Abstract
The influence of secondary hyperparathyroidism (2 HPT) on the set point of the parathyroid hormone (PTH)-Ca2+ curve is controversial. In vitro experiments have shown an increase in the set point. However, clinical studies with hemodialysis patients have provided a variety of results (increases, decreases and no changes in the set point have been reported). The present study was designed to investigate the influence of the progression of 2 HPT on the set point of the PTH-Ca2+ curve. The PTH-Ca2+ curve and the expression of parathyroid calcium receptor (CaR mRNA) and vitamin D receptor (VDR mRNA) have been studied in normal rabbits (group I, n=9) and in nephrectomized rabbits (group II, n=18) at two stages after inducing 2 HPT: 2–3 weeks (group IIA) and 5–6 weeks (group IIB). In group I, the set point of the PTH-Ca2+ curve was 1.63±0.03 mM. A progressive hypocalcemia was detected during the evolution of 2 HPT (groups IIA and IIB). Rabbits from group IIA had a significant (P<0.001) decrease in the set point to values of 1.45±0.02 mM. However, the set point increased significantly in group IIB (P<0.001) to 1.56±0.03 mM. CaR mRNA was similarly decreased in groups IIA (39±12%) and IIB (48±7%). No changes were detected in VDR mRNA. In conclusion, a reduction in the set point of the PTH-Ca2+ curve in response to decreased extracellular Ca2+ was detected in the early phases of 2 HPT. However, with the progression of 2 HPT the set point tended to increase even though extracellular Ca2+ was markedly decreased. The increase in the set point in the course of 2 HPT seems to be a complex process that cannot be fully explained by changes in parathyroid CaR mRNA or VDR mRNA.
Introduction
Secondary hyperparathyroidism (2 HPT) is a common complication of end-stage renal disease. The decrease in functioning renal mass results in hypocalcemia, hyperphosphatemia and reduced calcitriol levels which stimulate parathyroid hormone (PTH) secretion and synthesis and promote parathyroid gland hyperplasia (Silver et al. 2002).
The relationship between PTH and blood ionized calcium (Ca2+) is best represented by a sigmoidal curve: the PTH-Ca2+ curve. The set point of the PTH-Ca2+ curve, which is defined as the Ca2+ level at which PTH secretion equals 50% of maximal PTH, reflects the sensitivity of the parathyroid glands to changes in calcium (Felsenfeld & Llach 1993).
The influence of 2 HPT on the set point of the PTH-Ca2+ curve is controversial. In vitro experiments using parathyroid glands excised from azotemic patients with severe 2 HPT have shown an increase in the set point, which may represent an intrinsic abnormality of PTH secretion (Brown et al. 1982). However, clinical studies with hemodialysis patients have provided a variety of results. Ramirez et al.(1993) and Goodman et al.(1995) reported no changes in the set point of patients with 2 HPT. Malberti et al.(1993) and Goodman et al.(1998) did not find changes in the set point in patients with moderate 2 HPT but described an increase in the set point in patients with severe 2 HPT. By contrast, a decrease in the set point has been reported in uremic patients (Borrego et al. 1997, Cardinal et al. 1998).
On the other hand, several investigators have reported that the set point of the PTH-Ca2+ curve changes in concordance with variations in extracellular Ca2+ – i.e. decreases with hypocalcemia and increases with hypercalcemia (Pahl et al. 1996, Rodriguez et al. 1997, Hardy-Yverneau et al. 1998, Malberti et al. 1999).
Therefore, low extracellular Ca2+ seems to decrease the set point of the PTH-Ca2+ curve but persistently low Ca2+ levels are also known to generate parathyroid hyperplasia and 2 HPT which may increase the set point.
The work reported here was designed to investigate the influence of the progression of 2 HPT on the set point of the PTH-Ca2+ curve.
Materials and Methods
Animals
White New Zealand rabbits of both sexes, aged 9–15 months and weighing 3.8±0.1 kg, were used in the experiments. Rabbits were randomly assigned to any of the following groups.
Group I – control (n=9). These animals were used as a control, to obtain the PTH-Ca2+ curve in normal rabbits. They were fed ad libitum on a balanced diet containing Ca=1.2% and P=0.6% (normal diet). The hypercalcemic and hypocalcemic parts of the PTH-Ca2+ curve were obtained in the same animals with an interval of 1 week to avoid the phenomenom of hysteresis (Aguilera-Tejero et al. 1996, Bas et al. 2002, 2003).
