Characterization of the beta-adrenergic receptor mediating secretion of parathyroid hormone

In vitro incubation studies with bovine parathyroid gland slices compared the relative responsiveness of parathyroid hormone (PTH) secretion to isoprotherenol, epinephrine or norepinephrine. Isoproterenol was the most potent and norepinephrine the least potent of the three stimuli, suggesting a beta 2 type of an adrenergic response. However in this in vitro system, tazalol, a selective beta 1 adrenergic agonist significantly stimulated PTH secretion, whereas terbutaline, a selective beta 2 agonist had no effect. In addition, practolol, a selective beta 1 adrenergic antagonist blocked isoproterenol- or tazolol-stimulated PTH secretion. In vivo studies in normal human subjects showed that injection of te nonselective beta agonist, isoproterenol, (0.15 mg s.c.) significantly increased, whereas injection of the selective beta 2 agonist, terbulatine (0.3 mg s.c.) had no effect on serum PTH levels. These latter studies with putative selective beta adrenergic agents suggest that the beta adrenergic receptor mediating PTH secretion is of the beta 1 type (in contrast to the studies above with nonselective agents). The studies suggest that the beta adrenergic receptor mediating PTH secretion apparently differs from the classical beta 1 receptor described in th myocardium or the classical beta 2 receptor described in the bronchial smooth muscle.     

Parathyroid hormone 2 receptor is a functional marker of nociceptive myelinated fibers responsible for neuropathic pain

We have previously demonstrated that parathyroid hormone 2 (PTH2) receptors are expressed in dorsal root ganglion (DRG) neurons and that its endogenous agonist tuberoinfundibular peptide of 39 residues (TIP39) causes nociceptive paw flexor responses after intraplantar administration. Here we found that the PTH2 receptor is selectively localized on myelinated A‐, but not unmyelinated C‐fibers using immunohistochemical labeling, based on PTH2 receptor expression on antibody N52‐positive medium/large‐sized DRG neurons, but not on TRPV1, substance P, P2X₃ receptor or isolectin B4‐binding protein‐positive small‐sized DRG neurons. Pharmacological studies showed that TIP39‐induced nociceptive responses were mediated by activation of G_s and cAMP‐dependent protein kinase. We also found that nociceptive responses induced by TIP39‐ or the cAMP analog 8‐bromo‐cAMP were significantly greater following partial sciatic nerve injury induced neuropathic pain, without changes in PTH2 receptor expression. Together these data suggest that activation of PTH2 receptors stimulates nociceptive A‐fiber through G_s‐cAMP‐dependent protein kinase signaling, and this pathway has elevated sensitization following nerve injury.     

Effects of electrical stimulation of the parasympathetic nerve on the levels of cholinergic muscarinic and beta-adrenergic receptors and cyclic nucleotides in rat salivary glands

Density of muscarinic receptors of rat parotid gland, although unchanged after 5 or 10 min of stimulation of the parasympathetic nerve to the gland, showed a decrease of 23% following a 15-min period of stimulation. After 30 min the decrease was 19% but by 60 min density of receptors returned to within 5% of receptor density of the unstimulated gland; there was virtually no change in density of beta adrenoceptors at any time during the 60 min of stimulation. Markedly elevated (30-fold increase) levels of cyclic GMP appeared within 5 min after initiation of nerve stimulation and remained at this level at 10 min, but dropped from 90 to 46 pmol/mg total protein by 15 min, the time at which a decrease in muscarinic receptors first was evident. GMP levels continued to decrease but were still four times basal levels after 60 min of stimulation and did not return to normal concentration until 120 min. Cyclic AMP was generally unchanged. These changes in muscarinic receptors and cyclic GMP are apparently closely related to the kind of neural stimulation, unlike the condition when stimulation of the sympathetic nerve was employed.     

