiency and a form of renal osteodystrophy with low bone turnover known as adynamic bone. The goal of the present study was to determine whether a reduction in the dialysate calcium concentration would increase the predialysis (basal) PTH and maximal PTH level. Thirty-three diabetic maintenance hemodialysis patients with basal PTH values less than 300 pg/ml were randomized to be dialyzed with either a regular (3.0 mEq/liter or 3.5 mEq/liter, group I) or low (2.25 mEq/liter or 2.5 mEq/liter, group II) calcium dialysate for 1 year. At baseline and after 6 months and 12 months of study, low (1 mEq/liter) and high (4 mEq/liter) calcium dialysis studies were performed to determine parathyroid function. At baseline, basal (I, 126+/-20 vs. II, 108+/-19 pg/ml) and maximal (I, 269 pg/ml+/-40 pg/ml vs. II, 342 pg/ml+/-65 pg/ml) PTH levels were not different. By 6 months, basal (I, 98+/-18 vs. II, 200+/-34 pg/ml, p = 0.02) and maximal (I, 276 pg/ml+/-37 pg/ml vs. II, 529 pg/ml+/-115 pg/ml; p = 0.05) PTH levels were greater in group II. Repeated measures analysis of variance (ANOVA) of the 20 patients who completed the entire 12-month study showed that only in group II patients were basal PTH (p = 0.01), maximal PTH (p = 0.01), and the basal/maximal PTH ratio (p = 0.03) different; by post hoc test, each was greater (p < 0.05) at 6 months and 12 months than at baseline. When study values at 0, 6, and 12 months in all patients were combined, an inverse correlation was present between basal calcium and both the basal/maximal PTH ratio (r = -0.59; p < 0.001) and the basal PTH (r = -0.60; p < 0.001). In conclusion, in diabetic hemodialysis patients with a relative PTH deficiency (1) the use of a low calcium dialysate increases basal and maximal PTH levels, (2) the increased secretory capacity (maximal PTH) during treatment with a low calcium dialysate suggests the possibility of enhanced parathyroid gland growth, and (3) the inverse correlation between basal calcium and both the basal/maximal PTH ratio and the basal PTH suggests that the steady-state PTH level is largely determined by the prevailing serum calcium concentration.

The PTH-calcium curve and the set point of calcium in primary and secondary hyperparathyroidism

BACKGROUND

The regulation of PTH secretion by calcium is altered in patients with primary hyperparathyroidism (HPT). A similar abnormality may occur in secondary HPT, but comparisons of PTH secretion in normal subjects and those with secondary HPT have given contrasting results. Differences in baseline serum ionized calcium (ICa) may partly account for these conflicting results. The aim of the present study was to evaluate whether the regulation of PTH secretion by calcium differs from normal in patients with primary and secondary HPT and to determine whether serum calcium concentration per se can affect the set point of calcium and the PTH-calcium relationship.

METHODS

The PTH-ICa relationship and the set point of ICa were evaluated in 19 patients with primary HPT (1-HPT), 16 normocalcaemic patients with secondary HPT (2-HPT; PTH 344+/-191 pg/ml), 19 hypercalcaemic patients with secondary HPT (3-HPT; PTH 806+/-254 pg/ml) and 14 healthy volunteers, by inducing hypocalcaemia and hypercalcaemia in order to maximally stimulate or inhibit PTH secretion. In five 1-HPT patients the PTH-ICa curve was restudied after normalization of serum ICa by pamidronate. Parathyroid gland volume was determined by measuring gland size at parathyroidectomy or by means of high-resolution color Doppler ultrasonography.

RESULTS

In 1-HPT patients the PTH-ICa curve, constructed using maximal PTH secretion induced by hypocalcaemia as 100%, was shifted to the right, the set point of ICa was increased, and the slope of the curve was reduced when compared to normal subjects. After normalization of baseline serum ICa by pamidronate, a shift of the PTH-ICa curve towards normal and a reduction in the set point of ICa was observed. However, basal PTH and maximal PTH secretion induced by hypocalcaemia increased, minimal PTH secretion induced by hypercalcaemia remained increased and the slope of the curve did not change significantly. The alterations in the PTH-ICa relationship in hypercalcaemic patients with secondary HPT were similar to those found in 1-HPT patients. In normocalcaemic patients with secondary HPT baseline PTH, maximal and minimal PTH secretion and parathyroid gland size were reduced compared to 3-HPT patients. Compared to normal subjects, 2-HPT patients showed greater calcium-induced minimal PTH secretion. The increase in non-suppressible PTH secretion resulted in a rightward shift of the PTH-ICa curve and an increase in the set point of ICa. A strong correlation was found, in both primary and secondary HPT, between the set point of ICa and baseline serum ICa, and between parathyroid gland size and baseline PTH, maximal PTH and minimal PTH. Multivariate regression analysis showed that baseline serum ICa was the main determinant of the set point of ICa in both primary and secondary HPT.