Group II – 2 HPT (n=23). In these rabbits renal failure was induced by performing a five-sixth nephrectomy as previously described (Bas et al. 2004). Briefly, nephrectomy was completed in two stages: in the first stage, two-thirds of the cortical parenchyme of the left kidney was ablated and a week later a contralateral nephrectomy was performed. These rabbits were fed the normal diet for 2 weeks after performing the nephrectomy and then were switched to a pelleted synthetic diet containing Ca=0.6% and P=1.2% (low calcium–high phosphate diet (LC-aHPD)). The reason for feeding the normal diet during the first 2 weeks is that many rabbits do not survive if they are fed an LCaHPD diet immediately after nephrectomy. After 2 weeks on the LCaHPD diet the hypercalcemic part of the PTH-Ca2+ curve was obtained. One week later the hypocalcemic part of the curve was obtained in the same rabbits. Thus, the first PTH-Ca2+ curve was obtained, in 18 rabbits, after 2–3 weeks on the LCaHPD (short-term hyperparathyroidism; group IIA). Five rabbits from group IIA were killed at 3 weeks to study their parathyroid glands. Rabbits were maintained on the same diet and the PTH-Ca2+ curve was studied again, in nine rabbits, at 5–6 weeks (medium-term hyperparathyroidism; group IIB).
At the end of the experiments rabbits were killed by an overdose of barbiturates. Parathyroid glands were removed from five rabbits of each group (control, group IIA and group IIB).
All animals received humane care in compliance with the ‘Principles of Laboratory Animal Care’ formulated by the National Society for Medical Research and the ‘Guide for the Care and Use of Laboratory Animals’ prepared by the National Academy of Sciences and published by the National Institutes of Health. The experimental protocols of the present study were approved by the Ethics Commitee for Animal Research of the University of Cordoba.
PTH-Ca2+ curves
PTH-Ca2+ curves were obtained by i.v. infusion of disodium EDTA and CaCl2. In all experimental groups, rabbits were anesthetized by a combination of ketamine (40 mg/kg) and midazolam (1 mg/kg). The marginal auricular vein and the central auricular artery were cannulated with 24 gauge catheters. The venous port was used for disodium EDTA and CaCl2 infusion and the arterial side for blood sampling. The protocols for induction of hyper- and hypocalcemia were as follows. Hypercalcemia was achieved by i.v. infusion of CaCl2 which was started at a rate of 0.175 mmol Ca/kg per h. CaCl2 infusion was increased every 5 min up to a final rate of 1.75 mmol of Ca/kg per h at 45 min. Hypocalcemia was induced by an EDTA infusion which was initiated at a rate of 50 mg Na2EDTA/kg per h. To achieve a linear decrease in Ca2+, the rate of the EDTA infusion was progressively increased every 5 min, up to 190 mg/kg per h at the end of the experiment (45 min). When basal calcium was below normal, the duration of the hypocalcemic stimulus was adjusted to achieve a final Ca2+of 1 mM.
Three blood samples were obtained from each animal as baseline; thereafter, blood samples were collected every 5 min until the end of the experiments. Samples were immediately centrifuged. Plasma was separated and Ca2+ and pH were measured using selective electrodes (Bayer Diagnostics, Barcelona, Spain) and then plasma was frozen at −70 °C. PTH was measured on plasma samples within 3 months of collection using an immunoradiometric assay which has previously been validated for quantitation of rabbit PTH (Allegro Intact PTH; Nichols, San Juan Capistrano, CA, USA) (Warren et al. 1989, Bas et al. 2002, 2003).
Individual PTH-Ca2+ curves were constructed by adjusting the PTH and Ca2+ values of every rabbit to a sigmoidal equation using the software SPSS 8.0 for Windows (SPSS Inc., Chicago, IL, USA). The PTH concentrations at standardized Ca2+ levels (from Ca2+=2.1 mM to Ca2+=1mM, with an interval of 0.05 mM) were extrapolated from these individual curves. Mean PTH values at standardized Ca2+ concentrations were used to obtain the PTH-Ca2+ curve for each group.
The following parameters were derived from the PTH-Ca2+ curves (Felsenfeld & Llach 1993).
-
Basal PTH (PTHb) was the PTH concentration before initiating either hyper- or hypocalcemia.
-
Maximal PTH (PTHmax) was the highest PTH level observed in response to hypocalcemia and additional reduction in plasma Ca2+ did not further increase PTH value.
-
Minimal PTH (PTHmin) was the lowest PTH level during suppression by hypercalcemia and a further increase in plasma Ca2+ did not result in any additional increase in PTH.
