Canine renal receptors for parathyroid hormone. Down-regulation in vivo by exogenous parathyroid hormone

Profile of changes in bone turnover markers during once-weekly teriparatide administration for 24 weeks in postmenopausal women with osteoporosis

SUMMARY

Changes in bone turnover markers with weekly 56.5 μg teriparatide injections for 24 weeks were investigated in women with osteoporosis. Changes in bone turnover markers 24 h after each injection of teriparatide were constant. During the 24 week period, bone formation markers increased and baseline bone resorption marker levels were maintained.

INTRODUCTION

This study aimed to clarify the changes in bone turnover markers during 24 weeks of once-weekly teriparatide injections in postmenopausal women with osteoporosis.

METHODS

The 24 h changes in pharmacokinetics (PK), calcium metabolism, and bone turnover markers (serum osteocalcin, procollagen type I N-terminal propeptide (P1NP), urinary cross-linked N-telopeptide of type I collagen (NTX), deoxypiridinoline (DPD)) after each injection of 56.5 μg teriparatide at the data collection weeks (0, 4, 12, and 24 weeks) were investigated. The changes were evaluated by comparison with the data at 0 h in each data collection week.

RESULTS

Similar 24 h changes in each parameter after injection of teriparatide were observed in each data collection week. Serum calcium increased transiently, and intact PTH decreased 4-8 h after injection; serum calcium subsequently returned to baseline levels. Calcium and intact PTH levels decreased for 24 weeks. Although serum osteocalcin decreased at 24 h, it was significantly increased at 4 weeks. P1NP decreased transiently and then increased significantly at 24 h. P1NP was significantly increased at 4 weeks. Urinary NTX and DPD were significantly increased transiently and then decreased at 24 h. The urinary DPD level decreased significantly at 4 weeks.

CONCLUSIONS

Twenty-four hour changes in PK, calcium metabolism, and bone turnover markers showed the same direction and level after once-weekly teriparatide injections for 24 weeks, with no attenuation of the effect over time. After 24 weeks, the bone formation marker, serum osteocalcin, increased significantly, but the serum P1NP, did not. Bone resorption markers decreased or remained the same.

Up-regulation of parathyroid hormone-related protein in folic acid-induced acute renal failure

BACKGROUND

Parathyroid hormone (PTH)-related protein (PTHrP) is present in many normal tissues, including the kidney. Current evidence supports that PTHrP is involved in renal pathophysiology, although its role on the mechanisms of renal damage and/or repair is unclear. Our present study examined the changes in PTHrP and the PTH/PTHrP receptor (type 1) in folic acid-induced acute renal failure in rats. The possible role of PTHrP on the process of renal regeneration following folic acid administration, and potential interaction between angiotensin II (Ang II) and endothelin-1, and PTHrP, were examined in this animal model.

METHODS

PTHrP, PTH/PTHrP receptor, ACE, and preproendothelin-1 (preproET-1) mRNA levels in the rat kidney were analyzed by reverse transcription-polymerase chain reaction (RT-PCR) and/or RNase protection assay. Immunohistochemistry also was performed for PTHrP, the PTH/PTHrP receptor, and Ang II in the renal tissue of folic acid-injected rats. The role of PTHrP on tubular cell proliferation following folic acid injury was investigated in vitro in rat renal epithelial cells (NRK 52E). PTHrP secretion in the medium conditioned by these cells was measured by an immunoradiometric assay specific for the 1-36 sequence.

RESULTS

Using RT-PCR, PTHrP mRNA was rapidly (1 hour) and maximally increased (3-fold) in the rat kidney after folic acid, decreasing after six hours. At 72 hours, renal function was maximally decreased in these rats, associated with an increased PTHrP immunostaining in both renal tubules and glomeruli. In contrast, the PTH/PTHrP receptor mRNA (RNase protection assay) decreased shortly after folic acid administration. Moreover, PTH/PTHrP receptor immunostaining dramatically decreased in renal tubular cell membranes after folic acid. A single subcutaneous administration of PTHrP (1-36), 3 or 50 microg/kg body weight, shortly after folic acid injection increased the number of tubular cells staining for proliferating cell nuclear antigen by 30% (P < 0.05) or 50% (P < 0.01), respectively, in these rats at 24 hours, without significant changes in either renal function or calcemia. On the other hand, this peptide failed to modify the increase (2-fold over control) in ACE mRNA, associated with a prominent Ang II staining into tubular cell nuclei, in the kidney of folic acid-treated rats at this time period. The addition of 10 mmol/L folic acid to NRK 52E cells caused a twofold increase in PTHrP mRNA at six hours, without significant changes in the PTH/PTHrP receptor mRNA. The presence of two anti-PTHrP antibodies, with or without folic acid, in the cell-conditioned medium decreased (40%, P < 0.01) cell growth.