Beta adrenergic receptor repopulation of C6 glioma cells after irreversible blockade and down regulation

C6 glioma cells possess beta adrenergic receptors coupled with adenylate cyclase which can be irreversibly blocked by bromoacetylaminomethylpindolol (Br-AAM-pindolol), a beta adrenergic antagonist. With 1 microM Br-AAM-pindolol, more than 80% of beta adrenergic receptors, labeled by (3H)-dihydroalprenolol [3H)-DHA), were blocked. After this blockade, new beta adrenergic receptors were synthesized only during cell division. However, at cell confluency when the cell number was constant, turnover of beta adrenergic receptors was barely detectable. Cycloheximide (1 microgram/ml) inhibited cell growth as well as reappearance of beta adrenergic receptors. A 90% loss of beta adrenergic receptors in C6 glioma cells was obtained after down-regulation for 15 h with 10 microM isoproterenol, a beta adrenergic agonist. After removal of the agonist, recovery of beta-adrenergic-sensitive adenylate cyclase was complete within 2 to 3 days, whereas beta adrenergic receptors reached 90% of control value within 6 days. The half-life of the receptor recovery was 2 to 3 days. Pretreatment of C6 glioma cells by Br-AAM-pindolol and subsequent cell exposure to isoproterenol indicated that down regulation and recovery of unblocked beta adrenergic receptors did occur; however isoproterenol did not accelerate the biosynthesis of beta adrenergic receptors. The recovery of both biological response and beta adrenergic receptor occupancy was restored both in the presence or absence of cycloheximide (1 microgram/ml), a concentration which blocked 90% of protein synthesis. Our results suggest that reappearance of beta adrenergic receptors in C6 glioma cells, following isoproterenol-induced down regulation, was not due to synthesis of new receptors but to recycling of the beta adrenergic receptors.     

Dopamine D1 receptors in human parathyroid gland: In vitro and in vivo studies

Dopamine receptors in human parathyroid were studied in vitro using ligand binding techniques. With 3H-piflutixol as ligand, binding characteristic of the dopamine D1 receptor was observed. Administration of apomorphine, flupenthixol or metoclopramide to normal controls or acute schizophrenic patients at doses producing significant alterations in serum prolactin concentrations did not alter serum parathyroid hormone (PTH) concentrations. Whilst D1 binding sites are present in human parathyroid, the measurement of PTH after administration of dopaminergic drugs is unlikely to provide a test of D1 receptor function in man.     

Primary hyperparathyroidism in an adult female olive baboon (Papio anubis)

During an annual physical examination, a middle-aged adult female olive baboon (Papio anubis) in the time-mated breeding colony at the Biologic Resources Laboratory at the University of Illinois at Chicago was found to have a high serum calcium value (> 12 mg/dl). To determine the cause of the hypercalcemia, additional diagnostic tests, including thoracic and abdominal radiographs and a parathyroid panel (parathyroid hormone (PTH), ionized calcium, and parathyroid hormone-related protein (PTH-rp) assays), were performed. The radiographs did not reveal lesions suggestive of neoplasia. A parathyroid panel was obtained twice. Both times the PTH (23.4 and 46.4 pmol/L, normal = 2.91 to 4.57 pmol/L) and ionized calcium (1.68 and 2.10 mmol/L, normal = 1.31 to 1.37 mmol/L) were increased above values for adult females with normal calcium concentration. A tentative diagnosis of primary hyperparathyroidism was made. After a gamma-radiation scan and magnetic resonance imaging of the neck were done, exploratory surgery was performed to identify and remove the affected gland. After gland removal, the baboon's serum calcium, PTH (1.6 pmol/L), and ionized calcium (1.59 mmol/L) values decreased. Results of histologic examination confirmed the diagnosis of benign solitary parathyroid adenoma.     

