The set point of calcium and the reduction of parathyroid hormone in hemodialysis patients

Effect of Citrate-Acidified Dialysate on Intact Parathyroid Hormone in Prevalent Hemodialysis Patients: A Matched Retrospective Cohort Study

Background

It has been proposed that substituting citrate-acidified dialysate (CAD) solutions for acetate-acidified dialysate (AAD) could improve hemodynamics and dialysis tolerance and reduce the requirement for systemic anticoagulation. Citrate chelates ionized calcium, but long-term effects of CAD use during maintenance hemodialysis have not been well studied. While many studies of the effects of CAD on serum calcium and intact parathyroid hormone (iPTH) have been short-term or have been limited by sample size, we aimed to determine if there are any long-term (i.e., 6-month) changes from pre-dialysis iPTH levels when patients are switched from AAD to CAD.

Methods

This retrospective cohort study compared various clinical parameters, including pre-dialysis iPTH and serum calcium as well as single pool Kt/V, from eligible patients who received in-center hemodialysis thrice-weekly in geographically matched CAD (n=3) or AAD clinics (n=12). CAD clinics were defined as clinics converting from AAD to CAD if >85% of the patients were prescribed CAD after implementation of CAD within the clinic.

Results

Pre-dialysis iPTH was not significantly different from baseline to 6-month follow-up within either CAD or AAD clinics. Moreover, the mean change from baseline to month 6 in iPTH between patients (n=142) in CAD clinics (−17 pg/mL) and patients (n=671) in AAD clinics (13 pg/mL) was similar (p = 0.24). Likewise, the differences in the mean change in serum calcium concentrations and dialysis adequacy (single pool Kt/V) were not significant between CAD and AAD clinics. For subgroups of patients who were never prescribed cinacalcet or calcium-based phosphate binders, there were no significantly different categorical shifts in iPTH between CAD and AAD clinics.

Conclusion

Similar trends in single pool Kt/V, iPTH, and serum calcium levels were observed in clinics that switched from AAD to CAD versus the geographically matched AAD clinics. These results support CAD as a potential alternative to AAD in hemodialysis.

Calcimimetics for secondary hyperparathyroidism in chronic kidney disease patients

BACKGROUND

Calcimimetic agents lower abnormal serum parathyroid hormone (PTH) levels in people who have chronic kidney disease (CKD), but the benefits and harms on patient-level outcomes are uncertain. Since this review was first published in 2006 showing that evidence for calcimimetics was largely restricted to biochemical outcomes, additional studies have been conducted. This is an update of a review first published in 2006.

OBJECTIVES

To evaluate the benefits and harms of cinacalcet on patient-level outcomes in adults with CKD.

SEARCH METHODS

MEDLINE, EMBASE, CENTRAL and conference proceedings were searched for randomised controlled trials (RCTs) evaluating any calcimimetic against placebo or another agent in adults with CKD (persistent albuminuria > 30 mg/g with or without reduced glomerular filtration rate (GFR) (below 60 mL/min/1.73 m²)). We updated searches to 7 February 2013 including the Cochrane Renal Group's Specialised Register to complete this update.

SELECTION CRITERIA

We included all RCTs of a calcimimetic administered to patients with CKD for the treatment of elevated serum PTH levels.

DATA COLLECTION AND ANALYSIS

Data were extracted on all relevant patient-centred and surrogate outcomes. We summarised treatment estimates using random effects and expressed treatment effects as a risk ratio (RR) or mean difference (MD) with 95% confidence intervals (CI).