CONCLUSIONS

Renal PTHrP was rapidly and transiently increased in rats with folic acid-induced acute renal failure, featuring as an early response gene. In addition, changes in ACE and Ang II expression were also found in these animals. PTHrP induces a mitogenic response in folic acid-damaged renal tubular cells both in vivo and in vitro. Our results support the notion that PTHrP up-regulation participates in the regenerative process in this model of acute renal failure and is a common event associated with the mechanisms of renal injury and repair.

3',5'-Cyclic adenosine monophosphate activation in osteoblastic cells: effects on parathyroid hormone-1 receptors and osteoblastic differentiation in vitro

PTH has anabolic and catabolic effects in bone through activation of the PTH-1 (PTH/PTHrP) receptor and the cAMP/protein kinase A pathway. The effects of agents that regulate cAMP in nontransformed osteoblasts in relation to cell differentiation have not been described. The purpose of this study was to determine the effects of PTH fragments with differing cAMP-stimulating activity, and nonPTH cAMP regulators on PTH-1 receptor expression and activity, and osteoblast differentiation in vitro using MC3T3-E1 and primary rat calvarial cells. PTH (1-34), but not PTH (53-84), (7-34), or PTHrP (107-139) treatment (24 h) resulted in down-regulation of steady-state messenger RNA for the PTH-1 receptor. Forskolin (a stimulator of cAMP accumulation) also down regulated the PTH-1 receptor, whereas 9-(tetrahydro-2-furyl) adenine (THFA) (an inhibitor of adenylyl cyclase) had no effect. Similarly, PTH (1-34) treatment for 48 h abolished PTHrP binding to cell surface receptors; however, neither the PTH analogs nor the cAMP regulating agents altered PTH binding or numbers of binding sites on osteoblastic cells. Basal levels of cAMP were reduced in cultured cells treated for 6 days with PTH (7-34) or THFA compared with controls. In contrast, PTH-stimulated cAMP levels were significantly increased in cultures treated with PTH (7-34) and THFA for 6 days during osteoblast differentiation and were decreased in cultures treated with PTH (1-34) and forskolin compared with controls. To evaluate effects of the cAMP pathway on osteoblast differentiation, cultures were treated continuously with PTH analogs and cAMP regulators during an 18-day differentiation regime, total RNA was isolated at multiple time points, and Northern blot analysis for osteocalcin (OCN) was performed. THFA and PTH (7-34)-treated cultures had increased OCN expression; whereas, PTH (1-34) and forskolin reduced OCN expression. Interestingly, PTH (7-34) and THFA-treated cultures had increased mineralized nodule formation, in contrast to PTH (1-34) and forskolin treatment, which reduced nodule formation. Similarly, calcium accumulation in cultures was significantly increased in the PTH (7-34) and THFA-treated cultures and reduced in the PTH (1-34) and forskolin-treated cultures. These data demonstrate that agents that increase cAMP down regulate PTH-1 receptor messenger RNA and inhibit osteoblast differentiation in vitro. Agents that reduce or block adenylyl cyclase or cAMP activity do not alter PTH-1 receptor expression or binding, but have striking effects on promoting osteoblast differentiation. We conclude that many effects of PTH on osteoblasts may be mimicked or antagonized by agents that alter cAMP activity and bypass the PTH-1 receptor.

Megalin antagonizes activation of the parathyroid hormone receptor

Parathyroid hormone (PTH) is predominantly cleared from the circulation by glomerular filtration and degradation in the renal proximal tubules. Here, we demonstrate that megalin, a multifunctional endocytic receptor in the proximal tubular epithelium, mediates the uptake and degradation of PTH. Megalin was purified from kidney membranes as the major PTH-binding protein and shown in BIAcore analysis to specifically bind full-length PTH and amino-terminal PTH fragments (Kd 0.5 microM). Absence of the receptor in megalin knockout mice resulted in 4-fold increased levels of amino-terminal PTH fragments in the urine. In F9 cells expressing both megalin and the PTH/PTH-related peptide receptor (PTH/PTHrP receptor), uptake and lysosomal degradation of the hormone was mediated through megalin. Blocking megalin-mediated clearance of PTH resulted in 3-fold increased stimulation of the PTH/PTHrP receptor. These data provide evidence that megalin is involved in the renal catabolism of PTH and potentially antagonizes PTH/PTHrP receptor activity in the proximal tubular epithelium.