Disruption of parathyroid hormone and parathyroid hormone-related peptide receptor phosphorylation prolongs ERK1/2 MAPK activation and enhances c-fos expression

Previous studies have demonstrated that parathyroid hormone (PTH) binding to the PTH/PTH-related peptide receptor (PPR) stimulates G protein coupling, receptor phosphorylation, β-arrestin translocation, and internalization of the ligand/receptor complex. The extracellular signal-regulated mitogen-activated protein kinases 1/2 (ERK1/2 MAPK) are downstream effectors of PPR. In the current study, we investigated the role of PPR phosphorylation in the PTH regulation of the ERK1/2 MAPK pathway. Short treatment with PTH (0-40 min) of LLCP-K(1) cells stably expressing a wild-type (WT) or a phosphorylation-deficient (PD) PPR (WT-PPR or PD-PPR cells, respectively) results in similar activation of ERK1/2. Interestingly, PTH stimulation of ERK1/2 in the WT-PPR cells then decreases as a result of longer PTH (60 min) treatment, and inhibition of ERK1/2 by PTH is observed at 90 min. Strikingly, the PD-PPR cells exhibit prolonged ERK1/2 activation up to 90 min of PTH treatment. An ERK1/2-dependent increase in c-fos expression is observed in the PD-PPR cells. Subsequently, c-fos expression in the WT-PPR and PD-PPR cells was markedly attenuated by a specific ERK1/2 pathway inhibitor. Further investigations revealed that PTH treatment causes a robust recruitment of a green fluorescent protein-tagged β-arrestin2 (β-arrestin2-GFP) in the WT-PPR cells. In contrast, β-arrestin2 recruitment was reduced in the PD-PPR cells. Importantly, expression of a receptor phosphorylation-independent β-arrestin2 (R169E) in the PD-PPR cells restored the biphasic effect of PTH on ERK1/2 as in the WT-PPR cells. The study reports a novel role for receptor phosphorylation and β-arrestin2 in the subsequent inhibition of the ERK1/2 pathway and in control of gene expression.     

Transforming growth factor beta enhances parathyroid hormone stimulation of adenylate cyclase in clonal osteoblast-like cells

The effects of transforming growth factor beta (TGF beta) on parathyroid hormone (PTH)-responsive adenylate cyclase were examined in clonal rat osteosarcoma cells (UMR-106) with the osteoblast phenotype. Purified TGF beta incubated with UMR-106 cells for 48 hr produced a concentration-dependent increase in PTH stimulation of adenylate cyclase, with maximal increase in PTH response (37%) occurring at 1 ng/ml TGF beta. TGF beta also enhanced receptor-mediated activation of adenylate cyclase by isoproterenol and prostaglandin E2 (PGE2) and nonreceptor-mediated enzyme activation by cholera toxin and forskolin. In cells in which PTH-stimulated adenylate cyclase activity was augmented by treatment with pertussis toxin, the incremental increase in PTH response produced by TGF beta was reduced by 33%. However, TGF beta neither mimicked nor altered the ability of pertussis toxin to catalyze the ADP-ribosylation of a 41,000-Da protein, presumably the alpha subunit of the inhibitory guanine nucleotide-binding regulatory component (Gi) of adenylate cyclase, in cholate-extracted UMR-106 cell membranes. TGF beta also had no effect on the levels of alpha or beta subunits of Gi, as assessed by immunotransfer blotting. In time course studies, brief (less than or equal to 30 min) exposure of cells to TGF beta during early culture was sufficient to increase PTH response but only after exposed cells were subsequently allowed to grow for prolonged periods. TGF beta enhancement of PTH and isoproterenol responses was blocked by prior treatment of cells with cycloheximide but not indomethacin. The results suggest that TGF beta enhances PTH response in osteoblast-like cells by action(s) exerted at nonreceptor components of adenylate cyclase. The effect of TGF beta may involve Gi, although in a manner unrelated to either pertussis toxin-catalyzed ADP-ribosylation of the alpha subunit of Gi or changes in levels of Gi subunits. The regulatory action of TGF beta on adenylate cyclase is likely to be mediated by the rapid generation of cellular signals excluding prostaglandins, followed by a prolonged sequence of events involving protein synthesis. These observations suggest a mechanism by which TGF beta may regulate osteoblast responses to systemic hormones.     