MAIN RESULTS

Eighteen studies (7446 participants) compared cinacalcet in addition to standard therapy with no treatment or placebo plus standard therapy. In adults with GFR category G5 (GFR below 15 mL/min/1.73 m²) treated with dialysis, routine cinacalcet treatment had little or no effect on all-cause mortality (RR 0.97, 95% CI 0.89 to 1.05), imprecise effects on cardiovascular mortality (RR 0.67, 95% CI 0.16 to 2.87), and prevented surgical parathyroidectomy (RR 0.49, 95% CI 0.40 to 0.59) and hypercalcaemia (RR 0.23, 95% CI 0.05 to 0.97), but increased hypocalcaemia (RR 6.98, 95% CI 5.10 to 9.53), nausea (RR 2.02, 95% CI 1.45 to 2.81) and vomiting (RR 1.97, 95% CI 95% CI 1.73 to 2.24). Cinacalcet decreased serum PTH (MD -281.39 pg/mL, 95% CI -325.84 to -234.94) and calcium (MD -0.87 mg/dL, 95% CI -0.96 to -0.77) levels, but had little or no effect on serum phosphorous levels (MD -0.23 mg/dL, 95% CI -0.58 to 0.12).Data were sparse for adults with GFR categories G3a to G4 (GFR 15 to 60 mL/min/1.73 m²) and kidney transplant recipients.Overall, based on GRADE criteria, evidence for cinacalcet in adults with GFR category G5 treated with dialysis (mortality, parathyroidectomy, hypocalcaemia, and nausea) is of high or moderate quality. High quality evidence suggests "further research is very unlikely to change our confidence in the estimate of treatment effect" and moderate quality evidence is "further research is likely to have an important impact on our confidence in the estimate of effect and may change the estimate". Information for adults with less severe CKD GFR category G3a to G4 is of low or very low quality. This means that further research is very likely to have an important impact on our confidence in the estimate of effect and is likely to change the estimate

AUTHORS' CONCLUSIONS

Routine cinacalcet therapy reduced the need for parathyroidectomy in adults treated with dialysis and elevated PTH levels but does not improve all-cause or cardiovascular mortality. Cinacalcet increases risks of nausea, vomiting and hypocalcaemia, suggesting harms may outweigh benefits in this population.

Effects of pamidronate and calcitriol on the set point of the parathyroid gland in postmenopausal hemodialysis patients with secondary hyperparathyroidism

BACKGROUND/AIMS

Secondary hyperparathyroidism may worsen after the administration of pamidronate in postmenopausal hemodialysis (HD) patients. The aim of this study was to evaluate the short-term effect of coadministration of calcitriol and pamidronate on dynamic parathyroid hormone (PTH) secretion.

METHODS

Fifteen postmenopausal women undergoing regular HD with serum intact PTH levels of >200 pg/ml were enrolled. The PTH-ionized calcium (iCa) curve was evaluated by the response to hypo- and hypercalcemia induced with 1 and 4 mEq/l of dialysate calcium, respectively. Parameters were compared after pamidronate was administered and after coadministration of pamidronate and calcitriol. Changes in serum levels of maximal serum PTH (PTHmax), basal PTH (PTHbase) and minimal PTH (PTHmin) were evaluated.

RESULTS

Pamidronate therapy resulted in a decrease in predialysis basal plasma iCa (p < 0.05) and an increase in PTHmax (p < 0.01), PTHbase (p < 0.01) and PTHmin (p < 0.01). The change in serum iCa and PTH was reversed after the coadministration of calcitriol and pamidronate.

CONCLUSION

Our study demonstrated that pamidronate therapy is associated with a reduced plasma iCa and increased PTH secretion. These adverse effects may be reversed by calcitriol. These findings suggest that in considering pamidronate treatment in postmenopausal patients with osteoporosis receiving HD, it might be safer to add calcitriol to prevent the increased PTH secretion.

Left-shifted relation between calcium and parathyroid hormone in obesity

BACKGROUND

A condition resembling secondary hyperparathyroidism (HPT), including raised levels of PTH and normal levels of serum calcium, has been reported in obesity. A plausible reason may be vitamin D deficiency, but conflicting data have been reported.

OBJECTIVE

Our objective was to investigate calcium homeostasis in obese individuals with emphasis on the function of the parathyroid glands.

DESIGN AND INTERVENTION

Morbidly obese patients (mean body mass index=46.6+/-6) were examined for their status of calcium homeostasis. A subset was thoroughly investigated with calcium-citrate (CiCa) clamping.

PATIENTS

Of 108 morbidly obese patients, 11 underwent CiCa clamping as well as 21 healthy volunteers of normal weight and 15 with primary HPT (pHPT). Large patient cohorts of normal individuals and pHPT patients were also used as comparisons.

OUTCOME MEASURES AND RESULTS

All obese individuals had normal serum calcium and creatinine levels. Mean levels of 25-OH-vitamin D3 in serum were low, 53 nmol/liter (reference range 75-250 nmol/liter). Mean intact plasma PTH was 5.1 pmol/liter (reference range 1.1-6.9 pmol/liter). There was a significant positive correlation between PTH and duration of obesity. CiCa clamping in obese subjects revealed a remarkably high sensitivity for calcium and a left-shifted relation between plasma calcium and PTH (set point) compared with the normal population. CiCa clamping in pHPT patients demonstrated a right-shifted PTH-Ca curve.

