Influence of Ca2+ concentration on the clearance and circulating levels of intact and carboxy-terminal iPTH in pentobarbital-anesthetized dogs

Anesthetic effect of a combination of medetomidine-midazolam-butorphanol in cynomolgus monkeys (Macaca fascicularis)

The anesthetic effect of a combination of medetomidine, midazolam and butorphanol (Me-Mi-Bu) was evaluated in healthy cynomolgus monkeys. The Me-Mi-Bu combination was intramuscularly administered as follows: Dose 1, Me 0.015 mg/kg-Mi 0.1 mg/kg-Bu 0.15 mg/kg; Dose 2, Me 0.02 mg/kg-Mi 0.15 mg/kg-Bu 0.2 mg/kg; and Dose 3, Me 0.04 mg/kg-Mi 0.3 mg/kg-Bu 0.4 mg/kg. The combination rapidly induced immobilization, and lateral recumbency was reached within 15 min. The duration of anesthesia for each dose administered was follows: Dose 1, 47 ± 27 min; Dose 2, 113 ± 31 min; and Dose 3, 190 ± 24 min. The anesthetic effect of the combination was abolished by the α2-adrenoceptor antagonist atipamezole. No marked changes in the levels of hematologic or serum biochemical parameters were noted in cynomolgus monkeys administered the combination plus atipamezole. Taken together, these results suggest that the Me-Mi-Bu combination exhibits reversible anesthetic effect and may be useful for studies involving cynomolgus monkeys.

First- and second-generation immunometric PTH assays during treatment of hyperparathyroidism with cinacalcet HCl

BACKGROUND

First-generation immunometric assays for "intact" parathyroid hormone (iPTH) also measure large N-terminally truncated PTH fragments, whereas second-generation assays, such as the "bio-intact" PTH (biPTH) assay, measure only full-length biologically active PTH(1-84). This study compared iPTH and biPTH assays during cinacalcet treatment in subjects with secondary HPT receiving dialysis.

METHODS

Four hundred and ten subjects were enrolled in a 26-week randomized, double-blind, placebo-controlled trial of oral cinacalcet (or placebo), 30 to 180 mg once daily, and efficacy was assessed using biPTH and iPTH assays.

RESULTS

Compared with control treatment, cinacalcet improved the management of secondary HPT. Both biPTH and iPTH decreased by 38%+/- 3% during weeks 13 to 26 in the cinacalcet group; biPTH increased by 23%+/- 4% and iPTH increased by 9.5%+/- 3% in the control group (P < 0.001). Fifty-six percent of cinacalcet subjects and 10% of control subjects had a > or = 30% reduction in biPTH, and 61% and 11%, respectively, had a > or = 30% reduction in iPTH. Significant correlations between biPTH and iPTH levels were observed throughout the study. Both assays correlated similarly with bone-specific alkaline phosphatase levels. The ratio of biPTH to iPTH was maintained at 56% +/- 1% after treatment in both treatment groups. Increasing serum calcium levels were associated with a decreasing ratio of biPTH to (iPTH-biPTH).

CONCLUSION

These data show that PTH can be monitored with either iPTH or biPTH assays during therapy with cinacalcet, and that cinacalcet therapy does not exert a major influence on the ratio between PTH(1-84) and large, N-terminally truncated PTH fragments.

Parathyroid hormone secretion and action: evidence for discrete receptors for the carboxyl-terminal region and related biological actions of carboxyl- terminal ligands