-
The ratio of basal to maximal PTH (PTHb/max) was the basal PTH divided by the maximal PTH and this fraction was multiplied by 100 to obtain a percentage.
-
The basal plasma Ca2+ (Cab) was the plasma Ca2+ concentration before initiating either hyper- or hypocalcemia.
-
The plasma Ca2+ at maximal PTH (Camax) was the plasma Ca2+ concentration at which the PTH level was first observed to be maximal.
-
The plasma Ca2+ at minimal PTH (Camin) was the plasma Ca2+ concentration at which the PTH level was first observed to be minimal.
-
The set point of Ca2+ was calculated in two different ways: (a) as the plasma Ca2+ concentration at which maximal PTH secretion was reduced by 50% (SP1) and (b) as the plasma Ca2+ concentration at which the difference between PTHmax and PTHmin was reduced by 50% (SP2).
Assessment of parathyroid gland hyperplasia
Glands were dissected free of adipose tissue and wet glandular weight was measured. The mean of five consecutive measurements was obtained from each gland. Parathyroid cell cycle was studied by flow cytometry as previously described (Canalejo et al. 2000). Briefly, small pieces of parathyroid tissue were viewed under an inverted microscope (×10) and were stripped apart using sharp Dumont forceps. This was followed by gently pipetting. To maintain high cell concentrations, these manipulations were performed in a small volume (30 μl) of phosphate-buffered saline (PBS). Dispersed cells were first treated with PBS containing 1% Triton X-100 at 37 °C, then with DNase-free RNase (10 μg/ml for 10 min) and finally with propidium iodide (20 μg/ml for 30 min) at 37 °C in the dark. Cells were immediately acquired by flow cytometer (FACScan; Becton-Dickinson, San Jose, CA, USA). LYSYS II software (Becton-Dickinson) was used for data acquisition and analysis. Cell debris and clumps were excluded from analysis by gating. The cell cycle was analyzed using the CELLFIT software (Becton-Dickinson). This method measures the percentage of cells in the different phases of the cell cycle: cells in G0/G1 phase are diploid cells, cells in phase S show an increase in the synthesis of DNA that precedes cell duplication and cells in G2+M have doubled the DNA content or are undergoing mitosis. The percentage of cells in phase S was used as a marker of cell proliferation.
Determination of calcium receptor (CaR) mRNA and vitamin D receptor (VDR) mRNA: RNA isolation and RT-PCR
Parathyroid tissue samples were placed in nuclease-free 1.5 ml microcentrifuge tubes for total RNA extraction. The tissue was ultrasonicated for 5 min at 4 °C and immersed in liquid nitrogen (−196 °C) to allow for complete cell rupture. Then, 400 μl of a lysis buffer from a commercial kit for tissue total RNA extraction (Mammalian Total RNA miniprep kit; Sigma, St Louis, MO, USA) was added to the samples. Total RNA was extracted using the Mammalian Total RNA miniprep kit. Extracted total RNA was dissolved in nuclease-free water (Promega, Madison, WI, USA) and heated for 10 min at 70 °C. The CaR and VDR vs β-actin were amplified with the c-Master RT-PCR kit (Eppendorff, Hamburg, Germany). The sequences of the primers were: CaR: sense (6-carboxyfluorescein (6 FAM)): 5′-ATTGAGGG GGAGCCCACCTGCTGCT-3′, anti-sense: 5′-AAAG AGGGTGAGTGCGATCCCAAAGG-3′ and VDR: sense (6 FAM): 5′-TGAAGGCTGCAAAGGCTTCA GGC-3′, anti-sense: 5′-GGATGAACTCCTTCATCA TGCCGATG-3′. Each sense primer was marked with 6 FAM fluorocrome. DNA amplifications were processed by a Genetic Analyzer Abi Prism 310 (Perkin Elmer, Foster City, CA, USA). Data were analyzed using specific software, Gene Scan version 3.1/1998 (Perkin Elmer). The basepair standard (size standard) was marked with a different type F fluorochrome (Tamra-350; Perkin Elmer) and run in parallel. The amount of CaR/β-actin mRNA and VDR/β-actin mRNA was expressed as percent of the control group.
Biochemical measurements
In addition to the measurements of Ca2+ and PTH, the following analytes were measured in baseline samples: inorganic phosphate, creatinine and calcitriol. Phosphate and creatinine were measured using standard spectrophotometric techniques (Sigma Diagnostics). Calcitriol was measured using a radioreceptor assay (Immunodiagnostic Systems, Bolton, Lancs, UK).