CONCLUSIONS

(i) The regulation of PTH secretion by calcium is abnormal in secondary as well as in primary HPT. (ii) Parathyroid gland enlargement in secondary HPT is associated with reduced sensitivity to serum ICa and resistance of parathyroid gland to calcium-mediated PTH suppression, resulting ultimately in PTH hypersecretion, despite hypercalcaemia. (iii) The set point of calcium is strongly dependent on baseline serum calcium, and the PTH-ICa relationship can be affected by variations in serum ICa concentrations. Thus, when the set point of calcium and the PTH-ICa relationship are evaluated, possible differences in baseline serum ICa concentration among the patients should be taken into account.

Modulation of parathyroid cell function by calcium ion in health and uremia

Extracellular calcium ion concentration is the major determinant of parathyroid hormone (PTH) secretion from parathyroid cells. In dialysis patients with secondary hyperparathyroidism, higher calcium concentration is needed to suppress PTH secretion as demonstrated by the PTH-calcium curve. Such abnormal sensitivity to extracellular calcium ion has been recently explained by the decrease in number of calcium-sensing receptors, especially on cells in nodular hyperplasia, which is the advanced type of parathyroid hyperplasia in uremia. Modulation of the sensitivity of parathyroid cells to calcium has become partly possible through the use of newly developed calcimimetics.

Effect of rate of calcium reduction and a hypocalcemic clamp on parathyroid hormone secretion: a study in dogs

BACKGROUND

The parathyroid hormone (PTH) calcium curve is used to evaluate parathyroid function in clinical studies. However, unanswered questions remain about whether PTH secretion is affected by the rate of calcium reduction and how the maximal PTH response to hypocalcemia is best determined. We performed studies in normal dogs to determine whether (a) the rate of calcium reduction affected the PTH response to hypocalcemia and (b) the reduction in PTH values during a hypocalcemic clamp from the peak PTH value observed during the nadir of hypocalcemia was due to a depletion of stored PTH.

METHODS

Fast (30 min) and slow (120 min) ethylenediamine-tetraacetic acid (EDTA) infusions were used to induce similar reductions in ionized calcium. In the fast EDTA infusion group, serum calcium was maintained at the hypocalcemic 30-minute value for an additional 90 minutes (hypocalcemic clamp). To determine whether the reduction in PTH values during the hypocalcemic clamp represented depletion of PTH stores, three subgroups were studied. Serum calcium was rapidly reduced from established hypocalcemic levels in the fast-infusion group at 30 and 60 minutes (after 30 min of a hypocalcemic clamp) and in the slow-infusion group at 120 minutes.

RESULTS

At the end of the fast and slow EDTA infusions, serum ionized calcium values were not different (0.84 +/- 0.02 vs. 0.82 +/- 0.03 mM), but PTH values were greater in the fast-infusion group (246 +/- 19 vs. 194 +/- 13 pg/ml, P < 0.05). During the hypocalcemic clamp, PTH rapidly decreased (P < 0.05) to value of approximately 60% of the peak PTH value obtained at 30 minutes. A rapid reduction in serum calcium from established hypocalcemic levels at 30 minutes did not stimulate PTH further, but also PTH values did not decrease as they did when a hypocalcemic clamp was started at 30 minutes. At 60 minutes, the reduction in serum calcium increased (P < 0.05) PTH to peak values similar to those before the hypocalcemic clamp. The reduction in serum calcium at 120 minutes in the slow EDTA infusion group increased PTH values from 224 +/- 11 to 302 +/- 30 pg/ml (P < 0.05).

CONCLUSIONS

These results suggest that (a) the reduction in PTH values during the hypocalcemic clamp may not represent a depletion of PTH stores. (b) The use of PTH values from the hypocalcemic clamp as the maximal PTH may underestimate the maximal secretory capacity of the parathyroid glands and also would change the analysis of the PTH-calcium curve, and (c) the PTH response to similar reductions in serum calcium may be less for slow than fast reductions in serum calcium.