PTH is a major systemic regulator of the concentrations of calcium, phosphate, and active vitamin D metabolites in blood and of cellular activity in bone. Intermittently administered PTH and amino-terminal PTH peptide fragments or analogs also augment bone mass and currently are being introduced into clinical practice as therapies for osteoporosis. The amino-terminal region of PTH is known to be both necessary and sufficient for full activity at PTH/PTHrP receptors (PTH1Rs), which mediate the classical biological actions of the hormone. It is well known that multiple carboxyl-terminal fragments of PTH are present in blood, where they comprise the major form(s) of circulating hormone, but these fragments have long been regarded as inert by-products of PTH metabolism because they neither bind to nor activate PTH1Rs. New in vitro and in vivo evidence, together with older observations extending over the past 20 yr, now points strongly to the existence of novel large carboxyl-terminal PTH fragments in blood and to receptors for these fragments that appear to mediate unique biological actions in bone. This review traces the development of this field in the context of the evolution of our understanding of the "classical" receptor for amino-terminal PTH and the now convincing evidence for these receptors for carboxyl-terminal PTH. The review summarizes current knowledge of the structure, secretion, and metabolism of PTH and its circulating fragments, details available information concerning the pharmacology and actions of carboxyl-terminal PTH receptors, and frames their likely biological and clinical significance. It seems likely that physiological parathyroid regulation of calcium and bone metabolism may involve receptors for circulating carboxy-terminal PTH ligands as well as the action of amino-terminal determinants within the PTH molecule on the classical PTH1R.

Intact PTH assay overestimates true 1-84 PTH levels after maxacalcitol therapy in dialysis patients with secondary hyperparathyroidism

BACKGROUND

Although the so-called intact parathyroid hormone (iPTH) assay detects not only true 1-84 PTH (1-84PTH) but also large C-terminal PTH fragments, it remains inconclusive whether the 1-84PTH assay is more useful in clinical practice. Previous studies have shown that the results of these two PTH assays in dialysis patients are closely correlated.

METHODS

Chronic dialysis patients whose plasma iPTH levels were >400 pg/ml were selected for inclusion in the present study. Following a 4 week wash-out time during which all vitamin D administration was halted, maxacalcitol was intravenously injected at the end of dialysis sessions three times per week for 24 weeks, at an initial dosage of 10 micro g.

RESULTS

Ninety-seven patients with secondary hyperparathyroidism were included in our analysis. Their serum calcium levels were elevated from the start levels while phosphate levels remained unchanged. The plasma 1-84PTH levels constantly declined throughout the 24 weeks. Although the patients' plasma 1-84PTH and iPTH levels were closely correlated with each other both at the beginning of the study and after 24 weeks of maxacalcitol therapy, the ratio of 1-84PTH/iPTH consistently decreased throughout the study period (P<0.01). The changes in the ratio were significantly correlated with changes in serum calcium levels.

CONCLUSIONS

Twenty-four weeks of intravenous maxacalcitol injection therapy significantly reduced the 1-84PTH/iPTH ratio. Estimated 1-84PTH levels from iPTH levels using a conversion formula obtained before the treatment were 21.0+/-20.4% higher than measured 1-84PTH levels after the therapy. Thus, iPTH measurement has a potential risk to overestimate 1-84PTH levels when evaluating the efficacy of maxacalcitol therapy in dialysis patients with secondary hyperparathyroidism.

Dynamics of secretion and metabolism of PTH during hypo- and hypercalcaemia in the dog as determined by the 'intact' and 'whole' PTH assays

BACKGROUND

Recent evidence has shown that the assay for 'intact' parathyroid hormone (I-PTH) not only reacts with 1-84 PTH but also with large non-1-84 PTH fragments, most of which is probably 7-84 PTH. As a result, an assay specific for 1-84 PTH named 'whole' PTH (W-PTH) has been developed. The present study was designed: (i) to determine whether the W-PTH assay reliably measures PTH values in the dog; (ii) to evaluate differences between the W-PTH and I-PTH assays during hypo- and hypercalcaemia; and (iii) to assess the peripheral metabolism of W-PTH and I-PTH.

METHODS

In normal dogs, hypocalcaemia was induced by EDTA infusion and was followed with a 90 min hypocalcaemic clamp. Hypercalcaemia was induced with a calcium infusion.