CONCLUSION

Although vitamin D levels in the obese individuals were low, few displayed overt signs of secondary HPT. The CiCa clamping implied a disturbance in the calcium homeostasis comparable to early renal insufficiency, with a left-shifted Ca-PTH curve and a lower set point compared with the normal population.

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.

Dynamics of parathyroid hormone secretion in health and secondary hyperparathyroidism

This review examines the dynamics of parathyroid hormone secretion in health and in various causes of secondary hyperparathyroidism. Although most studies of parathyroid hormone and calcium have focused on the modification of parathyroid hormone secretion by serum calcium, the relationship between parathyroid hormone and serum calcium is bifunctional because parathyroid hormone also modifies serum calcium. In normal animals and humans, factors such as phosphorus and vitamin D modify the basal parathyroid hormone level and the maximal parathyroid hormone response to hypocalcemia. Certain medications, such as lithium and estrogen, in normal individuals and sustained changes in the serum calcium concentration in hemodialysis patients change the set point of calcium, which reflects the serum calcium concentration at which parathyroid hormone secretion responds. Hypocalcemia increases the basal/maximal parathyroid hormone ratio, a measure of the relative degree of parathyroid hormone stimulation. The phenomenon of hysteresis, defined as a different parathyroid hormone value for the same serum calcium concentration during the induction of and recovery from hypo- and hypercalcemia, is discussed because it provides important insights into factors that affect parathyroid hormone secretion. In three causes of secondary hyperparathyroidism--chronic kidney disease, vitamin D deficiency, and aging--factors that affect the dynamics of parathyroid hormone secretion are evaluated in detail. During recovery from vitamin D deficiency, the maximal parathyroid hormone remains elevated while the basal parathyroid hormone value rapidly becomes normal because of a shift in the set point of calcium. Much remains to be learned about the dynamics of parathyroid hormone secretion in health and secondary hyperparathyroidism.

New Therapies for Uremic Secondary Hyperparathyroidism

Secondary hyperparathyroidism (SHPT) is a common and serious complication of chronic kidney disease (CKD). It affects more than 300,000 end-stage renal disease patients treated by dialysis and probably more than 3 million patients with CKD worldwide. For a long time, traditional therapies for SHPT had consisted of correcting the hypocalcemia using calcium salts and vitamin D derivatives, preventing the hyperphosphatemia by calcium- or aluminum-containing intestinal phosphate binders, and recently by using no metal-containing intestinal phosphate binders; however, these therapies are limited by the occurrence of hypercalcemia, hyperphosphatemia, and the lack of specificity and long-term efficacy. Moreover, surgical parathyroidectomy (PTX), which remains the gold standard therapy, is not exempt from risk. PTX exposes patients to anesthesia risks, presurgical and postsurgical complications, and in many cases a permanent state of hypoparathyroidism. Thus, the medical treatment of SHPT became an ideal target for the development of new therapies and strategies. The purpose of this article is to provide an overview of these new therapies, including vitamin D analogs, intestinal phosphate binders, calcimimetics, parathyroidectomies, tyrosine kinase inhibitors, azydothymidine, anticalcineurins, N-terminal truncated parathyroid hormone fragments, bisphosphonates, calcitonin, osteoprotegerin, and others. The use of these new therapies alone or in combination may help to optimize the future treatment of SHPT in CKD patients.

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.

Relationship of predialytic intact parathyroid hormone on secondary hyperparathyroidism in chronic maintenance haemodialysis patients

METHODS AND RESULTS

In order to clarify the predialytic factors influencing the onset of secondary hyperparathyroidism (SHPT) in patients on chronic maintenance haemodialysis, the time-course changes of serum levels of intact-PTH (i-PTH) during haemodialysis for 5 years were investigated. The subjects were 69 non-diabetic patients who had a serum aluminium level of less than 1.85 nmol/L at the end of observation. Patients were divided into two groups based on i-PTH levels obtained at the start of dialysis; the high group (H group) consisted of patients whose i-PTH levels were more than 22.00 pmol/L, the low group (L group) had levels less than 22.00 pmol/L. In the H group, i-PTH was 41.46 +/- 2.87 pmol/L at the start of dialysis (vs L group, P < 0.0001) and 15.82 +/- 2.85 pmol/L after haemodialysis initiation. In the L group, i-PTH levels did not significantly change and was 11.69 +/- 2.50 pmol/L 12 months after the start of dialysis (at the 12th month). However, at the 60th month, the i-PTH level was 33.24 +/- 5.30 pmol/L in the H group, and 9.85 +/- 2.13 pmol/L in the L group (P < 0.005).