Prevention of renal osteodystrophy in peritoneal dialysis

BACKGROUND

Renal osteodystrophy (ROD) is still one of the major long-term complications in end-stage renal disease leading to considerable morbidity. Despite some progress in understanding the pathogenesis of secondary hyperparathyroidism (sHPT) during recent years, prevention and treatment of ROD is still suboptimal, requiring surgical parathyroidectomy in 6 to 10% of all patients on dialysis after 10 years. In addition, the spectrum of bone lesions has changed, with non-aluminum-related adynamic bone disease (ABD) found in up to 43% of peritoneal dialysis (PD) patients.

METHODS

Current recommendations concerning prevention of ROD in PD based on the literature and personal recent data were reviewed. The focus is on (i) the importance of early prophylactic intervention to prevent parathyroid gland hyperplasia, (ii) the pathogenesis of ABD, and (iii) the role of metabolic acidosis in ROD.

RESULTS

There is ample evidence that sHPT starts early during the course of renal failure and results from both hypersecretion of PTH by parathyroid cells and glandular hyperplasia. As shown by experimental and clinical studies, established parathyroid cell hyperplasia is hardly reversible by pharmacological means, and therefore prevention of parathyroid cell proliferation needs to start early. Recent data from randomized trials document the efficacy and safety of low dose active vitamin D (0.125 to 0.25 microgram/day) and/or an oral calcium substitute to prevent progression of sHPT in patients with mild to moderate renal failure. Since little is known about the pathogenesis, natural course and clinical impact of ABD in PD, specific therapeutic concepts have not yet been generated. Diabetes and advanced age are established risk factors, whereas the role of calcium and vitamin D overtreatment or the type of dialysis (PD vs. HD) are still controversial. Currently no evidence for different functional behavior of the parathyroids in ABD and sHPT has been found. The role of circulating or local factors such as cytokines, growth factors or the presence of advanced glycation end-product (AGE)-modified matrix proteins for the pathogenesis of either type of ROD deserves further investigation. Avoiding oversuppression of parathyroid gland and the use of low calcium dialysate may help prevent ABD. There is growing evidence that a correction of metabolic acidosis will influence ROD by both direct effects on the bone and on parathyroid cell function. New dialysate composition for CAPD with a high HCO3 concentration will allow normalization of acid-based metabolism in PD patients. Their effects on ROD under long term conditions remain to be determined.

CONCLUSION

Therapeutic efforts should aim to prevent the development of parathyroid gland hyperplasia and sHPT early during the course of renal failure, and should include the use of low dose vitamin D therapy and oral calcium substitution as well as correction of metabolic acidosis. Concerning ABD, more information is needed regarding the causes and consequences of this type of bone lesion to develop a more specific therapy.

Intermittent calcitriol therapy in secondary hyperparathyroidism: a comparison between oral and intraperitoneal administration

BACKGROUND

Intermittent oral or intravenous doses of calcitriol given two or three times per week are commonly used to treat secondary hyperparathyroidism (secondary HPT). This study was undertaken to compare the biochemical and skeletal responses to thrice weekly intraperitoneal (i.p.) versus oral doses of calcitriol in children with secondary HPT undergoing peritoneal dialysis (CCPD).

METHODS

Forty-six patients aged 12.5+/-4.8 years on CCPD for 22+/-25 months were randomly assigned to treatment with oral (p.o.) or i.p. calcitriol for 12 months; 17 subjects given p.o. calcitriol and 16 subjects given i.p. calcitriol completed the study. Bone biopsies were performed at the beginning and at the end of the study, while determinations of serum and total ionized calcium, phosphorus, alkaline phosphatase, parathyroid hormone (PTH) and calcitriol levels were done monthly.

RESULTS

Serum total and ionized calcium levels were higher in subjects treated with i.p. calcitriol, P < 0.0001, whereas serum phosphorus levels were higher in those given p.o. calcitriol, P < 0.0001. For the i.p. group, serum PTH levels decreased from pre-treatment values of 648+/-125 pg/ml to a nadir of 169+/-57 pg/ml after nine months. In contrast, serum PTH levels did not change from baseline values of 670+/-97 pg/ml in subjects given p.o. calcitriol, P < 0.0001 by multiple regression analysis. Serum alkaline phosphatase levels were also lower in patients treated with i.p. calcitriol, P < 0.0001, but there was no difference between groups in the average dose of calcitriol given thrice weekly. The skeletal lesions of secondary HPT improved in both groups, 33% of patients developed adynamic bone lesion.