RESULTS

I-PTH and W-PTH values increased from 36+/-8 and 13+/-3 pg/ml (P=0.01) at baseline to a maximum of 158+/-40 and 62+/-15 pg/ml (P=0.02 vs I-PTH) during hypocalcaemia. The W-PTH/I-PTH ratio, 38+/-4% at baseline, did not change during the induction of hypocalcaemia, but sustained hypocalcaemia increased (P<0.05) this ratio. During hypercalcaemia, maximal suppression for I-PTH was 2.0+/-0.5 and only 5.7+/-0.6 pg/ml for W-PTH, due to a decreased sensitivity of the W-PTH assay at values <5 pg/ml. The disappearance rate of PTH was determined in five additional dogs which underwent a parathyroidectomy (PTX). At 2.5 min after PTX, W-PTH was metabolized more rapidly, with a value of 25+/-2% of the pre-PTX value vs 30+/-3% for I-PTH (P<0.05).

CONCLUSIONS

(i) The W-PTH/I-PTH ratio is less in the normal dog than in the normal human, suggesting that the percentage of non-1-84 PTH measured with the I-PTH assay is greater in normal dogs than in normal humans; (ii) the lack of change in the W-PTH/I-PTH ratio during acute hypocalcaemia is different from the situation observed in humans; and (iii) the dog appears to be a good model to study I-PTH and W-PTH assays during hypocalcaemia.

Similar predictive value of bone turnover using first- and second-generation immunometric PTH assays in pediatric patients treated with peritoneal dialysis

BACKGROUND

Accurate measurements of the concentration of parathyroid hormone (PTH) in serum or plasma are essential for the proper assessment of renal osteodystrophy. The first-generation immunometric PTH assay (1st PTH-IMA) not only detects the intact hormone, but also additional PTH fragments truncated at the amino N-terminally truncated PTH-derived fragments [ntPTH(1-84)]. A second-generation immunometric PTH assay (2nd PTH-IMA) recognizes only PTH(1-84) and possibly PTH fragments that are truncated at the carboxyl-terminus but not PTH(7-84). Whether estimates of the ratio between PTH(1-84) and ntPTH(1-84) fragments are a better predictor of bone turnover remains controversial.

METHODS

Thirty-three patients aged 12.8 +/- 4.4 years treated with continuous cycling peritoneal dialysis (CCPD) for 13 +/- 9 months underwent iliac crest bone biopsy. PTH levels were measured by two newly developed first-generation and second-generation PTH-IMA. The ntPTH(1-84) fragments were calculated by subtracting PTH values determined using the 2nd PTH-IMA from values obtained using 1st PTH-IMA that detects both PTH(1-84) and relatively large ntPTH(1-84).

RESULTS

Determinations of PTH levels by both assays were highly correlated (r = 0.89, P < 0.001). The relationships between first-generation and second-generation PTH-IMA and bone formation were similar (r = 0.67, P < 0.0001 and r = 0.64, P < 0.0001, respectively). When patients were grouped according to the presence or absence of secondary hyperparathyroidism, the ratio PTH(1-84) to ntPTH(1-84) did not differ between groups.

CONCLUSION

PTH concentrations determined by either the first- or the second-generation PTH-IMA were found to be better predictors of bone formation than the PTH(1-84) to ntPTH(1-84) fragments ratio.