CONCLUSION

It is suggested that control of i-PTH levels in the predialytic period may be important to suppress SHPT throughout haemodialysis.

Acute effect of pamidronate on PTH secretion in postmenopausal hemodialysis patients with secondary hyperparathyroidism

BACKGROUND

The purpose of this study is to evaluate the acute effect of pamidronate on plasma ionized calcium (iCa) level reduction and dynamic parathyroid hormone (PTH) secretion in postmenopausal hemodialysis-dependent women with secondary hyperparathyroidism.

METHODS

Twelve postmenopausal women undergoing regular hemodialysis with serum intact PTH levels greater than 200 pg/mL (200 ng/L) were included in this study. Pamidronate was administered intravenously as a single dose of 15 mg in the last hour of hemodialysis. PTH responses to hypocalcemia and hypercalcemia induced with 1 mEq/L (0.5 mmol/L) and 4 mEq/L (2 mmol/L) of dialysate calcium, respectively, were evaluated before and 1 week after pamidronate therapy.

RESULTS

Pamidronate therapy resulted in a decrease in predialysis basal plasma iCa (iCa(base); P < 0.05) levels and an increase in maximal serum PTH (PTHmax; P < 0.001), basal PTH (PTHbase; P < 0.001), and minimal PTH levels (P < 0.001). The set point of serum calcium and the slope of the PTH-calcium curve were not altered by pamidronate therapy. An inverse correlation was present between iCa(base) and the PTHbase-PTHmax ratio before (r = -0.66; P < 0.05) and after (r = -0.84; P < 0.001) pamidronate therapy.

CONCLUSION

Our study shows that pamidronate therapy is associated with reduced plasma iCa levels and increased PTH secretion, resulting in aggravated secondary hyperparathyroidism. These findings suggest that secondary hyperparathyroidism may worsen after the administration of pamidronate, at least in the short term, in postmenopausal hemodialysis patients.

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.

Hysteresis and calcium set-point for the calcium parathyroid hormone relationship in healthy horses

Abnormalities in calcium (Ca(2+)) homeostasis are reported in horses with several pathological conditions; however, there is little information on Ca(2+) regulation in horses. The objectives of the present study were to determine the Ca(2+) set-point in healthy horses, to determine whether the Ca(2+)/parathyroid hormone (PTH) response curves were characterized by hysteresis, and to determine if the order of experimentally induced hypocalcemia or hypercalcemia had an effect on PTH secretion. The Ca(2+) set-point and hysteresis were determined in 12 healthy horses by infusing Na(2)EDTA and calcium gluconate. The Ca(2+) set-point was 1.37 +/- 0.05 mmol/L, which is higher than values reported for humans and dogs (1.0-1.2 mmol/L). Hysteresis was present during hypocalcemia and hypercalcemia. Horses in which hypocalcemia was followed by hypercalcemia secreted more PTH (7440 +/- 740 pmol min/L) than horses in which hypercalcemia was followed by hypocalcemia (5990 +/- 570 pmol min/L). This study has demonstrated that the Ca(2+) set-point in the horse is higher than in other domestic animals and man. We have shown that the Ca(2+)/PTH relationship in horses is sigmoidal and displays hysteresis during both hypocalcemia and hypercalcemia, and that extracellular Ca(2+) concentrations may affect the response of the parathyroid gland to hypocalcemia.

Rate dependence of acute PTH release and association between basal plasma calcium and set point of calcium-PTH curve in dialysis patients

BACKGROUND

In vivo, the control of calcium-mediated acute PTH release during induced hypo- or hypercalcaemia is linked not only to plasma calcium concentration per se but also to the rate and direction of calcium change. In fact, during induced hypocalcaemia, the predominant mechanism that causes PTH to be released is the reduction of plasma Ca(2+) irrespective of the absolute starting concentration of ionized calcium. This mechanism, which is rate-dependent and even activated in conditions of hypercalcaemia, may be involved in the association, reported in several papers, between the basal Ca(2+) and the set point of the calcium-PTH curve.