Episodic secretion of parathyroid hormone in postmenopausal women: assessment by deconvolution analysis and approximate entropy

In health young subjects, parathyroid hormone (PTH) is secreted presumptively in a dual fashion, with low amplitude pulses apparently superimposed on tonic secretion. In contrast, PTH secretion has not been as well characterized in postmenopausal women, and relationships among bone density, estrogen status, and PTH release have not been explored. It is possible that a pulsatile pattern of PTH secretion is important for bone remodeling, since exogenous PTH administered in a pulsatile manner stimulates bone formation. To assess the importance of pulsatile PTH secretion as a determinant of bone mass, we measured PTH in blood sampled every 2 minutes for 6 h in four groups of older women: (1) high bone density receiving estrogen (n = 6), (2) high bone density not receiving estrogen (n = 5), (3) low bone density receiving estrogen (n = 6), and (4) low bone density not receiving estrogen (n = 8). The plasma PTH release profiles were subjected to deconvolution analysis, which resolves measured hormone concentrations into secretion and clearance components, and to an approximate entropy (ApEn) estimate, which provides an ensemble measure of the serial regularity or orderliness of the release process. In postmenopausal subjects, PTH was secreted in a fashion similar to that observed in young adults, with significant tonic secretion and PTH pulse occurrences averaging every 18-19 minutes. Pulsatile PTH secretion accounted for approximately 25% of the total secreted PTH. There were no differences in the amplitude or frequency of pulsatile PTH secretory parameters or in ApEn values among the four groups or compared with young controls. We conclude that in postmenopausal women, PTH secretory patterns and temporal organization are similar to those in healthy young subjects and are not altered in states of low bone density or estrogen deficiency. This suggests that abnormalities in orderly pulsatile PTH secretion are unlikely to play a major role in established postmenopausal osteoporosis.

Changes in parathyroid hormone receptor binding affinity during egg laying: implications for calcium homeostasis in chicken

Parathyroid hormone (PTH) receptor bindings were examined in the membrane fraction of the calvaria and the kidney of the hen by the use of [125I]PTH-related protein (PTHrP) binding assays. The binding specificity, reversibility, and saturation of the receptor were demonstrated. The equilibrium dissociation constant (Kd) and the maximum binding capacity (Bmax) were obtained by Scatchard analyses. In both calvaria and kidney, Kd and Bmax values decreased at 3 h before oviposition in egg-laying hens, but not in nonlaying hens. Administration of 17 beta-estradiol or progesterone in vivo caused a decrease in the Kd and Bmax values. Ionized calcium concentrations in the blood plasma showed a decrease at 13 h before oviposition. The results suggest that the PTH receptor binding in the calvaria and the kidney is affected by ovarian steroid hormones and may play a role in maintaining the calcium homeostasis in the egg-laying hen.

The cytoplasmic tail of the G-protein-coupled receptor for parathyroid hormone and parathyroid hormone-related protein contains positive and negative signals for endocytosis

The present studies were done to evaluate the role of the cytoplasmic tail of the G-protein-coupled receptor for parathyroid hormone (PTH) and PTH-related protein (PTHrP) in the endocytosis of agonist-occupied receptors. PTH/PTHrP receptor mutants progressively truncated from the C terminus were expressed in COS-7 cells, and their ability to internalize 125I-PTHrP(1-34)amide was determined. Most of the C-terminal tail (91 of 127 residues) could be deleted without affecting internalization. However, further truncation removing residues 475-494 resulted in a 50-60% decrease in ligand internalization. A mutant with an internal deletion of these 20 amino acids showed a similar reduction in internalization, confirming the presence of a positive endocytic signal. No additional positive signals were found in the membrane-proximal region of the tail. However, alanine mutagenesis of the membrane-proximal residues 459-461 (EVQ-->AAA) resulted in a mutant PTH/PTHrP receptor displaying a 40% increase in ligand endocytosis, indicating that EVQ functions as a negative signal. Treatment of COS-7 cells with hypertonic sucrose (to disrupt clathrin lattices) markedly suppressed (by > 80%) PTH/PTHrP receptor internalization. These results demonstrate the presence of both positive and negative endocytic signals in the membrane-proximal cytoplasmic tail of the PTH/PTHrP receptor and suggest that these signals regulate the ability of the receptor to accumulate in clathrin-coated pits.