Parathyroid hormone (PTH), PTH-derived peptides, and new PTH assays in renal osteodystrophy

Parathyroid hormone (PTH), PTH-derived peptides, and new PTH assays in renal osteodystrophy. Reliable measurements of parathyroid hormone (PTH) concentrations in serum or plasma are critical for the appropriate diagnosis and management of patients with renal osteodystrophy. With the introduction of second generation immunometric assays for PTH, it is now possible to measure exclusively full-length, biologically active PTH(1-84). In contrast, first generation immunometric assays that have been used widely for many years detect not only PTH(1-84), but also other large amino-terminally-truncated, PTH-derived peptides. This development will require a careful re-evaluation of PTH measurements, as determined by either first or second generation immunometric assays, and their relationship to bone histology and bone remodeling rates in patients with end-stage renal disease (ESRD). Such information is essential for proper clinical management, but only limited bone biopsy data are available to guide the interpretation of PTH results using second generation PTH assays. The different performance characteristics of first and second generation immunometric PTH assays also makes it possible to quantify the plasma levels of amino-terminally-truncated, PTH-derived peptides, which may accumulate disproportionately in patients with ESRD. Recent experimental evidence indicates that one or more of these peptides can modify bone cell activity and skeletal remodeling, possibly by interacting with a PTH receptor distinct from the type I PTH receptor that binds to the amino-terminal portion of PTH and mediates the classical biological actions of the hormone. The putative C-PTH receptor interacts with mid- and/or carboxyterminal regions of PTH and other amino-terminally-truncated PTH-derived peptides; signaling through it may contribute to the skeletal resistance to PTH that characterizes ESRD. The current review discusses certain aspects of the molecular structure of PTH and its interaction with various receptors, briefly comments about selected components of PTH secretion, highlights recent technical advances in PTH assays, and summarizes the effects of various PTH-derived peptides on bone cells and on skeletal metabolism.

Extracellular calcium-sensing receptor is expressed in rat hepatocytes. coupling to intracellular calcium mobilization and stimulation of bile flow

Liver cells respond to changes in Ca(2+)(o). The hepatic functions affected include bile secretion, metabolic activity, liver regeneration, and the response to xenobiotics. In the present study, we demonstrate the presence, in the liver, of the extracellular calcium-sensing receptor (CASR), described previously in the parathyroid and thyroid glands and kidney. CASR mRNA was specifically expressed in hepatocytes and was absent in nonparenchymal liver cells (stellate, endothelial, and Kupffer cells). Western blot analysis using a specific CASR antibody showed staining in both whole liver and hepatocyte extracts. Immunohistochemistry and in situ hybridization of rat liver sections showed expression of CASR protein and mRNA by a subset of hepatocytes. The known agonists of the CASR, gadolinium (Gd(3+); 0.5-3.0 mm) and spermine (1.25-20 mm), in the absence of Ca(2+)(o), elicited dose-related increases in Ca(2+)(i) in isolated rat hepatocytes loaded with Fura-2/acetoxymethyl ester. There was a greatly attenuated response to a second challenge with either agonist. The response was also abrogated when inositol 1,4,5-trisphosphate (IP(3))-sensitive calcium pools had been depleted by pretreatment with either thapsigargin or phenylephrine, an alpha(1)-adrenergic receptor agonist known to mobilize Ca(2+)(i) from IP(3)-sensitive pools. Addition of the deschloro-phenylalkylamine compound, NPS R-467, but not the S enantiomer, NPS S-467, increased the sensitivity of the Ca(2+)(i) mobilization response to 1.25 mm spermine. Bile flow ceased after Ca(2+)(o) withdrawal, and its recovery was enhanced by spermine in isolated perfused liver preparations. The CASR agonists Ca(2+) and Gd(3+) increased bile flow, and the response to a submaximal Ca(2+) concentration was enhanced by NPS R-467 but not the S compound. Thus, the data indicate that rat hepatocytes harbor a CASR capable of mobilizing Ca(2+)(i) from IP(3)-sensitive stores and that activation of the CASR stimulates bile flow.

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.