CONCLUSION

Differences in the bioavailability of calcitriol and/or in phosphorus metabolism may account for the divergent biochemical response to p.o. and i.p. calcitriol.

Calcium-sensing by parathyroid glands in secondary hyperparathyroidism

Calcium-sensing by the parathyroids is abnormal in familial benign hypocalciuric hypercalcemia and in primary hyperparathyroidism (primary HPT), but the role of a calcium-sensing defect in uremic secondary hyperparathyroidism (secondary HPT) remains controversial. To study the regulation of PTH release by calcium, set point estimates were obtained using the four parameter model during in vivo dynamic tests of parathyroid gland function in 31 patients with secondary HPT, 8 patients with advanced secondary HPT studied shortly before undergoing parathyroidectomy (Pre-PTX), 3 patients with primary HPT, and 20 subjects with normal renal function (NL); the response to 2-h i.v. calcium infusions was also evaluated. Neither blood ionized calcium (iCa+2) levels nor the set point for calcium-regulated PTH release differed between secondary HPT and NL; iCa+2 levels and set point values were moderately elevated in Pre-PTX and markedly elevated in primary HPT. Compared with values obtained in NL, the lowest serum PTH levels achieved during calcium infusions, expressed as a percentage of pre-infusion values, were incrementally greater in secondary HPT, Pre-PTX, and primary HPT, whereas the slope of the relationship between iCa+2 and PTH, expressed as the natural logarithm (ln) of percent preinfusion values, decreased incrementally in secondary HPT, Pre-PTX, and primary HPT. The inhibitory effect of calcium on PTH release is blunted both in secondary HPT and primary HPT because of increases in parathyroid gland mass, but a calcium-sensing defect is a late, rather than early, consequence of renal secondary HPT.

Evidence for both abnormal set point of PTH stimulation by calcium and adaptation to serum calcium in hemodialysis patients with hyperparathyroidism

In vitro studies of parathyroid glands removed from dialysis patients with secondary hyperparathyroidism and hypercalcemia have demonstrated the presence of an increased set point of parathyroid hormone (PTH) stimulation by calcium (set point [PTHstim]), suggesting an intrinsic abnormality of the hyperplastic parathyroid cell. However, clinical studies on dialysis patients have not observed a correlation between the set point (PTHstim) and the magnitude of hyperparathyroidism. In the present study, 58 hemodialysis patients with moderate to severe hyperparathyroidism (mean PTH 780 +/- 377 pg/ml) were evaluated both before and after calcitriol treatment to establish the relationship among PTH, serum calcium, and the set point (PTHstim) and to determine whether changes in the serum calcium, as induced by calcitriol treatment, modified these relationships. Calcitriol treatment decreased serum PTH levels and increased the serum calcium and the setpoint (PTHstim); however, the increase in serum calcium was greater than the increase in the setpoint (PTHstim). Before treatment with calcitriol, the correlation between the set point (PTHstim) and the serum calcium was r = 0.82, p < 0.001, and between the set point (PTHstim) and PTH was r = 0.39, p = 0.002. After treatment with calcitriol, the correlation between the set point (PTHstim) and the serum calcium remained significant (r = 0.70, p < 0.001), but the correlation between the set point (PTHstim) and PTH was no longer significant (r = 0.09); moreover, a significant correlation was present between the change in the set point (PTHstim) and the change in serum calcium that resulted from calcitriol treatment (r = 0.73, p < 0.001). The correlation between the residual values (deviation from the regression line) of the set point (PTHstim), derived from the correlation between PTH and the set point (PTHstim), and serum calcium was r = 0.77, p < 0.001 before calcitriol and r = 0.72, p < 0.001 after calcitriol. In conclusion, the set point (PTHstim) increased after a sustained increase in the serum calcium, suggesting an adaptation of the set point to the existing serum calcium; the increase in serum calcium resulting from calcitriol treatment was greater than the increase in the set point (PTHstim); the set point (PTHstim) was greater in hemodialysis patients with higher serum PTH levels; and the correlation between PTH and the set point (PTHstim) may be obscured because the serum calcium directly modifies the set point (PTHstim).