Studies on the mechanism of desensitization of the parathyroid hormone-stimulated calcium signal in UMR-106 cells: reversal of desensitization by alkaline phosphatase but not by protein kinase C downregulation

The involvement of protein kinase C (PKC), cAMP-dependent protein kinase (PKA), and other phosphorylation mechanisms in the rapid desensitization of the [Ca2+]i response to parathyroid hormone (PTH) stimulation was investigated in osteoblast-like UMR-106 cells. A 5 minute preincubation of the cell suspension with phorbol 12,13-dibutyrate (PDB) decreased the response to PTH in a concentration-dependent manner. 1-Oleoyl-2-acetyl-r-glycerol (OAG) pretreatment likewise decreased the PTH response. Staurosporine, a potent protein kinase inhibitor, completely prevented the desensitization caused by PDB. These PDB and staurosporine effects were also observed in 3 mM EGTA-containing medium ([Ca2+]free < 10(-8) M). A 5 minute pretreatment of cells with 1 microM forskolin had no effect on the calcium response to PTH. Homologous and PDB-induced desensitizations differed in several respects. Staurosporine pretreatment resulted in only a slight restoration of the PTH response under conditions of homologous desensitization. Chronic treatment with phorbol ester prevented the desensitization of the PTH response by acute phorbol treatment but not the homologous desensitization. Both homologous and PDB-induced desensitization were relieved by alkaline phosphatase treatment, consistent with the involvement of phosphorylation in the desensitization. This alkaline phosphatase effect on desensitization was inhibited by L-phenylalanine. These results suggest that PTH receptor homologous desensitization involves phosphorylation process(es) other than or in addition to those of PKC.

Dynamics of parathyroid hormone release and serum calcium regulation after surgery for primary hyperparathyroidism

Analysis of 14 patients with primary hyperparathyroidism (HPT) prior to and during the first year after parathyroid surgery disclosed that the operation was associated with rapid reductions of intact serum parathyroid hormone (PTH) and total serum and ionized plasma calcium values. A decreased urinary calcium excretion, a gradual elevation of renal calcium reabsorption, a transient reduction of serum calcitriol, and a late increase in 25-hydroxycholecalciferol values were also noted. Dynamic tests of parathyroid function by EDTA infusion and an oral calcium load revealed a sigmoidal relationship between serum PTH and calcium levels, and that parathyroid surgery induced considerable changes in both the position and slope of the dose-response curve. It was also apparent that PTH release was submaximally stimulated event at periods of hypocalcemia. The findings substantiate that adjustments of PTH release to acute alterations of serum calcium occur along the prevailing dose-response relationship, while stimuli being maintained for longer periods of time induce compensatory shifts in the position and slope of this curve. It is further suggested that unknown factors with PTH-like function may participate in the calcium regulation after surgery for primary HPT.

Down-regulation of adenylate cyclase-coupled response to native bovine parathyroid hormone and fragments in the osteoblast-like cell line UMR-106

1. Parathyroid hormone-induced down-regulation was studied in the osteosarcoma cell line UMR-106. 2. A maximal priming does of bPTH (1-84) down-regulated PTH-responsiveness to 40% of its initial value; bPTH (1-41) was less effective than bPTH (1-84), whereas bPTH (42-84) had no effect, alone or in combination with bPTH (1-41). 3. A tentative model for the function of different domains of parathyroid hormone in down-regulation is suggested.

Effects of continuous and intermittent administration and inhibition of resorption on the anabolic response of bone to parathyroid hormone