Statistics
For the intra- or intergroup comparison of three or more samples, repeated ANOVA was used. If the ANOVA showed statistical differences, a post hoc test, the Fisher HSD test, was used to determine differences. The Pearson test was used in the analysis of correlation. A P value<0.05 was considered significant. Results are expressed as the means ± s.e.m.
Results
Table 1 shows plasma concentrations of phosphate, creati-nine and calcitriol in the three groups of rabbits. Phosphate was elevated in the 2 HPT groups. As expected, rabbits from group II (A and B) presented higher creatinine levels and lower calcitriol concentrations than the control group.
Figure 1 shows the PTH-Ca2+ curve in groups I, IIA and IIB. At baseline Ca2+, 1.71±0.05 mM, normal rabbits (group I) had a PTH concentration of 26.9±3.2 pg/ml. Induction of hypocalcemia resulted in an increase in PTH concentration that reached a maximum, 94.4±5.5 pg/ml, at Ca2+ levels of 1.4 mM. At 2–3 weeks, nephrectomized rabbits (group IIA) had a lower baseline Ca2+, 1.28±0.03 mM, than group I (P<0.001). Basal (318±22 pg/ml), maximal (366±22 pg/ml) and minimal (48±7 pg/ml) PTH concentrations in group IIA were significantly higher than in group I (P<0.001). After 5–6 weeks, nephrectomized rabbits (group IIB) had a baseline Ca2+, 1.22±0.08 mM, that was lower than group I (P<0.001) but not significantly different from group IIA (1.28±0.03 vs 1.22±0.08 mM, P=0.37). In group IIB, PTHb (739±155 pg/ml), PTHmax (801±169 pg/ml) and PTHmin (102±21 pg/ml) were significantly higher (P<0.05) than in the other groups.
The PTH-Ca2+ curves, with PTH values expressed as a percentage of PTHmax, are presented in Fig. 2. In normal rabbits (group I), the set point of the PTH-Ca2+ curve was 1.63±0.03 mM. Rabbits from group IIA had a significant (P<0.001) decrease in the set point to values of 1.45±0.02 mM. However, after 5–6 weeks of 2 HPT (group IIB), the set point increased significantly (P<0.001) to 1.56±0.03 mM and was not different from the control group. Set point values were similar when calculated as 50% of PTHmax (SP1) or as 50% of PTHmax−PTHmin (SP2) (Table 2).
The relationship between the set point of the PTH-Ca2+ curve and baseline Ca2+ in groups IIA and IIB is presented in Fig. 3. After 2–3 weeks of 2 HPT (group IIA), a significant correlation between baseline Ca2+ and the set point was found (r=0.84, P<0.001). However, after 5–6 weeks of 2 HPT (group IIB), the correlation between baseline Ca2+ and the set point decreased and statistical significance was lost (r=0.65, P=0.06). Although the difference in the number of observations from each group may have influenced the r values, it is interesting to note that, in a broad range of extracellular Ca2+, the set point was consistently higher in group IIB than in group IIA.
Parathyroid glands from normal rabbits weighed 6.2±0.3 mg and had 0.51±0.01% of cells in the S phase. A significant (P<0.05) increase in glandular weight (17.2±2.4 mg) and percent of cells in the S phase (3.35±0.8%) were identified after 5–6 weeks of 2 HPT (group IIB).
The CaR mRNA expression (CaR/β-actin mRNA) was markedly reduced, when compared with the control group (100±19%), in both rabbits with short-term hyperparathyroidism (group IIA=39±12%, P<0.05) and rabbits with medium-term hyperparathyroidism (group IIB=48±7%, P<0.05). CaR mRNA values in groups IIA and IIB were not significantly different. A significant negative correlation (P<0.05) was found between CaR mRNA expression and both glandular weight and maximal PTH secretion.
VDR mRNA/β-actin mRNA levels were slightly lower in both 2 HPT groups (group IIA=78.2±13.5% and group IIB=85.3±17.2%) when compared with the control group=100±8.2%. However, differences were not significant. In addition, VDR mRNA levels were not correlated with other indicators of hyperparathyroidism (glandular weight and maximal PTH secretion).
Discussion
The objective of the present study was to investigate the influence of the progression of 2 HPT on the set point of the PTH-Ca2+ curve. Our results showed a reduction in the set point which correlates with the decrease in extracellular Ca2+ 2–3 weeks after the induction of hyperparathyroidism. However, progression of parathyroid gland hyperplasia tended to increase the set point even though extracellular Ca2+ was markedly reduced.