In vivo assessments of calcium-regulated parathyroid hormone release in secondary hyperparathyroidism

In vivo dynamic tests of parathyroid gland function have provided useful information about the secretory behavior of parathyroids in various clinical disorders, but the limitations of this approach must be recognized when applied to studies of parathyroid gland physiology. Set point abnormalities have been documented in vivo both in primary hyperparathyroidism and in familial hypocalciuric hypercalcemia. Such findings are consistent with in vitro results obtained in studies of dispersed parathyroid cells from patients with primary hyperparathyroidism and with recently described alteration in calcium receptor expression in patients with FHH. The assessment of parathyroid gland function in patients with end-stage renal disease presents distinct methodological problems, however, because of marked variation in the degree of parathyroid gland enlargement. Neither the four parameter model originally used to describe set point abnormalities both in vitro and in vivo or alternative approaches to the assessment of PTH secretion in vivo adequately address this important issue. Results from recent in vivo studies of patients with chronic renal failure do not support the view that the set point for calcium-regulated PTH release is abnormal in secondary hyperparathyroidism or that treatment with calcitriol lowers the set point for calcium-regulated PTH release in patients with uremic secondary hyperparathyroidism. The concept of set point disturbances has strongly influenced discussions about the pathogenesis of secondary hyperparathyroidism, and it has served as a focal point for examining the therapeutic response to calcitriol in patients with this disorder. This matter requires careful reconsideration, however, in light of recent clinical findings and the development of techniques to directly assess the molecular mechanisms responsible for regulating calcium-mediated PTH release in renal failure and other disorders of mineral metabolism. Although knowledge in this area remains limited, the extent of parathyroid hyperplasia and the role of factors that influence the development of parathyroid gland enlargement may ultimately prove to be particularly important modifiers of parathyroid gland function in chronic renal failure.

Non-suppressible parathyroid hormone secretion is related to gland size in uremic secondary hyperparathyroidism

To determine the relative importance of parathyroid gland enlargement and alterations in calcium sensing (set-point changes) in the pathogenesis of uremic secondary hyperparathyroidism (2 degrees HPT), we investigated the relationship between estimates of parathyroid gland size and calcium-mediated parathyroid hormone (PTH) suppression in 19 normocalcemic 2 degrees HPT patients on chronic maintenance hemodialysis. We compared our results to calcium-mediated PTH suppression in 12 normal volunteers, 12 patients with familial benign hypocalciuric hypercalcemia (FBHH), a disorder of abnormal calcium sensing, and 9 subjects with primary hyperparathyroidism (1 degree HPT), which is characterized by both calcium set-point abnormalities and parathyroid gland enlargement. We found that the 2 degrees HPT group displayed a distinctive pattern of calcium-mediated PTH suppression characterized by a failure to normally suppress PTH at supraphysiologic ionized calcium concentrations, similar to 1 degree HPT, but without the rightward shift of the calcium-PTH suppression curve that characterizes calcium sensing abnormalities in FBHH and 1 degree HPT. In the patients with 2 degrees HPT, hypercalcemic suppression resulted in an ending PTH (as a percent of baseline) that was significantly higher (39.8 +/- 4.47%), and a slope of the calcium-PTH suppression curve that was significantly less negative (-4.8 +/- 0.53), compared to respective values of 19.4 +/- 1.81% (P = 0.0009) and -9.0 +/- 1.02 (P = 0.001) in normals and 19.1 +/- 2.49% (P = 0.001) and -9.6 +/- 1.11 (P = 0.0006) in FBHH. Values of ending PTH and slope in 2 degrees HPT patients, however, were similar to those found in 1 degree HPT (49.8 +/- 6.35%, P = 0.21 and -4.5 +/- 0.74, P = 0.72). The ionized calcium concentration required to attain half maximal PTH suppression (EC50) in 2 degrees HPT (1.20 +/- 0.02 mmol/liter) was not significantly different from normals (1.25 +/- 0.01 mmol/liter, P = 0.12) but was significantly less than in 1 degree HPT (1.52 +/- 0.02 mmol/liter, P < 0.0001) and in FBHH (1.44 +/- 0.02 mmol/liter, P < 0.0001). More importantly, we found a significant linear correlation between the natural logarithm of gland size and ending PTH suppression (r = 0.71, P < 0.001) and slope of the calcium-PTH curve (r = 0.67, P = 0.002) in 2 degrees HPT. Thus, calcium non-suppressible PTH secretion in 2 degrees HPT does not represent a simple set-point error, but rather correlates with the degree of parathyroid gland enlargement.