The role of resorption in the anabolic response of bone to parathyroid hormone (PTH) is not well understood. In contrast to the increase in bone mass induced by intermittent PTH in intact rats, continuous infusion of PTH into thyroparathyroidectomized (TPTX) rats failed to increase bone volume. The objective of this study were to determine if continuous infusions of low doses of PTH were anabolic in intact rats and if inhibition of resorption would enhance or block an anabolic action of PTH. Young male rats were treated with either continuous infusion or intermittent injections of hPTH-(1-34) for 12 days. In experiment 1, PTH, infused daily at 4 micrograms per 100 g, increased femur calcium and dry weight. Unlike infusion of 8 micrograms PTH, which did not alter bone mass, intermittent PTH at 8 micrograms was anabolic and increased bone mass by increasing trabecular thickness and number. Infusion of 16 micrograms induced hypercalcemia and death. In experiment 2, lower dose daily infusions of 0.25-4 micrograms PTH per 100 g did not increase bone mass. In experiment 3, in rats pretreated with dichloromethylene diphosphonate (Cl2MDP) to inhibit resorption and subsequently exhibiting decreased bone formation, PTH, irrespective of the method of administration, reversed the inhibitory effects of Cl2MDP on bone formation. Thus, intermittent and continuous PTH increase bone formation independently of effects on bone resorption, but only intermittent PTH increases bone mass consistently.

Binding and degradation of NH2-terminal parathyroid hormone by opossum kidney cells

The binding and cellular processing of NH2-terminal parathyroid (PTH) hormone by confluent monolayers of opossum kidney (OK) cells was characterized using radiolabeled PTH peptide analogues. Time- and temperature-dependent specific binding of 125I-labeled (Nle-8,18, Tyr-34)-NH2-bovine(b)PTH-(1-34) was accompanied by the appearance of degraded radiolabel in the cell medium. Degrading activity was observed to be a specific consequence of binding by PTH receptors. Degrading activity was inhibited by monensin, chloroquine, and NH4+ but not by chymotrypsin inhibitors. Acid washing demonstrated that greater than 80% of total cell-associated specific binding at equilibrium was located in a rapidly internalized (acid-resistant) pool. Monensin pretreatment led to increased acid-resistant binding, presumably through inhibition of turnover of internalized receptor ligand and indicated that the degradation of radiolabel was probably associated with processing of the receptor-ligand complex. Release of intact radiolabel from the acid-resistant pool indicated that some of the internalized peptide was recycled out of the cell in an undegraded form (retroendocytosis). Acid-resistant binding and degradation of 125I-(Nle-8,18, Tyr-34)-NH2-bPTH-(3-34) was minimal, indicating that this ligand was not internalized. It is concluded that the binding and internalization of PTH-(1-34) fragment by confluent OK cells is a specific receptor-mediated process. Cellular processing of PTH-(1-34) conforms to established models of internalization by receptor-mediated endocytosis.

Desensitization of parathyroid hormone receptors on cultured bone cells

Administration of excessive amounts of parathyroid hormone (PTH) in the treatment of osteoporosis can reverse the beneficial effects of a low-dose, intermittent regime. To investigate the direct actions and the possible cellular mechanisms of PTH in inducing desensitization of PTH receptors, we studied the effects of desensitization on rat osteoblastic UMR-106 cells. When the osteoblasts were preincubated with bPTH-(1-34), complete refractoriness to a subsequent challenge with the hormone developed within 1 h and at hormone concentrations as low as 5 nM. When osteoblasts thus desensitized were incubated in hormone-free medium, recovery of the cAMP responses began within 2 h and reached maximum after 16 h. Cycloheximide did not affect the process of desensitization. [Nle8,Nle18,Tyr34]bPTH-(3-34)amide significantly impaired the desensitization process by PTH-(1-34) but did not have stimulatory effect on cAMP responses. No significant heterologous desensitization was obvious after preincubation with isoprenaline (50 microM), prostaglandin E1 (50 microM), or prostaglandin E2 (50 microM) for 2 h. Binding experiments with [125I]PLP-(1-36)amide after desensitization revealed that there was an approximate twofold decrease in receptor affinities as analyzed by Scatchard analysis, showing that the decrease in affinity was prominent in the process of desensitization. When the cells were treated with monensin during desensitization, PTH challenge after desensitization produced significantly lower cyclic AMP responses. Recovery after desensitization occurred over a period of 16 h. Inclusion of monensin, but not cycloheximide, impaired the recovery. The results show that homologous desensitization of rat osteoblasts to PTH is brought about by the occupancy of receptors by PTH-(1-34) but not by cAMP generation itself.(ABSTRACT TRUNCATED AT 250 WORDS)

Desensitization to parathyroid hormone in renal cells from aged rats is associated with alterations in G-protein activity