The rabbit has been chosen, in the present study, as an animal model to investigate the effect of the progression of short and medium term 2 HPT on the set point of the PTH-Ca2+ curve. The rabbit model has the advantages of being an easily accessible laboratory animal whose body size allows the generation of detailed PTH-Ca2+ curves. Although normal rabbits on a standard diet have higher levels of extracellular Ca2+ than most mammals (around 1.7 mM), the PTH-Ca2+ relationship is comparable in rabbits, humans and other mammals (basal PTH, at Ca2+=1.7 mM, represents aproximately 25% of maximal PTH and both PTHmax and PTHmin are similar to the values obtained in humans). The rabbit model also has the advantage that extracellular Ca2+ levels are consistently and markedly decreased in the course of 2 HPT, thus facilitating study of the influence of hypocalcemia on the set point of the PTH-Ca2+ curve. In addition, rabbit PTH can be measured using the immunoradiometric assay for intact human PTH (Warren et al. 1989, Bas et al. 2002, 2003).
When obtaining the PTH-Ca2+ curves, the hypercalcemic part of the curve was always completed first to minimize the risk of death – uremic rabbits tolerate hypercalcemia better than hypocalcemia. To avoid any influence of the hypercalcemic stimulus on the subsequent hypocalcemia (hysteresis) a week was allowed before performing the hypocalcemic part of the curve (Aguilera-Tejero et al. 1996, Bas et al. 2002, 2003).
Nephrectomy plus the LCaHPD resulted in a progressive hypocalcemia and hyperphosphatemia. In addition, calcitriol levels were reduced in uremic rabbits. As a consequence, these rabbits developed parathyroid gland hyperplasia as demonstrated by an increase in glandular weight and an elevated cellular proliferation. The evolution of plasma PTH concentrations reflected progression of parathyroid gland hyperplasia. All PTH parameters (PTHb, PTHmax, PTHmin and the ratio PTHb/ PTHmax) were more elevated at 5–6 weeks (group IIB) than at 2–3 weeks (group IIA).
At 2–3 weeks after the onset of hyperparathyroidism, rabbits from group IIA experienced a decrease in the set point of the PTH-Ca2+ curve which was highly correlated with the decrease in Cab. Thus, in these rabbits it seems that there was an adaptation of the set point to the ambient Ca2+, even though they had already initiated parathyroid hyperplasia. These results are in agreeement with previous studies that have reported a decrease in the set point of the PTH-Ca2+ curve in uremic patients with decreased Ca2+ levels (Borrego et al. 1997). A decrease in the set point coincident with a reduction in extracellular Ca2+ has also been reported in vitamin D-deficient dogs fed a low calcium diet (Cloutier et al. 1992); however, it is difficult to compare these data with the present study since in the vitamin D-deficient dog model hypocalcemia is a late event that appears well after 2 HPT has started.
Rabbits from group IIB, which had been developing 2 HPT for 5–6 weeks, were also markedly hypocalcemic. However, in these rabbits the set point increased when compared with group IIA, approaching the set point of normal rabbits. Thus, the correlation between Cab and the set point decreased at 5–6 weeks of 2 HPT. The increase in the set point found at 5–6 weeks, when compared with 2–3 weeks, is in agreement with previous reports that have identified a tendency to an increased set point of the PTH-Ca2+ curve in the course of primary (Brown et al. 1979) and secondary (Brown et al. 1982, Malberti et al. 1993, Goodman et al. 1998) hyperparathyroidism. However, in these studies the progression of hyperparathyroidism was accompanied by an increase in extracellular Ca2+ while in our study plasma Ca2+ remained low.
The increase in the set point in the course of 2 HPT, which reflects a decreased sensitivity of the parathyroid cells to Ca2+, could be related to the reduction in CaR expression. A decrease in CaR has been described previously both in primary (Cetani et al. 2000) and secondary (Gogusev et al. 1997) hyperparathyroidism. Moreover, in patients with primary hyperparathyroidism the reduction in CaR has been reported to correlate with the increase in the set point (Cetani et al. 2000). Our results confirm a decrease in CaR in the course of 2 HPT. However, as previously reported (Ritter et al. 2001) the reduction in CaR expression occurs early in the course of 2 HPT (before the set point begins to increase); thus, other factors related to parathyroid hyperplasia may also influence the increase in the set point detected with the progression of 2 HPT.