METHODS

The calcium-PTH relationship was studied in 12 dialysis patients under conditions of induced low and high predialysis plasma Ca(2+). At each level of basal Ca(2+), dynamic tests were conducted using two methodological approaches. In method A patients underwent low (0.5 mmol/l) calcium dialysis in the stimulation test and high (2 mmol/l) calcium dialysis in the inhibition test, while the dialysate calcium (CaD) was kept constant during each test. In this way a higher but variable rate of change in plasma Ca(2+) was achieved. In method B, CaD was progressively decreased (stimulation test) and increased (inhibition test) during the tests in order to obtain a lower but more constant rate of change in plasma Ca(2+). Consequently, for each patient, four calcium-PTH curves were produced: low basal Ca(2+) with methods A and B, and high basal Ca(2+) with methods A and B.

RESULTS

Basal plasma Ca(2+) was similar in A and B at low (1.16+/-0.02 vs 1.15+/-0.02 mmol/l) and high (1.25+/-0.02 vs 1.26+/-0.02 mmol/l) basal plasma Ca(2+). The set point was higher in A than in B both at low (1.12+/-0.02 vs 1.10+/-0.02 mmol/l, P=0.01) and high (1.20+/-0.02 vs 1.16+/-0.02 mmol/l, P=0.03) basal Ca(2+) as was the slope (542+/-41 vs 426+/-44%/mmol, P=0.02; 615+/-73 vs 389+/-25%/mmol, P=0.01). No significant difference was found between A and B as regards minimal PTH and plasma Ca(2+) at minimal PTH (Camin) in both calcaemic states. Maximal PTH was slightly higher in B at low (510+/-97 vs 548+/-107 pg/ml, P=NS) and high basal plasma Ca(2+) (410+/-97 vs 464+/-108 pg/ml, P=0.02). Plasma calcium at maximal PTH (Camax) was significantly higher in A (1.1+/-0.03 vs 0.99+/-0.02 mmol/l, P=0.001) at high basal plasma Ca(2+). The set point was strictly related to basal plasma Ca(2+) in both methods, but the slope of the linear regression was significantly steeper with method A. The set point was predicted to increase by 0.881 (CI 0.772-0.990) mmol/l for each mmol/l of increase in basal plasma Ca(2+) with method A and by 0.641 (CI 0.546-0.737) mmol/l for each mmol/l of increase in basal plasma Ca(2+) with method B.

CONCLUSIONS

(i) Higher and variable rates of change in plasma Ca(2+) produce a higher set point value and a steeper slope of the calcium-PTH curve when compared to lower and more constant rates of calcium change. (ii) The different slope of the linear correlations between basal plasma Ca(2+) and set point in the two methods suggests that the rate-dependent mechanism of acute PTH release plays a significant role in the association between set point and basal plasma Ca(2+). (iii) The significance of the set point is questionable when the calcium-PTH curve is carried out in vivo.

Effect of calcitriol treatment and withdrawal on hyperparathyroidism in haemodialysis patients with hypocalcaemia

BACKGROUND

Calcitriol is used to treat secondary hyperparathyroidism in dialysis patients. For similarly elevated parathyroid hormone (PTH) levels, the PTH response to calcitriol treatment is believed to be better in hypocalcaemic dialysis patients than in dialysis patients with higher serum calcium values. Furthermore, few studies have evaluated the rapidity of the rebound in serum PTH values after prolonged treatment with calcitriol. Our goal was to evaluate (i) the PTH response to calcitriol treatment in hypocalcaemic haemodialysis patients, (ii) the rapidity of rebound in PTH after calcitriol treatment was stopped, and (iii) whether the effect of calcitriol treatment on PTH levels could be separated from those produced by changes in serum calcium and phosphate values.

METHODS

Eight haemodialysis patients (29+/-3 years) with hypocalcaemia and hyperparathyroidism were treated thrice weekly with 2 microg of intravenous calcitriol and were dialysed with a 3.5 mEq/l calcium dialysate. Parathyroid function (PTH-calcium curve) was determined before and after 30 weeks of calcitriol treatment and 15 weeks after calcitriol treatment was stopped.