Parathyroid hormone (PTH)-stimulated Na+/Ca2+ exchange activity, but not forskolin-sensitive Na+-dependent Ca2+ efflux, was blunted in renal cortical cells from aged rats. PTH-sensitive adenylate cyclase activity in renal membranes from senescent rats also declined, but forskolin-stimulated activity did not change. In addition, cholera toxin- and pertussis toxin-stimulated Na+-dependent Ca2+ efflux and cAMP formation were blunted in cells from aged animals. Further, cells from aged rats had decreased Gs-alpha and Gi-alpha proteins, as detected by ADP-ribosylation. These findings would be consistent with the proposal of an age-associated heterologous desensitization that involved the G-proteins. Serum concentrations of iPTH were increased in the old rat, suggesting that the desensitization to PTH in the aging rat represented an adaptive response to prolonged stimulation by the hormone. This hypothesis was supported by the findings that the attenuated PTH-sensitive Na+/Ca2+ exchange activity, cAMP formation, and adenylate cyclase activity in cells from old rats could be reversed by parathyroidectomy. The decreased label in cholera toxin-catalyzed ADP-ribosylated Gs-alpha and pertussis toxin catalyzed ADP-ribosylated Gi-alpha found in cells from aged rats was also largely negated by the surgery. In conclusion, the results suggest that the age-related blunting in the responses of renal cells to PTH was associated with a deficit in G-protein function and that this alteration could be reversed by removal of the parathyroid gland.

Parathyroid hormone receptors in human dermal fibroblasts: structural and functional characterization

Previous studies have established the presence of parathyroid hormone (PTH)-sensitive adenylate cyclase activity in cultured human skin fibroblasts. The present study was undertaken to identify and quantitate PTH receptors directly in such cells. Human dermal fibroblast cell line CRL 1564 was found to possess specific binding sites for [125I]PTH(1-34). These sites bound PTH selectively; bovine and human PTH(1-34) and PTH(1-84) competed for [125I]PTH(1-34) binding sites, whereas the unrelated peptides calcitonin, insulin, AVP, angiotensin II, and ACTH(1-24) were inactive even at micromolar concentrations. Competitive binding experiments demonstrated the presence of binding site heterogeneity. These data fit a "two-site" model (p less than 0.001) in which one binding component has high affinity (Kd = 2.5 ng/ml = 0.6 nM) and low capacity (10(4) sites/cell) while the other has low affinity (Kd = 5.9 micrograms/ml = 1.5 microM) and high capacity (greater than 10(7) sites/cell). Similar high- and low-affinity [125I]bPTH(1-34) binding sites were seen also in CRL 1564 membranes containing a PTH-responsive adenylate cyclase. The Kd of the high-affinity sites was identical to the concentration of unlabeled bPTH(1-34) (4.2 ng/ml = 1.0 nM) required to half-maximally elevated cyclic AMP in CRL 1564 cells. Affinity labeling of specific PTH binding sites revealed the presence of multiple components with Mrs of 85, 70, 40, 33, and 23 kD on SDS-PAGE. Competition experiments did not disclose structurally discrete high- and low-affinity sites. Thus, structurally homologous PTH receptors in human skin fibroblasts apparently can assume two affinity states: (i) a high-affinity state coupled to adenylate cyclase and (ii) a low-affinity state that may represent uncoupled receptors.

Properties of parathyroid hormone receptors on circulating bovine lymphocytes

Binding of parathyroid hormone (PTH) to circulating bovine lymphocytes was studied using, as the radioligand, a synthetic sulfur-free analog of bovine PTH, [Nle8,Nle18,Tyr34]bPTH-(1-34)amide, which was labeled to high specific activity with 125I and was purified by reverse-phase high-performance liquid chromatography. Binding of PTH to lymphocytes satisfies several criteria indicative of a specific interaction between the hormone and its receptor. Specific binding is saturable at 3.3 fmoles of radioligand bound per 10(7) cells, occurs more rapidly at 37 degrees C than at lower temperatures, and reaches equilibrium within 2 hr at 15 degrees C. Inhibition of specific binding occurs with intact PTH, with biologically active PTH analog or fragment, and with synthetic PTH antagonists, but not with biologically inactive PTH fragments, or peptide hormones unrelated to PTH antagonists, but not with biologically inactive PTH fragments, or peptide hormones receptors on lymphocytes and those previously reported with receptors in canine renal membranes, and on rat osteosarcoma cells. The dissociation constant (Kd) is approximately 10(-9) M, as calculated from the association and dissociation rate constants. This correlates very closely both with the apparent Kd, as estimated from Scatchard analysis of radioligand saturation and competition studies, and with previously reported Kd of PTH receptors in canine renal membranes and on intact rat osteosarcoma and opossum kidney cells. In addition, the relative binding affinity of intact hormone and a synthetic PTH agonist to to receptors on lymphocytes correlates closely with the relative biologic potency of these peptides in stimulating adenylate cyclase in membranes from these cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of 1,25-dihydroxyvitamin D on the skeletal resistance to parathyroid hormone