A decrease in VDR, which reduces the ability of calcitriol to inhibit parathyroid gland proliferation, is a well-documented etiopathogenic factor in both primary and secondary hyperparathyroidism (Carling et al. 2000, Lewin et al. 2002). The absence of changes in VDR expression in our study, which has also been reported in other models of experimental uremia (Szabo et al. 1996) and nutritional hyperparathyroidism (Hernandez et al. 1996), may be explained by the heterogenous distribution of VDR in the parathyroid glands (Fukuda et al. 1993, Valimaki et al. 2001). VDR mRNA has lower density in glands with chronic hyperparathyroidism showing nodular hyperplasia (Fukuda et al. 1993). However, in rabbits with short/medium term hyperparathyroidism, nodularity is not a significant feature (Bas et al. 2004). Thus the VDR mRNA results must be interpreted in the specific context of our study. Moreover, since in experimental models of uremia it is difficult to achieve the chronic hyperparathyroidism (nodular parathyroid hyperplasia) that is often found in patients receiving hemodialysis, care should be taken when extrapolating the results to humans with renal impairment.
In conclusion, a reduction in the set point of the PTH-Ca2+ curve has been detected in the early phases of 2 HPT. However, with the progression of parathyroid gland hyperplasia the set point tends to increase even though extracellular Ca2+ is still markedly reduced. The increase in the set point in the course of 2 HPT seems to be a complex process that cannot be fully explained by changes in parathyroid CaR mRNA or VDR mRNA.
Plasma concentrations (means±s.e.m.) of phosphate, creatinine and calcitriol in the three groups of rabbits: group I=control and group II=nephrectomized rabbits that had been fed a low Ca–high P diet for 2–3 weeks (IIA) and for 5–6 weeks (IIB)
| Phosphate (mmol/l) | Creatinine (3mol/l) | Calcitriol (pmol/l) | |
|---|---|---|---|
| aP<0.05 when compared with group I, bP<0.05 when compared with group IIA. | |||
| Group | |||
| I (n=9) | 1.06±0.06 | 79.5±8.8 | 125.7±9.8 |
| IIA (n=18) | 3.01±0.29a | 397.8±26.5a | 30.9±4.6a |
| IIB (n=9) | 4.17±0.45a,b | 468.5±53.1a | 30.5±5.0a |
Mean±s.e.m. analytes and parameters derived from the PTH-Ca2+ curve in the three groups of rabbits: group I=control and group II=nephrectomized rabbits that had been fed a low Ca–high P diet for 2–3 weeks (IIA) and for 5–6 weeks (IIB)
| PTHb (pg/ml) | PTHmax (pg/ml) | PTHmin (pg/ml) | PTHb/max (%) | Cab (mM) | Camax (mM) | Camin (mM) | SP1 (mM) | SP2 (mM) | |
|---|---|---|---|---|---|---|---|---|---|
| aP<0.05 when compared with group I, bP<0.05 when compared with group IIA. | |||||||||
| PTH (pmol/l)=PTH (pg/ml)× 0.105. | |||||||||
| Group | |||||||||
| I (n=9) | 26.9±3.2 | 94.4±5.5 | 3.2±0.3 | 28±3 | 1.71±0.02 | 1.49±0.03 | 1.97±0.02 | 1.63±0.02 | 1.62±0.2 |
| IIA (n=18) | 318.1±22.2a | 366.1±22.7a | 48.1±6.9a | 87±2a | 1.28±0.04a | 1.26±0.03a | 1.90±0.03a | 1.45±0.02a | 1.42±0.02a |
| IIB (n=9) | 738.6±155.8a,b | 801.1±169.4a,b | 102.2±22.2a,b | 93±2a | 1.22±0.08a,b | 1.31±0.04a | 1.93±0.02 | 1.56±0.03 | 1.53±0.03 |
The PTH-Ca2+ curve in control rabbits (group I (•), n=9) and in nephrectomized rabbits after 2–3 weeks (group IIA (▪), n=18) and 5–6 weeks (group IIB (Δ), n=9) of 2 HPT. Within each curve, baseline Ca2+ is marked by a diamond and the set point by a star. PTH (pmol/l)=PTH (pg/ml)× 0.105. Values are means±s.e.m.
Citation: Journal of Endocrinology 184, 1; 10.1677/joe.1.05565
The PTH-Ca2+ curve with PTH concentrations expressed as a percentage of maximal PTH in control rabbits (group I (•), n=9) and in nephrectomized rabbits after 2–3 weeks (group IIA (▪), n=18) and 5–6 weeks (group IIB (Δ), n=9) of 2 HPT. Within each curve, baseline Ca2+ is marked by a diamond and the set point by a star. PTH (pmol/l)=PTH (pg/ml)× 0.105. Values are means±s.e.m.