RESULTS

Pretreatment PTH and ionized calcium values were 907+/-127 pg/ml and 3.89+/-0.12 mg/dl (normal, 4.52+/-0.07 mg/dl). During calcitriol treatment, one patient did not respond, but basal (predialysis) PTH values in the other seven patients decreased from 846+/-129 to 72+/-12 pg/ml, P<0.001 and in all seven patients, the decrease exceeded 85%. During the 15 weeks after calcitriol treatment was stopped, a slow rebound in basal PTH values in the seven patients was observed, 72+/-12 to 375+/-44 pg/ml. Covariance analysis was used to evaluate the three tests of parathyroid function (0, 30, and 45 weeks), and showed that calcitriol treatment was associated with reductions in maximal PTH values while reductions in basal PTH were affected by ionized calcium and serum phosphate. The basal/maximal PTH ratio and the set point of calcium were associated with changes in ionized calcium.

CONCLUSIONS

In haemodialysis patients with hypocalcaemia, (i) moderate to severe hyperparathyroidism responded well to treatment with calcitriol, (ii) reductions in maximal PTH were calcitriol dependent while reductions in basal PTH were affected by the ionized calcium and serum phosphate concentrations, (iii) changes in the basal/maximal PTH ratio and the set point of calcium were calcium dependent, and (iv) the delayed rebound in basal PTH levels after withdrawal of calcitriol treatment may have been due to the long duration of treatment and the marked PTH suppression during treatment.

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.

Effect of calcitriol and age on recovery from hypocalcemia in hemodialysis patients

Calcitriol is used to treat hyperparathyroidism in hemodialysis patients. Calcitriol treatment, either through a reduction in parathyroid hormone (PTH) levels or direct effect on bone, decreases the osteoblast and osteoclast surface and bone formation rate. Our study of 13 hemodialysis patients was designed to evaluate whether calcitriol treatment changed the rate of spontaneous recovery from hypocalcemia induced by a low-calcium dialysis. Calcitriol treatment decreased basal PTH levels from 614 +/- 84 to 327 +/- 102 pg/mL (P < 0.001) and maximal PTH levels from 1,282 +/- 157 to 789 +/- 161 pg/mL (P < 0.001), but the rate of serum ionized calcium recovery from hypocalcemia did not change. When the 13 patients were separated based on the median age of 64 years, the predialysis serum ionized calcium level was less in the younger (group I, 44 +/- 6 years; n = 6) than older (group II, 68 +/- 1 years; n = 7) patients (1.05 +/- 0.03 v 1.22 +/- 0.03 mmol/L, respectively; P < 0.01) despite similar basal (group I, 595 +/- 122 pg/mL v group II, 629 +/- 96 pg/mL) and maximal (group I, 1,114 +/- 299 pg/mL v group II, 1,425 +/- 141 pg/mL) PTH levels. Before calcitriol treatment, the rate of serum ionized calcium recovery from induced hypocalcemia was greater (P < 0.05) for similar PTH levels in the older than younger patients. After calcitriol treatment, despite a similar reduction in PTH levels, the rate of calcium recovery increased (P < 0.05) in the younger patients but did not change in the older patients. We also observed that toward the end of the low-calcium hemodialysis, PTH values decreased even though serum ionized calcium level continued to decline when the rate of calcium reduction slowed. In addition, hysteresis, defined as a lower PTH value during the recovery from hypocalcemia than during the induction of hypocalcemia for the same serum calcium concentration, was present during the spontaneous recovery from hypocalcemia. In conclusion, in the hemodialysis patient: (1) age appeared to affect the bone response to PTH and calcitriol treatment, (2) the PTH response to hypocalcemia was affected by a deceleration in the rate of calcium decrease, and (3) hysteresis of the PTH response to hypocalcemia occurred during the spontaneous recovery from hypocalcemia.

Parathyroid function as a determinant of the response to calcitriol treatment in the hemodialysis patient

BACKGROUND

Bolus calcitriol (CTR) is used for the treatment of secondary hyperparathyroidism in dialysis patients. Although CTR treatment reduces parathyroid hormone (PTH) levels in many dialysis patients, a significant number fail to respond.

METHODS

To learn whether or not an analysis of parathyroid function could further illuminate the response to CTR, a PTH-calcium curve was performed before and after at least two months of CTR treatment in 50 hemodialysis patients with a predialysis intact PTH of greater than 300 pg/ml.