Hypocalcemia in chronic renal failure (CRF) has been attributed in part to a skeletal resistance (S.R.) to the calcemic action of parathyroid hormone (PTH) as a consequence of low levels of 1,25(OH)2D3. To further elucidate the role of 1,25(OH)2D3 in the genesis of S.R., the calcemic effect of infusion of synthetic b-PTH 1-34 was examined in dogs before and after 7, 90 and 180 days of CRF. The maximum increment in ionized calcium after the infusion of PTH in the normal dogs was 1.15 +/- 0.13 mg/dl, decreased to 0.84 +/- 0.09 after 7 days, 0.68 +/- 0.1 after 90 days and to 0.66 +/- 0.11 mg/dl after 180 days of CRF. Thereafter, the dogs received 1,25(OH)2D3, 0.5 microgram daily for seven days and the studies were repeated. No improvement in the calcemic response to PTH was observed (0.57 +/- 0.26 mg/100 ml). Subsequently, a parathyroidectomy (PTX) was performed and 24 hours later the studies were repeated. After the infusion of PTH the calcemic response returned to normal (0.93 +/- 0.14 mg/dl). Further studies were performed in a group of four uremic dogs receiving no 1,25(OH)2D3, in which a PTX was performed 24 hours prior to the infusion of PTH. For this group, the increase in serum ionized calcium was 0.99 +/- 0.11 mg/dl. These values were not statistically different from normal dogs. In summary: (1) renal insufficiency was characterized by an abnormal calcemic response to PTH; (2) the administration of 1,25(OH)2D3 did not correct the S.R. to PTH; and (3) PTX performed 24 hours before the infusion of PTH restored the calcemic response to normal. These data suggest that high levels of endogenous parathyroid hormone desensitized the skeleton to the administration of exogenous PTH. These studies indicate that low values of 1,25(OH)2D3 are not directly responsible for the skeletal resistance to PTH in chronic renal failure.

The effect of oral phosphate administration on major indices of skeletal metabolism in normal subjects

The effect of brief periods of phosphate administration on indices of human skeletal metabolism was investigated. Thirteen subjects (8 women, 5 men; 19-36 years old) received 2 g of oral phosphate daily for 5 days. Serum phosphorus rose 26% (3.8 +/- 0.1 mg/dl to 4.8 +/- 0.1 mg/dl; p less than .01) while total calcium fell (9.3 +/- 0.1 mg/dl to 8.9 +/- 0.1 mg/dl; p less than .01). Parathyroid hormone levels increased by 50% (14.1 +/- 2.0 pg/ml to 21.5 +/- 1.7 pg/ml; p less than .05) although values remained within the normal range. A persistent phosphaturia (0.64 +/- 0.10 g/g Cr to 1.8 +/- 0.4 g/g Cr; p less than .05) and a 69% fall in urinary calcium (80.8 +/- 10.0 mg/g Cr to 24.6 +/- 6.0 mg/g Cr; p less than .001) were observed. 1,25-dihydroxyvitamin D3 and urinary hydroxyproline concentrations did not change significantly but the bone gamma-carboxyglutamic acid protein (BGP) concentration rose 41% by day 2 (9.6 +/- 1.3 mg/ml to 13.5 +/- 2.2 mg/ml; p less than .005) and remained elevated throughout the study period. These results support the possibility that brief periods of phosphate administration may be useful in the therapy of disorders associated with low bone turnover, such as osteoporosis.