Morphological study of the parathyroid gland and thyroid C cell in senescence-accelerated mouse (SAMP6), a murine model for senile osteoporosis

SAMP6, a substrain of senescence-accelerated mouse, was developed as an animal model for senile osteoporosis. We investigated the morphology of the parathyroid gland and thyroid C cell, together with the serum parathyroid hormone (PTH) and calcitonin (CT) in SAMP6 and age-matched normal mice SAMR1. We did not find any significant differences between SAMR1 and SAMP6 at 1 month of age with regard to the serum PTH level and the morphology of the parathyroid glands. As compared with SAMR1, the serum PTH level was significantly higher in SAMP6 at 2, 5 and 12 months of age. In the parathyroid chief cells of SAMP6 at 2, 5 and 12 months of age, the Golgi complexes and the cisternae of the granular endoplasmic reticulum were well developed. Numerous secretory granules were located near the plasma membranes and mitoses were sometimes observed. There was no marked difference between SAMR1 and SAMP6 regarding the morphology of the thyroid C cells and the serum CT level. These findings suggest that the secretory activity of the parathyroid gland is stimulated in SAMP6 at 2, 5 and 12 months of age. The parathyroid follicle was sometimes found in SAMP6, and the significance of this structure was also discussed.     

Regulator of G protein signaling 5 is highly expressed in parathyroid tumors and inhibits signaling by the calcium-sensing receptor

The molecular mechanisms responsible for aberrant calcium signaling in parathyroid disease are poorly understood. The loss of appropriate calcium-responsive modulation of PTH secretion observed in parathyroid disease is commonly attributed to decreased expression of the calcium-sensing receptor (CaSR), a G protein-coupled receptor. However, CaSR expression is highly variable in parathyroid adenomas, and the lack of correlation between CaSR abundance and calcium-responsive PTH kinetics indicates that mechanisms independent of CaSR expression may contribute to aberrant calcium sensing in parathyroid disease. To gain a better understanding of parathyroid tumors and the molecular determinants that drive parathyroid adenoma development, we performed gene expression profiling on a panel of 64 normal and neoplastic parathyroid tissues. The microarray data revealed high-level expression of genes known to be involved in parathyroid biology (PTH, VDR, CGA, CaSR, and GCM2). Moreover, our screen identified regulator of G protein signaling 5 (RGS5) as a candidate inhibitor of CaSR signaling. We confirmed RGS5 to be highly expressed in parathyroid adenomas relative to matched-pair normal glands. Transient expression of RGS5 in cells stably expressing CaSR resulted in dose-dependent abrogation of calcium-stimulated inositol trisphosphate production and ERK1/2 phosphorylation. Furthermore, we found that RGS5-nullizygous mice display reduced plasma PTH levels, an outcome consistent with attenuated opposition to CaSR activity. Collectively, these data suggest that RGS5 can act as a physiological regulator of calcium sensing by CaSR in the parathyroid gland. The abnormally elevated expression of RGS5 observed in parathyroid adenomas could thus represent a novel mechanism of CaSR desensitization in patients with primary hyperparathyroidism.     

Substance P regulates PTH secretion through the neurokinin-1 receptor

The primary regulator of PTH secretion is serum ionized Ca(2+); however, neuropeptide-containing nerve fibers have been localized to the parathyroid gland. The purpose of this study was to determine whether or not substance P (SP) regulates PTH secretion. In dispersed porcine parathyroid cells, SP reversibly inhibited 0.5 mM CaCl(2)-induced PTH secretion (IC(50) = 0.29 nM) and had no effect at CaCl(2) concentrations of 1.5 mM and greater. At 0.5 mM CaCl(2), treatment with a NK-1 selective receptor agonist resulted in a concentration-dependent decrease in PTH secretion (IC(50) = 0.21 nM). In contrast, NK-2 and NK-3 receptor agonists were approximately 100-fold less active than SP or the NK-1 receptor selective agonist. An enantiospecific reversal of the effects of SP on PTH secretion was observed with LY306740, a potent selective NK-1 receptor antagonist (K(i) = 0.125 nM). In porcine parathyroid cells, expression of mRNA for the NK-1 receptor was observed using RT-PCR. In summary, a novel neuroendocrine pathway is described whereby the neuropeptide, SP, regulates PTH secretion through NK-1 receptors.     