RESULTS

For the entire group (N = 50), CTR treatment resulted in a 24% reduction in predialysis (basal) PTH from 773 +/- 54 to 583 +/- 71 pg/ml (P < 0.001), whereas ionized calcium increased from 1.10 +/- 0.02 to 1.22 +/- 0.02 mM (P < 0.001); however, maximal and minimal PTH did not change from pre-CTR values. Based on whether or not the basal PTH decreased by 40% or more during CTR treatment, patients were divided into responders (Rs, N = 25) and nonresponders (NRs, N = 25). Before CTR, the NR group was characterized by a greater basal (959 +/- 80 vs. 586 +/- 51 pg/ml, P < 0.001) and maximal (1899 +/- 170 vs. 1172 +/- 108 pg/ml, P < 0. 001) PTH and serum phosphorus (6.14 +/- 0.25 vs. 5.14 +/- 0.34 mg/dl, P < 0.01). Logistical regression analysis showed that the pre-CTR basal PTH was the most important predictor of the post-CTR basal PTH, and a pre-CTR basal PTH of 750 pg/ml represented a 50% probability of a response. Basal PTH correlated with the ionized calcium in the NR group (r = 0.59, P = 0.002) but not in the R group (r = 0.06, P = NS). In the R group, an inverse correlation was present between ionized calcium and the basal/maximal PTH ratio, an indicator of whether calcium is suppressing basal PTH secretion relative to the maximal secretory capacity (maximal PTH) r = -0.55, P = 0.004; in the NR group, this correlation approached significance but was positive (r = 0.34, P = 0.09). After CTR treatment, serum calcium increased in both groups, and despite marked differences in basal PTH (Rs, 197 +/- 25 vs. NRs, 969 +/- 85 pg/ml), an inverse correlation between ionized calcium and basal/maximal PTH was present in both groups (Rs, r = -0.61, P = 0.001, and NRs, r = -0.60, P = 0.001).

CONCLUSIONS

(a) Dynamic testing of parathyroid function provided insights into the pathophysiology of PTH secretion in hemodialysis patients. (b) The magnitude of hyperparathyroidism was the most important predictor of the response to CTR. (c) Before CTR treatment, PTH was sensitive to calcium in Rs, and serum calcium was PTH driven in NRs, and (d) after the CTR-induced increase in serum calcium, calcium suppressed basal PTH relative to maximal PTH in both groups.

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.

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.

Hysteresis of the PTH-calcium curve during hypocalcemia in the dog: effect of the rate and linearity of calcium decrease and sequential episodes of hypocalcemia

Several studies have shown the presence of hysteresis of the parathyroid hormone (PTH)-calcium relationship in both normal humans and hemodialysis patients; for hypocalcemia, hysteresis is defined as a lower PTH level for the same serum calcium during the recovery from than the induction of hypocalcemia. However, some have questioned whether hysteresis is only a function of the rate and/or direction of change in calcium, and others have proposed that hysteresis is due to depletion of PTH stores. To address these issues, two groups of dogs were studied. To induce hypocalcemia, sodium EDTA (50 mg/kg) was infused either over 60 (termed slow) or 30 (termed fast) minutes; immediately after the cessation of the ethylenediamine tetracetate (EDTA) infusion, calcium chloride (0.75 mEq/kg) was infused over 60 or 30 minutes, respectively, to correct the hypocalcemia. The EDTA infusion produced a linear decrease in serum calcium by progressively increasing the infusion rate at regular intervals. A second cycle of hypocalcemia and recovery using the same protocol was started immediately after the completion of the first cycle. To determine whether a nonlinear decrease in the serum calcium affected the PTH response to hypocalcemia, a third group of dogs, termed superfast, was studied; in this group, EDTA was infused for 30 minutes at a constant rate of 50 mg/kg. The hysteretic loops of PTH produced by the two sequential slow and fast cycles and the superfast cycle during the induction of and recovery from hypocalcemia were similar. Moreover, the maximal PTH level for the two sequential slow and fast cycles and the superfast cycle was not different even though the rates of calcium decrease varied and the calcium decrease was nonlinear in the superfast cycle. In conclusion, (1) since hysteresis was reproducible and the maximal PTH was not different during two sequential cycles of hypocalcemia, hysteresis is not due to PTH depletion; (2) the PTH-calcium curve is not affected by the rate at which hypocalcemia is induced; and (3) the maximal PTH level is not influenced by either the rate or linearity of the reduction in serum calcium.