The anesthetic isoflurane decreases ionized calcium and increases parathyroid hormone and osteocalcin in cynomolgus monkeys

The effects of anesthetics on calcium metabolism in cynomolgus monkeys were studied. Eight adult female cynomolgus monkeys were used in a crossover design. Blood was collected from each of the monkeys at four timepoints: (1) while conscious; (2) following induction of anesthesia with ketamine, ketamine and atropine, isoflurane, or no anesthetic; (3) at 30 min; and (4) 120 min thereafter. Four experiments were performed with a 1 week washout period between sessions, such that each monkey received each treatment. Potassium was lower in anesthetized monkeys than in those that remained conscious. Cortisol, although high, did not differ among anesthetic treatments. Ketamine and ketamine/atropine did not consistently affect ionized calcium or parathyroid hormone (PTH) concentrations. Isoflurane decreased ionized calcium (0.05 mmol/L), and increased PTH and osteocalcin twofold. The serum inorganic fluoride concentration was higher in monkeys anesthetized with isoflurane than with ketamine/atropine, which may partially account for the decrease in ionized calcium with isoflurane. The increases in PTH and osteocalcin are presumably secondary to the decrease in ionized calcium.

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.

Functional evidence for two types of parathyroid adenoma

OBJECTIVE

The carboxyterminal parathyroid hormone (C-PTH)/intact (I-) PTH ratio is influenced by serum calcium concentrations in man, increasing to a maximum value in hypercalcaemia and decreasing to a minimum value in hypocalcaemia. We decided to use this ratio to screen for parathyroid tumour with a normal sensitivity to calcium, symptomatic mainly through a mass effect.

DESIGN AND SUBJECTS

Nineteen patients with hypercalcaemia and elevated or inappropriate PTH, were studied in the basal state and during CaCl2 and Na2EDTA infusion and compared with 26 normal individuals. They all had one parathyroid adenoma removed surgically, and two remained hypercalcaemic.

RESULTS

In the basal state, the patients were hypercalcaemic (ionized calcium 1.44 +/- 0.12 vs. 1.23 +/- 0.03 mmol/l, P < 0.001) and had elevated PTH levels (I-PTH: 10.8 +/- 8.0 vs. 2.3 +/- 0.6 pmol/l, P < 0.001; C-PTH: 31.6 +/- 38.9 vs. 5.25 +/- 1.11 pmol/l, P < 0.001) when compared with normals. Their mean C-PTH/I-PTH ratio was similar to normals (2.7 +/- 1.3 vs. 2.4 +/- 0.6, NS) but, when individual values were considered, three patients had elevated values at 4.9, 5.3 and 5.8 (normal = 1.2-3.6). The regression line between basal C- and I-PTH revealed a significantly higher slope in these patients (P < 0.0001). The 16 patients with a normal basal C-PTH/I-PTH ratio had, as a group, an increased set point of I- or C-PTH stimulation by calcium and increased values of stimulated and non-suppressible I- and C-PTH, but these abnormalities were not all present in the smaller tumours (< or = 200 mg). Only three tumours in that group were larger than 1000 mg. Serum calcium concentration was related to the increased set point and non-suppressible fraction of I-PTH in these patients (r2 = 0.797). The three patients with a high basal C-PTH/I-PTH ratio had large tumours (2346, 4364 and 17,300 mg) and were more difficult to study, requiring a larger decrease in calcium concentration to achieve maximal stimulation. In the basal state, they were already expressing a non-suppressible level of I- or C-PTH and already had a maximal C-PTH/I-PTH ratio. Our data further suggest a normal set point of I- and C-PTH stimulation in the two patients who achieved sufficient hypocalcaemia and a normal set point of C-PTH/I-PTH ratio modulation in these three patients. Their hypercalcaemia was essentially related to the non-suppressible fraction of PTH. Furthermore, larger tumours were less active than smaller ones and produced less stimulated I-PTH/100 mg of tissue.

CONCLUSIONS

These data indicate two types of parathyroid tumours when calcium sensitivity is considered: (1) a majority of small tumours with abnormal sensitivity to calcium, symptomatic through an abnormal set point and an increased non-suppressible fraction and (2) a smaller number of larger tumours, with normal sensitivity to calcium and an increased non-suppressible fraction, of PTH.