Effects of parathyroid hormone on cyclic AMP, cyclic GMP, and efflux of calcium in isolated renal tubules.

The effects of parathyroid hormone (PTH) on concentrations of cyclic AMP and cyclic GMP were investigated in isolated renal cortical tubules from hamsters. Efflux of 45Ca from tubules was compared to temporal changes in both cyclic nucleotide concentrations. A rapid increase in cyclic AMP occurred following addition of PTH which was maximal by 1 min but decreased over the next 4 min period. Cyclic GMP concentrations were not significantly altered at 1 min but increased between 1 and 5 min from basal levels. Concentrations of both nucleotides remained significantly elevated from basal levels between 5 and 15 min following PTH. Efflux of 45Ca was increased by PTH with time-course changes closely paralleling changes in cyclic GMP concentrations. Changes in both cyclic AMP and cyclic GMP were related to PTH concentrations of the incubation media and were increased by addition of theophylline. Increasing the calcium concentration from 1 to 3 mM did not significantly alter the effect of PTH on cyclic AMP, however, cyclic GMP concentrations were further increased.     

Bone microenvironment-related growth factors, zoledronic acid and dexamethasone differentially modulate PTHrP expression in PC-3 prostate cancer cells.

Bone metastasis microenvironment-related growth factors such as insulin-like growth factor 1 (IGF-1), transforming growth factor beta 1 (TGF-beta1), basic fibroblast growth factor (bFGF) and interleukin 6 (IL-6) show survival factor activity, thereby inhibiting chemotherapy-induced apoptosis of PC-3 prostate cancer cells in vitro. Recently, zoledronic acid has been shown to induce apoptosis in PC-3 prostate cancer cells while overexpression of parathyroid hormone-related protein (PTHrP) inhibits serum deprivation-induced apoptosis in PC-3 cells. Consequently, we have investigated whether IGF-1, TGF-beta1, bFGF, IL-6, zoledronic acid and/or dexamethasone affect the expression of the PTHrP and type I PTH/PTHrP receptor (PTH.1R) in PC-3 prostate cancer cells using relative quantitative PCR and real-time PCR (expression at mRNA level) and immunocytochemical and immunofluorescence analysis (expression at protein level). Our data show that IGF-1, TGF-beta1, bFGF and IL-6 increase PTHrP mRNA expression and its perinuclear localization, while zoledronic acid (50 muM, 100 muM for 24 h and 48 h) and dexamethasone suppress PTHrP expression in PC-3 cells. We did not detect any appreciable change of the PTH.1R expression due to IGF-1, TGF- beta1, bFGF, IL-6, zoledronic acid or dexamethasone in PC-3 cells. Therefore, it is conceivable that bone metastasis microenvironment-related survival factor/anti-apoptotic activity and zoledronic acid anticancer action/pro-apoptotic activity on PC-3 cells is mediated, at least in part, by differential modulation of PTHrP expression.     

1,25-Dihydroxyvitamin D downregulation of TGFalpha/EGFR expression and growth signaling: a mechanism for the antiproliferative actions of the sterol in parathyroid hyperplasia of renal failure

Elevated serum levels of parathyroid hormone (PTH) contribute to the increased morbidity and mortality in renal failure patients. Parathyroid gland hyperplasia is a major cause of high serum PTH. The present studies used the rat model of renal failure to address the mechanisms underlying uremia-induced parathyroid hyperplasia and the antiproliferative properties of vitamin D therapy (1,25-dihydroxyvitamin D (1,25(OH)(2)D(3)) or its less calcemic analogs). Enhanced TGFalpha/EGFR co-expression is the major mitogenic signal in uremic parathyroid glands. At early stages of renal failure, vitamin D therapy efficiently counteracts uremia- and high phosphorus-induced hyperplasia by inhibiting the increases in parathyroid-TGFalpha/EGFR co-expression. In established hyperparathyroidism, characterized by highly enhanced-TGFalpha/EGFR co-expression, vitamin D therapy arrests growth by suppressing EGFR-growth signals from the plasma membrane and nuclear EGFR actions as a transactivator of the cyclin D1 gene, an important contributor to parathyroid hyperplasia in humans. In advanced renal failure, reduced-parathyroid vitamin D receptor levels limits the antiproliferative efficacy of vitamin D therapy. However, non-antiproliferative doses of 1,25-dihydroxyvitamin D enhance the anti-EGFR actions of EGFR-tyrosine kinase inhibitors (TKI). In fact, combined 1,25-dihydroxyvitamin D/TKI therapy inhibits parathyroid hyperplasia more efficiently than phosphorus restriction, the most powerful promoter of parathyroid growth arrest available at present.     