Long-term effects of intravenous calcitriol therapy on the control of secondary hyperparathyroidism

Although high-dose intravenous calcitriol has been shown to be effective in suppressing parathyroid hormone (PTH) secretion in dialysis patients with secondary hyperparathyroidism, an increasing number of patients is refractory to treatment. Only a few studies have evaluated the factors that can predict a favorable response to calcitriol, but contrasting results have been reported. This study was performed to evaluate the effect of high-dose intravenous calcitriol on parathyroid function and to investigate the factors that can predict a favorable response to treatment. Thirty-five dialysis patients were selected for intravenous calcitriol treatment (2 microg after dialysis for 12 months) because of increased PTH levels (>325 pg/mL). Before starting the treatment, the set point of calcium and the PTH-ionized calcium (ICa) curve was evaluated in each patient by inducing hypocalcemia and, 1 week later, hypercalcemia to maximally stimulate or inhibit PTH secretion. Parathyroid glands were assessed by high-resolution color Doppler ultrasonography. Throughout the study, calcium carbonate or acetate dosage was modified to maintain serum phosphate less than 5.5 mg/dL. Hypercalcemia was managed by reducing dialysate calcium to 5 mg/dL and, if necessary, calcitriol dose. The therapeutic goal was to reduce PTH levels below 260 pg/mL while maintaining normocalcemia. The patients who achieved the therapeutic goal were considered responders. Taking the data from the 35 patients together, we observed a significant decrease (P < 0.01) in alkaline phosphatase (from 252 +/- 106 IU/L to 194 +/- 81 IU/L) and PTH (from 578 +/- 231 pg/mL to 408 +/- 291 pg/mL), and a significant increase in serum ICa (from 5.1 +/- 0.2 mg/dL to 5.3 +/- 0.2 mg/dL; P < 0.001) after calcitriol therapy. PTH changes after therapy were not correlated to serum ICa changes, serum phosphate levels during treatment, and calcitriol dose. The response to therapy was heterogeneous because PTH levels markedly decreased over the treatment period in 18 responsive patients, whereas they increased or remained unchanged in 14 of 17 nonresponders. In three additional refractory patients, there was a decline in PTH of 20% to 35%, but this decline was associated with hypercalcemia. Pretreatment parathyroid gland size, serum ICa, PTH, maximal PTH induced by hypocalcemia, minimal PTH induced by hypercalcemia, the set point of ICa, and the ICa levels at which maximal PTH secretion and inhibition occurred were higher in the 17 refractory patients than in the 18 responsive patients. However, logistic regression analysis showed that among these parathyroid function parameters, the only significant predictors of a favorable response to calcitriol therapy were the parathyroid gland size and the set point of ICa. Throughout the study, serum phosphate and calcitriol dose were comparable in the two groups. In conclusion, the response to intravenous calcitriol therapy in dialysis patients with secondary hyperparathyroidism is heterogeneous, consisting of patients who are either responsive or refractory to treatment; refractoriness can be predicted by parathyroid volume and calcium set point.

Parathyroid gland function in secondary hyperparathyroidism

The parathyroid glands play a critical role in the maintenance of calcium homeostasis. It has been suggested that the set-point for calcium-regulated parathyroid hormone (PTH) release is higher in uremic patients than normal subjects. However, these assessments of parathyroid gland function have been performed using methods that differed from the original four-parameter model. Dynamic testing of the parathyroid glands has been performed with standardized infusions of calcium gluconate and sodium citrate in dialysis patients with secondary hyperparathyroidism and in normal volunteers. In addition, similar studies have been carried out before and after 4 months of intermittent calcitriol therapy. The derived values for the set-point were 1.21 + 0.04 mmol/l and 1.24 + 0.06 mmol/l, respectively in control and dialyzed patients (NS). Furthermore, the values for set-point were 1.21 + 0.01 and 1.22 + 0.01 mmol/l (NS), before and after calcitriol therapy. In addition, when subjects were grouped according to the severity of secondary hyperparathyroidism, the set-point calcium-regulated PTH release did not differ between the groups. Calcitriol therapy alters the secretory capacity of the parathyroid glands during hypocalcemia. The degree of parathyroid enlargement and the type of cell proliferation may be more important determinants of the severity of secondary hyperparathyroidism.