Citation: Journal of Endocrinology 184, 1; 10.1677/joe.1.05565
Correlation between baseline Ca2+ and the set point of the PTH-Ca2+ curve in nephrectomized rabbits after 2–3 weeks of 2 HPT (group IIA (▪), n=18) and after 5–6 weeks of 2 HPT (group IIB (Δ), n=9).
Citation: Journal of Endocrinology 184, 1; 10.1677/joe.1.05565
This work was supported by grants BFI2001-1901 and BFI2001-0350 from the Ministerio de Ciencia y Tecnología, the Plan Andaluz de Investigación (Grupo CTS-179) and the Fundación Reina Sofía-Cajasur. The authors declare that there is no conflict of interest that would prejudice the impartiality of this scientific work.
References
Aguilera-Tejero E, Sanchez J, Almaden Y, Mayer-Valor R, Rodriguez M & Felsenfeld AJ 1996 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. Journal of Bone and Mineral Research 11 1226–1233.
Bas S, Aguilera-Tejero E, Estepa JC, Garfia B, Lopez I & Rodriguez M 2002 The influence of acute and chronic hypercalcemia on the parathyroid hormone response to hypocalcemia in rabbits. European Journal of Endocrinology 146 411–418.
Bas S, Bas A, Almaden Y, Ballesteros E, Rodriguez M & Aguilera-Tejero E 2003 Both duration and degree of hypercalcemia influence the reduced parathyroid hormone response to hypocalcemia after hypercalcemia. Journal of Endocrinology 177 119–126.
Bas S, Bas A, Estepa JC, Mayer-Valor R, Rodriguez M & Aguilera-Tejero E 2004 Parathyroid gland function in the uremic rabbit. Domestic Animal Endocrinology 26 99–110.
Borrego MJ, Felsenfeld AJ, Martin-Malo A, Almaden Y, Concepcion MT, Aljama P & Rodriguez M 1997 Evidence for adaptation of the entire PTH-calcium curve to sustained changes in the serum calcium in hemodialysis patients. Nephrology Dialysis and Transplantation 12 505–513.
Brown EM, Broadus AE, Brennan MF, Gardner DG, Marx SJ, Spiegel AM, Downs RW, Attie M & Aurbach GD 1979 Direct comparison in vivo and in vitro of suppressibility of parathyroid function by calcium in primary hyperparathyroidism. Journal of Clinical Endocrinology and Metabolism 48 604–610.
Brown EM, Wilkson RE, Eastman RC, Pallota J & Marynick SP 1982 Abnormal regulation of parathyroid hormone release by calcium in secondary hyperparathyroidism due to chronic renal failure. Journal of Clinical Endocrinology and Metabolism 54 172–179.
Canalejo A, Almaden Y, Torregrosa V, Gomez-Villamandos JC, Ramos B, Campistol JM, Felsenfeld AJ & Rodriguez M 2000 The in vitro effect of calcitriol on parathyroid cell proliferation and apoptosis. Journal of the American Society of Nephrology 10 1865–1872.
Cardinal H, Brossard J-H, Roy L, Lepage R, Rousseau L & D’Amour P 1998 The set point of parathyroid hormone stimulation by calcium is normal in progressive renal failure. Journal of Clinical Endocrinology and Metabolism 83 3839–3844.
Carling T, Rastad J, Szabo E, Westin G & Akerstrom G 2000 Reduced parathyroid vitamin D receptor messenger ribonucleic acid levels in primary and secondary hyperparathyroidism. Journal of Clinical Endocrinology and Metabolism 85 2000–2003.
Cetani F, Picone A, Cerrai P, Vignali E, Borsari S, Pardi E, Viacava P, Naccarato AG, Miccoli P, Kifor O, Brown EM, Pinchera A & Marcocci C 2000 Parathyroid expression of calcium-sensing receptor protein and in vivo parathyroid hormone-Ca2+ set-point in patients with primary hyperparathyroidism. Journal of Clinical Endocrinology and Metabolism 85 4789–4794.
Cloutier M, Gascon-Barre M & D’Amour P 1992 Chronic adaptation of dog parathyroid function to a low-calcium-high sodium-vitamin D-deficient diet. Journal of Bone and Mineral Research 7 1021–1027.
Felsenfeld AJ & Llach F 1993 Parathyroid gland function in chronic renal failure. Kidney International 43 771–789.