Clinical use of calcimimetics in the treatment of hyperparathyroidisms

Recognition of the role of the extracellular calcium sensing receptor (CaR) in mineral metabolism has greatly improved our understanding of calcium homeostasis. The activation of this receptor by small changes in the extracellular ionized calcium concentration (Ca(2+)ec) regulates parathormone (PTH) and calcitonin secretion, urinary calcium excretion and ultimately bone turnover. Cloning of CaR and discovery of mutations making the receptor less or more sensitive to calcium allowed a better understanding of several hereditary disorders characterized either by hyperparathyroidism or hypoparathyroidism. CaR became an ideal target for the development of compounds able to modulate the activity of CaR, activators (calcimimetics) as well as inhibitors (calcilytics). The calcimimetics are able to amplify the sensitivity of the CaR to Ca(2+)ec, suppressing PTH levels with a resultant fall in blood Ca2+. They dose-dependently reduce the secretion of PTH in vitro in cultured parathyroid cells, in animal models and in humans. In uremic animals, these compounds prevent parathyroid cell hyperplasia, normalize plasma PTH levels and bone remodelling. In uremic patients undergoing hemodialysis, the calcimimetics reduce plasma PTH concentration at short-term (12 weeks) as well as at long-term (2 years), serum calcium-phosphorus product and bone remodelling. After one year of treatment, these patients show a gain of bone mass of 2-3% at the femoral neck and at the total body. Contrarily, the calcilytics, by inhibiting CaR, can intermittently stimulate the secretion and the serum concentration of PTH. This results in an skeletal anabolic effect with a substantial increase in bone mineral density. They are potentially very interesting for the treatment of post-menopausal osteoporosis.     

Serum parathyroid hormone concentration measured by highly sensitive assay in post-thyroidectomy hypocalcemia of patients with Graves' disease

To investigate the role of parathyroid function in transient hypocalcemia after subtotal thyroidectomy for Graves' disease, the serum parathyroid hormone (PTH) concentration and nephrogenous (N) cAMP were measured in 16 patients before and after surgery. Serum PTH was measured with two commercially available kits (PTH-M, PTH-C), PTH-M is a recently developed highly sensitive assay using an antibody recognizing the mid-portion of human PTH and a synthetic 125I-tyr45-human PTH (43-68) as a radioligand. One of the 16 patients had severe clinical tetany and had a markedly lower PTH-M concentration and NcAMP after thyroidectomy. However, no significant change in serum PTH-M, PTH-C and NcAMP were observed in the other patients, although their serum calcium (Ca) concentrations decreased significantly. The Data were analyzed by dividing the patients according to the change in serum Ca or PTH. Serum PTH-M and PTH-C significantly decreased in 4 patients whose serum Ca clearly decreased after surgery. Serum Ca on the first postoperative day was significantly lower in patients whose serum PTH decreased after thyroidectomy than in patients whose serum PTH did not. Furthermore, the serum Ca concentration was significantly correlated with PTH-M, and with NcAMP on the third postoperative day. These data proved that hypofunction of the parathyroid gland is important in transient hypocalcemia after subtotal thyroidectomy for Graves' disease. The pathogenetic mechanism of transient hypocalcemia was discussed in comparison with the data from a patient who had overt parathyroid injury.     