Renal cAMP and 1,25(OH)2D3 synthesis in estrogen-treated chick embryos and hens

Onset of sexual maturity in female chickens or administration of estrogen to mature males or to juveniles of either sex results in increased parathyroid hormone (PTH)-dependent adenylate cyclase activity and increased 25-hydroxyvitamin D3-1-hydroxylase activity in kidney. The relationship between estrogen-mediated alterations of these two enzyme systems was investigated in embryonic and mature, egg-laying chickens treated in vivo with 17 beta-estradiol (E2). Basal and PTH- and forskolin-stimulated adenylate cyclase activity in kidney plasma membrane preparations was not affected by E2 treatment of 19-day-old chick embryos or of 41-wk-old egg-laying females. High, possibly maximal, levels of catalytic activity in control embryos and hens may have precluded further stimulation by E2. In contrast, E2 significantly enhanced 25-hydroxyvitamin D3-1-hydroxylase activity of embryonic kidney up to 10-fold (P less than 0.005). In mature females, E2 caused cessation of egg laying accompanied by a significant reduction (P less than 0.005) of 25-hydroxyvitamin D3-1-hydroxylase activity. These results indicate that the PTH-dependent adenylate cyclase and the 25-hydroxyvitamin D3-1-hydroxylase systems of avian kidney can be regulated independently and suggest that factors in addition to estrogen are involved in their regulation.

Effect of renal function on renal responsiveness to parathyroid hormone in primary hyperparathyroidism and chronic renal failure

The effect of renal function on the cyclic AMP (cAMP) response to exogenous parathyroid hormone (PTH) was examined in patients with chronic renal failure (n = 22) and primary hyperparathyroidism (n = 19). In the patients with chronic renal failure there was marked resistance to the effect of exogenous PTH. In primary hyperparathyroidism the cAMP responses were variable; most of the patients with an abnormally small response having impaired renal function. After parathyroidectomy, responsiveness improved to varying degrees. In three patients repeatedly tested up to several months after parathyroidectomy, the recovery of responsiveness was a gradual process which began within days but did not, however, return to normal. Thus, there was an irreversible component to the resistance to PTH in these patients. A strong negative correlation between plasma creatinine and the cAMP response to PTH (P less than 0.001) was found in a group of patients, some with treated primary hyperparathyroidism and some with chronic renal failure. Thus, renal impairment is an important, but probably not the sole, contributory factor involved in the irreversible resistance to the action of PTH in hyperparathyroidism.

Vitamin D status regulates 25-hydroxyvitamin D3-1 alpha-hydroxylase and its responsiveness to parathyroid hormone in the chick

We asked this question: Under normal or near-normal metabolic conditions, does the prevailing normal or near-normal vitamin D status dampen the activity of 25-hydroxyvitamin-D3-1 alpha-hydroxylase (1 alpha-hydroxylase) such that it determines not only its "basal" activity but also its responsiveness to stimulation by increased circulating concentrations of parathyroid hormone (PTH)? To answer this question, we measured the activity of 1 alpha-hydroxylase in chicks, with and without administration of PTH, immediately before and during deprivation of vitamin D. Before deprivation of vitamin D, 1 alpha-hydroxylase activity increased only slightly with administration of PTH. With deprivation of vitamin D for 5 and 10 d, while the plasma concentrations of calcium and phosphorus persisted normal and unchanged, 1 alpha-hydroxylase activity not only increased progressively but also became sharply and increasingly responsive to stimulation by administration of PTH. But after 15 d of vitamin D deprivation, and the supervention of hypocalcemia, 1 alpha-hydroxylase activity was not further increased by the administration of PTH. With deprivation of vitamin D, the progressive increase in 1 alpha-hydroxylase correlated inversely with circulating levels of 1,25-dihydroxyvitamin D (1,25-[OH]2D), and the decreasing calcemic response to PTH correlated inversely with the responsiveness of 1 alpha-hydroxylase to PTH (in chicks deprived of vitamin D for 1-10 d). These results demonstrate that: under normal metabolic conditions, the normal vitamin D status regulates the activity of 1 alpha-hydroxylase so as to dampen both its "basal" activity and its responsiveness to stimulation by PTH; and vitamin D deprivation insufficient to cause hypocalcemia enhances both the "basal" activity of 1 alpha-hydroxylase and its responsiveness to stimulation by PTH. The results suggest that the normal dampening of 1 alpha-hydroxylase and both of the demonstrated enhancements of its activity are mediated by normal and reduced levels of circulating 1,25-(OH)2D, respectively. The finding that PTH fails to further stimulate 1 alpha-hydroxylase when vitamin D deprivation is sufficient in duration to cause hypocalcemia confirms the findings of other investigators and again demonstrates that observations made during abnormal metabolic circumstances may bear little on the physiologic regulation of 1 alpha-hydroxylase under normal or near-normal metabolic circumstances.