Fukuda N, Tanaka H, Tominaga Y, Fukugawa M, Kurokawa K & Seino Y 1993 Decreased 1,25-dihydroxyvitamin D3 receptor density is associated with a more severe form of parathyroid hyperplasia in chronic uremic patients. Journal of Clinical Investigation 92 1436–1443.
Gogusev J, Duchambon P, Hory B, Giovannini M, Goureau Y, Sarfati E & Drueke TB 1997 Depressed expression of calcium receptor in parathyroid gland tissue of patients with hyperparathyroidism. Kidney International 51 328–336.
Goodman WG, Belin T, Gales B, Juppner H, Segre GV & Salusky IB 1995 Calcium-regulated parathyroid hormone release in patients with mild or advanced secondary hyperparathyroidism. Kidney International 48 1553–1558.
Goodman WG, Veldhuis JD, Belin TR, Van Herle AJ, Juppner H & Salusky IB 1998 Calcium-sensing by parathyroid glands in secondary hyperparathyroidism. Journal of Clinical Endocrinology and Metabolism 83 2765–2772.
Hardy-Yverneau P, Shenouda M, Moriniere P, Legallais C, Brazier M, Achard J & Fournier A 1998 The dependency of calcium set point on basal plasma calcium in dialysis patients: a better explanation for the discrepancies regarding its link with PTH secretion than methodological differences. Clinical Nephrology 50 236–246.
Hernandez A, Concepcion MT, Rodriguez M, Salido E & Torres A 1996 High phosphorus diet increases preproPTH mRNA independent of calcium and calcitriol in normal rats. Kidney International 50 1872–1878.
Lewin E, Garfia B, Recio FL, Rodriguez M & Olgaard K 2002 Persistent downregulation of calcium-sensing receptor mRNA in rat parathyroids when severe secondary hyperparathyroidism is reversed by isogenic kidney transplantation. Journal of the American Society of Nephrology 13 2110–2116.
Malberti F, Corradi B, Pagliari, Romanini D, Gazo A, Sidoti A, Baretta A, Bellazi R & Imbasciati E 1993 The sigmoidal parathyroid hormone-ionized calcium curve and the set point of calcium in hemodialysis and continuous ambulatory peritoneal dialysis. Peritoneal Dialysis International 13 (Suppl 2) S476–S479.
Malberti F, Farina M & Imbasciati E 1999 The PTH-calcium curve and the set point of calcium in primary and secondary hyperparathyroidism. Nephrology Dialysis and Transplantation 14 2398–2406.
Pahl M, Jara A, Bover J, Rodriguez M & Felsenfeld AJ 1996 The set point of calcium and the reduction of parathyroid hormone in hemodialysis patients. Kidney International 49 226–231.
Ramirez JA, Goodman WG, Gornbein J, Menezes C, Moulton L, Segre GV & Salusky IB 1993 Direct in vivo comparison of calcium-regulated parathyroid hormone secretion in normal volunteers and patients with secondary hyperparathyroidism. Journal of Clinical Endocrinology and Metabolism 76 1489–1494.
Ritter CS, Finch JL, Slatopolsky EA & Brown AJ 2001 Parathyroid hyperplasia in uremic rats precedes down-regulation of the calcium receptor. Kidney International 60 1737–1744.
Rodriguez M, Caravaca F, Fernandez E, Borrego MJ, Lorenzo V, Cubero J, Martin-Malo A, Betriu A, Rodriguez AP & Felsenfeld AJ 1997 Evidence for both abnormal set point of PTH stimulation by calcium and adaptation to serum calcium in hemodialysis patients with hyperparathyroidism. Journal of Bone and Mineral Research 12 347–355.
Silver J, Kilav R & Naveh-Many T 2002 Mechanisms of secondary hyperparathyroidism. American Journal of Physiology 283 F367–F376.
Szabo A, Ritz E, Schmidt-Gayk H & Reichel H 1996 Abnormal expression and regulation of vitamin D receptor in experimental uremia. Nephron 73 619–628.
Valimaki S, Farnebo F, Forsberg L, Larsson C & Farnebo LO 2001 Heterogeneous expression of receptor mRNAs in parathyroid glands of secondary hyperparathyroidism. Kidney International 60 1666–1675.
Warren HB, Lausen NCC, Segre GV, El-Hajj G & Brown EM 1989 Regulation of calciotropic hormones in vivo in the New Zealand White rabbit. Endocrinology 125 2683–2690.