Chondrogenic differentiation of clonal mouse embryonic cell line ATDC5 in vitro: differentiation-dependent gene expression of parathyroid hormone (PTH)/PTH-related peptide receptor

The regulatory role of parathyroid hormone (PTH)/PTH-related peptide (PTHrP) signaling has been implicated in embryonic skeletal development. Here, we studied chondrogenic differentiation of the mouse embryonal carcinoma-derived clonal cell line ATDC5 as a model of chondrogenesis in the early stages of endochondral bone development. ATDC5 cells retain the properties of chondroprogenitor cells, and rapidly proliferate in the presence of 5% FBS. Insulin (10 micrograms/ml) induced chondrogenic differentiation of the cells in a postconfluent phase through a cellular condensation process, resulting in the formation of cartilage nodules, as evidenced by expression of type II collagen and aggrecan genes. We found that differentiated cultures of ATDC5 cells abundantly expressed the high affinity receptor for PTH (Mr approximately 80 kD; Kd = 3.9 nM; 3.2 x 10(5) sites/cell). The receptors on differentiated cells were functionally active, as evidenced by a PTH-dependent activation of adenylate cyclase. Specific binding of PTH to cells markedly increased with the formation of cartilage nodules, while undifferentiated cells failed to show specific binding of PTH. Northern blot analysis indicated that expression of the PTH/PTHrP receptor gene became detectable at the early stage of chondrogenesis of ATDC5 cells, preceding induction of aggrecan gene expression. Expression of the PTH/PTHrP receptor gene was undetectable in undifferentiated cells. The level of PTH/PTHrP receptor mRNA was markedly elevated parallel to that of type II collagen mRNA. These lines of evidence suggest that the expression of functional PTH/PTHrP receptor is associated with the onset of chondrogenesis. In addition, activation of the receptor by exogenous PTH or PTHrP significantly interfered with cellular condensation and the subsequent formation of cartilage nodules, suggesting a novel site of PTHrP action.     

Decreased mRNA expression of the PTH/PTHrP receptor and type II sodium-dependent phosphate transporter in the kidney of rats fed a high phosphorus diet accompanied with a decrease in serum calcium concentration

This study investigates the phosphorus (P) homeostasis in the process of an altered parathyroid hormone (PTH) action in the kidney of rats fed a high P diet. Four-week-old male Wistar strain rats were fed diets containing five different P levels (0.3, 0.6, 0.9, 1.2 and 1.5%) for 21 days. The serum PTH concentration and urinary excretion of P were elevated with increasing dietary P level. Compared to rats fed the 0.3% P diet, the serum calcium (Ca) concentration remained unchanged, while the serum 1,25(OH)(2)D(3) concentration and urinary excretion of cAMP were elevated with increasing dietary P level in rats fed the high P diets containing 0.6-0.9% P. On the other hand, a lower serum Ca concentration was observed in rats fed the high P diets containing 1.2% or greater P. The serum 1,25(OH)(2)D(3) concentration remained unchanged in rats fed the high P diets containing 1.2% or greater P, comparison with rats fed the 0.3% P diet. The urinary excretion of cAMP and PTH/PTH-related peptide (PTHrP) receptor and type II sodium-dependent phosphate transporter (NaPi-2) mRNA in the kidney were both decreased in rats fed the high P diets containing 1.2% or greater P. In conclusion, a high P diet with subsequent decrease in serum Ca concentration suppressed the PTH action in the kidney due to PTH/PTHrP receptor mRNA down-regulation. Furthermore, an increase in the urinary excretion of P might have been caused by decreased NaPi-2 mRNA expression without the effects of PTH and 1,25(OH)(2)D(3).     

Serum parathyroid hormone: a double antibody radioimmunoassay.

A radioimmunoassay for parathyroid hormone (PTH) using a double antibody system is described. Because of the immunolgoical heterogeneity of the hormone in human serum, the standard used has been serum from a patient with parathyroid carcinoma. With the use of the synthetic 34 amino acid N-terminal fragment of PTH, the anti-PTH antiserum was determined to react primarily with the N-terminal end of the molecule. PTH was detectable in the sera of 25% of normal subjects and elevated in 18 of 19 patients with parathyroid adenoma and carcinoma. Serum PTH levels were elevated in 3 of 5 patients with parathyroid hyperplasia.